Scipedia: Computational Geomechanics

Benchmarks for geotechnical modeling

María Jesús Samper — Tue, 11 Dec 2018 12:50:44 +0100

This letter represents an initiative started by a number of researchers signed below who are working in the field of numerical modelling of soil mechanics problems. We belive that the type of approach outlined is important in a serious numerical study of all problems in geomechanics and indeed is valid in other areas.

Physical modeling of a stepped spillway without sidewalls

María Jesús Samper — Mon, 29 Jun 2020 10:13:58 +0200

The interest of a consulting company in designing stepped spillways in roller compacted concrete (RCC) dams led us to propose the possibility of building this type of spillway without sidewalls. Previous research on stepped spillways has focused on characterizing the complex hydraulic behavior of flow on these structures, as well as design criteria. Such studies have usually been conducted on stepped spillways with a constant width along the spillway, that is, with sidewalls. In the present work, we report the results of the physical modeling of a generic stepped spillway without sidewalls (slope 1v:0.8h). In general terms, the lack of sidewalls produces a lateral expansion of water and therefore a non-uniform longitudinal and transversal discharge distribution. Consequently, the flow type, characteristic water depth, velocity, air concentration, and pressure fields change along and across the spillway. The resulting data demonstrate that the distribution of the different variables studied depend on the specific discharge at the entrance and the spillway height.

Preserving steady-state in one-dimensional finite-volume computations of river flow

María Jesús Samper — Fri, 26 Jun 2020 10:44:00 +0200

When using finite-volume methods and the conservative form of the Saint Venant equations in one-dimensional flow computations, it is important to establish the correct balance between the discretized flux vector and the geometric source terms. Over the last few years various improvements to numerical schemes have been presented to achieve this correct balance, focusing on the capability to simulate water at rest on irregular geometries (C-property). In this paper it is shown that common schemes can lead to energy-violating solutions in the case of steady flow. We present developments based on the Roe TVD finite-volume scheme for one-dimensional Saint Venant equations, which results in a method that not only satisfies the C-property, but also preserves the correct steady flow when stationary boundary conditions are used. We also present a totally irregular channel test case for the verification of the method.

Diseño hidráulico de aliviaderos escalonados en presas de HCR

María Jesús Samper — Thu, 25 Jun 2020 15:56:02 +0200

En el presente artículo se presenta un conjunto de criterios para el diseño de aliviaderos escalonados con pendientes típicas de presas de hormigón compactado con rodillo. El texto se basa en Información recogida en la bibliografía e incorpora los resultados del trabajo de Investigación desarrollado en Amador (2005). Se aborda el diseño de la cresta, la selección del ancho del aliviadero y la geometría de los peldaños. Se proponen criterios para determinar el tipo de flujo existente sobre la estructura y, en el caso del flujo rasante, se presentan expresiones que permiten obtener las principales características del flujo a lo largo de la rápida. Con ha.se en los resultados del análisis del campo de presiones se proponen un conjunto de ecuaciones para estimar las solicitaciones que el vertido ejerce sobre los peldaños a lo largo del aliviadero. Finalmente se hacen algunas consideraciones sobre los aspectos constructivos de los aliviaderos escalonados.

Presa de Caruachi. Estudio del comportmiento hidráulico de las compuertas de los conductos de desvío

María Jesús Samper — Thu, 25 Jun 2020 14:16:18 +0200

Se presenta el estudio desarrollado en el Laboratorio de Modelos Reducidos del Dpto. de Ing. Hidráulica, Marítima y Ambiental de la UPC, para el análisis del comportamiento hidráulico de las compuertas a utilizar en la operación de cierre de la presa de Caruachi en el río Caroní (Venezuela). Se han analizado la capacidad de desagüe, la aireación de los conductos de desvío, las presiones en los mismos durante la operación, así como las vibraciones y fuerza vertical descendente (down-pull) sobre las compuertas. The present article describes the study carried out at the Reduced Model Laboratory (Hydraulic, Maritime and Environmental Engineering Department, UPC) into the hydraulic behaviour of the gates employed in the closing operation of the Caruachi dam on the River Caroní (Venezuela). An analysis is made of the discharge and aeration capacity of the diversión conduits, the operation pressures in the same and the vibration and down-pull on the gates.

Soil-structure interaction simulation of landslides impacting a structure using an implicit material point method

María Jesús Samper — Tue, 09 Jun 2020 14:30:59 +0200

Global warming and climate changes have been one of the many causes that triggered numerous catastrophic landslide and mudflow disasters in the past twenty years. The increase of earth temperature has contributed to the increase of precipitation and undisputedly affected the soil slope stability, which by further may cause landslides in various scale and speed. This large soil deformation phenomenon carries along huge rocks and heavy materials that often results in extensive damage in civil infrastructures both directly or indirectly. Coming with this motivation, a soil-structure interaction simulation based on the implicit material point method (MPM) has been implemented within the Kratos Multiphysics framework for the objective of predicting structural deformation and, furthermore, structural failure caused by environmental flow problems such as landslides. In the current study, the soil is modeled using a non-associated Mohr-Coulomb-based elastoplastic law, while the structure is modeled as elastic and Neo-Hookean hyperelastic materials. In the numerical tests conducted, the equivalent stress and displacement measured on both rigid and flexible structures show a good qualitative agreement. In the future works, a more adequate consideration of the soil and structural model will be investigated before conducting a real-scale landslide simulation.

Coupled Soil-Structure Interaction Modeling and Simulation of Landslide Protective Structures

María Jesús Samper — Tue, 09 Jun 2020 13:35:40 +0200

Within the past two decades, mass movements hazards involving fast and large soil deformation have increased significantly in frequency and magnitude due to their strong relation to climate changes and global warming. These phenomena often bring along rocks, debris, and heavy materials that can extensively damage and destroy the landscape and infrastructures, causing devastating economic loss, and often, human casualties. The risk of future disasters continues to escalate with the increase of real estate development in suburban areas, including mountainous regions. Further assessment and prediction on such disasters and their countermeasures are, therefore, in high economic demands. One of the most intuitive ways is to install protective structures in mountain slopes and valleys that can hold the materials brought by the moving landslides. While the current state of the art of landslide prediction using numerical methods has been mainly dominated by the development of advanced geomechanical models suited for different types of soil materials, e.g. multi-phase unsaturated soil model, this study focuses more on the interaction of such phenomena with the installed protective structures. Here, an implicit formulation of material point method (MPM) is implemented to model the landslides considering finite strain assumption. Furthermore, a staggered coupling scheme with traditional Finite Element Method (FEM) is proposed to simulate accurately and robustly the dynamic force and displacement coupling of soil-structure interaction (SSI). All developments of the method are implemented within the Kratos-Multiphysics framework and available under the BSD license (https://github.com/KratosMultiphysics/Kratos/wiki). In the future works, more adequate consideration of coupling scheme and material models considering damage and fracture will be investigated before conducting a real-scale landslide simulation.

Characterisation of the multi-scale fabric features of high plasticity clays

María Jesús Samper — Mon, 04 May 2020 10:26:57 +0200

An investigation of the microstructural features of a high-plasticity clay, in both its natural conditions and reconstituted in the laboratory, is described. Scanning electron microscopy is used here to characterise the fabric at different magnifications, while image processing of the micrographs delivers a quantitative assessment of the fabric orientation. The results of energy-dispersive X-ray spectroscopy and swelling tests, as reported in a previous study by the authors, are used to characterise the bonding nature and strength, as well as mercury intrusion porosimetry to investigate clay porosimetry. Despite their identical composition, the natural and the reconstituted clay have experienced different deposition and loading history, generating different microstructural features that are shown to underlie their differences in state. For both clays, one-dimensional (1D) compression to medium–high pressures is seen to determine a well-oriented medium magnification fabric. However, larger-scale observations and the corresponding image processing results reveal non-uniform local fabric features, hence making fabric characterisation dependent on the scale of analysis and bringing about the issue of identifying the clay micro-scale representative element volume relating to the clay macro-behaviour. The micro-REV is identified for the clays under study and its connection with the macro-behaviour characterised. The microstructural evolution induced by 1D compression to very high pressures is shown to concern mainly the clay porosity and porosimetry, the fabric orientation being steady, thus explaining the isotropic hardening observed in laboratory tests.

Discussion on 'Effects of particle breakage and stress reversal on the behaviour of sand around displacement piles'

María Jesús Samper — Mon, 04 May 2020 10:02:46 +0200

Micromechanical inspection of incremental behaviour of crushable soils

María Jesús Samper — Mon, 04 May 2020 09:49:53 +0200

In granular soils grain crushing reduces dilatancy and stress obliquity enhances crushability. These are well-supported specimen-scale experimental observations. In principle, those observations should reflect some peculiar micromechanism associated with crushing, but which is it? To answer that question the nature of crushing-induced particle-scale interactions is here investigated using an efficient DEM model of crushable soil. Microstructural measures such as the mechanical coordination number and fabric are examined while performing systematic stress probing on the triaxial plane. Numerical techniques such as parallel and the newly introduced sequential probing enable clear separation of the micromechanical mechanisms associated with crushing. Particle crushing is shown to reduce fabric anisotropy during incremental loading and to slow fabric change during continuous shearing. On the other hand, increased fabric anisotropy does take more particles closer to breakage. Shear-enhanced breakage appears then to be a natural consequence of shear-enhanced fabric anisotropy. The particle crushing model employed here makes crushing dependent only on particle and contact properties, without any pre-established influence of particle connectivity. That influence does not emerge, and it is shown how particle connectivity, per se, is not a good indicator of crushing likelihood

Standard penetration testing in a virtual calibration chamber

María Jesús Samper — Thu, 30 Apr 2020 17:47:41 +0200

The virtual calibration chamber technique, based on the discrete element method, is here applied to study the standard penetration test (SPT). A macro-element approach is used to represent a rod driven with an impact like those applied to perform SPT. The rod is driven into a chamber filled with a scaled discrete analogue of a quartz sand. The contact properties of the discrete analogue are calibrated simulating two low-pressure triaxial tests. The rod is driven changing input energy and controlling initial density and confinement stress. Energy-based blowcount normalization is shown to be effective. Results obtained are in good quantitative agreement with well-accepted experimentally-based relations between blowcount, density and overburden. It is also shown that the tip resistance measured under impact dynamic penetration conditions is close to that under constant velocity conditions, hence supporting recent proposals to relate CPT and SPT results

Sphericity measures of sand grains

María Jesús Samper — Thu, 30 Apr 2020 17:27:42 +0200

The sphericity of a grain should measure the similitude of its shape with that of a sphere. Sphericity is a shape descriptor of long-standing interest for sedimentology. Now it has gained also interest to facilitate discrete element modelling of granular materials. True sphericity was initially defined by a surface ratio that requires three-dimensional (3D) grain surface measurement. That kind of measurement has been practically impossible until recently and, as a consequence, a number of alternative 3D measures and 2D proxies were proposed. In this work we present results from a study of grain shape based on x-ray tomography of two different sand specimens, containing more than 110.000 particles altogether. Sphericity measures were systematically obtained for all grains. 2D proxy measures were also obtained in samples of oriented and not-oriented grains. It is shown that the 2D proxy best correlated with true sphericity is perimeter sphericity, whereas the traditional Krumbein-Sloss chart proxy is poorly correlated. 2D measures acquired through minor axis projection are more closely related to 3D measures than those acquired using random projections.

Clays in natural and engineered barriers for nuclear waste disposal

María Jesús Samper — Thu, 30 Apr 2020 16:50:04 +0200

Argillaceous rocks and expansive clays play a key role in many scenarios of the deep geological disposal of nuclear waste.

Grading evolution and critical state in a discrete numerical model of Fontainebleau sand

María Jesús Samper — Thu, 30 Apr 2020 16:37:07 +0200

Granular materials reach critical states upon shearing. The position and shape of a critical state line (CSL) in the compression plane are important for constitutive models, interpretation of in situ tests and liquefaction analyses. It is not fully clear how grain crushing may affect the identification and uniqueness of the CSL in granular soils. Discrete-element simulations are used here to establish the relation between breakage-induced grading evolution and the CSL position in the compression plane. An efficient model of particle breakage is applied to perform a large number of tests, in which grading evolution is continuously tracked using a grading index. Using both previous and new experimental results, the discrete-element model is calibrated and validated to represent Fontainebleau sand, a quartz sand. The results obtained show that, when breakage is present, the inclusion of a grading index in the description of critical states is advantageous. This can be simply done using the critical state plane (CSP) concept. A CSP is obtained for Fontainebleau sand.

Numerical simulation of the undrained stability of slopes in anisotropic fine-grained soils

María Jesús Samper — Thu, 30 Apr 2020 16:26:24 +0200

The undrained stability of slopes in anisotropic fine-grained soils is studied in this paper using the finite element method (FEM). A constitutive model is presented, able to account for the observed variation of undrained strength with loading direction. The model is able to encompass the different strength distributions observed in normally, slightly overconsolidated and heavily overconsolidated soils. A series of stability analyses have been performed to explore the effect of the type of undrained strength anisotropy on the stability and failure mechanisms of slopes of different inclinations. In addition, a real case study of the failure of an underwater slope is analysed with the numerical approach presented. It suggests that, by considering undrained strength anisotropy, the failure can be satisfactorily explained.

SDMT-based numerical analyses of deep excavation in soft soil

María Jesús Samper — Thu, 30 Apr 2020 16:19:38 +0200

The paper explores the application of conventional (DMT) and seismic (SDMT) dilatometer tests to an important case of deep excavation design. The work presents finite-element analyses simulating a deep excavation close to Barcelona (Spain). A thick layer of soft interbedded sandy and silty soils made characterization based on laboratory testing very difficult. SDMT offered an alternative for estimating the soil stiffness and its stress-strain dependency. Numerical results and high-quality monitoring data show quite close agreement for most phases of the construction process, supporting the use of seismic dilatometer tests in numerical analyses of deep excavations. The paper also indicates the importance of incorporating stiffness data at low strains. FE analyses involved some uncertainties derived from the presence of jet-grouting soil treatments. On this point, a parametric study illustrates the effects of different modeling approaches.

Nonlocal plasticity modelling of strain localisation in stiff clays

María Jesús Samper — Thu, 30 Apr 2020 15:52:08 +0200

The paper addresses the numerical simulation of strain localisation in stiff clays that exhibit softening behaviour. An elastoplastic constitutive model developed to incorporate key features of stiff clay behaviour is described first. A non-local formulation is then introduced for the regularisation of the analysis of localisation. A series of analyses were conducted to explore relevant aspects of the numerical simulation of localisation. A 3D analysis was also performed to assess the suitability of the approach presented for 3D applications. Finally, application to the simulation of a laboratory test on Beaucaire marl results in an excellent reproduction of experimental observations.

On the choice of stress–strain variables for unsaturated soils and its effect on plastic flow

María Jesús Samper — Thu, 30 Apr 2020 15:43:52 +0200

The net stress plus suction and the average skeleton stress plus modified suction are two alternative sets of energetically consistent stress variables for modelling the hydro-mechanical behaviour of unsaturated soils. When used in conjunction with their work-conjugate strains, both sets of stress variables correctly calculate the first-order term of the hydro-mechanical work input into a soil element subjected to infinitesimal changes of deformation and water content. They therefore also correctly calculate the increment of internal energy along a given stress–strain path, that is the integral of the first-order term of the infinitesimal work input. This paper shows, however, that the above two sets of stress variables lead to different values of the second-order term of the hydro-mechanical work input. They are therefore no longer equivalent with respect to other aspects of material behaviour governed by the second-order work such as the flow rule of elasto-plastic models. The flow rule assumes the normality between plastic strains and equipotential surfaces defined in the conjugate stress–strain space. This normality is however lost when an elasto-plastic model originally formulated in terms of net stress plus suction is recast in terms of average skeleton stress plus modified suction (or vice versa) by means of standard mapping relationships between stress variables. To restore normality in both stress spaces, it is necessary to impose specific forms of elastic and plastic behaviour.

A time-dependent anisotropic model for argillaceous rocks: application to an underground excavation in Callovo-Oxfordian claystone

María Jesús Samper — Thu, 30 Apr 2020 15:15:32 +0200

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.

Numerical analysis of the hydromechanical response of Callovo-Oxfordian claystone to deep excavations

María Jesús Samper — Thu, 30 Apr 2020 15:07:40 +0200

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.