Approach for evaluating longitudinal deformation of underlying tunnels due to excavation of upper foundation pit

For most of the analysis methods for the longitudinal response of underlying tunnels, excavation of the foundation pits is treated as a simple unloading of the base and the additional stress of the greenfield soil at the tunnel axis is taken as the input condition. However, this force input condition is not the real load to which the tunnel is subjected, resulting in an unreasonable tunnel response. To this end, the reasons why the current force input condition is insufficient and the displacement input condition is more reasonable are firstly analyzed theoretically, and the Winkler subgrade moduli of the two input conditions are distinguished. It is proposed that the passive and active Winkler subgrade moduli should be used for the displacement and pressure input conditions, respectively. Secondly, by comparing the tunnel response obtained by the finite element model and the Winkler methods, it is verified that the displacement input condition is reasonable, while the current force input condition would underestimate the tunnel response. Finally, given the engineering practicality of the force-based method, it is proposed to adjust the magnitude of the force input condition by using the load amplification factor to obtain a reasonable tunnel response. Using a large number of finite element calculations for parametric studies, a simplified formulation for the load amplification factor is proposed. The feasibility of the proposed method is verified by comparing with the field measurements of a case study.