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Micromechanical analysis of granular ratcheting behind a retaining wall

The paper explores the response of granular media to both monotonic and cyclic loadings via 2D discrete element modelling (DEM). The case of a retaining wall is investigated where the granular backfill with a free surface is subjected to both monotonic and cyclic small amplitude active and passive displacements. In this study, a series of numerical simulations were performed on both dense and loose granular backfills in order to investigate (i) the volumetric and fabric changes during cyclic and monotonic displacements, (ii) the induced anisotropy of the granular material structure in response to applied loads and deformations, and in particular (iii) the ratcheting effect induced by gradual accumulations of small permanent deformations under cyclic loading. It is found that the response of the granular material cycle by cycle is marked by the formation of convective flux structures or vortices that evolve during the cycles due to grain ratcheting-like motion with respect to both active and passive failure wedges. An attempt has been made to describe the underlying microscopic and mesoscopic mechanisms of ratcheting in conjunction with stress escalation behind the retaining wall. In particular, attention is paid to the resulting changes in microstructure through the appearance of vortices and their link (if any) to local instabilities within the granular assembly during cyclic loading. The material instability analysis is performed at both microscopic and mesoscopic scales through the vanishing of the second order work at the contact level and minimal void loops with particle clusters exhibiting negative values of second order work, respectively.

Author(s):

Nejib Hadda    
Department of Civil Engineering, University of Calgary
Canada

Richard Wan    
Department of Civil Engineering, University of Calgary
Canada