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Documents published in Scipedia

• A time-dependent anisotropic model for argillaceous rocks: application to an underground excavation in Callovo-Oxfordian claystone

  M. Mánica, A. Gens, J. Vaunat, D. Ruiz

  Computers and Geotechnics (2017). Vol. 85, pp. 341-350

  
  

  Abstract

  The paper presents a constitutive model for argillaceous rocks, developed within the framework of elastoplasticity, that includes a number of features that are relevant for a satisfactory description of their hydromechanical behaviour: anisotropy of strength and stiffness, behaviour nonlinearity and occurrence of plastic strains prior to peak strength, significant softening after peak, time-dependent creep deformations and permeability increase due to damage. Both saturated and unsaturated conditions are envisaged. The constitutive model is then applied to the simulation of triaxial and creep tests on Callovo-Oxfordian (COx) claystone. Although the main objective has been the simulation of the COx claystone behaviour, the model can be readily used for other argillaceous materials. The constitutive model developed is then applied, via a suitable coupled hydromechanical formulation, to the analysis of the excavation of a drift in the Meuse/Haute-Marne Underground Research Laboratory. The pattern of observed pore water pressures and displacements, as well as the shape and extent of the damaged zone, are generally satisfactorily reproduced. The relevance and importance of rock anisotropy and of the development of a damaged zone around the excavations are clearly demonstrated.

  Abstract
  The paper presents a constitutive model for argillaceous rocks, developed within the framework of elastoplasticity, that includes a number of features that are relevant for [...]

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• Numerical analysis of the hydromechanical response of Callovo-Oxfordian claystone to deep excavations

  D. Seyedi, A. Gens

  Computers and Geotechnics (2017). Vol. 85, pp. 275-276 (Preprint)

  
  

  Abstract

  Deep geological disposal is considered as one of the promising options for the long-term management of the High-Level (HL) and Intermediate-Level Long-Lived (IL-LL) radioactive waste. Different argillaceous formations are studied in different countries for this purpose, such as, Opalinus clay in Switzerland, Boom clay in Belgium and Callovo-Oxfordian claystone in France.

  Abstract
  Deep geological disposal is considered as one of the promising options for the long-term management of the High-Level (HL) and Intermediate-Level Long-Lived (IL-LL) radioactive [...]

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• Performance of the Opalinus Clay under thermal loading: experimental results from Mont Terri rock laboratory (Switzerland)

  A. Gens, K. Wieczorek, I. Gaus, B. Garitte, J. Mayor, K. Schuster, G. Armand, J. García-Siñeriz, T. Trick

  Swiss Journal of Geosciences (2017). Vol. 110 (1), pp. 269-286

  
  

  Abstract

  The final publication is available at Springer via http://dx.doi.org/10.1007/s00015-016-0258-8 The paper presents an overview of the behaviour of Opalinus Clay under thermal loading as observed in three in situ heating tests performed in the Mont Terri rock laboratory: HE-B, HE-D and HE-E. The three tests are summarily described; they encompass a broad range of test layouts and experimental conditions. Afterwards, the following topics are examined: determination of thermal conductivity, thermally-induced pore pressure generation and thermally-induced mechanical effects. The mechanisms underlying pore pressure generation and dissipation are discussed in detail and the relationship between rock damage and thermal loading is examined using an additional in situ test: SE-H. The paper concludes with an evaluation of the various thermo-hydro-mechanical (THM) interactions identified in the heating tests.

  Abstract
  The final publication is available at Springer via http://dx.doi.org/10.1007/s00015-016-0258-8 The paper presents an overview of the behaviour of Opalinus Clay under thermal [...]

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• Numerical analysis of suction embedded plate anchors in structured clay

  T. Charlton, M. Rouainia, A. Gens

  Applied Ocean Research (2016). Vol. 61, pp. 156-166

  
  

  Abstract

  As offshore energy developments move towards deeper water, moored floating production facilities are increasingly preferred to fixed structures. Anchoring systems are therefore of great interest to engineers working on deep water developments. Suction embedded plate anchors (SEPLAs) are rapidly becoming a popular solution, possessing a more accurate and predictable installation process compared to traditional alternatives. In this paper, finite element analysis has been conducted to evaluate the ultimate pullout capacity of SEPLAs in a range of post-keying configurations. Previous numerical studies of anchor pullout capacity have generally treated the soil as an elastic-perfectly plastic medium. However, the mechanical behaviour of natural clays is affected by inter-particle bonding, or structure, which cannot be accounted for using simple elasto-plastic models. Here, an advanced constitutive model formulated within the kinematic hardening framework is used to accurately predict the degradation of structure as an anchor embedded in a natural soft clay deposit is loaded to its pullout capacity. In comparison with an idealised, non-softening clay, the degradation of clay structure due to plastic strains in the soil mass results in a lower pullout capacity factor, a quantity commonly used in design, and a more complex load–displacement relationship. It can be concluded that clay structure has an important effect on the pullout behaviour of plate anchors.

  Abstract
  As offshore energy developments move towards deeper water, moored floating production facilities are increasingly preferred to fixed structures. Anchoring systems are therefore [...]

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• Fully coupled thermo-hydro-mechanical double-porosity formulation for unsaturated soils

  M. Castilla, A. Gens, M. Villar, S. Olivella

  International journal of geomechanics (2016). Vol. 16 (6), pp. D4016015-1/D4016015-17 (Preprint)

  
  

  Abstract

  This work presents a fully coupled formulation developed to handle engineering problems in unsaturated (and saturated) soils that present two dominant void levels. The proposed framework assumes the presence of two porous media linked through a mass-transfer term between them. In its more general form, the proposed approach allows the consideration of nonisothermal multiphase flow coupled with the mechanical problem. The double-porosity formulation was implemented in a finite-element code and has been used to analyze a variety of engineering problems. The approach is especially suitable for cases in which the material exhibits a strong coupling between the mechanical and the hydraulic problems in both media, such as with swelling clays. For those types of problems, the proposed formulation is used in conjunction with the mechanical double-structure model already proposed by the authors. This paper presents the coupled formulation and the application of the proposed approach to problems involving expansive unsaturated clays. Very satisfactory results were obtained in these analyses

  Abstract
  This work presents a fully coupled formulation developed to handle engineering problems in unsaturated (and saturated) soils that present two dominant void levels. The proposed [...]

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• A numerical investigation of the incremental behavior of crushable granular soils

  M. Cintia, M. Arroyo, F. Calvetti, A. Gens

  International Journal for Numerical and Analytical Methods in Geomechanics (2016). Vol. 40 (13), pp. 1773-1798

  
  

  Abstract

  The mechanical behavior of granular materials is characterized by strong nonlinearity and irreversibility. These properties have been differently described by a variety of constitutive models. To test any constitutive model, experimental data relative to the nature of the incremental stress–strain response of the material is desirable. However, this type of laboratory data is scarce because of being expensive and difficult to obtain. The discrete element method has been used several times as an alternative to obtain incremental responses of granular materials. Crushable grains add one extra source of irreversibility to granular materials. Crushability has been variously incorporated into different constitutive models. Again, it will be helpful to obtain incremental responses of crushable granular materials to test these models, but the experimental difficulties are increased. Making use of a recently introduced crushing model for discrete element simulation, this paper presents a new procedure to obtain incremental responses in discrete analogs of granular crushable materials. The parallel probe approach, previously used for uncrushable discrete analogs, is here extended to account for the presence of crushable grains. The contribution of grain crushing to the incremental irreversible strain is identified and separately measured. Robustness of the proposed method is examined in detail, paying particular attention to aspects such as dynamic instability or crushing localization. The proposed procedure is later applied to map incremental responses of a discrete analog of Fontainebleau sand on the triaxial plane. The effect of stress ratio and granular state on plastic flow characteristics is highlighted.

  Abstract
  The mechanical behavior of granular materials is characterized by strong nonlinearity and irreversibility. These properties have been differently described by a variety of [...]

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• Exploring ice content on partially saturated frozen soils using dielectric permittivity and bulk electrical conductivity measurements

  Y. Mao, E. Romero, A. Gens

  E3S Web of Conferences (2016). Vol. 9, pp. 1-9

  
  

  Abstract

  The present paper explores the use of the measurement of bulk electric conductivity and relative dielectric permittivity during freezing and thawing to estimate the ice content of two different partially saturated soils. The soils (Castelldefels fine sand and Barcelona clayey silt) were prepared at different degrees of saturation and dry densities at room temperature. Different target temperatures were used, which were reached at varying freezing rates and controlled with a thermal bath. Models were used to fit electrical conductivity and relative dielectric permittivity results under full unfrozen liquid and maximum ice conditions. The extension of these models allows estimating ice content on partially saturated soils at different porosities.

  Abstract
  The present paper explores the use of the measurement of bulk electric conductivity and relative dielectric permittivity during freezing and thawing to estimate the ice content [...]

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• Anisotropic behaviour of compacted clayey silt subjected to hydromechanical paths

  C. Buenfil, E. Romero, A. Lloret, A. Gens

  E2S Web of Conferences (2016). Vol. 9, pp. 1-6

  
  

  Abstract

  Compaction induces anisotropy on soil deformational response due to preferential straining associated with the fabrication process. An experimental insight into the stress-strain response of Barcelona clayey silt is provided here. The material was statically compacted at low dry density to induce high collapsibility upon wetting (dry density 1.48 Mg/m3, water content 12%). Two types of controlled-suction tests were carried out: a) constant suction radial paths on as-compacted samples following different stress ratios (deviator stress / mean net stress: q/p Compaction induces anisotropy on soil deformational response due to preferential straining associated with the fabrication process. An experimental insight into the stress-strain response of Barcelona clayey silt is provided here. The material was statically compacted at low dry density to induce high collapsibility upon wetting (dry density 1.48 Mg/m3, water content 12%). Two types of controlled-suction tests were carried out: a) constant suction radial paths on as-compacted samples following different stress ratios (deviator stress / mean net stress: q/p¿, including isotropic and K0 stress paths; and b) the same type of radial stress paths but performed after saturation under low confining stress. These paths were intended to define the as-compacted and saturated yield surfaces. The experimental data on as-compacted state is satisfactorily interpreted adopting an anisotropic yield surface with an inclination linked to the compaction under oedometer conditions. Development of shear strain was observed during isotropic loading of the compacted samples. This anisotropy that the soil exhibits at the end of the compaction vanishes after collapse by saturation and as plastic volumetric straining develops

  Abstract
  Compaction induces anisotropy on soil deformational response due to preferential straining associated with the fabrication process. An experimental insight into the stress-strain [...]

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• Thrust and torque components on mixed-face EPB drives

  C. González, M. Arroyo, A. Gens

  Tunnelling and Underground Space Technology (2016). Vol. 57, pp. 47-54

  
  

  Abstract

  In this communication we propose a method to estimate net thrust and torque applied at the excavation front by cutting tools from EPB machines when they are working in mixed faces constituted by soils and soft rock. The method is inspired by similar simplifications used for the analysis of TBM drives in rock. The proposal is validated using a database of EPB registers which were gathered from more than 35 km of tunnel drives excavated in soils, soft rock and heterogeneous media. The results allow us to assess the influence of the type of ground excavated and its geotechnical properties on the net thrust and torque applied.

  Abstract
  In this communication we propose a method to estimate net thrust and torque applied at the excavation front by cutting tools from EPB machines when they are working in mixed [...]

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• Analysis of unsaturated materials hydration incorporating the effect of thermo-osmotic flow

  M. Castilla, C. Arson, A. Gens, F. Aponte

  Geomechanics for energy and the environment (2016). Vol. 6, pp. 101-115

  
  

  Abstract

  The geological disposal of a high level radioactive waste relies in a system composed of engineered and geological barriers. The soils and rocks involved in the design of this type of solution are generally initially unsaturated and subject to complex thermal, hydraulic and mechanical (THM) coupled phenomena triggered by the simultaneous heating and hydration of the barrier materials under confined conditions. Mathematical THM formulations are typically used to analyze the behavior and long term performance of the barriers system. These types of formulations generally do not include some coupled processes, for example thermo-osmosis (i.e. the movement of liquid water induced by gradient of temperature), because they are considered not significant when compared against the main or direct processes (e.g., Darcy’s, Fourier’s and Fick’s laws). In this work, the potential effects of thermo-osmotic phenomenon is studied in detail. Typical flow equations are modified to include thermo-osmotic flows and then they are implemented in numerical simulators. Two case studies are analyzed. The first one focuses on a simple and already proposed model to study the behavior of a geological barrier for nuclear waste when subjected to heating and hydration. The other case corresponds to the study of an engineered clay barrier material in the laboratory subjected to hydraulic and thermal gradients similar to the ones expected in real repository conditions. In both cases the analyses with and without thermo-osmotic flows are compared. From these comparisons it is observed that the effect of thermo-osmosis can be quite significant. Thermo-osmotic effects also assisted to explain the apparent low wetting observed in the hydration of a clayey barrier material.

  Abstract
  The geological disposal of a high level radioactive waste relies in a system composed of engineered and geological barriers. The soils and rocks involved in the design [...]

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