Analytical solution for vertical load transfer of cast-in-place pile considering shear-induced volumetric contraction across shaft-rock joints

In soft rock strata, the vertical load transfer behavior of cast-in-place pile is significantly influenced by the roughness of shaft-rock joints. It is particularly pronounced at the interface between the shaft and surrounding rock, where dislocations occur under load, leading to shear dilation and an increase in lateral constraint (normal stress). Existing modellings, such as the Patton’s model and its generalized form, can well predict the normal stress at the pre-peak, but there overlooks a critical aspect; in specific, the potential destruction of the asperity when shear dilation reaches a critical state due to local stress increasing, leading to the rapid release of accumulated energy. This destruction is macroscopically represented as volume shear contraction of the shaft-rock joints, causing a decrease in normal stress. This study identifies the geometries of the failure of a triangular asperity based on the upper-bound solution of a unilaterally compressed wedge and existing laboratory observations. After asperity failed, a newborn debris is separated from the original rock asperity and obliquely slides. Based on considerations of kinematic principles, the energy principle is used to determine the shear contraction angle and the sliding resistance at the post-peak. The modified shear model is verified using the observations of existing direct shear tests. On this basis, analytical solutions for the distribution of axial force are obtained. Parameter studies reveal that the half chord-length, shear dilation inclination, and internal friction angle of rock have a strong impact on the shear contraction angle and unit shaft resistance.