Abstract: When shaft boring machine is tunnelling in strata, the inadequate bearing capacity of the surrounding soil underlying gripper can lead to insufficient propulsion, inability of excavation direction control and even shaft instability, therefore, it is necessary to study the ultimate bearing capacity of the surrounding soil underlying gripper. Based on the first mine-type shaft boring machine developed in China, the interaction between the gripper and soil is herein investigated by numerical simulation and the failure mode of the soil underlying gripper is determined to be shear-slip failure. Based on the failure mode simulated and the assumption made to treat the fracture face of surrounding soil under gripper as a combination of logarithmic helix and linear rotating surface, the stress distribution on the fracture face is analysed. Then, the formula of ultimate bearing capacity of surrounding soil underlying gripper is derived using the plastic limit equilibrium theory. Finally, the formula of ultimate bearing capacity is compared and analysed with the numerical simulation results to show its rationality. The results of this investigation can provide important guidance to the gripper design of shaft boring machines and stability evaluation of surrounding soil underlying shaft boring machine’s gripper in different conditions.

Key words: shaft boring machine, gripper, soil, failure mode, ultimate bearing capacity