Documents published in 2020 : Scipedia

Marchetti flat dilatometer tests in a virtual calibration chamber
J. Butlanska, M. Arroyo, S. Amoroso, A. Gens

Geotechnical Testing Journal (2018). Vol. 41 (5), pp. 1-17 (Preprint)

Abstract
Calibration chambers are frequently used to verify, adapt, or both verify and adapt empirical relations between different state variables and in situ test results. Virtual calibration chambers (VCC) built with 3D discrete element models may be used to extend and partially substitute costly physical testing series. VCC are used here to explore the mechanics of flat dilatometer penetration and expansion. Results obtained for a simulation of physical tests in Ticino sand are presented. Blade tip resistance during penetration is in good agreement with the experiments. A piston-like design is used for the blade so that larger displacements may be applied than it is possible with a membrane. Initial piston pressures in the expansion curves are very low, strongly affected by the scaled-up grain sizes. Despite that difficulty, expansion curves may be easily interpreted to recover dilatometer moduli ED close to those observed in the physical experiments. Particle-scale examination of the results allows a firmer understanding of the current limitations and future potential of the technique. Copyright ASTM International. All rights reserved, 2018/16/07. Downloaded by UPC, pursuant to Author/Copyright Owner Agreement. No further reproduction authorized.
Abstract
Calibration chambers are frequently used to verify, adapt, or both verify and adapt empirical relations between different state variables and in situ test results. Virtual [...]
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A time-dependent anisotropic model for argillaceous rocks: application to an underground excavation in Callovo-Oxfordian claystone
M. Mánica, A. Gens, J. Vaunat, D. Ruiz

Computers and Geotechnics (2017). Vol. 85, pp. 341-350

Abstract
The paper presents a constitutive model for argillaceous rocks, developed within the framework of elastoplasticity, that includes a number of features that are relevant for a satisfactory description of their hydromechanical behaviour: anisotropy of strength and stiffness, behaviour nonlinearity and occurrence of plastic strains prior to peak strength, significant softening after peak, time-dependent creep deformations and permeability increase due to damage. Both saturated and unsaturated conditions are envisaged. The constitutive model is then applied to the simulation of triaxial and creep tests on Callovo-Oxfordian (COx) claystone. Although the main objective has been the simulation of the COx claystone behaviour, the model can be readily used for other argillaceous materials. The constitutive model developed is then applied, via a suitable coupled hydromechanical formulation, to the analysis of the excavation of a drift in the Meuse/Haute-Marne Underground Research Laboratory. The pattern of observed pore water pressures and displacements, as well as the shape and extent of the damaged zone, are generally satisfactorily reproduced. The relevance and importance of rock anisotropy and of the development of a damaged zone around the excavations are clearly demonstrated.
Abstract
The paper presents a constitutive model for argillaceous rocks, developed within the framework of elastoplasticity, that includes a number of features that are relevant for [...]
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Numerical analysis of the hydromechanical response of Callovo-Oxfordian claystone to deep excavations
D. Seyedi, A. Gens

Computers and Geotechnics (2017). Vol. 85, pp. 275-276 (Preprint)

Abstract
Deep geological disposal is considered as one of the promising options for the long-term management of the High-Level (HL) and Intermediate-Level Long-Lived (IL-LL) radioactive waste. Different argillaceous formations are studied in different countries for this purpose, such as, Opalinus clay in Switzerland, Boom clay in Belgium and Callovo-Oxfordian claystone in France.
Abstract
Deep geological disposal is considered as one of the promising options for the long-term management of the High-Level (HL) and Intermediate-Level Long-Lived (IL-LL) radioactive [...]
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Performance of the Opalinus Clay under thermal loading: experimental results from Mont Terri rock laboratory (Switzerland)
A. Gens, K. Wieczorek, I. Gaus, B. Garitte, J. Mayor, K. Schuster, G. Armand, J. García-Siñeriz, T. Trick

Swiss Journal of Geosciences (2017). Vol. 110 (1), pp. 269-286

Abstract
The final publication is available at Springer via http://dx.doi.org/10.1007/s00015-016-0258-8 The paper presents an overview of the behaviour of Opalinus Clay under thermal loading as observed in three in situ heating tests performed in the Mont Terri rock laboratory: HE-B, HE-D and HE-E. The three tests are summarily described; they encompass a broad range of test layouts and experimental conditions. Afterwards, the following topics are examined: determination of thermal conductivity, thermally-induced pore pressure generation and thermally-induced mechanical effects. The mechanisms underlying pore pressure generation and dissipation are discussed in detail and the relationship between rock damage and thermal loading is examined using an additional in situ test: SE-H. The paper concludes with an evaluation of the various thermo-hydro-mechanical (THM) interactions identified in the heating tests.
Abstract
The final publication is available at Springer via http://dx.doi.org/10.1007/s00015-016-0258-8 The paper presents an overview of the behaviour of Opalinus Clay under thermal [...]
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Numerical analysis of suction embedded plate anchors in structured clay
T. Charlton, M. Rouainia, A. Gens

Applied Ocean Research (2016). Vol. 61, pp. 156-166

Abstract
As offshore energy developments move towards deeper water, moored floating production facilities are increasingly preferred to fixed structures. Anchoring systems are therefore of great interest to engineers working on deep water developments. Suction embedded plate anchors (SEPLAs) are rapidly becoming a popular solution, possessing a more accurate and predictable installation process compared to traditional alternatives. In this paper, finite element analysis has been conducted to evaluate the ultimate pullout capacity of SEPLAs in a range of post-keying configurations. Previous numerical studies of anchor pullout capacity have generally treated the soil as an elastic-perfectly plastic medium. However, the mechanical behaviour of natural clays is affected by inter-particle bonding, or structure, which cannot be accounted for using simple elasto-plastic models. Here, an advanced constitutive model formulated within the kinematic hardening framework is used to accurately predict the degradation of structure as an anchor embedded in a natural soft clay deposit is loaded to its pullout capacity. In comparison with an idealised, non-softening clay, the degradation of clay structure due to plastic strains in the soil mass results in a lower pullout capacity factor, a quantity commonly used in design, and a more complex load–displacement relationship. It can be concluded that clay structure has an important effect on the pullout behaviour of plate anchors.
Abstract
As offshore energy developments move towards deeper water, moored floating production facilities are increasingly preferred to fixed structures. Anchoring systems are therefore [...]
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Fully coupled thermo-hydro-mechanical double-porosity formulation for unsaturated soils
M. Castilla, A. Gens, M. Villar, S. Olivella

International journal of geomechanics (2016). Vol. 16 (6), pp. D4016015-1/D4016015-17 (Preprint)

Abstract
This work presents a fully coupled formulation developed to handle engineering problems in unsaturated (and saturated) soils that present two dominant void levels. The proposed framework assumes the presence of two porous media linked through a mass-transfer term between them. In its more general form, the proposed approach allows the consideration of nonisothermal multiphase flow coupled with the mechanical problem. The double-porosity formulation was implemented in a finite-element code and has been used to analyze a variety of engineering problems. The approach is especially suitable for cases in which the material exhibits a strong coupling between the mechanical and the hydraulic problems in both media, such as with swelling clays. For those types of problems, the proposed formulation is used in conjunction with the mechanical double-structure model already proposed by the authors. This paper presents the coupled formulation and the application of the proposed approach to problems involving expansive unsaturated clays. Very satisfactory results were obtained in these analyses
Abstract
This work presents a fully coupled formulation developed to handle engineering problems in unsaturated (and saturated) soils that present two dominant void levels. The proposed [...]
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A numerical investigation of the incremental behavior of crushable granular soils
M. Cintia, M. Arroyo, F. Calvetti, A. Gens

International Journal for Numerical and Analytical Methods in Geomechanics (2016). Vol. 40 (13), pp. 1773-1798

Abstract
The mechanical behavior of granular materials is characterized by strong nonlinearity and irreversibility. These properties have been differently described by a variety of constitutive models. To test any constitutive model, experimental data relative to the nature of the incremental stress–strain response of the material is desirable. However, this type of laboratory data is scarce because of being expensive and difficult to obtain. The discrete element method has been used several times as an alternative to obtain incremental responses of granular materials. Crushable grains add one extra source of irreversibility to granular materials. Crushability has been variously incorporated into different constitutive models. Again, it will be helpful to obtain incremental responses of crushable granular materials to test these models, but the experimental difficulties are increased. Making use of a recently introduced crushing model for discrete element simulation, this paper presents a new procedure to obtain incremental responses in discrete analogs of granular crushable materials. The parallel probe approach, previously used for uncrushable discrete analogs, is here extended to account for the presence of crushable grains. The contribution of grain crushing to the incremental irreversible strain is identified and separately measured. Robustness of the proposed method is examined in detail, paying particular attention to aspects such as dynamic instability or crushing localization. The proposed procedure is later applied to map incremental responses of a discrete analog of Fontainebleau sand on the triaxial plane. The effect of stress ratio and granular state on plastic flow characteristics is highlighted.
Abstract
The mechanical behavior of granular materials is characterized by strong nonlinearity and irreversibility. These properties have been differently described by a variety of [...]
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Exploring ice content on partially saturated frozen soils using dielectric permittivity and bulk electrical conductivity measurements
Y. Mao, E. Romero, A. Gens

E3S Web of Conferences (2016). Vol. 9, pp. 1-9

Abstract
The present paper explores the use of the measurement of bulk electric conductivity and relative dielectric permittivity during freezing and thawing to estimate the ice content of two different partially saturated soils. The soils (Castelldefels fine sand and Barcelona clayey silt) were prepared at different degrees of saturation and dry densities at room temperature. Different target temperatures were used, which were reached at varying freezing rates and controlled with a thermal bath. Models were used to fit electrical conductivity and relative dielectric permittivity results under full unfrozen liquid and maximum ice conditions. The extension of these models allows estimating ice content on partially saturated soils at different porosities.
Abstract
The present paper explores the use of the measurement of bulk electric conductivity and relative dielectric permittivity during freezing and thawing to estimate the ice content [...]
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Anisotropic behaviour of compacted clayey silt subjected to hydromechanical paths
C. Buenfil, E. Romero, A. Lloret, A. Gens

E2S Web of Conferences (2016). Vol. 9, pp. 1-6

Abstract
Compaction induces anisotropy on soil deformational response due to preferential straining associated with the fabrication process. An experimental insight into the stress-strain response of Barcelona clayey silt is provided here. The material was statically compacted at low dry density to induce high collapsibility upon wetting (dry density 1.48 Mg/m3, water content 12%). Two types of controlled-suction tests were carried out: a) constant suction radial paths on as-compacted samples following different stress ratios (deviator stress / mean net stress: q/p Compaction induces anisotropy on soil deformational response due to preferential straining associated with the fabrication process. An experimental insight into the stress-strain response of Barcelona clayey silt is provided here. The material was statically compacted at low dry density to induce high collapsibility upon wetting (dry density 1.48 Mg/m3, water content 12%). Two types of controlled-suction tests were carried out: a) constant suction radial paths on as-compacted samples following different stress ratios (deviator stress / mean net stress: q/p¿, including isotropic and K0 stress paths; and b) the same type of radial stress paths but performed after saturation under low confining stress. These paths were intended to define the as-compacted and saturated yield surfaces. The experimental data on as-compacted state is satisfactorily interpreted adopting an anisotropic yield surface with an inclination linked to the compaction under oedometer conditions. Development of shear strain was observed during isotropic loading of the compacted samples. This anisotropy that the soil exhibits at the end of the compaction vanishes after collapse by saturation and as plastic volumetric straining develops
Abstract
Compaction induces anisotropy on soil deformational response due to preferential straining associated with the fabrication process. An experimental insight into the stress-strain [...]
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Thrust and torque components on mixed-face EPB drives
C. González, M. Arroyo, A. Gens

Tunnelling and Underground Space Technology (2016). Vol. 57, pp. 47-54

Abstract
In this communication we propose a method to estimate net thrust and torque applied at the excavation front by cutting tools from EPB machines when they are working in mixed faces constituted by soils and soft rock. The method is inspired by similar simplifications used for the analysis of TBM drives in rock. The proposal is validated using a database of EPB registers which were gathered from more than 35 km of tunnel drives excavated in soils, soft rock and heterogeneous media. The results allow us to assess the influence of the type of ground excavated and its geotechnical properties on the net thrust and torque applied.
Abstract
In this communication we propose a method to estimate net thrust and torque applied at the excavation front by cutting tools from EPB machines when they are working in mixed [...]
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Analysis of unsaturated materials hydration incorporating the effect of thermo-osmotic flow
M. Castilla, C. Arson, A. Gens, F. Aponte

Geomechanics for energy and the environment (2016). Vol. 6, pp. 101-115

Abstract
The geological disposal of a high level radioactive waste relies in a system composed of engineered and geological barriers. The soils and rocks involved in the design of this type of solution are generally initially unsaturated and subject to complex thermal, hydraulic and mechanical (THM) coupled phenomena triggered by the simultaneous heating and hydration of the barrier materials under confined conditions. Mathematical THM formulations are typically used to analyze the behavior and long term performance of the barriers system. These types of formulations generally do not include some coupled processes, for example thermo-osmosis (i.e. the movement of liquid water induced by gradient of temperature), because they are considered not significant when compared against the main or direct processes (e.g., Darcy’s, Fourier’s and Fick’s laws). In this work, the potential effects of thermo-osmotic phenomenon is studied in detail. Typical flow equations are modified to include thermo-osmotic flows and then they are implemented in numerical simulators. Two case studies are analyzed. The first one focuses on a simple and already proposed model to study the behavior of a geological barrier for nuclear waste when subjected to heating and hydration. The other case corresponds to the study of an engineered clay barrier material in the laboratory subjected to hydraulic and thermal gradients similar to the ones expected in real repository conditions. In both cases the analyses with and without thermo-osmotic flows are compared. From these comparisons it is observed that the effect of thermo-osmosis can be quite significant. Thermo-osmotic effects also assisted to explain the apparent low wetting observed in the hydration of a clayey barrier material.
Abstract
The geological disposal of a high level radioactive waste relies in a system composed of engineered and geological barriers. The soils and rocks involved in the design [...]
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Artificial ground freezing of a volcanic ash: laboratory tests and modelling
F. Casini, A. Gens, S. Olivella, G. Viggiani

Environmental geotechnics (2016). Vol. 3 (3), pp. 1-14 (Preprint)

Abstract
The use of artificial ground freezing (AGF) to form earth support systems has had applications worldwide. These cover a variety of construction problems, including the formation of frozen earth walls to support deep excavations, structural underpinning for foundation improvement and temporary control of ground water in construction processes. On one hand, the main advantage of AGF as a temporary support system in comparison to other support methods, such as those based on injections of chemical or cement grout into the soil, is the low impact on the surrounding environment as the refrigerating medium required to obtain AGF is circulated in pipes and exhausted in the atmosphere or re-circulated without contamination of the ground water. On the other hand, the available methods may vary significantly in their sustainability and complexity in terms of times and costs required for their installation and maintenance. The ability to predict the effects induced by AGF on granular materials is therefore crucial to assessing construction time and cost and to optimising the method. In this work, the thermo-hydro-mechanical processes induced by artificial freezing of a soil body are studied using a constitutive model that encompasses frozen and unfrozen behaviour within a unified effective-stress-based framework. It makes use of a combination of ice pressure, liquid water pressure and total stress as state variables. The model is validated and calibrated using the results of a series of laboratory tests on natural samples of a volcanic ash (Pozzolana) retrieved during construction of Napoli underground, where the technique of AGF was used extensively to stabilise temporarily the ground and control the ground water.
Abstract
The use of artificial ground freezing (AGF) to form earth support systems has had applications worldwide. These cover a variety of construction problems, including the [...]
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A cross-anisotropic formulation for elasto-plastic models
M. Mánica, A. Gens, J. Vaunat, D. Ruiz

Géotechnique Letters (2016). Vol. 6 (2), pp. 156-162

Abstract
A cross-anisotropic formulation for elasto-plastic constitutive models based on a non-uniform scaling of the stress tensor is described. Taking advantage of the material symmetries characterising cross-anisotropy, only two scaling factors, one for the normal stress components and one for the shear stress components, are required. It is shown that the formulation can be easily introduced in already implemented models with minor modifications. The performance of this formulation is investigated by reproducing the strength variation of anisotropic rocks in triaxial tests. The numerical simulation of an unsupported excavation is also presented to show the effect of different scaling factors and bedding plane orientations.
Abstract
A cross-anisotropic formulation for elasto-plastic constitutive models based on a non-uniform scaling of the stress tensor is described. Taking advantage of the material symmetries [...]
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DEM modelling of cone penetration tests in a double-porosity crushable granular material
M. Ciantia, M. Arroyo, J. Butlanska, A. Gens

Computers and Geotechnics (2016). Vol. 73, pp. 109-127 (Preprint)

Abstract
A three-dimensional discrete element model is used to investigate the effect of grain crushing on the tip resistance measured by cone penetration tests (CPT) in calibration chambers. To do that a discrete analogue of pumice sand, a very crushable microporous granular material, is created. The particles of the discrete model are endowed with size-dependent internal porosity and crushing resistance. A simpli- fied Hertz–Mindlin elasto-frictional model is used for contact interaction. The model has 6 material parameters that are calibrated using one oedometer test and analogies with similar geomaterials. The calibration is validated reproducing other element tests. To fill a calibration chamber capable of containing a realistic sized CPT the discrete analogue is up-scaled by a factor of 25. CPT is then performed at two different densities and three different confinement pressures. Cone tip resistance in the crushable material is practically insensitive to initial density, as had been observed in previous physical experiments. The same CPT series is repeated but now particle crushing is disabled. The ratios of cone tip resistance between the two types of simulation are in good agreement with previous experimental comparisons of hard and crushable soils. Microscale exploration of the models indicates that crushing disrupts the buttressing effect of chamber walls on the cone. DEM modelling of cone penetration tests in a double-porosity crushable granular material.
Abstract
A three-dimensional discrete element model is used to investigate the effect of grain crushing on the tip resistance measured by cone penetration tests (CPT) in calibration [...]
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Wear and abrasivity: observations from EPB drives in mixed soft – rock sections
C. González, M. Arroyo, A. Gens

Geomechanik and Tunnelbau (2015). Vol. 8 (3), pp. 258-264

Abstract
This paper summarizes the main observations related to geotechnical properties and tool changes that characterize the different tunnel drives in a database of 33 km of EPB tunnel records from the Barcelona area. The data have been examined in detail with regard to cutting tool replacement and performance of the mechanized excavation. The database includes tunnels in soft soils, in hard and medium rocks and in mixed soil‐rock conditions; tool changing records and geotechnical properties. Transverse and longitudinal geotechnical heterogeneity are accounted for systematically. Longitudinal heterogeneity is used to subdivide the drives in the database into homogeneous units. Transverse (within section) heterogeneity is described by a set of newly developed impact factors FI.
Abstract
This paper summarizes the main observations related to geotechnical properties and tool changes that characterize the different tunnel drives in a database of 33 km of EPB [...]
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An approach to enhance efficiency of DEM modelling of soils with crushable grains
M. Ciantia, M. Arroyo, F. Calvetti, A. Gens

Géotechnique (2015). Vol. 65 (2), pp. 91-110 (Preprint)

Abstract
In this study oedometric compression tests of hydrocarbon coke, Fontainebleau sand and silica sand are simulated in three dimensions using breakable particles. The method adapts a rigorous breakage criterion for elasto-brittle spheres to represent failure of grains isolated between platens or within granular masses. The breakage criterion allows for the effect of particle bulk and contact properties to be treated separately. A discrete fragmentation multigenerational approach is applied as a spawning procedure. The number of particles quickly increases during the simulation, but is kept manageable by systematic fine exclusion and upscaling. Fine exclusion leads to mass losses between generations, but that loss is accounted for outside the mechanical model. Sensitivity analysis shows that it is enough to keep 53% of the crushed particle mass within the mechanical model to correctly reproduce experimental macroscopic behaviour. Practical upscaling rules are proposed for (a) contact stiffness, (b) breakage criteria and (c) grain size distribution, and validated simulating the same test, reducing by half the initial number of particles. The results are promising as both the mechanical and grading evolution are well captured with two orders of magnitude savings in computing efficiency.
Abstract
In this study oedometric compression tests of hydrocarbon coke, Fontainebleau sand and silica sand are simulated in three dimensions using breakable particles. The method [...]
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Relative performance of two unsaturated soil models using different constitutive variables
S. Wheeler, J. Pineda, D. Sheng, A. Gens

Canadian Geotechnical Journal (2014). Vol. 51 (12), pp. 1423-1437

Abstract
Mechanical and water retention behaviour of unsaturated soils is investigated in the context of two well established coupled constitutive models, each of which is formulated in terms of a different set of stress state variables or constitutive variables. Incremental relationships describing the volume change and variation of the degree of saturation are derived for each model. These incremental relationships are used to simulate a set of experimental tests on compacted Speswhite kaolin previously reported in the literature. Six individual tests, involving isotropic compression and various forms of shearing, are analyzed in the context of the incremental forms developed, and the model predictions are then compared against experimental results. The results show that, although each constitutive model uses a different set of constitutive variables and a different scheme for coupling mechanical and water retention behaviour, the two sets of model predictions are similar and both sets provide a reasonable match to the experimental results, suggesting that both models are able to capture the relevant features of unsaturated soil behaviour, despite expressing the constitutive laws in different ways.
Abstract
Mechanical and water retention behaviour of unsaturated soils is investigated in the context of two well established coupled constitutive models, each of which is formulated [...]
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Shear strength of rock joints influenced by compacted infill
B. Indraratna, W. Premadasa, E. Brown, A. Gens, A. Heitor

International Journal of Rock Mechanics and Mining Sciences (2014). Vol. 70, pp. 296-307

Abstract
Discontinuities such as fault planes, joints and bedding planes in a rock mass may be filled with different types of fine-grained material that are either transported or accumulated as gouge due to weathering or joint shearing. Previous laboratory studies have mainly examined the role of saturated infill that exhibits the minimum shear strength. However, in practice, the infill materials are often partially saturated generating matric suction within the joint that can contribute to increased shear strength. To the authors' knowledge this is the first study to examine the influence of compacted (unsaturated) infill on the joint shear strength. A series of laboratory triaxial tests on idealised model joints and imprinted natural joint profiles was carried out, with constant water contents of the infill being maintained. From the laboratory results, it is observed that the peak shear strength of infilled joints increased with the decrease of degree of saturation from 85% to 35% for both idealised joints and replicated natural joints. Based on the laboratory observations an empirical model for describing the infilled joint shear strength was developed.
Abstract
Discontinuities such as fault planes, joints and bedding planes in a rock mass may be filled with different types of fine-grained material that are either transported or accumulated [...]
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Multi-scale analysis of cone penetration test (CPT) in a virtual calibration chamber
J. Butlanska, M. Arroyo, A. Gens, C. O'Sullivan

Canadian Geotechnical Journal (2014). Vol. 51 (1), pp. 51-66

Abstract
A virtual calibration chamber was developed using a three-dimensional (3D) discrete element method (DEM) to perform cone penetration tests (CPTs) on a discrete analogue of Ticino sand. The macroscale response of the DEM model was previously shown to be in good quantitative agreement with that of analogous physical models. In the current study the performance of the model at meso and microscale levels of resolution is examined. The microscale response is examined using particle displacements and contact force distributions. The mesoscale behaviour is examined using stress and strain fields obtained through appropriate averaging and interpolating procedures. Four CPTs are examined at the steady-state penetration stage. The effects of radial boundary conditions, initial stress state, initial average density, and particle rotational inertia are examined. The ability of the micro and mesoscale data to identify and explain the relevant mechanisms underlying the significant differences in the macroscale response of the models is discussed. Comparisons with similar phenomena observed in physical tests are also highlighted.
Abstract
A virtual calibration chamber was developed using a three-dimensional (3D) discrete element method (DEM) to perform cone penetration tests (CPTs) on a discrete analogue of [...]
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Interpretation of dam monitoring data combining visualisation tools and machine learning. Eberlaste Dam Case Study
S. F., . Kohler, . Landstorfer, A. Conde

Abstract
'''The installation of automatic data acquisition systems, together with the use of machine learning, allow obtaining useful information on the behaviour of dams. In this contribution, an example of application for a machine learning based predictive model is presented. Specifically, the level in a piezometer and its association with the reservoir level is studied for an embankment dam. The results show the model's ability to identify changes in dam response by taking full advantage of the available monitoring data. The flexibility of the algorithm allows different types of variables to be analysed without the need to determine a priori which are the most influential loads or how they affect the target value. The model has been implemented in a software tool that includes additional functionalities, specific for the treatment and exploration of dam monitoring data. It can be applied to different dam types and response variables.''' '''
Abstract
'''The installation of automatic data acquisition systems, together with the use of machine learning, allow obtaining useful information on the behaviour of dams. [...]
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Identification of dam behavior by means of machine learning classification models
F. Salazar, A. Conde, D. Vicente

(2021). Numerical Analysis of Dams, DOI: 10.1007/978-3-030-51085-5_48

Abstract
'''The improvements in monitoring devices result in databases of increasing size showing dam behaviour. Advanced tools are required to extract useful information from such large amounts of data. Machine learning is increasingly used for that purpose worldwide: data-based models are built to estimate the dam response in front of a given combination of loads. The results of the comparison between model predictions and actual measurements can be used for decision support in dam safety evaluations. However, most of the works to date consider each device separately. A different approach is used in this contribution: a set of displacement records are jointly considered to identify patterns using a classification model. First, potential anomaly scenarios are defined and the response of the dam for each of them is obtained with numerical models under a realistic load combination. Then, the resulting displacements are used to generate a machine learning classifier. This model is later used to predict the most probable class of dam behavior corresponding to a new set of records. The methodology is applied to a double-curvature arch dam, showing great potential for anomaly detection.
Abstract
'''The improvements in monitoring devices result in databases of increasing size showing dam behaviour. Advanced tools are required to extract useful information [...]
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Sand production simulation coupling DEM with CFD
N. Climent, M. Arroyo, C. O'Sullivan, A. Gens

European Journal of Environmental and Civil Engineering (2014). Vol. 18 (9), pp. 983-1008

Abstract
Sand production in oil wells is often predicted using continuum fluid-coupled models. However, a continuum approach cannot capture important features of the sanding problem, such as erosion and localised failure. This shortcoming of continuum-based analyses can be overcome using the particulate discrete element method (DEM). However, these models, apart from issues of computational cost, have the disadvantage of being difficult to calibrate. One way forward is to calibrate DEM models to capture the response observed in continuum models, where the material parameters can be selected with greater confidence. Adopting this philosophy here, a 3D numerical model based on DEM coupled with Computational Fluid Dynamics was built to simulate sand production around perforations. In the first instance, the basic DEM model (i.e. a dry case) is calibrated against a well-known poro-elastoplastic analytical solution by Risnes et al. (1982). Subsequently, a range of hydrostatic scenarios involving different levels of pore pressure and effective stress are considered. The numerical model shows an asymmetry of the eroded zone that is related to initial microscale inhomogeneity. The stress peak of the analytical solution at the elastic-plastic interface is smoothed because of that asymmetry. The presence of hydrostatic fluid decreases the plastic region and reduces the amount of sand produced. This is not due to changes in effective stress but rather by the particle-scale stabilizing effect of the fluid drag.
Abstract
Sand production in oil wells is often predicted using continuum fluid-coupled models. However, a continuum approach cannot capture important features of the sanding problem, [...]
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Combination of advanced numerical methods and machine learning for dam safety assessment
F. Salazar, B. Crookston, M. Hariri-Ardebili, D. Vicente, A. Conde

Andre Conde's personal collection (2021). 4

Abstract
The availability of machine learning techniques opens up possibilities in different fields of civil engineering. Their application in conjunction with numerical simulations overcomes the limitations in traditional approaches and pave the road for some new horizons. This communication presents several applications of such a hybrid tool in design and safety assessment of dams and hydraulic structures. They include the generation of behavior prediction models from monitoring data, the identification of behavior patterns with classification models, the analysis of the seismic response of gravity dams with heterogeneous concrete in a probabilistic framework, the investigation of the performance of arch dams, and the estimation of the discharge capacity of arched labyrinth spillways.
Abstract
The availability of machine learning techniques opens up possibilities in different fields of civil engineering. Their application in conjunction with numerical simulations [...]
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Homogenization in clay barriers and seals: two case studies
A. Gens, B. Vallejan, M. Zandarin, M. Sanchez

Journal of Rock Mechanics and Geotechnical Engineering (2013). Vol. 5 (3), pp. 191-199

Abstract
The paper presents two case studies that provide information on the process of homogenization of initially heterogeneous clay barriers and seals. The first case is the canister retrieval test performed in the Aspö Hard Rock Laboratory (Sweden). The heterogeneity arises from the use of a combination of blocks and pellets to construct the engineered barrier. The degree of homogenization achieved by the end of the tests is evaluated from data obtained during the dismantling of the test. To assist in the interpretation of the test, a fully coupled thermo-hydro-mechanical (THM) analysis has been carried out. The second case involves the shaft sealing test performed in the HADES underground research laboratory (URL) in Mol (Belgium). Here the seal is made up of a heterogeneous mixture of bentonite pellets and bentonite powders. In addition to the full scale test, the process of homogenization of the mixture has also been observed in the laboratory using X-ray tomography. Both field test and laboratory tests are successfully modelled by a coupled hydro-mechanical (HM) analysis using a double structure constitutive law. The paper concludes with some considerations on the capability of highly expansive materials to provide a significant degree of homogenization upon hydration.
Abstract
The paper presents two case studies that provide information on the process of homogenization of initially heterogeneous clay barriers and seals. The first case is the canister [...]
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AN INTERACTIVE TOOL FOR DATA ANALYSIS AND MACHINE-LEARNING MODEL FITTING WITH APPLICATION TO HYDRO-ENVIRONMENT SYSTEMS
F. Salazar, A. Conde, D. Vicente

Abstract
The advances in sensors and communication technologies open great possibilities in the management and maintenance of engineering systems. In general, the performance of monitoring devices has undergone relevant improvements in terms of both accuracy and reliability, which have resulted in more information available on the behaviour of the structure under consideration. However, the investments made in the modernization of the monitoring systems are not recovered unless complemented by applications capable of handling such large and diverse information. In this contribution, we present a software tool for importing, exploring, cleaning and analysing monitoring data. Also, it allows for fitting machine-learning behaviour models, as well as interpreting the response of the system to the actions or loads in operation. It was initially developed for dam safety assessment, but can be used -with minor changes- for other engineering systems. The methodology and the overall structure can be categorized in two main sections: (i) the monitoring data can be uploaded, cleaned, completed and analysed and (ii) the machine-learning model can be fitted to predict the variables of interest of the system. The same model can then be used for online detection of anomalies by comparing predictions with recorded behaviour. For example, this allows the identification of abnormal displacements in dams for a given load combination. The software can be equally run locally or in the cloud, with appropriate safe access. It has been written in the R language using the Shiny package for interactivity with the following functionalities: zooming and showing information for data exploration, selecting time periods to interpolate and choosing the training parameters to fit behaviour models.
Abstract
The advances in sensors and communication technologies open great possibilities in the management and maintenance of engineering systems. In general, the performance of monitoring [...]
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Compacted soil behaviour: initial state, structure and constitutive modelling
E. Alonso, N. Pinyol, A. Gens

Géotechnique (2013). Vol. 63 (6), pp. 463-478

Abstract
The paper explores the behaviour of compacted soils throughout the (dry density–water content) compaction plane by means of a conceptual framework that incorporates microstructural information. The engineering properties of compacted soils are described by an initial state in terms of a yielding stress, soil suction and a microstructural state variable. Microstructure is defined by the ratio of microvoid volume to total void volume. The pattern of variation of the microstructural parameter within the compaction plane has been determined, for some compacted soils, by analysing mercury intrusion porosimetry data. The microstructure of wet and dry compaction conditions can then be quantified. To ensure consistency, the framework is cast in the form of a constitutive model defined in terms of an effective suction and a constitutive stress that incorporate the microstructural variable. The model is shown to be consistent with a number of experimental observations and, in particular, it explains the intrinsic collapse potential of compacted soils. It predicts, for a common initial suction, a higher collapse potential for dry of optimum conditions than for wet compaction. It also predicts in a natural manner the observed evolution of soil compressibility during drained or undrained loading. Model capabilities are illustrated by application to a testing programme on statically compacted samples of low-plasticity silty clay. The compression behaviour of samples compacted wet and dry of optimum and the variation of collapse strains with confining stress have been successfully reproduced by the model.
Abstract
The paper explores the behaviour of compacted soils throughout the (dry density–water content) compaction plane by means of a conceptual framework that incorporates [...]
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A chemo-mechanical constitutive model accounting for cation exchange in expansive clays
L. Guimarães, A. Gens, M. Sanchez, S. Olivella

Geotechnique (2013). Vol. 63 (3), pp. 221-234

Abstract
The paper presents a chemo-mechanical model for expansive clays that takes into account the effects of cation content and cation exchange. These factors play a key role in the mechanical behaviour of very active clays, particularly with regard to volumetric behaviour. The model is based on an existing double-structure formulation that distinguishes specifically between microstructure and macrostructure. Chemical effects are defined at the microstructural level, the seat of the basic physico-chemical phenomena affecting highly swelling clays. The microstructural model accounts for changes in both osmotic suction and in cation content. Microstructural strains are considered to be reversible; material irreversibility arises from the interaction between the two structural levels. The formulation is developed for general unsaturated conditions; saturation is considered as a limiting case. The model is successfully applied to the reproduction of experimental behaviour observed in oedometer tests on saturated bentonite subjected to chemo-mechanical loadings, and in hydration tests of unsaturated bentonite performed using different solute concentrations.
Abstract
The paper presents a chemo-mechanical model for expansive clays that takes into account the effects of cation content and cation exchange. These factors play a key role [...]
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Analysis of hydro-mechanical processes in a ventilated tunnel in an argillaceous rock on the basis of different modelling approaches
B. Garitte, A. Bond, A. Millard, C. Zhang, C. McDermott, S. Nakama, A. Gens

Journal of rock mechanics and geotechnical engineering (2013). Vol. 5 (1), pp. 1-17

Abstract
In this paper, a modelling benchmark exercise from the DECOVALEX-2011 project is presented. The benchmark is based on the performance and results of a laboratory drying test and of the ventilation experiment (VE) carried out in the Mont Terri Underground Rock Laboratory (URL). Both tests involve Opalinus clay. The work aims at the identification, understanding and quantification of mechanisms taking place during the ventilation of a gallery in argillaceous host rocks on one hand and at investigating the capacity of different codes and individuals to reproduce these processes on the other hand. The 4-year in situ VE took place in a 1.3 m diameter unlined tunnel and included two resaturation–desaturation cycles. The test area was equipped with over one hundred sensors (including the global water mass balance of the system, relative humidity (RH), water content, liquid pressure, relative displacement and concentration of some chemical species) to monitor the rock behaviour during ventilation. The laboratory drying experiment, carried out before the VE, was designed to mimic the in situ conditions. The work was organized in a progressive manner in terms of complexity of the computations to be performed, geared towards the full hydro-mechano-chemical (HMC) understanding of the VE, the final objective. The main results from the modelling work reported herein are that the response of the host rock to ventilation in argillaceous rocks is mainly governed by hydraulic processes (advective Darcy flow and non-advective vapour diffusion) and that the hydro-mechanical (TM) back coupling is weak. A ventilation experiment may thus be regarded as a large scale-long time pump test and it is used to determine the hydraulic conductivity of the rock mass.
Abstract
In this paper, a modelling benchmark exercise from the DECOVALEX-2011 project is presented. The benchmark is based on the performance and results of a laboratory drying [...]
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Steady state of solid-grain interfaces during simulated CPT
J. Butlanska, M. Arroyo, A. Gens

Studia Geotechnica et Mechanica (2013). Vol. XXXV (4), pp. 13-22

Abstract
It has recently been shown (Arroyo et al. [1]) that 3D DEM models are able to reproduce with reasonable accuracy the macroscopic response of CPT performed in calibration chambers filled with sand. However, the cost of each simulation is an important factor. Hence, to achieve manageable simulation times the discrete material representing the sand was scaled up to sizes that were more typical of gravel than sand. A side effect of the scaled-up discrete material size employed in the model was an increased fluctuation of the macro-response that can be filtered away to observe a macroscopic steady-state cone resistance. That observation is the starting point of this communication, where a series of simulations in which the size ratio between penetrometer and particles is varied are systematically analyzed. A micromechanical analysis of the penetrometer-particle interaction is performed. These curves reveal that a steady state is arrived also at the particle-cone contact level. The properties of this dynamic interface are independent of the initial density of the granular material.
Abstract
It has recently been shown (Arroyo et al. [1]) that 3D DEM models are able to reproduce with reasonable accuracy the macroscopic response of CPT performed in calibration chambers [...]
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Abrasividad y su influencia en el rendimiento de una excavación mecanizada
C. González, M. Arroyo, A. Gens

Obras y proyectos. Revista de ingeniería civil (2013). Vol. 13, pp. 6-18

Abstract
El empleo de tuneladoras TBM (Tunnel Boring Machine) es cada vez más frecuente aunque su optimización requiere la evaluación precisa de varios riesgos geotécnicos entre ellos, el efecto de la abrasividad del suelo en el coste y planificación de un proyecto dado. El desgaste de las herramientas es más difícil de calcular que el resto de parámetros de la maquinaria, debido a la complejidad de la interacción entre herramienta de corte y terreno. En este trabajo se usan datos de un caso real de túnel en terrenos mixtos suelo-roca para examinar el resultado de varias metodologías de predicción de la penetración de TBM. También se examinan algunas predicciones de desgaste y se describe y aplica un nuevo método para la evaluación de la abrasividad de geomateriales que puede aplicarse tanto a materiales sueltos como a rocas.
Abstract
El empleo de tuneladoras TBM (Tunnel Boring Machine) es cada vez más frecuente aunque su optimización requiere la evaluación precisa de varios riesgos [...]
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Thermal conductivity of argillaceous rocks: determination methodology using in situ heating tests
B. Garitte, A. Gens, J. Vaunat, G. Armand

Rock Mechanics and Rock Engineering (2012). Vol. 47, pp. 111-129

Abstract
This study focuses on the characterisation of thermal conductivity for three potential host rocks for radioactive waste disposal. First, the heat conduction process is reviewed on the basis of an analytical solution and key aspects related to anisotropic conduction are discussed. Then the existing information on the three rocks is summarised and a broad uncertainty range of thermal conductivity is estimated based on the mineralogical composition. Procedures to backanalyse the thermal conductivity on the basis of in situ heating tests are assessed and a methodology is put forward. Finally, this methodology is used to estimate the impact of experimental uncertainties and applied to the four in situ heating tests. In the three potential host rocks, a clear influence of the bedding planes was identified and anisotropic heat conduction was shown to be necessary to interpret the observed temperature field. Experimental uncertainties were also shown to induce a larger uncertainty on the anisotropy ratio than on the equivalent thermal conductivity defined as the geometric mean of the thermal conductivity in the three principal directions.
Abstract
This study focuses on the characterisation of thermal conductivity for three potential host rocks for radioactive waste disposal. First, the heat conduction process is reviewed [...]
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Thermal-hydraulic-mechanical (THM) behaviour of a large-scale in situ heating experiment during cooling and dismantling
M. Castilla, A. Gens, L. Guimarães

Canadian Geotechnical Journal (2012). Vol. 49 (10), pp. 1169-1195

Abstract
A geological disposal facility for high-level radioactive waste (HLW) encompasses both natural (host rock) and (generally clay-based) engineered barriers. Many processes can influence, either positively or negatively, the effectiveness of the safety functions of isolation and retardation. This paper focuses on the analysis of a large-scale heating test when subjected to cooling and subsequent partial dismantling. The experiment reproduces the conditions of an HLW repository at full scale under realistic conditions. Key thermal, hydraulic, and mechanical (THM) variables, such as temperature, relative humidity, stresses, and fluid pressures, were measured in the clay barrier and surrounding rock. The experimental observations recorded during the cooling down and clay barrier excavation are analyzed in light of a fully coupled THM finite element formulation. This analysis has provided the opportunity to explore the behaviour of the clay and natural barriers under conditions very relevant for the repository performance but not analyzed previously. Overall, the model predictions are quite satisfactory when compared against experimental observations. Furthermore, model predictions for a period of 20 years, including the transient phase induced by the partial dismantling, are also presented. This additional analysis has allowed a better understanding of the effect of thermal gradient on long-term clay hydration.
Abstract
A geological disposal facility for high-level radioactive waste (HLW) encompasses both natural (host rock) and (generally clay-based) engineered barriers. Many processes can [...]
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Simulation of cement-improved clay structures with a bonded elasto-plastic model: A practical approach
M. Arroyo, M. Ciantia, R. Castellanza, A. Gens, R. Nova

Computers and Geotechnics (2012). Vol. 45, pp. 140-150

Abstract
Engineering practice has usually dealt with the treated soil bodies using simplistic constitutive models (e.g. elastic perfectly-plastic Mohr–Coulomb). In this paper, a more refined bonded elasto-plastic model is here applied, with emphasis on the ease of calibration. Empirical studies have identified the ratio of cement content to the cured mixture void ratio as a controlling variable for mechanical response. This observation is elaborated upon to show that measuring porosity and unconfined compressive strength is enough to initialize the state variables of a bonded elasto-plastic model. Data from cement-improved Bangkok clay is employed to illustrate and validate the calibration procedure proposed. The structure-scale consequences of the constitutive model choice for the soil–cement are explored through the parametric analysis of an idealized excavation problem. A treated soil–cement slab is characterized by increasing cement contents in the clay–cement mixture. Two sets of parametric analysis are run characterizing the clay–cement either with a linear elastic-perfectly plastic model or with the bonded elasto-plastic model. The same values of unconfined compressions strength (UCS) are specified for the two models to make comparisons meaningful. Results from both series of analysis are compared highlighting the differences in predicted behaviour of the retaining wall and the excavation stability.
Abstract
Engineering practice has usually dealt with the treated soil bodies using simplistic constitutive models (e.g. elastic perfectly-plastic Mohr–Coulomb). In this [...]
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Analysis of tunnel excavation in London Clay incorporating soil structure
N. González, M. Rouainia, M. Arroyo, A. Gens

Géotechnique (2012). Vol. 62 (12), pp. 1095-1109

Abstract
Recent studies on London Clay have identified a number of different units in the geological profile, and have highlighted the role of soil structure in mechanical behaviour. In fact, structure is the dominant factor determining the differences in the mechanical response of different units. In the paper, numerical analyses simulating the undrained excavation of a tunnel in St James's Park are presented. London Clay behaviour is characterised by a kinematic-hardening structured soil model incorporating structure and stiffness degradation. The parameters and initial conditions are based on a careful calibration that takes into account the presence of different units within the London Clay formation and the different degrees of soil structure. The analyses performed result in a very satisfactory reproduction of the magnitude and patterns of short-term surface and subsurface displacements, as well as pore pressures. The paper concludes with a discussion of the results in the context of other analyses performed previously, and puts forward some considerations concerning design issues.
Abstract
Recent studies on London Clay have identified a number of different units in the geological profile, and have highlighted the role of soil structure in mechanical behaviour. [...]
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Interpretation of unsaturated soil behaviour in the stress-saturation space. II: Constitutive relationships and validations
A. Zhou, D. Sloan, S. Sloan, A. Gens

Computers and Geotechnics (2012). Vol. 43, pp. 111-123

Abstract
Based on the equations for volume change and saturation variation proposed in the companion paper [37], an alternative constitutive framework is presented for interpreting coupled hydro-mechanical behaviour for unsaturated soils. In this new framework, all constitutive laws are built in the space of stress vs. degree of saturation. Suction is not involved explicitly in the constitutive model for unsaturated soils. The loading-collapse yield surface is derived based on the proposed volume change equation in the plane of the effective degree of saturation and the Bishop effective stress. The proposed volume change equation and the corresponding yield surface are generalised to three-dimensional stress states by incorporating with the Modified Cam-clay model, following the same procedure introduced in the Sheng–Fredlund–Gens (SFG) model. The basic properties and performance of the proposed constitutive model are then illustrated through numerical examples with various drying/wetting/loading paths. Finally, the proposed model is validated against a variety of experimental data including drained and undrained tests, isotropic and triaxial tests and reconstituted and compacted soils.
Abstract
Based on the equations for volume change and saturation variation proposed in the companion paper [37], an alternative constitutive framework is presented for interpreting [...]
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Interpretation of unsaturated soil behaviour in the stress. Saturation space, I: volume change and water retention behaviour
N. Zhou, D. Sheng, S. Scott, A. Gens

Computers and Geotechnics (2012). Vol. 43, pp. 178-187

Abstract
Unsaturated soil behaviour, such as volume change, shear strength and yield stress, is usually interpreted and modelled in terms of stress and suction. This approach is consistent with laboratory tests where suction is a controllable variable. However, it also suffers some limitations. This paper (Parts I and II) presents an alternative approach for interpreting unsaturated soil behaviour, which is built in the space of stress versus degree of saturation. In Part I, a new volume change equation is proposed in terms of stress and degree of saturation, to give a better explanation to the non-linear change of soil compressibility under constant suctions. The soil compression index is assumed to be a function of the effective degree of saturation and is interpolated from the known compressibility at the fully saturated state and that at a dry state. An alternative approach to simulate hydraulic hysteresis and hydro-mechanical interaction is then introduced, which enables the calculation of the effective degree of saturation under complex stress and suction paths. The proposed volume change equation and the approach to describe saturation variation, which are two fundamental aspects to establish constitutive laws for unsaturated soils, are validated against a variety of experimental data in literature.
Abstract
Unsaturated soil behaviour, such as volume change, shear strength and yield stress, is usually interpreted and modelled in terms of stress and suction. This approach [...]
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THM analysis of a large-scale heating test incorporating material fabric changes
M. Castilla, A. Gens, S. Olivella

International Journal of Numerical and Analytical Methods in Geomechanics (2012). Vol. 36 (4), pp. 391-421

Abstract
Engineered barriers are basic elements in the design of repositories for the isolation of high‐level radioactive waste. This paper presents the thermo‐hydro‐mechanical (THM) analysis of a clay barrier subjected to heating and hydration. The study focuses on an ongoing large‐scale heating test, at almost full scale, which is being carried out at the CIEMAT laboratory under well‐controlled boundary conditions. The test is intensely instrumented and it has provided the opportunity to study in detail the evolution of the main THM variables over a long period of time. Comprehensive laboratory tests carried out in the context of the FEBEX and NF‐PRO projects have allowed the identification of the model parameters to describe the THM behaviour of the compacted expansive clay. A conventional THM approach that assumes the swelling clay as a single porosity medium has been initially adopted to analyse the evolution of the test. The model was able to predict correctly the global THM behaviour of the clay barrier in the short term (i.e. for times shorter than three years), but some model limitations were detected concerning the prediction of the long‐term hydration rate. An additional analysis of the test has been carried out using a double structure model to describe the actual behaviour of expansive clays. The double structure model explicitly considers the two dominant pore levels that actually exist in the FEBEX bentonite and it is able to account for the evolution of the material fabric. The simulation of the experiment using this enhanced model provides a more satisfactory reproduction of the long‐term experimental results. It also contributes to a better understanding of the observed test behaviour and it provides a physically based explanation for the very slow hydration of the barrier.
Abstract
Engineered barriers are basic elements in the design of repositories for the isolation of high‐level radioactive waste. This paper presents the thermo‐hydro‐mechanical [...]
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Hydromechanical behaviour of a heterogeneous compacted soil: experimental observations and modelling
A. Gens, B. Vallejan, M. Castilla, C. Imbert, M. Villar, M. Geetl

Géotechnique (2011). Vol. 61 (5), pp. 367-386

Abstract
The paper describes a theoretical and experimental study of the coupled hydromechanical behaviour of a compacted mixture of bentonite powder and bentonite pellets intended as sealing material in underground repositories for nuclear waste. One of the main advantages of the use of powder/pellets mixtures is the reduction of the compaction effort required to achieve the value of average dry density necessary to attain the required swelling potential. However, the heterogeneous fabric of the material requires special approaches in order to describe adequately its behaviour during hydration. A double porosity formulation is presented to account for the presence of two distinct structural levels in the material. Hydraulic equilibrium between the two porosities is not assumed; instead a water exchange term between them is postulated. The formulation is applied to the modelling of a number of one-dimensional swelling pressure tests performed in the CEA (Commisariat à l'Énergie Atomique, France) and CIEMAT (Spain) laboratories. A very satisfactory quantitative description of the experimental observations is obtained that includes a number of complex behaviour features such as size effects and non-monotonic development of swelling pressures. Some microfabric observations using X-ray tomography and mercury intrusion porosimetry lend support to the conceptual approach adopted. The formulation is then applied to the analysis of a long-term large-scale sealing test performed at the Hades underground facility in Belgium, using the same set of hydraulic and mechanical parameters employed in the modelling of the laboratory tests. Although the field observations exhibit a much higher degree of scatter, the basic behaviour of the field sealing test is satisfactorily simulated. A formulation that incorporates basic features of the microfabric of the mixture is thus able to span successfully over a large range of space and time scales.
Abstract
The paper describes a theoretical and experimental study of the coupled hydromechanical behaviour of a compacted mixture of bentonite powder and bentonite pellets intended [...]
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Thermo-hydro-mechanical model of the Canister Retrieval Test
M. Zandarin, A. Gens, S. Olivella, E. Alonso

Physics and Chemistry of the Earth (2011). Vol. 36 (17-18), pp. 1806-1816

Abstract
The paper presents a 2-D axisymmetric numerical simulation of the Canister Retrieval Test (CRT). The main objectives of this analysis are the study of the canister–buffer and buffer–host rock interfaces; and the evaluation of the mathematical models used to predict the THM behaviour of the buffer and interface. The THM simulation was performed using the finite element program Code-Bright. The constitutive law adopted to represent the mechanical behaviour of bentonite and pellets is the Barcelona Basic Model (BBM) with a non-linear elastic component accounting for swelling. A joint element with a non-linear elastic mechanical law was implemented in the code for the simulation of the opening–closure of the interface canister–buffer. The formulation of this element also incorporates appropriate thermal and hydraulic laws. The evolution of temperatures, relative humidities and stresses recorded during the test were compared with the simulation results. Comparing the results measured in situ with the results of numerical analysis shows that the formulation used is able to reproduce satisfactorily the phenomena involved in the test as well as their interactions. In addition, the final dry density and degree of saturation measured in samples of bentonite and pellets extracted during the test dismantling were also in agreement with the calculated values.
Abstract
The paper presents a 2-D axisymmetric numerical simulation of the Canister Retrieval Test (CRT). The main objectives of this analysis are the study of the canister–buffer [...]
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Cone penetration tests in a virtual calibration chamber
M. Arroyo, J. Butlanska, A. Gens, F. Calvetti, M. Jamiolkowski

Géotechnique (2011). Vol. 61 (6), pp. 525-531

Abstract
A virtual calibration chamber was built using a threedimensional model based on the discrete-element method. The chamber was then filled with a scaled granular equivalent of Ticino sand, the material properties of which were selected by curve-fitting triaxial tests. Cone penetration tests were then performed under different initial densities and isotropic stresses. Penetration resistance in the virtual calibration chamber was affected by the same cone/chamber size effect that affects physical calibration chambers and was corrected accordingly. The corrected cone resistance obtained from the virtual calibration chamber cone penetration tests shows good quantitative agreement with correlations that summarise previous physical results.
Abstract
A virtual calibration chamber was built using a threedimensional model based on the discrete-element method. The chamber was then filled with a scaled granular equivalent [...]
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Heating pulse tests under constant volumen on natural Boom clay.
A. Lima, E. Romero, A. Gens, J. Muñoz, X. Li

Journal of rock mechanics and geotechnical engineering (2010). Vol. 2 (2), pp. 124-128

Abstract
Boom clay formation is a potential natural host rock for geological disposal of high-level nuclear waste in Belgium. Heating pulse tests with controlled power supply (maximum temperature was limited to 85°C) and controlled hydraulic boundary conditions were performed under nearly constant volume conditions to study the impact of thermal loading on the clay formation. Selected test results of intact borehole samples retrieved in horizontal direction are presented and discussed. The study focuses on the time evolution of temperature and pore water pressure changes along heating and cooling paths, i.e. pore pressure build-up during quasi-undrained heating and later dissipation at constant temperature.
Abstract
Boom clay formation is a potential natural host rock for geological disposal of high-level nuclear waste in Belgium. Heating pulse tests with controlled power supply (maximum [...]
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Soil-environment interactions in geotechnical engineering
A. Gens

Géotechnique (2010). Vol. 60 (1), pp. 3-74

Abstract
The range of problems that geotechnical engineers must face is increasing in complexity and scope. Often, complexity arises from the interaction between the soil and the environment – the topic of this lecture. To deal with this type of problem, the classical soil mechanics formulation is progressively generalised in order to incorporate the effects of new phenomena and new variables on soil behaviour. Recent advances in unsaturated soil mechanics are presented first: it is shown that they provide a consistent framework for understanding the engineering behaviour of unsaturated soils, and the effects of suction and moisture changes. Building on those developments, soil behaviour is further explored by considering thermal effects for two opposite cases: high temperatures, associated with the problem of storage and disposal of high-level radioactive waste; and low temperatures in problems of freezing ground. Finally, the lecture examines some issues related to chemical effects on soils and rocks, focusing in part on the subject of tunnelling in sulphate-bearing rocks. In each case new environmental variables are identified, enhanced theoretical formulations are established, and new or extended constitutive laws are presented. Particular emphasis is placed on mechanical constitutive equations, as they are especially important in geotechnical engineering. The lecture includes summary accounts of a number of case histories that illustrate the relevance and implications of the developments described for geotechnical engineering practice.
Abstract
The range of problems that geotechnical engineers must face is increasing in complexity and scope. Often, complexity arises from the interaction between the soil and [...]
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Effect of thermo-coupled processes on the behaviour of a clay barrier submitted to heating and hydration
M. Castilla, A. Gens, S. Olivella

Anais da Academia Brasileira de Ciências (2010). Vol. 82 (1), pp. 153-168

Abstract
The storage of high level radioactive waste is still an unresolved problem of the nuclear industry, being geological disposal the most favoured option and, naturally, the one requiring the strongest geo-mechanical input. Most conceptual designs for the deep geological disposal of nuclear waste envisage placing the canisters containing the waste in horizontal drifts or vertical boreholes. The empty space surrounding the canisters is filled by an engineered barrier often made up of compacted swelling clay. In the barrier and the near field, significant thermo-hydro-mechanical (THM) phenomena take place that interact in a complex way. A good understanding of THM issues is, therefore, necessary to ensure a correct performance of engineered barriers and seals. The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal are being simulated in a mock-up heating test at almost scale, at the premises of CIEMAT in Madrid. The evolution of the main Thermo-Hydro-Mechanical (THM) variables of this test are analysed in this paper by using a fully coupled THM formulation and the corresponding finite element code. Special emphasis has been placed on the study of the effect of thermo-osmotic flow in the hydration of the clay barrier at an advanced staged of the experiment
Abstract
The storage of high level radioactive waste is still an unresolved problem of the nuclear industry, being geological disposal the most favoured option and, naturally, the [...]
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Modelling thermo-hydro-mechano-chemical interactions for nuclear waste disposal
A. Gens, L. Guimarães, S. Olivella, M. Sanchez

Journal of rock mechanics and geotechnical engineering (2010). Vol. 2 (2), pp. 97-102

Abstract
A fully coupled thermo-hydro-mechano-chemical (THMC) formulation is described in this paper. Special attention is paid to phenomena likely to be encountered in clay barriers used as engineered barriers in the disposal of nuclear radioactive waste. The types of processes considered in the chemical formulation include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetics- and equilibrium-controlled reactions are incorporated. The formulation is implemented in a numerical code. An application is presented concerning the performance of a large-scale in-situ heating test simulating high-level radioactive waste repository conditions.
Abstract
A fully coupled thermo-hydro-mechano-chemical (THMC) formulation is described in this paper. Special attention is paid to phenomena likely to be encountered in clay barriers [...]
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A review of expansive phenomena in Wagenburg North Tunnel
I. Berdugo, E. Alonso, A. Gens, M. Albis

Revista de la academia Colombiana de Ciencias Exactas, Físicas y Naturales (2009). Vol. 31 (129), pp. 455-469

Abstract
The paper deals on tunnelling and swelling in anhydritic-gypsiferous claystones. The geology of the Gipskeuper in Baden-Württemberg (Germany) and the phenomenology of expansions in Wagenburg North tunnel are described. Consistent swelling triggering events, mechanisms and exhaustion causes were identified using a thermo-hydro-chemo-mechanical theoretical analysis. It is suggested that long-term swelling in tunnels excavated through anhydritic-gypsiferous claystones is a result of solvent-way gypsum crystal growth due to ventilation induced groundwater evaporation and rock drying; an opposite concept to the usual consequences of drying in argillaceous materials, which causes shrinkage strains.
Abstract
The paper deals on tunnelling and swelling in anhydritic-gypsiferous claystones. The geology of the Gipskeuper in Baden-Württemberg (Germany) and the phenomenology of [...]
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Reply to Comments on “Unsaturated soils: From constitutive modelling to numerical algorithms” by Daichao Sheng, Antonio Gens, Delwyn G. Fredlund and Scott W. Sloan
D. Sheng, A. Gens, D. Fredlund, S. Sloan

Computers and Geotechnics (2009). Vol. 36 (6), pp. 1100
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Identification of bonded clay parameters in SBPM tests: a numerical study
N. González, M. Arroyo, A. Gens

Soils and Formulation (2009). Vol. 49 (3), pp. 329-340

Abstract
A bonded elasto-plastic soil model is adopted to simulate self-boring pressuremeter tests. Calibration of the model with results from several natural clays identifies a meaningful parameter range for the simulations. Then a systematic sensitivity analysis of the loading SBPM test curve is carried out with an infinite cylindrical cavity analogue, focusing on the variables and parameters that are specific to the bonded material. This analysis reveals that the effects of mechanical overconsolidation and bonding on the pressuremeter loading curves are very similar. Unloading and strain-holding phases of the test show a greater capability to separate mechanical overconsolidation from bonding. The limited effects of finite pressuremeter length and partial drainage on the simulated curves do not change those conclusions.
Abstract
A bonded elasto-plastic soil model is adopted to simulate self-boring pressuremeter tests. Calibration of the model with results from several natural clays identifies [...]
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Tunnelling and swelling in triassic sulphate–bearing rocks: part I: case studies from Baden–Württemberg
I. Berdugo, E. Alonso, E. Romero, A. Gens

Epsilon (2009). Vol. 12, pp. 13-37

Abstract
Cases of swelling in tunnels excavated through Triassic sulphate-bearing rocks from the Gipskeuper and the Anhydritgruppe are presented and discussed in this series of papers in order to gain a better understanding of the expansive phenomena in these materials and to identify the most relevant ones. Part I deals with main features of swelling in tunnels and alternative supports designs for their control an mitigation, as well as with the phenomenology of swelling in tunnels from Baden-Württemberg (Germany) excavated through the Gipskeuper.
Abstract
Cases of swelling in tunnels excavated through Triassic sulphate-bearing rocks from the Gipskeuper and the Anhydritgruppe are presented and discussed in this series of papers [...]
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Tunnelling and swelling in triassic sulphate-bearing rocks: part II: case studies from Jura Mountains
I. Berdugo, E. Alonso, E. Romero, A. Gens

Epsilon (2009). Vol. 12, pp. 39-53

Abstract
Part II of this series of paper deals with the phenomenology of swelling in tunnels from Jura Mountains (Switzerland) excavated through the Gipskeuper and the Anhydritgruppe. The main features of expansions observed in the presented case studies are summarized and the performance of resisting and yielding support systems is compared. Evidences presented in this series of papers indicate that the transformation of anhydrite into gypsum is not a reasonable explanation for long-term expansive phenomena occurring in sulphate-bearing rocks. It is suggested that these phenomena are strongly related to rock degradation due to both tunneling induced drainage towards the bottom of excavations and tunneling induced ventilation.
Abstract
Part II of this series of paper deals with the phenomenology of swelling in tunnels from Jura Mountains (Switzerland) excavated through the Gipskeuper and the Anhydritgruppe. [...]
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A full-scale in situ heating test for high-level nuclear waste disposal: observations, analysis and interpretation
A. Gens, M. Sanchez, L. Guimarães, E. Alonso

Geotechnique (2009). Vol. 59 (4), pp. 377-399

Abstract
The paper describes the performance, observations and interpretation of a large-scale in situ heating test that simulates a disposal concept for heat-emitting, high-level nuclear waste. In the experiment, heaters are emplaced in the axis of a tunnel excavated in granite to simulate the heat production of radioactive waste. The test is fully instrumented, and attention is focused on the thermo-hydro-mechanical (THM) behaviour of the near-field region constituted by the compacted bentonite barrier surrounding the heater and the immediately adjacent rock. Interpretation of the test is assisted by the performance of a coupled numerical analysis based on a formulation that incorporates the relevant THM phenomena. Initial and boundary conditions for the analysis as well as material parameters are determined from an extensive programme of field and laboratory experiments. The paper presents and discusses the thermal, hydraulic and mechanical observations in the bentonite barrier and in the host rock. Special attention is paid to the progress of hydration in the barrier, to the effects of heating and vapour transport, and to the development of swelling pressures in the barrier. After five years of heating, one of the heaters was switched off and the experiment was partially dismantled, allowing the final state of the barrier to be observed directly. The numerical analysis performed has proved able to represent the progress of the experiment very satisfactorily. In addition, predictions concerning the final state of the clay barrier are very close to the observations obtained during dismantling. The performance and analysis of the in situ test have significantly enhanced the understanding of a complex THM problem and have proved the capability of the numerical formulation to provide adequate predictive capacity
Abstract
The paper describes the performance, observations and interpretation of a large-scale in situ heating test that simulates a disposal concept for heat-emitting, high-level [...]
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THM-coupled finite element analysis of frozen soil: formulation and application
S. Nishimura, A. Gens, S. Olivella, R. Jardine

Géotechnique (2009). Vol. 59 (3), pp. 159-171

Abstract
A fully coupled thermo-hydro-mechanical (THM) finite element (FE) formulation is presented that considers freezing and thawing in water-saturated soils. The formulation considers each thermal, hydraulic and mechanical process, and their various interactions, through fundamental physical laws and models. By employing a combination of ice pressure, liquid pressure and total stress as state variables, a new mechanical model has been developed that encompasses frozen and unfrozen behaviour within a unified effective-stress-based framework. Important frozen soil features such as temperature and porosity dependence of shear strength are captured inherently by the model. Potential applications to geotechnics include analysis of frost heave, foundation stability or mass movements in cold regions. The model's performance is demonstrated with reference to the in situ pipeline frost heave tests conducted by Slusarchuk et al. Detailed consideration is given to FE mesh design, the influence of hydraulic parameters, and the treatment of air/ground interface boundary conditions. The THM simulation is shown to reproduce, with fair accuracy, the observed pipeline heave and the porosity growth driven by water migration.
Abstract
A fully coupled thermo-hydro-mechanical (THM) finite element (FE) formulation is presented that considers freezing and thawing in water-saturated soils. The formulation considers [...]
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Applications of multiphysical geomechanics in underground nuclear waste storage
A. Gens, B. Garitte, S. Olivella, J. Vaunat

European Journal of Environmental and Civil Engineering (2009). Vol. 13 (7-8), pp. 937-962

Abstract
Deep geological disposal in suitable host rocks is the favoured strategy for the storage and disposal of heat-emitting high level nuclear waste. A rational design of repositories requires a good understanding of the interacting thermo-hydro-mechanical phenomena that occur in the engineered barrier and adjacent rock. To this end, a multiphysical formulation is described that allows the performance of coupled THM analyses capable of reproducing observed phenomena. The formulation and computer code is applied to the simulation of two large scale tests: a mine-by test involving the excavation of a shaft in an argillaceous rock and a large-scale high- temperature heating test in fractured rock.
Abstract
Deep geological disposal in suitable host rocks is the favoured strategy for the storage and disposal of heat-emitting high level nuclear waste. A rational design of repositories [...]
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Reply to the discussion by Zhang and Lytton on “A new modelling approach for unsaturated soils using independent stress variables”
D. Sloan, D. Fredlund, A. Gens

Canadian Geotechnical Journal (2008). Vol. 45 (12), pp. 1784-1787

Abstract
We would like to thank the discussers for their interest in our paper and also for their challenging questions, which show their thorough knowledge on the subject. Their comments have enhanced our understanding of the subject area and hopefully may lead to further constitutive model developments.
Abstract
We would like to thank the discussers for their interest in our paper and also for their challenging questions, which show their thorough knowledge on the subject. Their comments [...]
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Unsaturated soils: from constitutive modelling to numerical algorithms
D. Sloan, A. Gens, D. Fredlund, S. Sloan

Computers and Geotechnics (2008). Vol. 35 (6), pp. 810-824

Abstract
This paper presents an overview of constitutive modelling of unsaturated soils and the numerical algorithms for solving the associated boundary value problems. It first discusses alternative stress and strain variables that can be used in constitutive models for unsaturated soils. The paper then discusses the key issues in unsaturated soil modelling and how these issues can be incorporated into an existing model for saturated soils. These key issues include (1) volumetric behaviour associated with saturation or suction changes; (2) strength behaviour associated with saturation and suction changes, and (3) hydraulic behaviour associated with saturation or suction changes. The paper also shows how hysteresis in soil–water characteristics can be incorporated into the elasto-plastic framework, leading to coupled hydro-mechanical models. Finally, the paper demonstrates the derivation of the incremental stress–strain relations for unsaturated soils and discusses briefly the new challenges in implementing these relations into the finite element method.
Abstract
This paper presents an overview of constitutive modelling of unsaturated soils and the numerical algorithms for solving the associated boundary value problems. It first discusses [...]
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Tunnelling and geotechnics: new horizons. Vote of thanks
R. Mair, A. Gens

Géotechnique (2008). Vol. 58 (9), pp. 736-736

Abstract
New developments in both the theory and the practice of tunnelling are covered in the lecture. The important relationship between tunnelling and geotechnics is highlighted, and recent advances in research and practice are described, drawing on model studies, theoretical developments and field measurements from case histories from around the world. Simplified plasticity models are presented that can be used by designers to assess ground movements and tunnel lining loads in complex ground conditions. The important role of pilot tunnels and in situ measurements to validate such models, drawing on a case history from Bolu, Turkey, and on other tunnelling projects, is described. Recent technical advances in earth pressure balance tunnelling are considered, illustrated by measurements from the Channel Tunnel Rail Link project, with emphasis on key factors influencing volume loss, such as face pressure, soil conditioning and effective screw conveyor operation. A recent case history in Bologna is described, in which the innovative use of directional drilling to install curved grout tubes was employed for a compensation grouting project in granular soils. Time-dependent ground movements and tunnel lining distortions occurring after tunnelling are discussed, their magnitude depending on the relative permeability of the tunnel lining and soil, the degree of anisotropy of the soil permeability, and the initial pore pressure prior to tunnelling. The effects of tunnelling-induced settlements on pipelines are considered, drawing on centrifuge tests and analytical solutions, and a new design approach is presented, taking into account the reduction of soil stiffness with increasing shear strain as a result of tunnel volume loss. The lecture concludes with a description of a distributed strain sensing technique using fibre optic technology, based on Brillouin optical time domain reflectometry (BOTDR), and its innovative application to field monitoring of a masonry tunnel subjected to new tunnel construction beneath it.
Abstract
New developments in both the theory and the practice of tunnelling are covered in the lecture. The important relationship between tunnelling and geotechnics is highlighted, [...]
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Unsaturated soils: models algorithm and applications
A. Gens, D. Sloan

Computers and Geotechnics (2008). Vol. 35 (6), pp. 809
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Implementation algorithm of a generalised plasticity model for swelling clays
M. Sanchez, A. Gens, L. Guimarães, S. Olivella

Computers and Geotechnics (2008). Vol. 35 (6), pp. 860-871

Abstract
A multi-mechanism generalisation of Sloan’s integration scheme for elasto-plastic laws has been developed in order to implement a double structure model for expansive clays into a FE code. The constitutive model is built on a conceptual approach for unsaturated expansive soils in which the fundamental characteristic is the explicit consideration of the two pore levels often present in expansive clays. The distinction between macro and microstructure provides the opportunity to take into account the dominant phenomena that affect the behaviour of each structural level and the main interactions between them. The model is formulated using concepts of classical and generalised plasticity theories. The integration scheme proposed can deal with the two plastic mechanisms defined in the model and can incorporate the effects of strains, suction and temperature in the stress integration process. A large scale heating test is analysed to check the capabilities of the implemented model to simulate an actual problem involving complex thermo-hydro-mechanical stress paths. The performance of the model has been very satisfactory and the proposed integration scheme has proved to be robust and efficient in solving a highly non-linear coupled problem.
Abstract
A multi-mechanism generalisation of Sloan’s integration scheme for elasto-plastic laws has been developed in order to implement a double structure model for expansive [...]
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Modelling unsaturated soil behaviour during normal consolidation and at critical state
D. Gallipoli, A. Gens, G. Chen, F. D'Onza

Computers and Geotechnics (2008). Vol. 35 (6), pp. 852-834

Abstract
An analysis of results from published laboratory tests on Jossigny silt and Barcelona clayey silt is presented to confirm the existence of a unique capillary bonding function linking the quotient between unsaturated and saturated void ratio, at the same mean average skeleton stress, to a single capillary bonding scalar variable. The analysis confirms that the same capillary bonding function applies to both normally consolidated and critical stress states. The above two experimental sets with the addition of further published data for Speswhite Kaolin are also used to study the relationship between unsaturated critical shear strength, mean average skeleton stress and capillary bonding variable. The results of such analysis are assessed in the light of a similar modelling framework proposed in the literature.
Abstract
An analysis of results from published laboratory tests on Jossigny silt and Barcelona clayey silt is presented to confirm the existence of a unique capillary bonding function [...]
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The soilmodels.info project
G. Gudehus, A. Amorosi, A. Gens, I. Herle, D. Kolymbas, D. Masin, D. Miui-Wood

International Journal for Numerical and Analytical Methods in Geomechanics (2008). Vol. 32 (12), pp. 1571-1572
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A mechanism contributing to subsidence above gas reservoirs
A. Menin, V. Salomoni, R. Santagiuliana, L. Simoni, A. Gens, B. Schrefler

International Journal for Computational Methods in Engineering Science and Mechanics (2008). Vol. 9 (5), pp. 270-287

Abstract
This paper aims to demonstrate that capillary effects and structural collapse can not be ruled out as significant factors in the development of subsidence occurring above gas fields. These phenomena provide sound explanations for continuing surface settlements when reservoir pore pressures stabilize and for additional settlements occurring even after the end of gas production. Conventional subsidence models fail to simulate this settlement behavior. Capillary effects also explain the lower rock compressibilities observed in gas-bearing strata as compared to the values obtained in the laboratory from fully saturated samples. Taking into account these aspects, the observed subsidence above a reservoir in the North Adriatic basin, Italy, is studied in detail.
Abstract
This paper aims to demonstrate that capillary effects and structural collapse can not be ruled out as significant factors in the development of subsidence occurring above [...]
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A new modelling approach for unsaturated soils using independent stress variables
D. Sheng, D. Fredlund, A. Gens

Canadian Geotechnical Journal (2008). Vol. 45 (4), pp. 511-534

Abstract
Although a number of constitutive models for unsaturated soils exist in the literature, some fundamental questions have not been fully answered. There are questions related to (i) the change of the yield stress with soil suction, (ii) modelling slurry soils, and (iii) the smooth transition between saturated and unsaturated soil states. This paper addresses these questions by proposing an alternative modelling approach. The paper first presents a volumetric model for unsaturated soils. This volumetric model is then used to derive the yield surface in the suction – mean stress space. Hysteresis associated with soil-water characteristic curves is then formulated in the same framework of elastoplasticity. It is shown that volume collapse during wetting and plastic shrinkage during initial drying are both direct results of a suction-dependent hardening law. The proposed model seems to be more flexible in modelling different types of unsaturated soils than most models in the literature. The model can be applied to soils that are dried or loaded from initially slurry conditions, for soils that have low to high air-entry values, and for compacted soils as well. R.M. Quigley Award 2009 (to authors of the best paper published in the Canadian Geotechnical Journal in the preceding year)
Abstract
Although a number of constitutive models for unsaturated soils exist in the literature, some fundamental questions have not been fully answered. There are questions related [...]
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Chemical impact on the hydro-mechanical behaviour of high-density Febex bentonite
E. Castellanos, M. Villar, E. Romero, A. Lloret, A. Gens

Physics and Chemistry of the Earth (2008). Vol. 33 (S1), pp. S516-S526

Abstract
The effect of the salinity of the saturating fluid on the hydro-mechanical properties of the FEBEX compacted bentonite was investigated by means of swelling, compressibility and permeability tests in which deionised water and solutions of different concentrations and compositions were used as saturating fluids. The solutions were chosen to simulate natural and extreme conditions in a high-level radioactive waste repository excavated in crystalline or clay host rocks. The swelling capacity of the bentonite decreases with the increase in salinity of the pore water, although this difference becomes less patent for high vertical loads and high densities and when the salinity of the solution is very low. The samples saturated with solutions containing high concentration of ions are also less deformable and consolidate more rapidly than the samples saturated with low-salinity solutions. The hydraulic conductivity of the highly compacted saturated FEBEX bentonite increases when high-salinity permeants are used, especially for low densities and when the stress level is low. The influence of the composition of the solution on the hydro-mechanical properties of the bentonite was also checked.
Abstract
The effect of the salinity of the saturating fluid on the hydro-mechanical properties of the FEBEX compacted bentonite was investigated by means of swelling, compressibility and [...]
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Behaviour of a bentonite barrier in the laboratory: experimental results up to 8 years and numerical simulation
M. Villar, M. Sanchez, A. Gens

Physics and Chemistry of the Earth (2008). Vol. 33 (S1), pp. S476-S485

Abstract
The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal were simulated in a series of tests performed in cylindrical cells (length 60 cm, diameter 7 cm). Inside the cells, six blocks of FEBEX bentonite compacted to dry density 1.65 g/cm3 were piled up, giving rise to a total length similar to the thickness of the clay barrier in a repository according to the Spanish concept. The bottom surface of the material was heated at 100 °C and the top surface was injected with granitic water. The duration of the tests was 6, 12, 24 and 92 months. The temperatures inside the clay and the water intake were measured during the tests and, at the end, the cells were dismounted and the dry density, water content and hydro-mechanical properties were measured at different positions. The injection of water provokes, near the hydration surface, a decrease of the dry density due to the increase of the water content and the clay swelling, while heating gives rise to an increase of the dry density and a reduction of the water content in the hottest areas. A fully coupled thermo-hydro-mechanical (THM) formulation has been adopted as a general framework to analyse these experiments. This work presents the comparisons between the variables recorded online during the tests (water intake and temperature) and the model results. The main results of the postmortem analysis (water content and dry density) are also modelled.
Abstract
The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal were simulated in a series of tests performed in cylindrical cells (length [...]
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In situ behaviour of a stiff layered clay subject to thermal loading: Observations and interpretation
A. Gens, J. Vaunat, B. Garitte, Y. Wileveau

Geotechnique (2007). Vol. 57 (2), pp. 207-228

Abstract
The paper presents an interpretation of an in situ heating test carried out on Opalinus clay in the Mont Terri underground laboratory. Opalinus clay is a stiff, strongly bedded, Mesozoic clay of marine origin. When subjected to thermal loading, saturated stiff clays exhibit a strong pore pressure response that significantly affects the hydraulic and mechanical behaviour of the material. The observations gathered in the in situ test have provided an opportunity to examine the integrated thermo-hydro-mechanical (THM) response of this sedimentary clay. Coupled THM numerical analyses have been carried out to provide a structured framework for interpretation, and to enhance understanding of THM clay behaviour. Numerical analyses have been based on a coupled theoretical formulation that incorporates a constitutive law especially developed for this type of material. The law includes degradation of bonding by damage. By performing three-dimensional computations, it has been possible to incorporate anisotropy of material parameters and of in situ stresses. The 3D simulation has proved able to furnish a satisfactory representation of the development of the in situ test and of the main observed patterns of behaviour. A sensitivity analysis has also been carried out to examine the potential effect of various key or uncertain parameters. The critical examination of test observations and the results of the numerical analyses have allowed the classification, by differing degrees of significance, of the various coupled phenomena present in the problem
Abstract
The paper presents an interpretation of an in situ heating test carried out on Opalinus clay in the Mont Terri underground laboratory. Opalinus clay is a stiff, strongly bedded, [...]
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Coupled Thermo-Hydro-Mechanical and Chemical Analysis of Expansive Clay Subjected to Heating and Hydration
L. Guimarães, A. Gens, S. Olivella

Transport in Porous Media (2007). Vol. 66 (3), pp. 341-372

Abstract
A fully coupled formulation combining reactive transport and an existing thermo-hydro-mechanical (THM) code is presented. Special attention has been given to phenomena likely to be encountered in clay barriers used as part of containment systems of nuclear waste. The types of processes considered include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetically-controlled and equilibrium-controlled reactions have been incorporated. The total analytical concentrations (including precipitated minerals) are adopted as basic transport variables and chemical equilibrium is achieved by minimizing Gibbs Free Energy. The formulation has been incorporated in a general purpose computer code capable of performing numerical analysis of engineering problems. A validation exercise concerning a laboratory experiment involving the heating and hydration of an expansive compacted clay is described.
Abstract
A fully coupled formulation combining reactive transport and an existing thermo-hydro-mechanical (THM) code is presented. Special attention has been given to phenomena likely [...]
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Determinación de la masa modal en estructuras civiles mediante el acoplamiento de un sistema dinámico oscilante en la situación de amortiguamiento proporcional independiente de la frecuencia
L. VILLA GARCIA

Rev. int. métodos numér. cálc. diseño ing. (2020). Vol. 36, (3), 43

Abstract
A new alternative is presented for the estimation of modal mass, in order to scale the vibration modes obtained from operational modal analysis tests, using optimization techniques, in the frequency-independent proportional damping situation (structural or hysteretic), achieving optimal results even with the presence of a high level of noise in the signal. After a brief exposition of the theoretical foundations, and the limitations in the current state of the art, the determination of modal parameters is analyzed in a coupled dynamic system, using optimization techniques. To illustrate the procedure, two examples of application are presented: first, for the case of an elementary beam model of small dimensions; and then, to confirm the capacity of the proposed method to be applied to real civil structures, said capacity is illustrated by a numerical simulation, on a very slender bridge currently in service, and with a very complex internal geometry of its main beam.
Abstract
A new alternative is presented for the estimation of modal mass, in order to scale the vibration modes obtained from operational modal analysis tests, using optimization techniques, [...]
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Efficiency of a borehole seal by means of pre-compacted bentonite blocks
M. Geet, G. Volckaert, W. Bastiaens, N. Maes, E. Weetjens, X. Sillen, B. Vallejan, A. Gens

Physics and Chemistry of the Earth (2007). Vol. 32 (1-7), pp. 123-134

Abstract
The backfilling and sealing of shafts and galleries is an essential part of the design of underground repositories for high-level radioactive waste. Part of the EC funded project RESEAL studied the feasibility of sealing off a borehole in plastic Boom Clay by means of pre-compacted bentonite blocks. Two bentonites, namely the FoCa and Serrata clay, have been used. Based on laboratory tests, the bentonite blocks had an initial dry density of about 1.8 g/cm3 to obtain a swelling pressure of about 4.4 MPa, corresponding to the in situ lithostatic stress, at full saturation. The set-up was equipped with several sensors to follow-up the behaviour of the seal and the surrounding host rock during hydration. Full saturation was reached after five months and was mainly reached by natural hydration. Swelling pressure was lower than originally foreseen due to the slow reconsolidation of the host rock. Later on, the efficiency of the seal with respect to water, gas and radionuclide migration was tested. The in situ measured permeability of the seals was about 5 × 10-13 m/s. A gas breakthrough experiment did not show any preferential gas migration through the seal. No evidences of a preferential pathway could be detected from 125I tracer test results
Abstract
The backfilling and sealing of shafts and galleries is an essential part of the design of underground repositories for high-level radioactive waste. Part of the EC funded [...]
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On constitutive modelling of unsaturated soils
A. Gens, M. Sánchez, D. Sloan

Acta Geotechnica (2006). Vol. 1 (3), pp. 137-147

Abstract
The paper presents a review of constitutive modelling of unsaturated soils. After a brief historical perspective, a number of existing constitutive models are classified and discussed according to the type of stress variables adopted in their formulation. Afterwards, attention is given to recent developments in the proposal of coupled hydraulic-mechanical models and the possibility of casting them in a sound thermodynamical framework. Finally, a double structure model for expansive soils is described. The incorporation of microstructural considerations and its use as a platform for incorporating the influence of new variables are highlighted.
Abstract
The paper presents a review of constitutive modelling of unsaturated soils. After a brief historical perspective, a number of existing constitutive models are classified and [...]
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THM and reactive transport analysis of expansive clay barrier in radioactive waste isolation
L. Guimarães, A. Gens, M. Sanchez, S. Olivella

Communications in numerical methods in engineering (2006). Vol. 22 (8), pp. 849-859

Abstract
A fully coupled formulation combining reactive transport and an existing thermo‐hydro‐mechanical (THM) code is briefly described. Special attention has been given to phenomena likely to be encountered in clay barriers used as part of containment systems of nuclear waste. The types of processes considered in the chemical formulation include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetically controlled and equilibrium‐controlled reactions have been incorporated. The formulation has been implemented in the finite element code CODE_BRIGHT. An application is presented concerning the performance of a large scale in situ heating test simulating high‐level nuclear waste repository conditions.
Abstract
A fully coupled formulation combining reactive transport and an existing thermo‐hydro‐mechanical (THM) code is briefly described. Special attention has been given to phenomena [...]
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Coupled thermo-hydro-mechanical analysis of engineered barriers for high level radioactive waste
A. Gens, S. Olivella

Chinese journal of rock mechanics and engineering (2006). Vol. 25 (4), pp. 670-680

Abstract
A general formulation for the performance of coupled thermo-hydro-mechanical (THM) analysis is presented. A multi-phase and multi-species approach has been adopted for the development of the formulation. The analysis considers a porous material with three phases (solid, liquid and gas) and three species (mineral, water and air). The formulation is presented in terms of governing equations, constitutive laws and equilibrium restrictions. An extensive programme of coupled THM analyses simulating the behaviour of the near field in a scheme for the underground disposal of high level radioactive waste is presented. The analyses are able to take into account the main interactions between the various processes that occur in the engineered barrier and immediately adjacent rock associated with heating and hydration phenomena. Examination of the results leads to a series of significant conclusions concerning the thermal, hydraulic and mechanical aspects of the problem.
Abstract
A general formulation for the performance of coupled thermo-hydro-mechanical (THM) analysis is presented. A multi-phase and multi-species approach has been adopted for the [...]
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Aznalcóllar dam failure. Part 3: dynamics of the motion
E. Alonso, A. Gens

Géotechniques (2006). Vol. 56 (3), pp. 203-210

Abstract
The motion of the Aznalcóllar dam slide, after the initiation of the failure, is examined in the paper. The moving mass remained essentially rigid, and Newton's second law provides the basic equation to determine the resulting dam displacement, velocity and acceleration. Resisting forces have been derived from a previous analysis of the conditions leading to failure. Pore water pressures induced by the forward motion of the dam are approximated as an undrained loading of the clay. Driving forces and their evolution in time derive from the liquefaction of tailings and the condition of constant volume of liquefied tailings pushing the moving dam forward. The stable ‘cliff’ left by the motion inside the tailings lagoon supports this hypothesis. The model is able to reproduce closely the distance travelled by the dam. It shows a low sensitivity with respect to reasonable changes of the main parameters. The model computes that the slide motion lasted about 15 s, and that the maximum acceleration experienced by the dam was 0·14g.
Abstract
The motion of the Aznalcóllar dam slide, after the initiation of the failure, is examined in the paper. The moving mass remained essentially rigid, and Newton's [...]
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Aznalcóllar dam failure. Part 2: stability conditions and failure mechanisms
A. Gens, E. Alonso

Géotechniques (2006). Vol. 56 (3), pp. 185-201

Abstract
An analysis of the conditions that led to the failure of the Aznalcóllar dam is made in the paper. Pore water pressures prevailing in the foundation clay at the time of the failure have been established on the basis of piezometric observations made after the failure. A simplified consolidation analysis is first presented to interpret pore water pressure generation and dissipation and to evaluate the evolution of shear stresses along the potential failure plane. Both limit equilibrium and finite element analysis are used to back-analyse the failure and to derive representative strength parameters along the failure surface. Three-dimensional effects introduced by the layered nature of the foundation clay are then described. They provide an explanation for the location of the slide, which affected only the south-eastern portion of the embankment. The failure is examined in the final part of the paper from the perspective offered by the current knowledge of the strength behaviour of brittle clays. The significance of progressive failure is discussed, together with other factors that played a significant role in this case: the homogeneous nature and very low permeability of the foundation clay and the natural state of the clay, probably affected by some initial damage, which reduced the available strength along bedding planes.
Abstract
An analysis of the conditions that led to the failure of the Aznalcóllar dam is made in the paper. Pore water pressures prevailing in the foundation clay at the time [...]
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Aznalcóllar dam failure. Part 1: field observations and material properties
E. Alonso, A. Gens

Géotechniques (2006). Vol. 56 (3), pp. 165-183

Abstract
The paper describes the failure of Aznalcóllar dam, in southern Spain, in April 1998. The rockfill dam slid forward and released a flow of acid-saturated tailings. The geology and geomorphology of the site are described. The results of a detailed laboratory testing programme on the tailings material and on the overconsolidated high-plasticity foundation clay are given. Special attention is paid to the identification of cementation of the tailings and to the strength and consolidation properties of the foundation clay. Failure features are interpreted on the basis of the field evidence, taking into account the measured geotechnical properties of the materials involved.
Abstract
The paper describes the failure of Aznalcóllar dam, in southern Spain, in April 1998. The rockfill dam slid forward and released a flow of acid-saturated tailings. [...]
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Analysis of the hydration of a bentonite seal in a deep radioactive waste repository
J. Vaunat, A. Gens

Engineering Geology (2005). Vol. 81 (3), pp. 317-328

Abstract
A deep geological repository for nuclear waste requires the backfilling and sealing of shafts and galleries to block any preferential path for radioactive contaminants. The paper presents the coupled hydromechanical analyses of an in situ test carried out in the HADES underground laboratory in Mol, Belgium. The test examines the effectiveness of an expansive clay seal in a horizontal borehole specifically drilled for this purpose. The analysis covers the phase of seal hydration up to saturation and subsequent pore pressure equilibration. Hydraulic and mechanical constitutive laws suited to expansive clay materials have been chosen for the analyses with all the parameters determined independently of the in situ test. A quite good agreement has been found between test observations and computed results suggesting that the numerical formulation employed is able to reproduce the main features of a real sealing system
Abstract
A deep geological repository for nuclear waste requires the backfilling and sealing of shafts and galleries to block any preferential path for radioactive contaminants. The [...]
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A hydro-geochemical analysis of the saturation process with salt water of a bentonite crushed granite rock mixture in an engineered nuclear barrier
C. Mata, L. Guimarães, A. LEDESMA, A. Gens, S. Olivella

Engineering Geology (2005). Vol. 81 (3), pp. 227-245

Abstract
A MX-80 sodium bentonite crushed granite rock mixture is being saturated in the Äspö Hard Rock Laboratory managed by SKB (the Swedish Company of nuclear waste management) as part of the “Backfill and Plug Test Project”. The Äspö Hard Rock Laboratory is placed in the Swedish Island of Äspö, an underground full-scale laboratory where different testing construction procedures and handling techniques are being studied. The groundwater of Äspö has a variable salt concentration according to the zone considered. The added water during the mixing process of both materials had an average salt concentration of 6 g/L. However, backfill is being saturated with salt water containing higher salt content (up to 16 g/L, 50 / 50 of NaCl and CaCl2 by mass) to speed up the saturation process. The mixture swelling capacity is small due to the low backfill bentonite content, but its activity is still large if compared with natural clayey soils. A coupled hydro-chemical approach was used to simulate the backfill hydration process, comparing the results with in situ measurements. An intrinsic permeability law, depending on salt concentration in the liquid phase, was incorporated into the model. The retention curve was also determined taking into account the effect of the chemical species on backfill behaviour. The simulation of the saturation process shows the importance of studying these problems with a HC (hydro-chemical) formulation, especially if long term behaviour of such mixtures is going to be reproduced.
Abstract
A MX-80 sodium bentonite crushed granite rock mixture is being saturated in the Äspö Hard Rock Laboratory managed by SKB (the Swedish Company of nuclear waste management) [...]
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A double generalized plasticity model for expansive materials
M. Sánchez, A. Gens, L. Guimarães, S. Olivella

International Journal for Numerical and Analytical Methods in Geomechanics (2005). Vol. 29 (8), pp. 751-787

Abstract
The constitutive model presented in this work is built on a conceptual approach for unsaturated expansive soils in which the fundamental characteristic is the explicit consideration of two pore levels. The distinction between the macro- and microstructure provides the opportunity to take into account the dominant phenomena that affect the behaviour of each structural level and the main interactions between them. The microstructure is associated with the active clay minerals, while the macrostructure accounts for the larger-scale structure of the material. The model has been formulated considering concepts of classical and generalized plasticity theories. The generalized stress–strain rate equations are derived within a framework of multidissipative materials, which provides a consistent and formal approach when there are several sources of energy dissipation. The model is formulated in the space of stresses, suction and temperature; and has been implemented in a finite element code. The approach has been applied to explaining and reproducing the behaviour of expansive soils in a variety of problems for which experimental data are available. Three application cases are presented in this paper. Of particular interest is the modelling of an accidental overheating, that took place in a large-scale heating test. This test allows the capabilities of the model to be checked when a complex thermo-hydro-mechanical (THM) path is followed.
Abstract
The constitutive model presented in this work is built on a conceptual approach for unsaturated expansive soils in which the fundamental characteristic is the explicit consideration [...]
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Coupled thermal-hydrological-mechanical analyses of the Yucca Mountain Drift Scale Test-Comparison of field measurements to predictions of four different numerical models
J. Rutqvist, D. Barr, R. Datta, A. Gens, A. Millard, S. Olivella, C. Tsang, Y. Tsang

International Journal of Rock Mechanics and Mining Sciences (2005). Vol. 42 (5-6), pp. 680-697

Abstract
The Yucca Mountain Drift Scale Test (DST) is a multiyear, large-scale underground heating test designed to study coupled thermal–hydrological–mechanical–chemical behavior in unsaturated fractured and welded tuff. As part of the international cooperative code-comparison project DEvelopment of COupled models and their VALidation against EXperiments, four research teams used four different numerical models to simulate and predict coupled thermal–hydrological–mechanical (THM) processes at the DST. The simulated processes included heat transfer, liquid and vapor water movements, rock-mass stress and displacement, and stress-induced changes in fracture permeability. Model predictions were evaluated by comparison to measurements of temperature, water saturation, displacement, and air permeability. The generally good agreement between simulated and measured THM data shows that adopted continuum model approaches are adequate for simulating relevant coupled THM processes at the DST. Moreover, thermal-mechanically induced rock-mass deformations were reasonably well predicted using elastic models, although some individual displacements appeared to be better captured using an elasto-plastic model. It is concluded that fracture closure/opening caused by change in normal stress across fractures is the dominant mechanism for thermal-stress-induced changes in intrinsic fracture permeability at the DST, whereas fracture shear dilation appears to be less significant. This indicates that such changes in intrinsic permeability at the DST, which are within one order of magnitude, are likely to be mostly reversible.
Abstract
The Yucca Mountain Drift Scale Test (DST) is a multiyear, large-scale underground heating test designed to study coupled thermal–hydrological–mechanical–chemical [...]
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Double structure THM analysis of a heating test in a fractured tuff incorporating intrinsic permeability variations
S. Olivella, A. Gens

International Journal of Rock Mechanics and Mining Sciences (2005). Vol. 42 (5-6), pp. 667-679

Abstract
This paper presents thermo-hydro-mechanical (THM) analyses simulating the Drift Scale Test (DST) performed at Yucca Mountain. A double structure approach based on two superimposed domains is adopted. Intrinsic permeability changes with deformations imply full THM coupling. Temperatures and gas permeabilities were measured during 4 years and are used to validate the model. Measured gas permeability variations show patterns that are successfully explained by the model calculations. These gas permeability variations may be attributed to thermo-hydraulic effects, and also to mechanical effects. Different cases of intrinsic permeability variations have been considered in the model and their influence on the calculated temperatures, degree of saturations and gas permeabilities are presented. Volumetric deformation, in contraction or dilatancy, implies changes in the aperture of rock fractures that in turn lead to changes in intrinsic permeability. Dilatancy, caused by shear stresses, increases intrinsic permeability. Consideration of this factor contributes significantly to improve the agreement of calculated gas permeability with the measured values obtained during the DST experiment.
Abstract
This paper presents thermo-hydro-mechanical (THM) analyses simulating the Drift Scale Test (DST) performed at Yucca Mountain. A double structure approach based on two [...]
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A constitutive model for unsaturated soils: thermomechanical and computational aspects
D. Sheng, S. Sloan, A. Gens

Computational Mechanics (2004). Vol. 33 (6), pp. 453-465

Abstract
This paper first presents a complete formulation of a constitutive model that deals with the irreversible behaviour of unsaturated soils under various loading and drying/wetting conditions. A standard form of incremental stress-strain relations is derived. The constitutive model is then cast into the thermodynamical theories and verified using the thermomechanical principles. It is shown that hydraulic hysteresis does not contribute to the plastic dissipation, though it contributes to the plastic work. All plastic work associated with a plastic increment of the degree of saturation is stored and can be recovered in a reversed plastic increment of saturation. The incremental constitutive equations are also reformulated for implementation in finite element codes where displacements and pore pressures are primary unknowns. Qualitative predictions of the constitutive model show that incorporating two suction related yield surfaces and non-associated flow rules into the Barcelona Basic Model opens a full range of possibilities in modelling unsaturated soil behaviour
Abstract
This paper first presents a complete formulation of a constitutive model that deals with the irreversible behaviour of unsaturated soils under various loading and drying/wetting [...]
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Finite element formulation and algorithms for unsaturated soils. Part II: verification and application
D. Sheng, D. Smith, S. Sloan, A. Gens

International Journal for Numerical and Analytical Methods in Geomechanics (2003). Vol. 27, pp. 767-790

Abstract
The finite-element formulation and integration algorithms developed in Part I are used to analyse a number of practical problems involving unsaturated and saturated soils. The formulation and algorithms perform well for all the cases analysed, with the robustness of the latter being largely insensitive to user-defined parameters such as the number of coarse time steps and error control tolerances. The efficiency of the algorithms, as measured by the CPU time consumed, does not depend on the number of coarse time steps, but may be influenced by the error control tolerances. Based on the analyses presented here, typical values for the error control tolerances are suggested. It is also shown that the constitutive modelling framework presented in Part I can, by adjusting one constitutive equation and one or two material parameters, be used to simulate soils that expand or collapse upon wetting. Treating the suction as a strain variable instead of a stress variable proves to be an efficient and robust way of solving suction-dependent plastic yielding. Moreover, the concept of the constitutive stress is a particularly convenient way of handling the transition between saturation and unsaturation.
Abstract
The finite-element formulation and integration algorithms developed in Part I are used to analyse a number of practical problems involving unsaturated and saturated soils. [...]
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Finite element formulation and algorithms for unsaturated soils. Part I: theory
D. Sheng, S. Sloan, A. Gens, D. Smith

International Journal for Numerical and Analytical Methods in Geomechanics (2003). Vol. 27 (9), pp. 745-765

Abstract
This paper presents a complete finite-element treatment for unsaturated soil problems. A new formulation of general constitutive equations for unsaturated soils is first presented. In the incremental stress–strain equations, the suction or the pore water pressure is treated as a strain variable instead of a stress variable. The global governing equations are derived in terms of displacement and pore water pressure. The discretized governing equations are then solved using an adaptive time-stepping scheme which automatically adjusts the time-step size so that the integration error in the displacements and pore pressures lies close to a specified tolerance. The non-linearity caused by suction-dependent plastic yielding, suction-dependent degree of saturation, and saturation-dependent permeability is treated in a similar way to the elastoplasticity. An explicit stress integration scheme is used to solve the constitutive stress–strain equations at the Gauss point level. The elastoplastic stiffness matrix in the Euler solution is evaluated using the suction as well as the stresses and hardening parameters at the start of the subincrement, while the elastoplastic matrix in the modified Euler solution is evaluated using the suction at the end of the subincrement. In addition, when applying subincrementation, the same rate is applied to all strain components including the suction
Abstract
This paper presents a complete finite-element treatment for unsaturated soil problems. A new formulation of general constitutive equations for unsaturated soils is first presented. [...]
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The role of structure in the chemically induced deformations of FEBEX bentonite
G. Musso, E. Morales, A. Gens, E. Castellanos

Applied Clay Science (2003). Vol. 23 (1-4), pp. 229-237

Abstract
The paper presents the experimental results of an investigation aimed at understanding the role played by the soil structure in the evolution of chemo-mechanical-coupled processes in bentonite. The objective was pursued by subjecting samples with different structures to similar salinisation–desalinisation cycles. Samples were prepared at approximately the same void ratio by means of either static compaction at hygroscopic humidity or starting from slurries consolidated under mechanical loads. The investigation was carried out both from a macroscopic point of view (measuring the induced deformations) and from a microscopic point of view (ESEM analysis, mercury intrusion porosimetry). It is evidenced how the behaviour of the compacted samples differs from the one in the consolidated samples, especially as far as the evolution in time of the deformations is considered. An attempt to explain the differences by means of structural considerations is then provided.
Abstract
The paper presents the experimental results of an investigation aimed at understanding the role played by the soil structure in the evolution of chemo-mechanical-coupled processes [...]
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Suction effects on a compacted clay under non-isothermal conditions
E. Romero, A. Gens, A. Lloret

Geotechnique (2003). Vol. 53 (1), pp. 65-81

Abstract
The paper presents the results of an experimental study on the effects of suction and temperature on the volumetric behaviour of an unsaturated soil. Statically compacted Boom clay samples have been used in the testing programme, which has been performed using an oedometer apparatus with simultaneous control of suction and temperature. Suctions up to 0·45 MPa and temperatures up to 80°C have been applied. Special measures have been taken to control and minimise the problems associated with phase changes and vapour migration at high temperatures. A comprehensive experimental programme has been carried out, including isothermal tests with control of vertical stress, suction and temperature, non-isothermal tests under constant suction, and isothermal swelling pressure tests. Testing samples at two compacted soil densities provides observations concerning a wide scope of behaviour ranging from collapse to swelling behaviour on wetting. Test results obtained at 80°C have been compared with those of tests performed at room temperature (22°C). Based on these comparisons, thermal effects on a variety of features of behaviour such as swelling potential, swelling pressure, collapsibility, shrinkage strains, compressibility, stress path dependence and degree of strain reversibility are presented and discussed. Although the basic behaviour is similar at high and low temperatures, various significant differences are identified concerning mainly swelling strains developed during wetting and compressibility with respect to loading. Swelling pressure test results indicate that the high-temperature samples yield at a lower stress than that observed in room-temperature samples, suggesting that the stress domain bounded by the main yield surface reduces as temperature increases—a behaviour feature shared with saturated soils.
Abstract
The paper presents the results of an experimental study on the effects of suction and temperature on the volumetric behaviour of an unsaturated soil. Statically compacted [...]
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An elasto-plastic model for unsaturated soil incorporating the effects of suction and degree of saturation on mechanical behaviour
D. Gallipoli, A. Gens, A. Sharma, J. Vaunat

Géotechniques (2003). Vol. 53 (1), pp. 123-135

Abstract
The paper presents an elasto-plastic model for unsaturated soils that takes explicitly into account the mechanisms with which suction affects mechanical behaviour as well as their dependence on degree of saturation. The proposed model is formulated in terms of two constitutive variables directly related to these suction mechanisms: the average skeleton stress, which includes the average fluid pressure acting on the soil pores, and an additional scalar constitutive variable, ξ, related to the magnitude of the bonding effect exerted by meniscus water at the inter-particle contacts. The formulation of the model in terms of variables closely related to specific behaviour mechanisms leads to a remarkable unification of experimental results of tests carried out with different suctions. The analysis of experimental isotropic compression data strongly suggests that the quotient between the void ratio, e, of an unsaturated soil and the void ratio es, corresponding to the saturated state at the same average soil skeleton stress, is a unique function of the bonding effect due to water menisci at the inter-particle contacts. The same result is obtained when examining critical states at different suctions. Based on these observations, an elasto-plastic constitutive model is developed using a single yield surface the size of which is controlled by volumetric hardening. In spite of this simplicity, it is shown that the model reproduces correctly many important features of unsaturated soil behaviour. It is especially remarkable that, although only one yield surface is used in the formulation of the model, the irreversible behaviour in wetting–drying cycles is well captured. Because of the behaviour normalisation achieved by the model, the resulting constitutive law is economical in terms of the number of tests required for parameter determination.
Abstract
The paper presents an elasto-plastic model for unsaturated soils that takes explicitly into account the mechanisms with which suction affects mechanical behaviour as [...]
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Influence of rainfall on the deformation and stability of a slope in overconsolidated clays: a case study
E. Alonso, A. Gens, C. Delahaye

Hydrogeology journal (2003). Vol. 11 (1), pp. 174-192

Abstract
The behaviour of an instrumented unstable slope in a profile of weathered overconsolidated clay has been analysed. Available field investigation data and laboratory tests were integrated in a coupled hydromechanical model of the slope. Particular attention was given to the unsaturated soil conditions above the water table and to the influence of the rainfall record. Recorded pore-water pressures helped to identify the hydrogeological conditions of the slope. The coupled model was used to compute slope deformations and the variation of safety with time. Actual rainfall records were also integrated into the analysis. Comparison of measurements and calculations illustrate the nature of the slope instability and the complex relationships between mechanical and hydraulic factors.
Abstract
The behaviour of an instrumented unstable slope in a profile of weathered overconsolidated clay has been analysed. Available field investigation data and laboratory tests [...]
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Mechanical behaviour of heavily compacted bentonite under high suction changes
A. Lloret, M. Villar, M. Sanchez, A. Gens, X. Pintado, E. Alonso

Géotechniques (2003). Vol. 53 (1), pp. 27-40

Abstract
The paper reports the results of an experimental study carried out on a bentonite compacted to a dry density of up to 1·7 Mg=m3, a high value for this type of soil. The soil fabric has been studied using a variety of techniques, revealing a clear bimodal pore distribution that corresponds to two distinct structural levels: a microstructural one and a macrostructural one. The main testing programme has been performed using oedometers especially designed to apply a very large range of suctions. By applying the axis-translation technique (using nitrogen as the gas fluid), it has been possible to reach suctions up to 15 MPa. The higher suction range has been achieved by applying a controlled atmosphere where the relative humidity has been fixed by a solution of sulphuric acid or salts. In this way suctions up to 550 MPa could be reached. The maximum vertical stress that could be applied in the apparatus was 10 MPa. Two types of test have been carried out: (a) tests in which a combination of loading paths at constant suction and drying/wetting paths at constant load were applied; (b) swelling tests under constant-volume conditions in order to determine the swelling pressure and the stress path followed during wetting. The results of the experimental programme are examined, taking into account the role of the soil fabric in controlling observed mechanical behaviour. In addition, the results of the laboratory tests are reproduced and interpreted using a generalised plasticity model that considers explicitly the interaction between macrostructure and microstructure. In this way, it is possible to achieve a more complete understanding of the mechanisms that underlie observed behaviour, and in particular the interplay between the two structural levels
Abstract
The paper reports the results of an experimental study carried out on a bentonite compacted to a dry density of up to 1·7 Mg=m3, a high value for this type of soil. [...]
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Ensayos de suelos parcialmente saturados con control simultáneo de succión y temperatura
E. Romero, A. Gens, A. Lloret

Ingeniería Civil (Madrid) (2002). Vol. 127, pp. 115-120

Abstract
En el artículo se presentan algunos avances en técnicas experimentales relacionadas con la caracterización termo-hidro-mecánica de suelos parcialmente saturados. Se describen los principales problemas asociados con la aplicación de la técnica de traslación de ejes a temperaturas elevadas. Se presentan algunos resultados experimentales de trayectorias no isotérmicas a succión matricial impuesta efectuadas sobre diferentes muestras de arcilla preparadas artificialmente.
Abstract
En el artículo se presentan algunos avances en técnicas experimentales relacionadas con la caracterización termo-hidro-mecánica de suelos parcialmente [...]
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Evaluation of a constitutive model for clays and sands: Part II - clay behaviour
J. Pestana, A. Whittle, A. Gens

International Journal for Numerical and Analytical Methods in Geomechanics (2002). Vol. 26 (11), pp. 1123-1146

Abstract
The undrained response of cohesive soils is of paramount importance in geomechanics and it has beenmodelled extensively for the last 50 years. In comparison, drained behaviour of clays has received onlymodest attention. Drained and undrained behaviour is signi¿cantly affected by past consolidation stresshistory. This paper evaluates the capabilities of the MIT-S1 effective stress model, described in acompanion paper, for predicting the anisotropic stress–strain–strength behaviour of clays. The paperillustrates the selection of model parameters for Lower Cromer Till, using data from standard types oflaboratory tests. Comparison of model simulations with measured response for Lower Cromer Till andBoston Blue Clay illustrate model capabilities. The work focuses initially on comparisons of modelpredictions with measurements from undrained triaxial and plane strain tests on initially K0-consolidatedspecimens. Comparisons with measured data from undrained shear tests performed in different modes ofshearing for LCT and BBC show that the model: (a) gives excellent predictions of maximum shear stressconditions and accurately describes the non-linear shear stress–strain behaviour; (b) accurately describesthe anisotropic shear stress–strain–strength conditions for different consolidation stress histories; and(c) gives realistic description of mobilized friction angles, especially at large OCR’s. The paper then focuseson the effects of consolidation stress history for isotropically consolidated specimens of resedimentedLower Cromer Till and Boston Blue Clay. Finally, the paper compares model predictions for drained sheartests on K0and isotropically consolidated specimens with overconsolidation ratios, OCR410; used toevaluate particular aspects of the critical state framework of soil behaviour. Overall, the model givesexcellent predictions of the effect of initial anisotropy and overconsolidation stress history on the shearstress–strain and volumetric behaviour of clays.
Abstract
The undrained response of cohesive soils is of paramount importance in geomechanics and it has beenmodelled extensively for the last 50 years. In comparison, drained behaviour [...]
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A constitutive model for crushed salt
S. Olivella, A. Gens

International Journal for Numerical and Analytical Methods in Geomechanics (2002). Vol. 26 (7), pp. 719-746

Abstract
A constitutive model for crushed salt is presented in this paper. A creep constitutive model is developed¿rst and compared with test results. The constitutive model presented here concentrates on creepdeformation because saline media behave basically in a ductile and time-dependent way. An idealizedgeometry is used as a common framework to obtain stress–strain macroscopic laws based on twodeformation mechanisms: ¿uid-assisted diffusional transfer creep and dislocation creep. The model is ableto predict strain rates that compare well with results from laboratory tests under isotropic and oedometricconditions. Macroscopic laws are written using a non-linear viscous approach, which incorporates also aviscoplastic component, based on critical state theory. The viscoplastic term is intended for non-creepdeformation mechanisms such as grain reorganization and crushing.
Abstract
A constitutive model for crushed salt is presented in this paper. A creep constitutive model is developed¿rst and compared with test results. The constitutive model [...]
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Factors controlling rock-clay buffer interaction in a radioactive clay repository
A. Gens, L. Guimareas, A. García-Molina, E. Alonso

Engineering Geology (2002). Vol. 64 (2-3), pp. 297-308

Abstract
The paper examines the interaction between host rock and a clay-based engineered barrier in the context of deep underground disposal of high-level radioactive waste. A large scale “in situ” heating test currently under way in the underground Grimsel Rock Laboratory (GRL; Switzerland) is adopted as a representative case for study. The main features of the behaviour of the rock/barrier system have been examined using a comprehensive programme of coupled thermo–hydro–mechanical analyses. A parametric study has been performed to assess the effects of individual phenomena and parameters. The characterisation activities carried out in relation with the in situ test provide the information on material properties and test conditions required. Results highlighting the influence of vapour diffusion, permeability, and retention curve of rock and bentonite are presented. The interaction between clay barrier and host medium exhibits a high degree of complexity.
Abstract
The paper examines the interaction between host rock and a clay-based engineered barrier in the context of deep underground disposal of high-level radioactive waste. [...]
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Temperature effects on the hydraulic behaviour of an unsaturated clay
E. Romero, A. Gens, A. Lloret

Geotechnical and Geological Engineering (2001). Vol. 19 (3/4), pp. 311-332

Abstract
The influence of temperature on the hydraulic properties of unsaturated clays is of major concern in the design of engineered barriers in underground repositories for high-level radioactive waste disposal. This paper presents an experimental study centred on the investigation of the influence of temperature on soil hydraulic properties related to water retention and permeability. Laboratory tests were conducted on artificially prepared unsaturated fabrics obtained from a natural kaolinitic-illitic clay. Special attention is given to the testing procedures involving controlled suction and temperature oedometer cells and the application of the vapour equilibrium technique at high temperatures. Retention curves at different temperatures show that total suction tends to reduce with increasing temperatures at constant water content. Temperature influence on water permeability is more relevant at low matric suctions corresponding to bulk water preponderance (inter-aggregate zone). Below a degree of saturation of 75% no clear effect is detected. Experimental data show that temperature dependence on permeability at constant degree of saturation and constant void ratio is smaller than what could be expected from the thermal change in water viscosity. This behaviour suggests that phenomena such as porosity redistribution and thermo-chemical interactions, which alter clay fabric and pore fluid, can be relevant.
Abstract
The influence of temperature on the hydraulic properties of unsaturated clays is of major concern in the design of engineered barriers in underground repositories for [...]
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Artificial neural network to estimate an index of water quality
L. Quiñones Huatangari, L. Ochoa Toledo, N. Kemper Valverde, O. Gamarra Torres, J. Bazán Correa, J. Delgado Soto

Enfoque UTE - Scientific Engineering Journal (2020). Vol. 11, (2), 40

Abstract
The artificial neural network (RNA) is a computational model that emulates the biological neural system in information processing. The originating models are suitable for the purpose of describing long-term specifics, in addition to nonlinear relationships. This tool is used to predict physical chemical and microbiological parameters that influence water quality. The United States National Sanitation Foundation proposed a water quality index, known as the NSF WQI. This article describes the design, training and use of the three-layer neural perceptron neural model for the calculation of the NSF WQI of the Utcubamba River and its tributaries. Using the Matlab software and applying the Levenberg-Marquardt training algorithm, the optimal RNA architecture was found to be 6-12-1, plus the percentage for the training, validation, and test sets of 70 %, 10 %, and 20 % respectively. RNA performance has been evaluated using the root of the root mean square error (RMSE) and the correlation coefficient (R). High correlations (greater than 0.94) were made between the measured and predicted values. Finally, the RNA proposal offers a useful alternative for the calculation and prediction of the water quality index in relation to dissolved oxygen (DO), biochemical demand for oxygen (BOD), nitrates, fecal coliforms, potential for hydrogen ions (pH) and turbidity.
Abstract
The artificial neural network (RNA) is a computational model that emulates the biological neural system in information processing. The originating models are suitable for [...]
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Third-party management in software development: proposal of a methodology
Y. Núñez-Sánchez, A. González-Torres

Enfoque UTE - Scientific Engineering Journal (2020). Vol. 11, (2), 38

Abstract
Outsourcing allows organizations to reduce costs, optimize the use of resources, provide added value in services and goods, and concentrate on achieving the strategic objectives of the business. However, many software projects have failed due to the lack of knowledge of the factors involved in its planning. Some of these elements are poor communication with the supplier, weak monitoring of the status of tasks, lack of active participation of users and stakeholders at all stages of the process, and software quality assurance. Consequently, this research aims to aid the management of the outsourcing process and proposes a methodology for its management during system development. The methodology comprises 7 phases, each one with its inputs, tasks, and outputs, which are used by the following stages. In summary, it synthesizes the best practices for the management of third parties.
Abstract
Outsourcing allows organizations to reduce costs, optimize the use of resources, provide added value in services and goods, and concentrate on achieving the strategic objectives [...]
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CFD Analysis of a solar flat plate collector with different cross sections
A. Andrade Cando, W. Quitiaquez Sarzosa, L. Toapanta

Enfoque UTE - Scientific Engineering Journal (2020). Vol. 11, (2), 36

Abstract
Low and medium solar heating systems used for domestic and industrial applications, such as water and space heating, usually utilize solar flat plate collectors in order to absorb solar thermal energy converting it into heat and then transferring the heat to a fluid (usually water or air) that flows through it. The aim of this study is to evaluate the solar flat plate collector’s efficiency and the fluid behavior inside the pipeline with three different cross sections, whose hydraulic diameters are 10, 5.12 and 6.16 mm, by using ANSYS Fluent. The results obtained from the Computational Fluid Dynamics (CFD) tool showed that the collector with the Type I cross section reached temperatures up to 330 K at the pipe outlet obtaining an efficiency of 68 %, higher than those of Types II and III, whose efficiencies were 51 % and 60 %, respectively. Type I cross section also presented the lowest values in both speed and pressure drop, these being 0.266 m/s and 108.3 Pa, respectively.
Abstract
Low and medium solar heating systems used for domestic and industrial applications, such as water and space heating, usually utilize solar flat plate collectors in order to [...]
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Optimal Location of Transformers in Electrical Distribution Networks Using Geographic Information Systems
S. Pinzón, S. Yánez, M. Ruiz

Enfoque UTE - Scientific Engineering Journal (2020). Vol. 11, (1), 34

Abstract
This research shows a heuristic model for the design of scalable and reliable electrical distribution networks. The algorithms presented allow to optimize the location of transformation centers using on their database geographic information systems from which it is possible to define user locations, candidate sites, possible routes for the deployment of the electricity grid and, in general, data for the reconstruction of the scenario. The model employs clustering and triangulation methods, as well as algorithms for creating a minimally expanding tree and the consequent site assignment for transformer placement. After setting the optimal locations for the transformer site, the algorithms compute voltage drops in secondary circuits, required transformation capability, execution times, and coverage achieved. The results obtained are adjusted to the requirements of an actual distribution power grid and show a good performance on the proposed scenario.
Abstract
This research shows a heuristic model for the design of scalable and reliable electrical distribution networks. The algorithms presented allow to optimize the location of [...]
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Alternative Under Frecuency Load Disconection Methodology Based on Semi-Adaptative Model
D. Carrión, A. Ayo, J. González

Enfoque UTE - Scientific Engineering Journal (2020). Vol. 11, (1), 32

Abstract
Stability analysis in electrical power systems is based on the study in dynamic state of the voltage and frequency, since at the moment that there is some contingency it fluctuates drastically due to the primary and secondary controls of the voltage and frequency to the power systems that act on the generators. In order to solve the possible stability problems that may arise in power systems, various techniques have been developed that act on the generating machines for their protection as well as on the loads for the power cut. The present investigation proposes an alternative methodology for load disconnection by low frequency as an option to save the power system from a possible blackout due to instability due to a fall in the frequency, managing to improve the results affected by other improved techniques, the frequency change range, frequency deviation and the effects of demand disconnection. The proposed methodology was tested in the IEEE 14 bus system.
Abstract
Stability analysis in electrical power systems is based on the study in dynamic state of the voltage and frequency, since at the moment that there is some contingency it fluctuates [...]
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Outlier detection with data mining techniques and statistical methods
M. Orellana Cordero, P. Cedillo

Enfoque UTE - Scientific Engineering Journal (2020). Vol. 11, (1), 30

Abstract
The detection of outliers in the field of data mining (DM) and the process of knowledge discovery in databases (KDD) is of great interest in areas that require support systems for decision making. A straightforward application can be found in the financial area, where DM can potentially detect financial fraud or find errors produced by the users. Thus, it is essential to evaluate the veracity of the information, through the use of methods for the detection of unusual behaviors in the data. This paper proposes a method to detect values that are considered outliers in a database of nominal type data. The method implements a global algorithm of "k" closest neighbors, a clustering algorithm called k-means and a statistical method called chi-square. These techniques have been implemented on a database of clients who have requested a financial credit. The experiment was performed on a data set with 1180 tuples, where, outliers were deliberately introduced. The results showed that the proposed method is able to detect all the outliers entered.
Abstract
The detection of outliers in the field of data mining (DM) and the process of knowledge discovery in databases (KDD) is of great interest in areas that require support systems [...]
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Estimated cost of electricity with time horizon for micro grids based on the policy response of demand for real price of energy
E. GARCIA TORRES

Enfoque UTE - Scientific Engineering Journal (2020). Vol. 11, (1), 28

Abstract
The intelligent microgrids are an efficient alternative, which allows to supply the demand decreasing the losses of the electrical system and at the same time; the environment and the consumers are the main beneficiaries. This article develops a heuristic based on an energy management model based on the real price of electricity, which will allow end users to encourage the implementation of a policy of response to demand, in order to optimize their consumption, for which a micro smart grid is analyzed, with conventional and non-conventional renewable generation, In addition, a mechanism of "real energy price" will be implemented as a policy of response to demand, with the aim of optimizing the costs of energy that will be transferred to users depending on the stratum to which it belongs, these costs will have a short-term horizon with hourly intervals, achieving a reduction in the purchase of energy from the system
Abstract
The intelligent microgrids are an efficient alternative, which allows to supply the demand decreasing the losses of the electrical system and at the same time; the environment [...]
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Novel Approach for Teaching English Language using Emerging Information and Communication Technologies for Visual Impairment Students
J. Cardenas, E. Inga

Enfoque UTE - Scientific Engineering Journal (2020). Vol. 11, (1), 26

Abstract
Nowadays, Higher Education Institutions (HEI) need to be more inclusive from the methodological vision appropriate used in the classroom. Visual impairment students (VIS) become a challenge for teachers who look for incorporating different learning environments. This challenge increases when teaching and learning process to VIS is related to the English language; due to the absence of educational didactic resources available at the school and the lack of knowledge of appropriate teaching methodologies and strategies to develop the different language skills: listening, speaking, reading and writing that go with the educational process in the acquisition of the English language. In accordance with this problem, this research proposes a novel methodological strategy, based on the usage of meaningful learning with emerging technologies as a main guide for students with visual impairment. Thus, the road map of good practices in the use of this methodology will allow establishing a clear process to achieve students’ active, meaningful and autonomous participation, using the least time in the teaching-learning process of the English language.
Abstract
Nowadays, Higher Education Institutions (HEI) need to be more inclusive from the methodological vision appropriate used in the classroom. Visual impairment students (VIS) [...]
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Evaluation of video traffic behavior metrics in an experimental multicast network
H. Facchini, S. Pérez, A. Dantiacq, F. Hidalgo

Enfoque UTE - Scientific Engineering Journal (2020). Vol. 11, (1), 24

Abstract
Currently, there is a significant change in data traffic on networks, both local and Internet, across the spectrum of possible customers, whether fixed or mobile. The consumption of video traffic increases day by day in all its formats. Service providers are required more bandwidth, quality and performance, for all products, but mainly those based on video. This article presents an improved version of our laboratory tests for generic networks with four sub-scenarios, which allow injecting multicast video traffic, represented in different codecs. The topology of network used includes a streaming server and up to 20 desktop computers as clients. The most important metrics to be analyzed have been defined, based on performance and quality of service (QoS) measurements under limited bandwidth conditions. Experimental numerical results can be useful for administrators, simulation analysts, designers, and data network planners. But, also, this document aims to serve as a guide for the management of similar networks and the efficiently use available resources, without compromising performance and QoS.
Abstract
Currently, there is a significant change in data traffic on networks, both local and Internet, across the spectrum of possible customers, whether fixed or mobile. The consumption [...]
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