Compared with straight tunnels, over-excavation occurs on the inner side of the curved section during shield construction of small radius curved tunnels, and the disturbance to the ground surface and mechanical properties of surrounding rock and segment are more severe. This paper establishes the numerical models of small radius curve tunnels and straight tunnels to study the characteristics of surface deformation caused by the shield excavation of small radius curved tunnels and the influence of shield construction parameters on ground settlement, surrounding rock deformation, and segment force. The maximum error between the numerical simulation results and the measured surface settlement curve is 7.3%, which is in good agreement. The results show that: (1) The maximum value of the surface settlement of the small radius curve tunnel appears inside the curve section, and with the decrease in the curve radius, the surface settlement increases, and the distance between the peak settlement point and the tunnel center is larger. (2) When the curve radius of the tunnel is smaller, the lateral displacement of the ground surface moves farther to the inner side, and the range of soil mass with lateral displacement in the inner side is also wider. (3) Increasing the heading face pressure and grouting pressure can reduce surface settlement, but the heading face pressure should not exceed 350 kPa, and the grouting pressure should not exceed 250 kPa. (4) When the curve radius is smaller, the deformation of surrounding rock and the segment stress is larger.

This paper establishes a three-dimensional symmetrical shield model to investigate the influence of a double-line shield tunnel undercrossing an existing foundation pit and of changed grouting pressure on the deformation and mechanical characteristics of both the foundation pit and the tunnel itself, and it proposes a method of symmetrical segmented pressure, in which different grouting pressure is applied in different sections of the tunnel. The monitoring data are used to verify the reliability of the model, and the maximum relative error is 5.44%. The numerical results show that the maximum subsidence of the retaining pile and anchor are 3.76 mm and 10.33 mm, respectively, and the maximum tensile stress of the anchor is increased by 32.4%. The subsidence shape of the foundation pit raft is an arch with four corners warping upward and the maximum subsidence difference is 3.17 mm. Uneven subsidence of the tunnel occurs along the longitudinal direction, and large and small subsidences are located at the outside and underpart of the foundation pit, respectively, and the maximum and minimum values are 11.15 mm and 2.13 mm, respectively, and the maximum subsidence difference is 9.02 mm. The deformation and mechanical characteristics of both the foundation pit and the tunnel are significantly decreased by appropriately increasing the grouting pressure, and it is recommended that the grouting pressure should not exceed 300 kPa. The proposed method of segmented pressure can reduce the differential subsidence by 47.2% and the maximum tensile stress by 27.2%, so it can significantly reduce the uneven subsidence of the tunnel and improve the tunnel stress condition. The research results can provide a theoretical basis for the safe construction of shield tunnels under the existing foundation pit.