Sand-sand and sand-steel interface grain-scale behavior under shearing

The behavior of a sand-steel interface can be different than the shearing behavior of a sand. This study focuses on the effect of shear loading on a sand-to-sand and a sand-steel interface as well as on particle breakage. A series of ring shear and simple shear tests were conducted to measure the effects of particle shape and surface roughness characteristics before and after shearing at the shear zones within a sand-to-sand and at a sand-steel plate interface. It was found that sand density had a large effect on the interface shearing behavior. While interface sliding began at a shear displacement of 0.9 mm (corresponding to 22% of the total shear displacement) in dense specimens, this occurred at a much higher displacement of 5.6 mm (62% of total displacement) in loose sand at rough steel interface. Sliding displacement prevailed at the sand-steel plate interface in simple shear tests as the sand sample became denser. Critical states were observed in the sand-to-sand ring shear tests after shear displacements of 65-79 mm and 87-98 mm in medium-dense and loose samples, respectively. Comparing sand-tosand and sand-steel interface results indicated that sand-steel interface behaves less dilative with the critical state occurring at much smaller shear displacements of about 12-14 mm at the interface compared to that occurring within the sand at 65-79 mm. Both loose and dense sands reached a residual strength of 98 kPa at a shear displacement of 7 mm in simple shear tests. The inter-particle shear resistance and particle crushing, combined with particle rearrangement/rotation were the mechanisms which developed shear resistance. An interface friction angle of 27 degrees was subsequently mobilized at the critical states of the simple shear tests, while particle crushing and abrasion resulted in a slightly elevated (by 2 degrees) interface friction angle in the ring shear tests. The difference between sand-steel interface and sand-to-sand interaction mechanism highly influenced particle damage induced by particle fracturing and abrasion. As a result of particle crushing, roughness and aspect ratio of sand particles sheared at the interface with a steel plate reduced by respectively 39.7% and 12-14% sheared at normal stresses of 50 to 200 kPa. In particular, particles' circularity increased from 0.685 to 0.817, indicating symmetrically rounded particles. The maximum size of particles in all tests reduced to about 16.9-18.1% of the original maximum particle size (D-max = 0.85 mm). While the skewness of particles sheared within the sand increased from -0.231 to 0.656 (indicating increased surface roughness), those of particles collected from the sand-steel interface decreased to -1.31 reflecting smoother particle surfaces.