Calculation of pressures on after incomplete arch cutterhead removal in earth-rock composite formation by in-situ junction method for shield tunneling

It is the key to ensure the stability of the free face after the cutterhead is disassembled by using the shield junction method of "junction, shell abandoning and disintegration in opposite tunneling". For a shield tunneling project in Qingdao, a numerical model is established by COMSOL Multiphysics. Based on the streamline deflection of the principal stress vector and change of stresses in various directions, the range of the main arch ring of the pressure arch in the junction section of the tunneling is determined, and the formula for calculating the overlying pressures on the surrounding rock is derived by considering the incomplete deflection of the major principal stress and the incomplete call of the internal friction angle. The results show that the major principal stress along the tunnel axis does not arch along the vector streamline, but the arching effects along the tunnel axis are more significant. Due to the loose area at the vault, the inner boundary does not coincide with the upper edge of the tunnel, but is a certain height away from the vault. The boundaries at both sides of the pressure arch are inclined upward, and the main arch ring is generally in the shape of a basin. By comparing the actual deflection of the major principal stress of incomplete arch, it is found that the arc, linear and parabolic arch traces all overestimate the deflection angle of the principal stress, resulting in a large lateral pressure coefficient, and it is unsafe to calculate the overlying earth pressures, while the internal friction angle increases with the decrease of the distance from the arch vault. Through the numerical calculation and theoretical analysis, it is concluded that under the action of pressure arch, the loads on the free surface drop to 1/3 of the initial confining pressures.