Triaxial Shear Test for Strength Behavior of Saturated Sand–Steel Interface Based on Preformed Failure Plane

An innovative test method for analyzing the strength characteristics of a saturated sand–steel interface by triaxial apparatus is presented. A two-part triaxial test specimen was utilized incorporating a preformed interface along which surface sliding took place. The triaxial apparatus is used to control the drainage state and stress path of the specimen to implement expected interface boundary condition. Four steel specimens with various surface roughness (R1 to R4, smooth to noticeably rough) were used to analyze the effect of surface roughness on the sand–steel interface shear behavior. Typical drained and undrained interface behavior are demonstrated, and the influence of boundary conditions, initial confining pressure, and roughness on drained sand-steel interface are discussed. Results show that the strength of specimen with preformed failure plane is less than that of pure sand specimen. The strain hardening occurs in R1, R2, and R3 roughness during the tests, but the deviator stress has the tendency to decrease after reaching the turning point in R4 roughness. In addition, the normalized efficiency parameter E and the interface friction angle ϕint increased approximately linearly with Ra in a semi-logarithmic scale. Comparison with direct interface shear tests indicates that friction angles obtained by triaxial tests are only slightly higher than those derived from direct shear tests, implying that the triaxial interface shear test can reasonably obtain the index of the saturated soil–steel interface shear strength.