Discrete element modelling of granular materials incorporating realistic particle shapes

Abstract This paper proposes an approach to generate realistic particle shapes considering the major plane of orientation of particles in discrete element modelling (DEM). The particle generation framework includes capturing high-quality scanning electron microscope (SEM) images, followed by image processing and generation of clumps using a commonly used multi-sphere (MS) approach in particle flow code (PFC3D). A set of experimental direct shear tests (DST) and subsequent DEM simulations were performed by incorporating realistic particle shapes. The simulation results show a good agreement with those obtained in the laboratory. In addition, the normal stress showed a significant effect on the structural anisotropy of the granular materials

Influence of randomly mixed coir fibres and fly ash on stabilization of clayey subgrade

Abstract This paper discusses the stabilization of clayey subgrade when mixed with coir fibres (CF) randomly and fly ash (FA). The tests were carried out on the locally available clay soil with different proportions of randomly mixed CF and FA. The CF and FA proportions used in the study are expressed in percentage by dry weight of soil. The CF and FA proportions used in the present study are 1%, 1.5% and 2% and 0%, 5%, 10%, 20% and 30% respectively. The tests results revealed that the clayey subgrade stabilized with FA resulted in improvement of strength and California bearing ratio (CBR). Further, it is observed that randomly mixed 1% CF and 20% FA mixture resulted in improvement of CBR of subgrade as compared to clay subgrade treated with 20% FA alone and this increase is noticed as 1.5 times. Overall, it is concluded that 20% FA + 1% CF resulted in improvement of subgrade in terms of CBR

Effect of RBI Grade 81 on strength characteristics of clayey subgrade

Abstract The present study deals with the effect of Road Building International Grade 81 (RBI Grade 81) on strength characteristics of the clayey subgrade. A wide range of tests was conducted on various percentages of RBI Grade 81 subgrade mixtures such as Atterberg limits, compaction characteristics, California Bearing Ratio (CBR) and Unconfined Compression Strength tests (UCS). It can be noticed that the CBR of clayey subgrade has increased significantly with the addition of RBI Grade 81. Similarly, the maximum dry density (MDD) has decreased and the optimum moisture content (OMC) has increased with RBI Grade 81 addition to subgrade. The strength of clay subgrade at 8% of RBI Grade 81 has improved 3–3.5 times as compared to the natural clayey subgrade. Overall, it can be concluded that RBI Grade 81 is suggested as a potential stabilizing agent especially for clayey subgrades

DEM simulations of critical state behaviour of granular materials under various drained triaxial stress path tests

The present study investigates the critical state behaviour of granular assemblies composed of clumped particles under four different drained axisymmetric triaxial stress paths, using the discrete element method (DEM). A series of numerical samples were prepared at initial states with different density indexes (ID) and different initial confining pressures (po′). These samples were sheared to large strains, at which constant stresses and volumes were maintained to reach the critical state. The evolution of stress ratio under the same loading mode (for the same intermediate principal stress ratio, b) is shown to yield an almost identical behaviour independent of stress paths, whereas the stress-strain response depends on the stress paths. Four different axisymmetric stress paths all share the same unique friction angle at critical state, indicating the Mohr-Coulomb failure criterion is the appropriate critical state strength criterion, which is at least true for the axisymmetric stress conditions. A unique coordination number (CN) is achieved at the critical state for a given po′, which is independent of the stress path. The critical state CN is found to increase with the increase in po′, which could be attributed to the decrease in the critical state void ratio (ec) as mean effective stress (p′) increases. Interestingly, a unique linear functional relationship is found between the critical state values of CN and ec, and a unique polynomial functional relationship is found between the critical state values of CN and p′. These functional relationships indicate no dependency on the stress paths or loading modes, thus characterizing unique features at critical states at both macroscopic and microscopic levels for a given type of granular material

Exploring the undrained behaviour of granular clumps after isotropic and Ko-consolidation paths using DEM

This paper investigates the undrained behaviour of granular clumps after isotropic and Ko-consolidation paths using a three-dimensional discrete element method (3D-DEM). Four randomly chosen clumped particles with a wide range of densification indexes, ID, and mean confining stresses, p\u27 were considered. The specimens were sheared to the deviatoric strain, εq of 40 % to reach the critical state (CS) conditions. It was found from the results that a unique critical state line (CSL) was achieved, irrespective of consolidation paths. The micro-mechanical quantities such as the average coordination number (CN) and von Mises fabric in terms of the second invariant of deviatoric fabric, FvM, also reached CS values. Irrespective of the consolidation paths, unique relationships were found between e−log(p′)and CN−log(p′). The stress-fabric joint invariant, KF established a unique relationship with p′and e, which forms a relationship in the KF−p′−e space and the projection of this relationship in the e−log(p′) plane confirms the classical CSL. Moreover, the flow potential (uF), stress ratio at instability (ηIS), and average coordination number at instability (CNIS) showed no dependency on the consolidation paths, while a dependency was observed for the second-order deviator fabric, FvM

Strength and fabric anisotropy of granular materials under true triaxial configurations using DEM

This paper investigates the strength and fabric anisotropy of granular materials under true triaxial configurations using the discrete element method. A clump logic based on the multi-sphere approach was utilized to replicate realistic particle shapes. The evolutions of deviatoric stress and volumetric strains were found to be independent of mean effective stress, and intermediate principal stress parameter (b). From a macroscopic viewpoint, all samples exhibited initial strain hardening followed by softening behaviour, stabilizing at large deviatoric strains due to the dense state of assemblies. The peak state friction angles showed dependency on the b-value, aligning closely with experimental findings. Microscopic parameters such as mechanical coordination number and sliding fraction were found to be approximately independent of b-values. Additionally, non-coaxiality was observed for various b-values except for specific shear modes, aligning with previous findings using spherical particles. These results highlight the capability of clumped particles to simulate the true mechanical behaviour of granular materials and contribute to our understanding of their complex response under different loading conditions

Discrete element modelling of strength and critical state characteristics of granular materials under axial compression and axial extension stress path tests

The critical state soil mechanics (CSSM) framework has been widely used across a range of problems in geomechanics involving complex loading conditions. However, the uniqueness of the critical state has been disputed for many years and it remains a controversial issue. Motivated by previous investigations, a series of discrete element method (DEM) simulations were performed under both axial compression (AC) and axial extension (AE) stress paths. All samples were isotropically compressed at varying mean normal effective stresses (confining pressures) and sheared to a large axial strain of approximately 60%. It is found that there exist unique values of critical void ratios and stress ratios under critical state, which are independent of the samples’ initial packings but dependent on stress paths. And the critical strength (stress ratio) for the AC stress path tests is higher than that for the AE stress path. The critical state lines (CSLs) are found to path-dependent but unique for each stress path. A unique linear relationship between the critical coordination numbers and critical void ratios is identified under the AC and AE stress paths respectively, but such a relationship depends on the stress paths. It is also found that there exist unique values of microscopic parameters in terms of deviator fabric under critical state, which are independent of the samples’ initial packings but dependent on stress paths. All these simulation results lead to the conclusion of non-uniqueness of CSLs from both macroscopic and microscopic viewpoints

The influence of particle elongations on direct shear behaviour of granular materials using DEM

This study examines the influence of particle elongation on the direct shear behaviour of granular materials using the discrete element method. A series of numerical direct shear test simulations were performed, and both the macroscopic and microscopic behaviour of elongated assemblies at the critical state were examined. The macroscopic response of elongated particles exhibits an initial hardening followed by post-peak strain softening, prior to reaching the critical state. The peak state friction angles initially increase and stay stable as the dimensionless elongation parameter (η) increases, whereas the critical state friction angles increase with the increase of η. Independent of the applied normal stresses, all samples reach a critical state at a unique normalized stress ratio (i.e., τ/ σ= 0.51) after ~ 25% shear strain. The stress-fabric relationship is mainly governed by the strong force subnetwork which is more affected by the change of η than the weak force subnetwork. Particle elongation generates a downward shifting of critical state lines (CSLs) in e- p′ space. Furthermore, the correlations between CSL parameters and η are well-fitted by a second-order polynomial function. These findings highlight the significance of particle elongation on direct shear behaviour of granular materials