COLLECTIONS

Documents published in Scipedia

• Conceptual uncertainties in modelling the interaction between engineered and natural barriers of nuclear waste repositories in crystalline rocks

  S. Finsterle, B. Lanyon, M. Akesson, A. Gens

  Geological Society, London, special publications (2019). Vol. 482, pp. 261-283

  
  

  Abstract

  Nuclear waste disposal in geological formations relies on a multi-barrier concept that includes engineered components – which, in many cases, include a bentonite buffer surrounding waste packages – and the host rock. Contrasts in materials, together with gradients across the interface between the engineered and natural barriers, lead to complex interactions between these two subsystems. Numerical modelling, combined with monitoring and testing data, can be used to improve our overall understanding of rock–bentonite interactions and to predict the performance of this coupled system. Although established methods exist to examine the prediction uncertainties due to uncertainties in the input parameters, the impact of conceptual model decisions on the quantitative and qualitative modelling results is more difficult to assess. A Swedish Nuclear Fuel and Waste Management Company Task Force project facilitated such an assessment. In this project, 11 teams used different conceptualizations and modelling tools to analyse the Bentonite Rock Interaction Experiment (BRIE) conducted at the Äspö Hard Rock Laboratory in Sweden. The exercise showed that prior system understanding along with the features implemented in the available simulators affect the processes included in the conceptual model. For some of these features, sufficient characterization data are available to obtain defensible results and interpretations, whereas others are less supported. The exercise also helped to identify the conceptual uncertainties that led to different assessments of the relative importance of the engineered and natural barrier subsystems. The range of predicted bentonite wetting times encompassed by the ensemble results were considerably larger than the ranges derived from individual models. This is a consequence of conceptual uncertainties, demonstrating the relevance of using a multi-model approach involving alternative conceptualizations.

  Abstract
  Nuclear waste disposal in geological formations relies on a multi-barrier concept that includes engineered components – which, in many cases, include a bentonite buffer [...]

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• Standard penetration testing in a virtual calibration chamber

  N. Zhang, M. Arroyo, M. Ciantia, A. Gens, J. Butlanska

  Computers and Geotechnics (2019). Vol. 111, pp. 277-289

  
  

  Abstract

  The virtual calibration chamber technique, based on the discrete element method, is here applied to study the standard penetration test (SPT). A macro-element approach is used to represent a rod driven with an impact like those applied to perform SPT. The rod is driven into a chamber filled with a scaled discrete analogue of a quartz sand. The contact properties of the discrete analogue are calibrated simulating two low-pressure triaxial tests. The rod is driven changing input energy and controlling initial density and confinement stress. Energy-based blowcount normalization is shown to be effective. Results obtained are in good quantitative agreement with well-accepted experimentally-based relations between blowcount, density and overburden. It is also shown that the tip resistance measured under impact dynamic penetration conditions is close to that under constant velocity conditions, hence supporting recent proposals to relate CPT and SPT results

  Abstract
  The virtual calibration chamber technique, based on the discrete element method, is here applied to study the standard penetration test (SPT). A macro-element approach is [...]

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• Sphericity measures of sand grains

  R. Rorato, M. Arroyo, E. Andò, A. Gens

  Engineering Geology (2019). Vol. 254, pp. 43-53

  
  

  Abstract

  The sphericity of a grain should measure the similitude of its shape with that of a sphere. Sphericity is a shape descriptor of long-standing interest for sedimentology. Now it has gained also interest to facilitate discrete element modelling of granular materials. True sphericity was initially defined by a surface ratio that requires three-dimensional (3D) grain surface measurement. That kind of measurement has been practically impossible until recently and, as a consequence, a number of alternative 3D measures and 2D proxies were proposed. In this work we present results from a study of grain shape based on x-ray tomography of two different sand specimens, containing more than 110.000 particles altogether. Sphericity measures were systematically obtained for all grains. 2D proxy measures were also obtained in samples of oriented and not-oriented grains. It is shown that the 2D proxy best correlated with true sphericity is perimeter sphericity, whereas the traditional Krumbein-Sloss chart proxy is poorly correlated. 2D measures acquired through minor axis projection are more closely related to 3D measures than those acquired using random projections.

  Abstract
  The sphericity of a grain should measure the similitude of its shape with that of a sphere. Sphericity is a shape descriptor of long-standing interest for sedimentology. [...]

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• Clays in natural and engineered barriers for nuclear waste disposal

  A. Gens

  Geomechanics for energy and the environment (2019). Vol. 17, pp. 1-2

  
  

  Abstract
  Argillaceous rocks and expansive clays play a key role in many scenarios of the deep geological disposal of nuclear waste.

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• Grading evolution and critical state in a discrete numerical model of Fontainebleau sand

  M. Ciantia, M. Arroyo, C. O'Sullivan, A. Gens, T. Liu

  Géotechnique (2019). Vol. 69 (1), pp. 1-15 (Preprint)

  
  

  Abstract

  Granular materials reach critical states upon shearing. The position and shape of a critical state line (CSL) in the compression plane are important for constitutive models, interpretation of in situ tests and liquefaction analyses. It is not fully clear how grain crushing may affect the identification and uniqueness of the CSL in granular soils. Discrete-element simulations are used here to establish the relation between breakage-induced grading evolution and the CSL position in the compression plane. An efficient model of particle breakage is applied to perform a large number of tests, in which grading evolution is continuously tracked using a grading index. Using both previous and new experimental results, the discrete-element model is calibrated and validated to represent Fontainebleau sand, a quartz sand. The results obtained show that, when breakage is present, the inclusion of a grading index in the description of critical states is advantageous. This can be simply done using the critical state plane (CSP) concept. A CSP is obtained for Fontainebleau sand.

  Abstract
  Granular materials reach critical states upon shearing. The position and shape of a critical state line (CSL) in the compression plane are important for constitutive models, [...]

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• Numerical simulation of the undrained stability of slopes in anisotropic fine-grained soils

  S. Conesa, M. Mánica, A. Gens, Y. Huang

  Geomechanics and Geoengineering (2019). Vol. 14 (1), pp. 18-29

  
  

  Abstract

  The undrained stability of slopes in anisotropic fine-grained soils is studied in this paper using the finite element method (FEM). A constitutive model is presented, able to account for the observed variation of undrained strength with loading direction. The model is able to encompass the different strength distributions observed in normally, slightly overconsolidated and heavily overconsolidated soils. A series of stability analyses have been performed to explore the effect of the type of undrained strength anisotropy on the stability and failure mechanisms of slopes of different inclinations. In addition, a real case study of the failure of an underwater slope is analysed with the numerical approach presented. It suggests that, by considering undrained strength anisotropy, the failure can be satisfactorily explained.

  Abstract
  The undrained stability of slopes in anisotropic fine-grained soils is studied in this paper using the finite element method (FEM). A constitutive model is presented, able [...]

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• SDMT-based numerical analyses of deep excavation in soft soil

  A. Marino, S. Amoroso, M. Arroyo, P. Monaco, A. Gens

  Journal of Geotechnical and Geoenvironmental Engineering (2019). Vol. 145 (1), pp. 04018102-1/04018102-12

  
  

  Abstract

  The paper explores the application of conventional (DMT) and seismic (SDMT) dilatometer tests to an important case of deep excavation design. The work presents finite-element analyses simulating a deep excavation close to Barcelona (Spain). A thick layer of soft interbedded sandy and silty soils made characterization based on laboratory testing very difficult. SDMT offered an alternative for estimating the soil stiffness and its stress-strain dependency. Numerical results and high-quality monitoring data show quite close agreement for most phases of the construction process, supporting the use of seismic dilatometer tests in numerical analyses of deep excavations. The paper also indicates the importance of incorporating stiffness data at low strains. FE analyses involved some uncertainties derived from the presence of jet-grouting soil treatments. On this point, a parametric study illustrates the effects of different modeling approaches.

  Abstract
  The paper explores the application of conventional (DMT) and seismic (SDMT) dilatometer tests to an important case of deep excavation design. The work presents finite-element [...]

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• Nonlocal plasticity modelling of strain localisation in stiff clays

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

  Computers and Geotechnics (2018). Vol. 103, pp. 138-150

  
  

  Abstract

  The paper addresses the numerical simulation of strain localisation in stiff clays that exhibit softening behaviour. An elastoplastic constitutive model developed to incorporate key features of stiff clay behaviour is described first. A non-local formulation is then introduced for the regularisation of the analysis of localisation. A series of analyses were conducted to explore relevant aspects of the numerical simulation of localisation. A 3D analysis was also performed to assess the suitability of the approach presented for 3D applications. Finally, application to the simulation of a laboratory test on Beaucaire marl results in an excellent reproduction of experimental observations.

  Abstract
  The paper addresses the numerical simulation of strain localisation in stiff clays that exhibit softening behaviour. An elastoplastic constitutive model developed to incorporate [...]

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• On the choice of stress–strain variables for unsaturated soils and its effect on plastic flow

  D. Gallipoli, P. Grassi, S. Wheeler, A. Gens

  Geomechanics for energy and the environment (2018). Vol. 15, pp. 3-9 (Preprint)

  
  

  Abstract

  The net stress plus suction and the average skeleton stress plus modified suction are two alternative sets of energetically consistent stress variables for modelling the hydro-mechanical behaviour of unsaturated soils. When used in conjunction with their work-conjugate strains, both sets of stress variables correctly calculate the first-order term of the hydro-mechanical work input into a soil element subjected to infinitesimal changes of deformation and water content. They therefore also correctly calculate the increment of internal energy along a given stress–strain path, that is the integral of the first-order term of the infinitesimal work input. This paper shows, however, that the above two sets of stress variables lead to different values of the second-order term of the hydro-mechanical work input. They are therefore no longer equivalent with respect to other aspects of material behaviour governed by the second-order work such as the flow rule of elasto-plastic models. The flow rule assumes the normality between plastic strains and equipotential surfaces defined in the conjugate stress–strain space. This normality is however lost when an elasto-plastic model originally formulated in terms of net stress plus suction is recast in terms of average skeleton stress plus modified suction (or vice versa) by means of standard mapping relationships between stress variables. To restore normality in both stress spaces, it is necessary to impose specific forms of elastic and plastic behaviour.

  Abstract
  The net stress plus suction and the average skeleton stress plus modified suction are two alternative sets of energetically consistent stress variables for modelling the hydro-mechanical [...]

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• Marchetti flat dilatometer tests in a virtual calibration chamber

  J. Butlanska, M. Arroyo, S. Amoroso, A. Gens

  Geotechnical Testing Journal (2018). Vol. 41 (5), pp. 1-17 (Preprint)

  
  

  Abstract

  Calibration chambers are frequently used to verify, adapt, or both verify and adapt empirical relations between different state variables and in situ test results. Virtual calibration chambers (VCC) built with 3D discrete element models may be used to extend and partially substitute costly physical testing series. VCC are used here to explore the mechanics of flat dilatometer penetration and expansion. Results obtained for a simulation of physical tests in Ticino sand are presented. Blade tip resistance during penetration is in good agreement with the experiments. A piston-like design is used for the blade so that larger displacements may be applied than it is possible with a membrane. Initial piston pressures in the expansion curves are very low, strongly affected by the scaled-up grain sizes. Despite that difficulty, expansion curves may be easily interpreted to recover dilatometer moduli ED close to those observed in the physical experiments. Particle-scale examination of the results allows a firmer understanding of the current limitations and future potential of the technique. Copyright ASTM International. All rights reserved, 2018/16/07. Downloaded by UPC, pursuant to Author/Copyright Owner Agreement. No further reproduction authorized.

  Abstract
  Calibration chambers are frequently used to verify, adapt, or both verify and adapt empirical relations between different state variables and in situ test results. Virtual [...]

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