Shear Strength of Sand–Clay Interfaces Through Large-Scale Direct Shear Tests

Shear failures of sand–clay interfaces are frequently encountered in engineering practice. It is of great concern to investigate the shear strength of the sand–clay interfaces. In this work, a series of large-scale direct shear tests have been conducted on the sand–clay interfaces. Considering the influences of the interface roughness, the preloading and the stiffness ratio, the interface shear stresses and the vertical displacements are evaluated and compared under different normal stresses. Results indicate that the interface shear stresses and the vertical displacement of the sand–clay samples are quasi-dependent of normal stresses. The shear stress curves feature hardening characteristics with no peak values followed by softening. Light dilation at the shear beginning followed by continuous contraction show in the vertical displacement curves. The interface shear strength is reduced, and the ultimate vertical displacement increases when the preloading is applied before shearing. Larger stiffness ratio leads to smaller ultimate shear stresses and larger ultimate vertical displacements. The values of the ultimate interface shear stresses and vertical displacements of sand–clay samples are between those of the sand–sand and clay–clay samples. Additionally, the movement pattern of the upper sand particles is observed by particle image velocimetry analysis.