Analysis of subgrade settlement in large-diameter-section curved shield tunnel undercrossing construction

Tunnel excavation will trigger stratum loss and overlying soil settlement. The other buildings (structures), if any, in the affected area of tunnel excavation will also be subjected to settlement. Too large settlement will lead to the shear failure of the existing structures. To explore the settlement problem incurred when the existing expressway subgrade was crossed during the large-diameter-section curved shield tunnel construction, a modeling analysis of the construction process was performed using the numerical simulation method, and the settlement laws and morphologies of the expressway subgrade were acquired. Results showed that the lateral settlement curve of ground surface presented a unipolar type. The maximum settlement was located above the tunnel axis, while the settlement decreased gradually at a long distance from the tunnel axis, and the maximum settlement was about 23 mm. Due to the settlement of soil, the expressway subgrade experienced compatibility of deformation with the soil, and finally, the lateral settlement form was consistent with that of the ground surface. The scope of influence of longitudinal settlement induced by tunnel excavation was about 30 m. When the tunnel excavation face was rightly below the settlement monitoring point, the subgrade settlement rate was high. During the construction, close attention should be paid to the subgrade settlement amount and settlement rate in the rapid settlement stage. In the face of a high settlement rate and large settlement amount, the reinforcement countermeasures should be adopted timely for the stratum and subgrade to reduce the settlement rate and settlement amount during construction, protect the expressway subgrade from large deformation-induced failure and ensure expressway operation safety during the tunnel construction.