Research on the Optimization of Reinforcement Measures and the Deformation Mechanism of the Lower Tunnel in the Construction of the Overlapping Tunnels

Taking a completely stacked section of Beijing Metro Line 22 as the research background, a three-dimensional finite element model was established to study and analyze the displacement and stress variation laws of the existing lower tunnel under different working conditions. The results show that the combined reinforcement measures of radial grouting and trolley support can effectively reduce the adverse effects of the upper tunnel on the lower tunnel during the construction of the overlapping tunnel. It cut the vault vertical displacement from 5.31 mm to 2.67 mm and reduced the stress range from 2.30 MPa to 0.71 MPa, reducing vertical displacement by 50% and maximum principal stress changes by 40% compared to the unreinforced condition. Furthermore, a parametric study indicated an optimal grouting scheme with a 2 m thickness, 120° angle, and 200 MPa modulus, which achieved similar reinforcement effectiveness with a 50% reduction in grout volume.