Coupled effects of particle overall regularity and sliding friction on the shear behavior of uniformly graded dense sands

Particle morphology has been regarded as an important factor affecting shear behaviors of sands,and covers three important aspects,i.e.global form(overall shape),local roundness(large-scale smoothness),and surface texture(roughness)in terms of different observation scales.Shape features of different aspects can be independent of each other but might have coupled effects on the bulk behavior of sands,which has been not explored thoroughly yet.This paper presents a systematic investigation of the coupled effects of the particle overall regularity(OR)and sliding friction on the shear behavior of dense sands using three-dimensional(3D)discrete element method(DEM).The representative volume elements consisting of ideal spheres and irregular clumps of different mass proportions are prepared to conduct drained triaxial compression simulations.A well-defined shape descriptor named OR is adopted to quantify particle shape differences of numerical samples at both form and roundness aspects,and the particle sliding friction coefficient varies from 0.001 to 1 to consider the surface roughness effect equivalently in DEM.The stress-strain relationships as well as peak and critical friction angles of these assemblies are examined systematically.Moreover,contact network and anisotropic fabric characteristics within different granular assemblies are analyzed to explore the microscopic origins of the multi-scale shape-dependent shear strength.This study helps to improve the current understanding with respect to the influence of the particle shape on the shear behavior of sands from different shape aspects.

Shear strength criteria for rock,rock joints,rockfill and rock masses:Problems and some solutions

Although many intact rock types can be very strong,a critical confining pressure can eventually be reached in triaxial testing,such that the Mohr shear strength envelope becomes horizontal.This critical state has recently been better defined,and correct curvature or correct deviation from linear Mohr-Coulomb（MC） has finally been found.Standard shear testing procedures for rock joints,using multiple testing of the same sample,in case of insufficient samples,can be shown to exaggerate apparent cohesion.Even rough joints do not have any cohesion,but instead have very high friction angles at low stress,due to strong dilation.Rock masses,implying problems of large-scale interaction with engineering structures,may have both cohesive and frictional strength components.However,it is not correct to add these,following linear M-C or nonlinear Hoek-Brown（H-B） standard routines.Cohesion is broken at small strain,while friction is mobilized at larger strain and remains to the end of the shear deformation.The criterion ＇c then σn tan φ＇ should replace ＇c plus σn tan φ＇ for improved fit to reality.Transformation of principal stresses to a shear plane seems to ignore mobilized dilation,and caused great experimental difficulties until understood.There seems to be plenty of room for continued research,so that errors of judgement of the last 50 years can be corrected.