Effect of grout conditions on the mechanical behaviors of unloading sand-concrete interface for reinforcing bored pile foundation

•The grouting interface shear behavior is mainly improved by the cohesive force.•The grouting mode transition is particularly affected by the grout pressure and volumes.•The cohesive force of the grouting interface is 2–4 times that of the non-grouting interface.•Diffusion process of compaction and fracture grouting are revealed. The present study analyzes the mechanical behaviors and law of the grouting mode transition of unloading grout sand-concrete interface with different grout conditions. The direct shear test results show that the mechanical behavior of the grout sand-concrete interface is mainly improved by improving the cohesive force of the interface when the grouting pressure is lower, and the grout cement improves the shear behavior of the interface by improving both the interface cohesive force and friction angle for the high grouting pressure. The diffusion process of compaction and fracture grouting are revealed and the transition of the grouting mode is particularly affected by the grouting pressure as well as the grouting volume.