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Strength anisotropy in circular-particle deposits

The strength anisotropy of granular materials deposited under gravity has mostly been attributed to elongated particles’ tendency of long axis alignment in the bedding plane normal to the gravity direction. However, some recent direct shear experiments (Tong et al., 2014, DOI:10.1007/s11440-014-0303-6) on glass beads for which preferred particle alignment is inapplicable have exhibited surprisingly strong strength anisotropy. The current study tests the hypothesis that a significant amount of fabric anisotropy caused by the anisotropic stress during the deposition, can be locked in the material. DEM (discrete element method) simulation provides an ideal tool for the study because the evolution of stresses and fabrics during deposition and loading can be continuously monitored. We start with simulating the deposition process of 2D circular particles under gravity that induces the appropriate fabric. Direct shear tests along various directions with respect to the deposition plane were performed on the virtual specimens. Because the strength anisotropy is expected to be weak, extreme care was taken to minimize strength variations among different tests due to factors other than fabric anisotropy. The results show modest yet clear anisotropy in shear strength, thereby proving the hypothesis. When using a contact normal-based fabric tensor to quantify fabric anisotropy in the material, we find that the amount of fabric anisotropy locked in the material varies with particle size distributions. The anisotropic fabric locked in mono-sized particles can withstand the isotropic consolidation process prior to direct shearing, whereas that in continuously graded particles largely diminish as we apply the isotropic consolidation stress. However, both materials show clear strength anisotropy, suggesting that the contact normal-based fabric measurement might be insensitive to the fabric anisotropy responsible for some of the observed strength anisotropy.

Author(s):

Rui Wang    
Department of Hydraulic Engineering, Tsinghua University
China

Pengcheng Fu    
Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory
United States

Zhaoxia Tong    
School of Transportation Science and Engineering, Beihang University
China

Jian-Min Zhang    
Department of Hydraulic Engineering, Tsinghua University
China

Yannis Dafalias    
Department of Civil and Environmental Engineering, University of California, Davis; Department of Mechanics, School of Applied Mathematical and Physical Sciences, National Technical University of Athens, Zographou Campus
United States