Investigations of the effects of particle morphology on granular material behaviors using a multi-sphere approach

This article studies the influences of particle morphology on the behaviors of granular materials at both macroscopic and microscopic levels based on the discrete element method(DEM).A set of numerical tests under drained triaxial compression was performed by controlling two morphological descriptors,i.e.ratio of the smallest to the largest pebble diameter,x,and the maximum pebbleepebble intersection angle,b.These descriptors are vital in generating particle geometry and surface textures.It was found that the stress responses of all assemblies exhibited similar behavior and showed post-peak strainsoftening.The normalized stress ratio and volumetric strains flatten off and tended to reach a steady value after an axial strain of 40%.While the friction angles at peak state varied with different morphological descriptors,the friction angles at critical state showed no significant variation.Moreover,evolution of the average coordination numbers showed a dramatic exponential decay until an axial strain of about 15%after which it stabilized and was unaffected by further increase of axial strain.In addition,stress ratio q/p and strong fabric parameter fs d=fs m were found to follow an approximately linear relationship for each assembly.These findings emphasized the significance of the influences of particle morphology on the macroscopic and microscopic responses of granular materials.

DEM investigation of strength and critical state behaviours of granular materials under true triaxial loadings

This paper investigates the strength and critical state(CS)behaviours of granular materials via DEM simulations of true triaxial drained tests under three different loading modes including constant b(in-termediate stress ratio)tests with constantσ'1(major principal stress),constant p(mean pressure)and constantσ'3(minor principal stress)respectively.To this end,a series of samples are generated with the same particle size distribution,and with the confining stresses ranging from 100 kPa to 900 kPa.The CS is achieved for all samples.Both the macroscopic behaviours and the microscopic behaviours are examined and compared considering different loading modes,confining stresses and intermediate stress ratios(b).The critical state lines(CSLs)are found to be unique and independent of the loading modes,but dependent on the b values.The CSLs with b=0 and b=1 form the two boundaries of CSLs respectively beyond which CSLs under all other b tests cannot go beyond.Six different strength criteria are examined and compared in terms of both peak and CS failures.The Mohr-Coulomb strength criterion is found to be only suitable for axisymmetric loading conditions.The Lade-Duncan criterion is only suitable for describing peak strengths,which is dependent on the loading modes and confining stresses.The Satake criterion and Matsuoka-Nakai criterion are the more appropriate strength criteria for describing CS failures,indicating that the CS values of both the Satake parameter and the Matsuoka-Nakai parameter describe an inherent property that characterizes the CS failure for a given type of soil.The CS mechanical coordination number is fitted by a curved line for a given b value,which is unique regardless of the loading modes.The peak and CS values of both major and minor principal fabric tensors decrease with increasing b values,while they increase with increasing b values for the intermediate principal fabric tensor.

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(1D)and different initial confining pressures(ρ′0).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(ρ′0)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 andρ′.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.

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.

MICP as a potential sustainable technique to treat or entrap contaminants in the natural environment:A review

In the last two decades,developments in the area of biomineralization has yielded promising results making it a potentially environmentally friendly technique for a wide range of applications in engineering and wastewater/heavy metal remediation.Microbially Induced Carbonate Precipitation(MICP)has led to numerous patented applications ranging from novel strains and nutrient sources for the precipitation of biominerals.Studies are being constantly published to optimize the process to become a promising,cost effective,ecofriendly approach when compared with the existing traditional remediation technologies which are implemented to solve multiple contamination/pollution issues.Heavy metal pollution still poses a major threat towards compromising the ecosystem.The removal of heavy metals is of high importance due to their recalcitrance and persistence in the environment.In that perspective,this paper reviews the current and most significant discoveries and applications of MICP towards the conversion of heavy metals into heavy metal carbonates and removal of calcium from contaminated media such as polluted water.It is evident from the literature survey that although heavy metal carbonate research is very effective in removal,is still in its early stages but could serve as a solution if the microorganisms are stimulated directly in the heavy metal environment.