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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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