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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 e_s, 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.

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.