COLLECTIONS

Documents published in Scipedia

• 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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