Numerical Analysis on the Behavior of Existing Tunnels Subjected to the Undercrossed Shield Tunneling at a Small Proximity

Shield excavation in a proximity to existing tunnels inevitably causes deformation and stress change of the adjacent tunnels. Based on the project of new metro line in Beijing where the shield tunnel was constructed under existing tunnel, a modified equivalent stiffness model for the existing tunnel is proposed. Based on this, a three-dimensional finite element is established, and subsequently the proposed modified equivalent stiffness model and the previous model are implemented into the finite element model (FEM), respectively, and then the effectiveness of it is verified through comparison with the field measurements. The results indicate that the modified equivalent stiffness model has excellent agreement with the monitoring value. The impacts of key parameters including undercrossing angle, vertical spacing, and soil parameters on the crown settlement of the existing tunnel are investigated. The maximum settlement of the existing tunnel crown decreases as the undercrossing angle increases. The decease of vertical spacing between the newly constructed tunnel and the existing tunnel leads to an increase of the peak value of crown settlement. As for the soil parameters, friction angle φ is the dominant parameter affecting crown settlement of the existing tunnel whereas the cohesion c is least influential. During the process of route selection, it is suggested to keep the vertical spacing between new tunnel and the existing tunnel greater than the external diameter of the new metro tunnel. For the situation of undercrossing an existing tunnel within an extremely small clearance, it is recommended to strengthen the soil as necessary.