Influence of depositional fabric on mechanical properties of naturally deposited sands

Abstract

Fabric characteristics and shear deformation behaviour were analysed in four intact small-scale samples, including two disturbed and two undisturbed samples of naturally deposited sand from a shoal in the San Francisco Bay. Triaxial compression tests were performed on each sample while incrementally imaging with X-ray micro-computed tomography (XRCT). The grains in each intact sample exhibited a nearly unidirectional orientation with their long axes oriented within a narrow range of horizontal directions, in stark contrast to the randomly oriented grains of a reconstituted sample. The disturbed intact samples exhibited lower peak strengths and lower yet positive rates of dilation as compared with undisturbed samples. Higher rates of dilation corresponded with larger quantifiable decreases in the average number of grain-to-grain contacts and contact areas per grain within the shear band. The XRCT images of the shear bands were quantified by way of large incremental deviatoric strains and large incremental grain rotations. The grain orientations within the fully developed shear band of each sample reached a consistent orientation independent of their initial fabric. These results are a promising step toward a systematic study of granular deposits from different depositional environments needed for the development of more advanced models of their mechanical behaviour.