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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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• Precompression design for secondary settlement reduction

  E. Alonso, A. Gens, A. Lloret

  Geotechnique (2000). Vol. 50 (6), pp. 645-656

  
  

  Abstract

  A large water treatment plant is to be built on soft deltaic deposits. Precompression has been selected as the method for achieving the required ground improvement. An instrumented preload test has been carried out to obtain reliable information on precompression performance. Distributions of displacements throughout the foundation depth, obtained using sliding micrometer extensometers, have proved extremely useful for identifying the mechanisms of behaviour controlling ground deformation. The magnitudes of displacements are largely dependent on the overconsolidation state of the soil. As primary consolidation settlements take place rather quickly, they can be largely controlled by applying a preload over a limited period. The main design criterion therefore concerns secondary settlements. Laboratory and field data indicate clearly that overconsolidating the soil, even in moderate amounts, significantly reduces the secondary compression rate. The performance of an unloading stage in the preload test provides crucial information in this regard. Therefore applying a preload surcharge larger than the final structure load is quite effective in controlling the magnitude of subsequent secondary settlements. The information collected during the site investigation and in the preload test provides the basis for the development of a ground deformation model that can be used for computing settlement histories. The model is used as a design tool for the final proposal of the precompression treatment required for the various structures of the plant

  Abstract
  A large water treatment plant is to be built on soft deltaic deposits. Precompression has been selected as the method for achieving the required ground improvement. An instrumented [...]

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• A stress point algorithm for an elastoplastic model in unsaturated soils

  J. Vanuat, J. Cante, A. LEDESMA, A. Gens

  International Journal of Plasticity (2000). Vol. 16, pp. 121-141

  
  

  Abstract

  Two stress fields, combination of total stresses, liquid pressure and gas pressure have to be considered to explain the deformational behaviour of unsaturated media. Elastoplastic models developed for these materials consider generally two yield surfaces, each one associated to a stress field, and whose intersection produces a corner in the space of generalised stress components. In this paper, a stress point algorithm is proposed to cope with the integration of such constitutive laws, which can be seen as non smooth multisurface plastic models in the space of the two stress fields. The basic model developed by Alonso et al. (Alonso, E.E., Gens, A., 1990. A constitutive model for partially saturated soils. Géotechnique 40 (3), 405–430), which will be used to test the algorithm, is first described. Generalised stress and strain variables are then defined. Implementation of the return mapping algorithm, based on an implicit integration scheme, is presented with special attention devoted to the problem of mixed control imposed by the F.E. formulation generally used to analyse the hydromechanical behaviour of unsaturated media. Validation results on distinct generalised stress paths are given at the end.

  Abstract
  Two stress fields, combination of total stresses, liquid pressure and gas pressure have to be considered to explain the deformational behaviour of unsaturated media. Elastoplastic [...]

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• Vapour transport in low permeability unsaturated soils with capillary effects

  S. Olivella, A. Gens

  Transport in Porous Media (2000). Vol. 40 (2), pp. 219-241

  
  

  Abstract

  A discussion of water phase change in unsaturated soils that develop capillary effects is first carried out in the paper. A distinction between the GR (geothermal reservoir) and the NUS (nonisothermal unsaturated soil) approaches is performed. Several aspects concerning advective and nonadvective fluxes of vapour are described secondly and some relationships concerning the case of mass motion in a closed system subjected to temperature gradients derived. Since the structure of unsaturated clays changes with moisture content, in order to correctly simulate the coupled phenomena induced by temperature gradients a model for intrinsic permeability as a function of humidity is required. A preliminary version of the model is presented and applied to interpret a laboratory test by means of a numerical simulation using CODE BRIGHT

  Abstract
  A discussion of water phase change in unsaturated soils that develop capillary effects is first carried out in the paper. A distinction between the GR (geothermal reservoir) [...]

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• Water permeability, water retention and microstructure of unsaturated compacted Boom clay

  E. Alonso, J. Vaunat, A. Gens

  Engineering Geology (1999). Vol. 54 (1-2), pp. 117-127

  
  

  Abstract

  Three classes of experiments are considered in this paper to provide information for two artificially prepared Boom clay fabrics: mercury intrusion/extrusion tests; main wetting/drying paths; and water inflow/outflow transient (permeability) tests. These tests, which are usually treated separately, are joined in a common reference frame to provide information about the morphology of the porous medium and factors influencing Boom clay unsaturated hydraulic states with reference to water retention curves and relative water permeability values. The main objective is to interpret mercury intrusion porosimetry results in order to define an entrance pore size region at ca 130 to 180 nm separating intra-aggregate and inter-aggregate zones. This pore size region is further associated to a delimiting zone in the retention curve separating regions of ‘intra-aggregate governing suction’ at gravimetric water contents lower than 13–15% (gravimetric water content is not affected by mechanical effects) and ‘inter-aggregate governing suction’ (gravimetric water content is sensible to mechanical actions). This water content is further used to define a threshold zone around a relative water permeability of kw/kws=0.01 delimiting a zone of greater water relative permeability from others that present a restricted flow in a generalised Darcian sense. All these results are consistent with the existence of two main pore size regions: an intra-aggregate porosity with quasi-immobile water that is little affected by loading processes and an inter-aggregate porosity for which the loading mechanism results in a reduction of interconnected macropores affecting free water. Testing results show that intra-aggregate water represents between 54 and 59% of the total volume of water in soil in a low-porosity packing compacted at a dry unit weight of 16.7 kN m-3, whereas it corresponds to ca 28 and 38% in the case of a high-porosity packing compacted at a dry unit weight of 13.7 kN m-3.

  Abstract
  Three classes of experiments are considered in this paper to provide information for two artificially prepared Boom clay fabrics: mercury intrusion/extrusion tests; main wetting/drying [...]

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• Modelling the mechanical behaviour of expansive clays

  E. Alonso, J. Vaunat, A. Gens

  Engineering Geology (1999). Vol. 54 (1-2), pp. 173-183

  
  

  Abstract

  A simple formalism is presented to model the behaviour of expansive clays. Two levels of structure are considered. The behaviour of the macrostructure follows the model developed for unsaturated materials by Alonso et al. [Géotechnique 40 (3) (1990) 405–430]. The behaviour of the microstructure is adapted from the work of Gens and Alonso [Can. Geotech. J. 29 (1992) 1013–1032] in order to include the possibility of the micropores being partially saturated. Mechanical coupling between both levels of structure are defined through two functions, one for wetting and the other for drying. They express the change in macrostructural void ratio due to a change in microstructural void ratio, and their value depends on the state of compaction of the macrostructure. The general shape of these curves is discussed on the basis of experimental evidence. Phenomena such as the dependency of strain on stress–suction path, accumulation of expansion strain during suction cycles at low confining stress, accumulation of compression strain during suction cycles at high confining stress, strain fatigue during drying–wetting cycles, macropore invasion by expanded microstructure and development of macroporosity during strong drying can be represented. A mathematical formulation of the model is described and its performance finally assessed by comparison with laboratory tests.

  Abstract
  A simple formalism is presented to model the behaviour of expansive clays. Two levels of structure are considered. The behaviour of the macrostructure follows the model developed [...]

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