Research on arch forming mechanism and overburden pressure calculation theory of shield butt section

When the shield tunneling method of "face to face excavation, underground docking, and discard shell and disassemble shield" is adopted, ensuring the stability of the free surface after cutter head disassembly is the key to ensure the safe docking. Based on a shield butt joint project, a numerical model was established through COMSOL Multiphysics, and the range of the main arch circle of the pressure arch in the butt joint section of the shield was determined based on the streamline deflection of the principal stress vector and the change of the stress in every direction. The overlying pressure calculation formula was derived by considering the incomplete deflection of the large principal stress and the incomplete call of the internal friction angle. The results show that the deflection degree of the maximum principal stress along the tunnel axis is small, the three-dimensional arch effect is not obvious, and the pressure arch of surrounding rock is mainly manifested as the radial arch of the tunnel. The outer boundary of the pressure arch is located at the interface between soft and hard surrounding rock, and the inner boundary is located at the position of 0.5m of the arch top. The boundary on both sides is inclined upward, and the Angle between the inclination and the vertical plane is 5.7°. The main arch ring is in the shape of a basin. Comparing the actual deflection of large principal stress of incomplete arch, it is found that the arc, linear and parabolic arch trace overestimated the deflection angle of principal stress, resulting in a large lateral pressure coefficient, and it is unsafe to calculate the overlying earth pressure, while the internal friction angle increases with the decrease of the distance from the arch top. Through numerical model calculation and theoretical analysis, it is concluded that under the action of pressure arch, the load on the free surface drops to 1/3 of the initial confining pressure.