Abstract: In soft rock strata, the vertical load transfer behavior of cast-in-place piles is significantly influenced by the roughness of shaft-rock joints. It is particularly pronounced at the interface between the shaft and the surrounding rock, where dislocations occur under loads, leading to shear dilation and an increase in the lateral constraint (normal stress). The existing models, such as the Patton's model and its generalized form, can well predict the normal stress at the pre-peak, but they ignore a critical aspect: in specific, the potential destruction of the asperity when shear dilation reaches the critical state due to increasing local stress, leading to the rapid release of accumulated energy. This destruction is macroscopically represented as the volume shear contraction of the shaft-rock joints, causing a decrease in the normal stress. This study identifies that a newborn debris will be separated from the original rock asperity and obliquely slides after the asperity fails based on the upper-bound solution of a unilaterally compressed wedge and the existing laboratory observations. Considering the 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 the existing direct shear tests. On this basis, the analytical solutions for the distribution of axial force are obtained. The 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.