Face stability analysis of EPB shield tunnel in dense sand stratum considering the evolution of failure pattern

摘要

Face support pressure is a crucial index to guarantee the stability at the earth pressure balance (EPB) shield tunnel face. Based on the limit equilibrium theory and the failure pattern observed in existing model tests, a novel three-dimensional failure mechanism is proposed to consider the evolution of failure pattern in dense sand stratum. The failure zone is divided into two parts: a failure slip zone and an upper loosen zone. The failure slip zone is formed by rotating a straight line and a logarithmic spiral, whilst the upper loosen zone is defined as a cone or a cylinder for different stage during the failure zone evolution process. The input parameters of the proposed mechanism are determined based on the results of existing model tests, where the inclination angle of the slip surface alpha varies from 51 degrees + 0.5 phi (phi is the internal friction angle) to 53 degrees + 0.5 phi, and the height coefficient of the logarithmic spiral is lambda = 0.3. The proposed solution can provide reasonable calculations compared with experimental measurements and numerical simulations, suggesting the efficacy of the method for use in design.