| 1. |
- Abed, Ayman, 1976, et al.
(författare)
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Finite element method algorithm for geotechnical applications based on Runge-Kutta scheme with automatic error control
- 2020
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Ingår i: Computers and Geotechnics. - : Elsevier BV. - 0266-352X .- 1873-7633. ; 128
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Tidskriftsartikel (refereegranskat)abstract
- This paper introduces a novel explicit algorithm to solve the finite element equation linking the nodal displacements of the elements with the external forces applied via means of non-linear global stiffness matrix. The proposed method solves the equation using Runge-Kutta scheme with automatic error control. The method allows any Runge-Kutta scheme, with the paper demonstrating the algorithm efficiency for Runge-Kutta schemes of second to fifth order of accuracy. The paper discusses the theoretical background, the implementation steps and validates the proposed algorithm. The numerical tests show that the proposed method is robust and stable. In comparison to the iterative implicit methods (e.g. Newton-Raphson method), the new algorithm overcomes the problem of occasional divergence. Furthermore, considering the computation time, the fifth-order accurate scheme proves to be competitive with the iterative method. It seems that the proposed algorithm could be a powerful alternative to the standard solution procedures for the cases with strong nonlinearity, where the typical algorithms may diverge.
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| 2. |
- Gramegna, Liliana, et al.
(författare)
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A constitutive framework for the chemo-mechanical behaviour of unsaturated non-expansive clays
- 2023
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Ingår i: E3S Web of Conferences. - 2555-0403 .- 2267-1242. ; 382
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Konferensbidrag (refereegranskat)abstract
- Both osmotic and matric suction changes have a significant influence on the mechanical behaviour of clays. Despite the different types of interactions at the microstructural level, both suctions havea relevant effect on the fabric of non-expansive clays. Starting from experimental observations at the laboratory scale, it is possible to identify some common features characterizing the mechanical response of non-expansive clays to salinity and degree of saturation changes. This paper presents an elastoplasticframework to reproduce the behaviour of unsaturated clayey soils upon changes in the salt concentration of the pore fluid. In particular, it presents a strategy to include osmotic suction induced by pore fluid salinity in BBM-like models [1]. The model was implemented in the Thebes code and it was calibrated on experimental data performed on Boom clay [2] and remoulded loess [3].
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| 3. |
- Gramegna, Liliana, et al.
(författare)
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An Elastoplastic Framework Accounting for Changes in Matric and Osmotic Suction in Unsaturated Non-expansive Clays
- 2023
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Ingår i: Springer Series in Geomechanics and Geoengineering. - 1866-8763 .- 1866-8755. ; , s. 311-318
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Konferensbidrag (refereegranskat)abstract
- The mechanical behaviour of clays is significantly influenced by their salinity and degree of saturation, which are expected to change in both natural and anthropogenic environments. This influence is triggered by fabric changes. Capillary forces, related to changes in the degree of saturation, and electrochemical interactions, related to changes in salinity, affect differently the interaction between particles. However, at the continuum scale, it is possible to pinpoint some common characteristics. This paper focuses on the modelling capabilities of an elasto-plastic framework formulated to reproduce the behaviour of unsaturated non-expansive clays exposed to changes in matric and osmotic suction, by introducing osmotic suction on BBM-like models in terms of its effects on the normal compression line. The model, calibrated on experimental data on Boom clay (Mokni et al., 2014) and remoulded loess (Zhang et al., 2022), has been implemented in the Thebes code and used to predict material response under selected chemo-hydro-mechanical paths.
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| 4. |
- Gupta, Abhishek, et al.
(författare)
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Identification of key thermal couplings affecting the bentonite behaviour in a deep geological nuclear waste repository
- 2023
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Ingår i: Engineering Geology. - 0013-7952. ; 324
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Tidskriftsartikel (refereegranskat)abstract
- Deep geological nuclear waste repositories use the multi-layer Engineered Barrier System (EBS) to isolate nuclear waste from the environment. The key component of the barrier is densely compacted bentonite, closely resembling claystone. Therefore, to ensure safety, we need a numerical model for the bentonite and the barrier that predicts EBS behaviour during transient thermal, hydraulic, mechanical and chemical conditions. The paper identifies key mechanisms and processes affecting the bentonite in the barrier due to temperature changes (thermal couplings) based on advanced fully-coupled Finite Element Method simulations. The paper investigates 1) non-isothermal infiltration experiment on FEBEX bentonite (Villar and Gomez-Espina, 2009) and, 2) Centro de Investigaciones Energeticas Medioambientales y Tecnologicas (Ciemat) test (Martin et al., 2006), presenting 10 simulation configurations that are set up by inactivating one thermal coupling/variable at a time. The difference between these simulations and the baseline model results, examined in terms of the net mean stress (swelling pressure), suction and fluid flow, give insights into the significance of investigated coupling. Results suggest that thermal couplings related to vapour density, viscosity, water retention curve, and molecular diffusivity are among the most influential. The study additionally highlights the importance of water transport as liquid and gas, and water evaporation and condensation.
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| 5. |
- Gupta, Abhishek, et al.
(författare)
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Implementation and validation of pressure-dependent gas permeability model for bentonite in FEM code Thebes
- 2023
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Ingår i: E3S Web of Conferences. - 2555-0403 .- 2267-1242. ; 382
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Konferensbidrag (refereegranskat)abstract
- In an Engineered Barrier System of a nuclear waste repository, gas migrates through: a) diffusion/advection of dissolved gases, b) two-phase continuum flow, c) dilatant pathway flow and d) single-phase gas flow through macro-fractures in the soil. The gas production rate and the corresponding gas pressure accumulation affect the clay material behaviour and its properties such as air entry value. For the safe design of the EBS system, computational models need to account for the identified transport mechanisms. This study presents an enhancement in the finite element code Thebes [1, 2] that replicates the observed increase in permeability at higher gas pressures, e.g. due to pore dilatancy and gas fracture as proposed by Xu et al. [3]. The formulation links permeability to gas pressure and threshold/critical pressure. For model validation, the study utilizes a gas injection experiment carried out in IfG (Institute for Rock Mechanics, Germany) on Opalinus Clay [4]. The results show a good fit against the measurements while giving insight into gas flow through clays.
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| 6. |
- Pintado, X., et al.
(författare)
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Shear strength and shear stiffness analysis of compacted Wyoming-type bentonite
- 2023
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Ingår i: Geomechanics for Energy and the Environment. - 2352-3808. ; 34
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Tidskriftsartikel (refereegranskat)abstract
- Wyoming-type bentonite is one of the materials to be used as part of the Engineered Barrier System (EBS) in deep geological disposal facilities for the safe disposal of spent nuclear fuel. In the KBS-3 type repository, the canisters containing the spent fuel assemblies are surrounded by bentonite buffer. Therefore, it is necessary to understand the bentonite behaviour in shearing conditions for the long-term performance assessment of the canisters. Bentonite shearing could arise, for example, due to potential post-glacial earthquakes. The deviatoric stress at failure and the shear modulus of a compacted Wyoming-type bentonite were measured using different shearing tests. Tests were carried out on unsaturated samples at three laboratories. The shearing tests on unsaturated samples were unconfined and triaxial compression tests, hollow cylinder tests and simple shear tests. Unconfined and triaxial compression tests were also carried out on saturated samples. The failure was quite ductile when tests were carried out with confining pressure. The samples were compacted at different dry densities and degrees of saturation and tested at different confining pressures and strain levels to study their influence on the shear properties. Shear strength and shear modulus were studied based on models that consider the double porosity structure of the bentonites. Previously published triaxial tests and resonant column test results were also included in this study. In combination with these models, several alternatives for the definition of the constitutive stress were checked for the case of bentonite. A novel, approach, involving the results of mercury intrusion porosimetry in estimating the constitutive stress, was introduced. The shear strength calculated by the models showed a good agreement with the measured values. In some cases, unrealistic large tensile stresses were predicted. Furthermore, the models can assess the tendency of the shear modulus as a function of the shear strain, degree of saturation and dry density.
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| 7. |
- Scelsi, Giulia, et al.
(författare)
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Modelling the behaviour of unsaturated non-active clays in saline environment
- 2021
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Ingår i: Engineering Geology. - : Elsevier BV. - 0013-7952. ; 295
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Tidskriftsartikel (refereegranskat)abstract
- The chemical composition of pore fluid and matric suction rule the mechanical behaviour of soils. In case of clays, their fabric changes in line with those variables. Since both the increase in matric suction and salinity cause a transition from an open to a close microstructure of low and medium activity clayey materials, a unique framework could tackle problems where salinity and saturation changes are expected. This paper presents a simple elasto-plastic model capable of reproducing the behaviour of unsaturated clayey soils in saline environments. Changes in the pore fluid composition are addressed through the use of osmotic suction as a variable. The proposed model extends the Barcelona Basic Model for partially saturated soils to consider the effect of osmotic suction. The model, implemented in the Thebes code, is calibrated for Boom Clay. The reproduced tests include mechanical loading at different matric and osmotic suctions in oedometric conditions, as well as more complex chemo-mechanical stress paths. Despite the simplicity of the formulation, the agreement between the experimental results and the simulations is encouraging. It seems that the modelling approach addresses the most important features of partially saturated soils with saline pore fluid which are slightly or moderately expansive.
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| 8. |
- Tran, Q.A., et al.
(författare)
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Modelling of fall-cone tests with strain-rate effects
- 2017
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Ingår i: Procedia Engineering. - : Elsevier BV. - 1877-7058 .- 1877-7058. ; 175, s. 293-301
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Konferensbidrag (refereegranskat)abstract
- © 2017 The Authors. Published by Elsevier Ltd.Material Point Method (MPM) is a numerical method, which is well suited for large displacement simulations. Large displacements problems are relatively common in geotechnics, including post-failure behaviour of landslides as well as a wide range of problems involving penetration into the soil body. One of those problems is the fall-cone test, commonly used to establish the undrained shear strength and the sensitivity of saturated fine grained soils. This paper shows a Generalized Interpolation Material Point Method (GIMP) simulation replicating published free-fall cone experiment performed on a kaolin clay. In the fall-cone tests, the penetration characteristics of the cone, such as velocity and total penetration depth depend on the soil properties. Those properties are affected greatly by the strain-rate which must be accounted for in a numerical simulation. Hence, the simulations shown uses a Mohr-Coulomb / Tresca material extended with strain-rate effects. The presentednumerical simulations are compared with the published fall-cone experiment in whichdisplacement and force were measured. The comparison indicates that Generalized Interpolation Material Point Method and Mohr-Coulomb / Tresca model extended with strain-rate effects areableto replicate the fall-cone penetration test very well.
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| 9. |
- Tran, Q.A., et al.
(författare)
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Modelling of the quickness test of sensitive clays using the generalized interpolation material point method
- 2017
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Ingår i: Advances in Natural and Technological Hazards Research. - Cham : Springer International Publishing. - 1878-9897 .- 2213-6959. ; 46, s. 323-336
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The presence of the sensitive clays in Scandinavia and Canada creates quick clay landslide hazards. The ability to predict the likelihood of such landslides occurrence, as well as their outreach would reduce the damage to the infrastructures and loss of life. Recently, a simple experimental technique named a quickness test (Thakur V, Degago S, Geotech Lett 2:87-95, 2012) has been employed to investigate the susceptibility of the clay to create large retrogression landslides. In this paper, we applied Generalized Interpolation Material Point Method (a numerical method suitable for large displacement dynamic problems) to replicate the quickness test experiment. The primary goal of the presented simulations is to further validate the modelling technique and the constitutive model used. In particular, the computations suggest the importance of the strain rates for the prediction of the run-out distance of the remoulded sensitive clays.
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