Estimation of granular flow impact force on rigid wall using material point method

Landslides and avalanches cause loss of lives, as well as generate significant economic cost. Protection barriers help reduce the impact of such events. However, the design of the barriers requires the prediction of the landslide flow trajectory and the estimation of impact force. Material Point Method appears to have great potential for estimating those, since it can account for large displacement nature of sediment flows and their nonlinear behaviour. Therefore, it may be able to capture the complex interaction of landslides or avalanches with the ground and structures. This study focuses on simulating granular flows with Generalized Interpolation Material Point Method. The calculations use a constitutive model inspired by the Bagnold theory of granular flow [1] to model sand landslide / avalanche experiment [2] with sand treated as a linear elasto-plastic material. Shown simulations aim was to replicate the experiment. In particular, the paper focuses on estimation of the impact force of sand flow on a fixed rigid wall. Such force estimation is a first step to validate the Generalized Interpolation Material Point Method for use as a tool for the design of barriers defending against landslides and avalanches

Implementation and validation of pressure-dependent gas permeability model for bentonite in FEM code Thebes

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

Inclusion of chemical effect in a fully coupled THM finite element code

Bentonite-rich clays can be used as a buffer / backfill material in deep geological repositories for nuclear waste. The prediction of the long-term performance of a buffer / backfill in such a complex environment, where the temperature, humidity and chemistry of water change, requires a fully thermo-hydro-mechanical-chemical (THMC) coupled numerical code. This paper presents a simple extension of a THM coupled finite element code to include chemical effects. After deriving the governing salt mass balance equation and discussing its implementation into the code, the paper verifies the extended framework against an analytical solution for 1D salt transport. In addition, the article presents a validation example in which the code replicates experimental data. The numerical results obtained from the extended THMC coupled finite element code encourage further investigation of the chemical effects on the mechanical and thermal behaviour of the material, which would serve the ultimate goal of achieving a safer design of the nuclear waste storage facility.Postprint (published version

Identification of key thermal couplings affecting the bentonite behaviour in a deep geological nuclear waste repository

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

Hydro-mechanical coupled dual domain material point method stabilized with a null-space filter

The Material Point Method (MPM) is a continuum-based numerical method especially suitable for solving large deformation problems. In this paper, we investigate the null-space errors present in MPM solutions. The paper establishes a null-space stability condition which is used to examine the null-space errors in different versions of the MPM. This analysis shows that a B-splines MPM satisfies the null-space stability condition and therefore reduces greatly the errors associated with the null-space. In contrast, the MPM, the Generalized Interpolation Material Point Method (GIMP) and the Dual Domain Material Point Method (DDMP) show non-trivial null-spaces in the mapping. To remove the null-space errors, this paper utilizes QR factorization method, which is similar to the Single Value Decomposition (SVD) method, but requires fewer computations. This paper simulates several problems with hydro-mechanical coupled Dual Domain Material Point Method (DDMP) formulation both with and without null-space error reduction. The simulations indicate that the null-space filter can improve significantly the accuracy of the pore water pressure for both gravity loading and consolidation in large strain simulation

A constitutive framework for the chemo-mechanical behaviour of unsaturated non-expansive clays

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]

Estimation of granular flow impact force on rigid wall using material point method

Landslides and avalanches cause loss of lives, as well as generate significant economic cost. Protection barriers help reduce the impact of such events. However, the design of the barriers requires the prediction of the landslide flow trajectory and the estimation of impact force. Material Point Method appears to have great potential for estimating those, since it can account for large displacement nature of sediment flows and their nonlinear behaviour. Therefore, it may be able to capture the complex interaction of landslides or avalanches with the ground and structures. This study focuses on simulating granular flows with Generalized Interpolation Material Point Method. The calculations use a constitutive model inspired by the Bagnold theory of granular flow [1] to model sand landslide / avalanche experiment [2] with sand treated as a linear elasto-plastic material. Shown simulations aim was to replicate the experiment. In particular, the paper focuses on estimation of the impact force of sand flow on a fixed rigid wall. Such force estimation is a first step to validate the Generalized Interpolation Material Point Method for use as a tool for the design of barriers defending against landslides and avalanches

Modelling of sand column collapse with material point method

The paper shows numerical analysis of sand column collapse. The simulation was performed with the material point method and the results are compared to experiment. The problem considered involves extreme deformations and is difficult to model with more traditional numerical approaches like the finite element method. In the analysis, the sand is modelled with a rate-independent Mohr-Coulomb model. Despite the use of a simple constitutive model, the computed results agree with the experimental observations reasonably well. This agreement is satisfactory both during and after the collapse

Explicit stress integration with reduced drift for Barcelona Basic Model

The paper studies the effectiveness of a drift reduction method in integrating unsaturated soil models. The drift reduction is based on the NICE (Next Increment Corrects Error) method. The NICE method can be applied to any explicit stress integration method based on Runge-Kutta method. The performance of the stress integration enhanced by the NICE scheme is demonstrated with the Barcelona Basic Model. Numerical studies in this paper involve the integration of the Barcelona Basic Model with different numbers of strain increments and different sets of model parameters and initial conditions. The improved algorithm does not require any significant computations, thus the drift reduction is virtually accomplished without significant cost. The reduction in the yield surface drift appears to be rather significant, suggesting that under normal circumstances no additional drift correction method is required