Prediction formula for surface settlement in double-line tunnel based on trapdoor tests

The disturbance caused by the excavation of urban tunnels is inevitable. In order to investigate the mutual influence of a double-line tunnel and the reliable method for prediction of ground surface subsidence, a multi-trapdoor model test setup and analogical soil are adopted to create the two-dimensional test conditions to simulate the tunnel excavation. The deformation caused by tunnel excavation is modeled by the trapdoors sinking. First of all, three single trapdoor model tests are conducted to obtain the fitting parameters of Peck's formula for a single-line tunnel. Then, the fitting parameters are substituted into the superposition formula to calculate the surface settlement which is compared to the measured curves of the double-trapdoor tests. The results reflect that in the case of large buried depth and small tunnel spacing, the surface settlement will turn out to be the single-peak form earlier, then the application conditions of the two forms are determined. For the single peak, a peak value correction factor and a spacing influence correction factor are introduced to correct the Peck’s formula, and then an equivalent prediction formula for the settlement of the double-line tunnel based on model test data is proposed.