3D Analysis on Surrounding Soil Stability of Shaft Boring Machine Based on Plastic Limit Equilibrium Theory

Shaft boring machine (SBM) plays a significant role in mechanized construction of deep and large shafts. However, surrounding soil instability of SBM pose great threats to the shaft construction. Therefore, it is necessary to analyse the soil stability underlying SBM’s gripper. On the basis of the first SBM (MSJ5.8/1.6D) that independently developed in China, the interaction between the SBM’s gripper and surrounding soil is investigated. According to the failure mode of the shaft obtained by numerical simulation, the geometry of the failure surfaces is first derived, and the stresses distribution on the failure surfaces are clearly presented. Then, the ultimate bearing capacity formula of the surrounding soil underlying SBM’s gripper is derived based on plastic limit equilibrium theory with Mohr-Coulomb criterion. The change rules of bearing capacity factors and the influence rules of basic parameters in this formula are obtained. Finally, the theoretical values in typical soils are compared with those from the numerical simulation to illustrate the rationality of the formula. The results of this investigation not only provide an advanced method for evaluating the surrounding soil stability of SBM in shaft construction, but also provide guidelines for the design and construction risk control of SBM.