Time-dependent behaviour of clay-concrete interfaces with different contact surface roughnesses under shear loading

Mechanical properties of the interface between a pile and soil greatly affect the bearing capacity of a pile. The creep of soil causes a long-term strength of the pile-soil interface to be smaller than its instantaneous shear strength. It is helpful to understand the settlement deformation and strength characteristics of friction piles from the micromechanism by studying the shear creep mechanical properties of the clay-concrete interface. In this work, we carry out shear creep tests on the interfaces between clay and concrete slabs with different roughnesses under different normal stresses. We analyze the influence of the joint roughness coefficient (JRC) on the long-term cohesion and long-term friction angle of the interface. We propose an instantaneous damage factor of the shear modulus and improve the Nishihara creep model with the instantaneous damage effect of the shear modulus. The experimental results show: (1) the whole process test curve of the shear creep shows that the specimen is rapidly destroyed after the transient creep stage and the steady-state creep stage, but the tertiary creep stage is not obvious. (2) The long-term shear strength of the clay-concrete interface is greatly affected by the change in the normal stress under the same JRC conditions, and the long-term shear strength of the interface increases linearly with increasing normal stress. (3) The relationship between the long-term shear strength and normal stress of the interface can be described by the Mohr-Coulomb failure criterion, and the long-term cohesion and long-term friction angle under different JRC conditions can be calculated accordingly. There exists a critical value JRC(cr)for the long-term cohesion, and the long-term cohesion of the interface first increases and then decreases as the JRC increases. When the long-term cohesion of the interface decreases to a certain extent, it fluctuates around the long-term cohesion value of clay. Moreover, the long-term friction angle of the interface fluctuates slightly with varying JRC, and there is no obvious change rule. (4) Considering the instantaneous damage of the shear modulus, we establish a relationship between the instantaneous shear modulus and the damage factor and introduce the damage factor into the Nishihara creep model. The shear creep mechanical parameters of the clay-concrete specimens under multistage loading are inversed.