COLLECTIONS

Documents published in Scipedia

• Interpretation of unsaturated soil behaviour in the stress. Saturation space, I: volume change and water retention behaviour

  N. Zhou, D. Sheng, S. Scott, A. Gens

  Computers and Geotechnics (2012). Vol. 43, pp. 178-187

  
  

  Abstract

  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.

  Abstract
  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 [...]

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• THM analysis of a large-scale heating test incorporating material fabric changes

  M. Castilla, A. Gens, S. Olivella

  International Journal of Numerical and Analytical Methods in Geomechanics (2012). Vol. 36 (4), pp. 391-421

  
  

  Abstract

  Engineered barriers are basic elements in the design of repositories for the isolation of high‐level radioactive waste. This paper presents the thermo‐hydro‐mechanical (THM) analysis of a clay barrier subjected to heating and hydration. The study focuses on an ongoing large‐scale heating test, at almost full scale, which is being carried out at the CIEMAT laboratory under well‐controlled boundary conditions. The test is intensely instrumented and it has provided the opportunity to study in detail the evolution of the main THM variables over a long period of time. Comprehensive laboratory tests carried out in the context of the FEBEX and NF‐PRO projects have allowed the identification of the model parameters to describe the THM behaviour of the compacted expansive clay. A conventional THM approach that assumes the swelling clay as a single porosity medium has been initially adopted to analyse the evolution of the test. The model was able to predict correctly the global THM behaviour of the clay barrier in the short term (i.e. for times shorter than three years), but some model limitations were detected concerning the prediction of the long‐term hydration rate. An additional analysis of the test has been carried out using a double structure model to describe the actual behaviour of expansive clays. The double structure model explicitly considers the two dominant pore levels that actually exist in the FEBEX bentonite and it is able to account for the evolution of the material fabric. The simulation of the experiment using this enhanced model provides a more satisfactory reproduction of the long‐term experimental results. It also contributes to a better understanding of the observed test behaviour and it provides a physically based explanation for the very slow hydration of the barrier. 

  Abstract
  Engineered barriers are basic elements in the design of repositories for the isolation of high‐level radioactive waste. This paper presents the thermo‐hydro‐mechanical [...]

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• Hydromechanical behaviour of a heterogeneous compacted soil: experimental observations and modelling

  A. Gens, B. Vallejan, M. Castilla, C. Imbert, M. Villar, M. Geetl

  Géotechnique (2011). Vol. 61 (5), pp. 367-386

  
  

  Abstract

  The paper describes a theoretical and experimental study of the coupled hydromechanical behaviour of a compacted mixture of bentonite powder and bentonite pellets intended as sealing material in underground repositories for nuclear waste. One of the main advantages of the use of powder/pellets mixtures is the reduction of the compaction effort required to achieve the value of average dry density necessary to attain the required swelling potential. However, the heterogeneous fabric of the material requires special approaches in order to describe adequately its behaviour during hydration. A double porosity formulation is presented to account for the presence of two distinct structural levels in the material. Hydraulic equilibrium between the two porosities is not assumed; instead a water exchange term between them is postulated. The formulation is applied to the modelling of a number of one-dimensional swelling pressure tests performed in the CEA (Commisariat à l'Énergie Atomique, France) and CIEMAT (Spain) laboratories. A very satisfactory quantitative description of the experimental observations is obtained that includes a number of complex behaviour features such as size effects and non-monotonic development of swelling pressures. Some microfabric observations using X-ray tomography and mercury intrusion porosimetry lend support to the conceptual approach adopted. The formulation is then applied to the analysis of a long-term large-scale sealing test performed at the Hades underground facility in Belgium, using the same set of hydraulic and mechanical parameters employed in the modelling of the laboratory tests. Although the field observations exhibit a much higher degree of scatter, the basic behaviour of the field sealing test is satisfactorily simulated. A formulation that incorporates basic features of the microfabric of the mixture is thus able to span successfully over a large range of space and time scales.

  Abstract
  The paper describes a theoretical and experimental study of the coupled hydromechanical behaviour of a compacted mixture of bentonite powder and bentonite pellets intended [...]

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• Thermo-hydro-mechanical model of the Canister Retrieval Test

  M. Zandarin, A. Gens, S. Olivella, E. Alonso

  Physics and Chemistry of the Earth (2011). Vol. 36 (17-18), pp. 1806-1816

  
  

  Abstract

  The paper presents a 2-D axisymmetric numerical simulation of the Canister Retrieval Test (CRT). The main objectives of this analysis are the study of the canister–buffer and buffer–host rock interfaces; and the evaluation of the mathematical models used to predict the THM behaviour of the buffer and interface. The THM simulation was performed using the finite element program Code-Bright. The constitutive law adopted to represent the mechanical behaviour of bentonite and pellets is the Barcelona Basic Model (BBM) with a non-linear elastic component accounting for swelling. A joint element with a non-linear elastic mechanical law was implemented in the code for the simulation of the opening–closure of the interface canister–buffer. The formulation of this element also incorporates appropriate thermal and hydraulic laws. The evolution of temperatures, relative humidities and stresses recorded during the test were compared with the simulation results. Comparing the results measured in situ with the results of numerical analysis shows that the formulation used is able to reproduce satisfactorily the phenomena involved in the test as well as their interactions. In addition, the final dry density and degree of saturation measured in samples of bentonite and pellets extracted during the test dismantling were also in agreement with the calculated values.

  Abstract
  The paper presents a 2-D axisymmetric numerical simulation of the Canister Retrieval Test (CRT). The main objectives of this analysis are the study of the canister–buffer [...]

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• Cone penetration tests in a virtual calibration chamber

  M. Arroyo, J. Butlanska, A. Gens, F. Calvetti, M. Jamiolkowski

  Géotechnique (2011). Vol. 61 (6), pp. 525-531

  
  

  Abstract

  A virtual calibration chamber was built using a threedimensional model based on the discrete-element method. The chamber was then filled with a scaled granular equivalent of Ticino sand, the material properties of which were selected by curve-fitting triaxial tests. Cone penetration tests were then performed under different initial densities and isotropic stresses. Penetration resistance in the virtual calibration chamber was affected by the same cone/chamber size effect that affects physical calibration chambers and was corrected accordingly. The corrected cone resistance obtained from the virtual calibration chamber cone penetration tests shows good quantitative agreement with correlations that summarise previous physical results.

  Abstract
  A virtual calibration chamber was built using a threedimensional model based on the discrete-element method. The chamber was then filled with a scaled granular equivalent [...]

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• Heating pulse tests under constant volumen on natural Boom clay.

  A. Lima, E. Romero, A. Gens, J. Muñoz, X. Li

  Journal of rock mechanics and geotechnical engineering (2010). Vol. 2 (2), pp. 124-128

  
  

  Abstract

  Boom clay formation is a potential natural host rock for geological disposal of high-level nuclear waste in Belgium. Heating pulse tests with controlled power supply (maximum temperature was limited to 85°C) and controlled hydraulic boundary conditions were performed under nearly constant volume conditions to study the impact of thermal loading on the clay formation. Selected test results of intact borehole samples retrieved in horizontal direction are presented and discussed. The study focuses on the time evolution of temperature and pore water pressure changes along heating and cooling paths, i.e. pore pressure build-up during quasi-undrained heating and later dissipation at constant temperature.

  Abstract
  Boom clay formation is a potential natural host rock for geological disposal of high-level nuclear waste in Belgium. Heating pulse tests with controlled power supply (maximum [...]

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• Soil-environment interactions in geotechnical engineering

  A. Gens

  Géotechnique (2010). Vol. 60 (1), pp. 3-74

  
  

  Abstract

  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.

  Abstract
  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 [...]

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• Effect of thermo-coupled processes on the behaviour of a clay barrier submitted to heating and hydration

  M. Castilla, A. Gens, S. Olivella

  Anais da Academia Brasileira de Ciências (2010). Vol. 82 (1), pp. 153-168

  
  

  Abstract

  The storage of high level radioactive waste is still an unresolved problem of the nuclear industry, being geological disposal the most favoured option and, naturally, the one requiring the strongest geo-mechanical input. Most conceptual designs for the deep geological disposal of nuclear waste envisage placing the canisters containing the waste in horizontal drifts or vertical boreholes. The empty space surrounding the canisters is filled by an engineered barrier often made up of compacted swelling clay. In the barrier and the near field, significant thermo-hydro-mechanical (THM) phenomena take place that interact in a complex way. A good understanding of THM issues is, therefore, necessary to ensure a correct performance of engineered barriers and seals. The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal are being simulated in a mock-up heating test at almost scale, at the premises of CIEMAT in Madrid. The evolution of the main Thermo-Hydro-Mechanical (THM) variables of this test are analysed in this paper by using a fully coupled THM formulation and the corresponding finite element code. Special emphasis has been placed on the study of the effect of thermo-osmotic flow in the hydration of the clay barrier at an advanced staged of the experiment

  Abstract
  The storage of high level radioactive waste is still an unresolved problem of the nuclear industry, being geological disposal the most favoured option and, naturally, the [...]

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• Modelling thermo-hydro-mechano-chemical interactions for nuclear waste disposal

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

  Journal of rock mechanics and geotechnical engineering (2010). Vol. 2 (2), pp. 97-102

  
  

  Abstract

  A fully coupled thermo-hydro-mechano-chemical (THMC) formulation is described in this paper. Special attention is paid to phenomena likely to be encountered in clay barriers used as engineered barriers in the disposal of nuclear radioactive waste. The types of processes considered in the chemical formulation include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetics- and equilibrium-controlled reactions are incorporated. The formulation is implemented in a numerical code. An application is presented concerning the performance of a large-scale in-situ heating test simulating high-level radioactive waste repository conditions.

  Abstract
  A fully coupled thermo-hydro-mechano-chemical (THMC) formulation is described in this paper. Special attention is paid to phenomena likely to be encountered in clay barriers [...]

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• A review of expansive phenomena in Wagenburg North Tunnel

  I. Berdugo, E. Alonso, A. Gens, M. Albis

  Revista de la academia Colombiana de Ciencias Exactas, Físicas y Naturales (2009). Vol. 31 (129), pp. 455-469

  
  

  Abstract

  The paper deals on tunnelling and swelling in anhydritic-gypsiferous claystones. The geology of the Gipskeuper in Baden-Württemberg (Germany) and the phenomenology of expansions in Wagenburg North tunnel are described. Consistent swelling triggering events, mechanisms and exhaustion causes were identified using a thermo-hydro-chemo-mechanical theoretical analysis. It is suggested that long-term swelling in tunnels excavated through anhydritic-gypsiferous claystones is a result of solvent-way gypsum crystal growth due to ventilation induced groundwater evaporation and rock drying; an opposite concept to the usual consequences of drying in argillaceous materials, which causes shrinkage strains.

  Abstract
  The paper deals on tunnelling and swelling in anhydritic-gypsiferous claystones. The geology of the Gipskeuper in Baden-Württemberg (Germany) and the phenomenology of [...]

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