Mechanism of shear stress fluctuation and dropping of the soil-rock mixture

The shear stress-dropping (jump) of the soil-rock mixture (S-RM) exists under direct shear conditions. In this paper the main factors affecting the stress-dropping are explored with the large scale direct shear test. Artificial soil-rock mixture specimens are prepared for different rock block proportions (0%, 30%, 50% and 70%), normal pressures (50, 200, 300 and 400 kPa), and rock sizes ranging from 9.5 to 19.0 mm, from 19.0 to 31.5 mm and from 31.5 to 53.0 mm. Meanwhile, the thickness of the shear band is monitored by putting the aluminum wires and dry ash into the hole inside the specimens. The thickness of the shear band will contribute to further understand the stress jump and rock blocks deformation. The experimental results show that the soil-rock mixture sample with skeleton structure and larger rock blocks is inclined to form the interlocking rock block. Furthermore, the shear strength of the sample is dominated by interlocking rock blocks until the breakage of the rock block. The rotation, slippage, and breakage of interlocking rock block are the direct causes of the stress-dropping. The high content of the rock blocks, rock blocks of oversize, and high normal pressure are the necessary conditions for the formation of the stress locked patch. When the sample rock content is 50%, the rotation of shear band rock blocks in the shear direction is observed, and the deformation of rock blocks are more obvious close to the sidewall. However, the change of relative spatial arrangement of rock blocks is not more obvious, when the rock content of the sample is 70%, the behaviors of sliding, climbing, and the change of relative spatial arrangement of rock blocks are more notable than those of the sample with rock content of 50%. When the rock block size is less than the shear band thickness, the shear stress curve shows a wave shape. However, when the rock block size is close to the shear band thickness, the shear stress curve shows obvious brittle stress-dropping, and the vertical displacement will change sharply accordingly. Under the large-scale direct shear test of soil-rock mixture with high-level normal stress, the specimen with the rock content of above 70% and the size of rock block larger than the thickness of shear band will be apt to form the stress locked patch.