Micromechanical inspection of incremental behaviour of crushable soils

Matteo O Ciantia,Antonio Gens,Catherine O’Sullivan,Marcos Arroyo

doi:10.1007/s11440-019-00802-0

Abstract

In granular soils grain crushing reduces dilatancy and stress obliquity enhances crushability. These are well-supported specimen-scale experimental observations. In principle, those observations should reflect some peculiar micromechanism associated with crushing, but which is it? To answer that question the nature of crushing-induced particle-scale interactions is here investigated using an efficient DEM model of crushable soil. Microstructural measures such as the mechanical coordination number and fabric are examined while performing systematic stress probing on the triaxial plane. Numerical techniques such as parallel and the newly introduced sequential probing enable clear separation of the micromechanical mechanisms associated with crushing. Particle crushing is shown to reduce fabric anisotropy during incremental loading and to slow fabric change during continuous shearing. On the other hand, increased fabric anisotropy does take more particles closer to breakage. Shear-enhanced breakage appears then to be a natural consequence of shear-enhanced fabric anisotropy. The particle crushing model employed here makes crushing dependent only on particle and contact properties, without any pre-established influence of particle connectivity. That influence does not emerge, and it is shown how particle connectivity, per se, is not a good indicator of crushing likelihood.

Highlights

Grain fragmentation is significant for several important geotechnical problems including side friction on driven piles, railway ballast durability or rockfill dam settlement
Two well-established observations arising from earlier research are that (a) soil crushability is enhanced by stress obliquity, and (b) plastic deformation in crushable soils is associated with less dilatancy, or, equivalently, that plastic flow is more volumetric when crushing is present
This study aims to fill that gap, by investigating how particle breakage modifies the micromechanics underlying the incremental stress response of granular specimens

Summary

Introduction

Grain fragmentation is significant for several important geotechnical problems including side friction on driven piles, railway ballast durability or rockfill dam settlement. To address these problems, granulometric evolution has been incorporated into constitutive models for soils [12, 23, 36, 37, 39, 44, 50, 56, 62]. Two well-established observations arising from earlier research are that (a) soil crushability is enhanced by stress obliquity, and (b) plastic deformation in crushable soils is associated with less dilatancy, or, equivalently, that plastic flow is more volumetric when crushing is present. This general principle has not been yet systematically applied to crushable soils, and several aspects of their micromechanics remain relatively obscure

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