Stability analysis of shield excavation surface in saturated silt strata considering seepage

When the earth pressure balance shield crosses the high water-level Qiantang River silt layer, the high water pressure difference between the groundwater and the sealing chamber will generate excessive infiltration force toward the excavation surface. The excessive infiltration force will cause large deformation of the soil and thus leading to destabilization of the excavation surface because of the low cohesion of Qiantang River silt, poor self-supporting of the excavation surface and small modulus of the soil. The reasonable selection of soil constitutive relationship and model parameters in numerical simulations are very important for the reliability of numerical results. In order to study the stability of the excavation surface under seepage conditions, the parameters of the Mohr-Coulomb model and its strain-softening model of the Qiantang River silt were investigated based on laboratory tests. It was found that the shear strain of the Qiantang River silt was 1.5 times of the axial strain. According to the triaxial unloading test, the relationship between the strain softening internal friction angle and the shear strain is obtained. The internal friction angle reaches a peak value of 25 degrees when the shear strain is 4.7%; the internal friction angle reaches a residual value of 21 degrees when the shear strain is 7.4%; the internal friction angle decreases linearly when the shear strain is between 4.7% and 7.4%. Then the finite difference method was used to establish 39 groups of three-dimensional analytical models. By comparing with the ultimate support force and surface settlement of the centrifuge test, it was found that under the same water level, the minimum error between the ultimate support pressure calculated by the Mohr-Coulomb strain-softening model and the centrifuge test was 3.1%, and the error of the maximum surface settlement was 5.67%. It proves the feasibility of the Mohr-Coulomb strain-softening model and the reliability of the model parameters in the Qiantang River silt formation.