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• Compacted soil behaviour: initial state, structure and constitutive modelling

  E. Alonso, N. Pinyol, A. Gens

  Géotechnique (2013). Vol. 63 (6), pp. 463-478

  
  

  Abstract

  The paper explores the behaviour of compacted soils throughout the (dry density–water content) compaction plane by means of a conceptual framework that incorporates microstructural information. The engineering properties of compacted soils are described by an initial state in terms of a yielding stress, soil suction and a microstructural state variable. Microstructure is defined by the ratio of microvoid volume to total void volume. The pattern of variation of the microstructural parameter within the compaction plane has been determined, for some compacted soils, by analysing mercury intrusion porosimetry data. The microstructure of wet and dry compaction conditions can then be quantified. To ensure consistency, the framework is cast in the form of a constitutive model defined in terms of an effective suction and a constitutive stress that incorporate the microstructural variable. The model is shown to be consistent with a number of experimental observations and, in particular, it explains the intrinsic collapse potential of compacted soils. It predicts, for a common initial suction, a higher collapse potential for dry of optimum conditions than for wet compaction. It also predicts in a natural manner the observed evolution of soil compressibility during drained or undrained loading. Model capabilities are illustrated by application to a testing programme on statically compacted samples of low-plasticity silty clay. The compression behaviour of samples compacted wet and dry of optimum and the variation of collapse strains with confining stress have been successfully reproduced by the model.

  Abstract
  The paper explores the behaviour of compacted soils throughout the (dry density–water content) compaction plane by means of a conceptual framework that incorporates [...]

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• A chemo-mechanical constitutive model accounting for cation exchange in expansive clays

  L. Guimarães, A. Gens, M. Sanchez, S. Olivella

  Geotechnique (2013). Vol. 63 (3), pp. 221-234

  
  

  Abstract

  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.

  Abstract
  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 [...]

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• Analysis of hydro-mechanical processes in a ventilated tunnel in an argillaceous rock on the basis of different modelling approaches

  B. Garitte, A. Bond, A. Millard, C. Zhang, C. McDermott, S. Nakama, A. Gens

  Journal of rock mechanics and geotechnical engineering (2013). Vol. 5 (1), pp. 1-17

  
  

  Abstract

  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.

  Abstract
  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 [...]

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• Steady state of solid-grain interfaces during simulated CPT

  J. Butlanska, M. Arroyo, A. Gens

  Studia Geotechnica et Mechanica (2013). Vol. XXXV (4), pp. 13-22

  
  

  Abstract

  It has recently been shown (Arroyo et al. [1]) that 3D DEM models are able to reproduce with reasonable accuracy the macroscopic response of CPT performed in calibration chambers filled with sand. However, the cost of each simulation is an important factor. Hence, to achieve manageable simulation times the discrete material representing the sand was scaled up to sizes that were more typical of gravel than sand. A side effect of the scaled-up discrete material size employed in the model was an increased fluctuation of the macro-response that can be filtered away to observe a macroscopic steady-state cone resistance. That observation is the starting point of this communication, where a series of simulations in which the size ratio between penetrometer and particles is varied are systematically analyzed. A micromechanical analysis of the penetrometer-particle interaction is performed. These curves reveal that a steady state is arrived also at the particle-cone contact level. The properties of this dynamic interface are independent of the initial density of the granular material.

  Abstract
  It has recently been shown (Arroyo et al. [1]) that 3D DEM models are able to reproduce with reasonable accuracy the macroscopic response of CPT performed in calibration chambers [...]

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• Abrasividad y su influencia en el rendimiento de una excavación mecanizada

  C. González, M. Arroyo, A. Gens

  Obras y proyectos. Revista de ingeniería civil (2013). Vol. 13, pp. 6-18

  
  

  Abstract

  El empleo de tuneladoras TBM (Tunnel Boring Machine) es cada vez más frecuente aunque su optimización requiere la evaluación precisa de varios riesgos geotécnicos entre ellos, el efecto de la abrasividad del suelo en el coste y planificación de un proyecto dado. El desgaste de las herramientas es más difícil de calcular que el resto de parámetros de la maquinaria, debido a la complejidad de la interacción entre herramienta de corte y terreno. En este trabajo se usan datos de un caso real de túnel en terrenos mixtos suelo-roca para examinar el resultado de varias metodologías de predicción de la penetración de TBM. También se examinan algunas predicciones de desgaste y se describe y aplica un nuevo método para la evaluación de la abrasividad de geomateriales que puede aplicarse tanto a materiales sueltos como a rocas.

  Abstract
  El empleo de tuneladoras TBM (Tunnel Boring Machine) es cada vez más frecuente aunque su optimización requiere la evaluación precisa de varios riesgos [...]

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• Thermal conductivity of argillaceous rocks: determination methodology using in situ heating tests

  B. Garitte, A. Gens, J. Vaunat, G. Armand

  Rock Mechanics and Rock Engineering (2012). Vol. 47, pp. 111-129

  
  

  Abstract

  This study focuses on the characterisation of thermal conductivity for three potential host rocks for radioactive waste disposal. First, the heat conduction process is reviewed on the basis of an analytical solution and key aspects related to anisotropic conduction are discussed. Then the existing information on the three rocks is summarised and a broad uncertainty range of thermal conductivity is estimated based on the mineralogical composition. Procedures to backanalyse the thermal conductivity on the basis of in situ heating tests are assessed and a methodology is put forward. Finally, this methodology is used to estimate the impact of experimental uncertainties and applied to the four in situ heating tests. In the three potential host rocks, a clear influence of the bedding planes was identified and anisotropic heat conduction was shown to be necessary to interpret the observed temperature field. Experimental uncertainties were also shown to induce a larger uncertainty on the anisotropy ratio than on the equivalent thermal conductivity defined as the geometric mean of the thermal conductivity in the three principal directions.

  Abstract
  This study focuses on the characterisation of thermal conductivity for three potential host rocks for radioactive waste disposal. First, the heat conduction process is reviewed [...]

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• Thermal-hydraulic-mechanical (THM) behaviour of a large-scale in situ heating experiment during cooling and dismantling

  M. Castilla, A. Gens, L. Guimarães

  Canadian Geotechnical Journal (2012). Vol. 49 (10), pp. 1169-1195

  
  

  Abstract

  A geological disposal facility for high-level radioactive waste (HLW) encompasses both natural (host rock) and (generally clay-based) engineered barriers. Many processes can influence, either positively or negatively, the effectiveness of the safety functions of isolation and retardation. This paper focuses on the analysis of a large-scale heating test when subjected to cooling and subsequent partial dismantling. The experiment reproduces the conditions of an HLW repository at full scale under realistic conditions. Key thermal, hydraulic, and mechanical (THM) variables, such as temperature, relative humidity, stresses, and fluid pressures, were measured in the clay barrier and surrounding rock. The experimental observations recorded during the cooling down and clay barrier excavation are analyzed in light of a fully coupled THM finite element formulation. This analysis has provided the opportunity to explore the behaviour of the clay and natural barriers under conditions very relevant for the repository performance but not analyzed previously. Overall, the model predictions are quite satisfactory when compared against experimental observations. Furthermore, model predictions for a period of 20 years, including the transient phase induced by the partial dismantling, are also presented. This additional analysis has allowed a better understanding of the effect of thermal gradient on long-term clay hydration.

  Abstract
  A geological disposal facility for high-level radioactive waste (HLW) encompasses both natural (host rock) and (generally clay-based) engineered barriers. Many processes can [...]

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• Simulation of cement-improved clay structures with a bonded elasto-plastic model: A practical approach

  M. Arroyo, M. Ciantia, R. Castellanza, A. Gens, R. Nova

  Computers and Geotechnics (2012). Vol. 45, pp. 140-150

  
  

  Abstract

  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.

  Abstract
  Engineering practice has usually dealt with the treated soil bodies using simplistic constitutive models (e.g. elastic perfectly-plastic Mohr–Coulomb). In this [...]

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• Analysis of tunnel excavation in London Clay incorporating soil structure

  N. González, M. Rouainia, M. Arroyo, A. Gens

  Géotechnique (2012). Vol. 62 (12), pp. 1095-1109

  
  

  Abstract

  Recent studies on London Clay have identified a number of different units in the geological profile, and have highlighted the role of soil structure in mechanical behaviour. In fact, structure is the dominant factor determining the differences in the mechanical response of different units. In the paper, numerical analyses simulating the undrained excavation of a tunnel in St James's Park are presented. London Clay behaviour is characterised by a kinematic-hardening structured soil model incorporating structure and stiffness degradation. The parameters and initial conditions are based on a careful calibration that takes into account the presence of different units within the London Clay formation and the different degrees of soil structure. The analyses performed result in a very satisfactory reproduction of the magnitude and patterns of short-term surface and subsurface displacements, as well as pore pressures. The paper concludes with a discussion of the results in the context of other analyses performed previously, and puts forward some considerations concerning design issues.

  Abstract
  Recent studies on London Clay have identified a number of different units in the geological profile, and have highlighted the role of soil structure in mechanical behaviour. [...]

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• Interpretation of unsaturated soil behaviour in the stress-saturation space. II: Constitutive relationships and validations

  A. Zhou, D. Sloan, S. Sloan, A. Gens

  Computers and Geotechnics (2012). Vol. 43, pp. 111-123

  
  

  Abstract

  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.

  Abstract
  Based on the equations for volume change and saturation variation proposed in the companion paper [37], an alternative constitutive framework is presented for interpreting [...]

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