Investigation into the influence of particle size distribution on the face stability of EPB shield tunnels in granular soils

Face failure of earth pressure balance (EPB) shield tunnels in granular soils is a great threat to the surroundings. This paper aims at investigating the influence of particle size distribution (PSD) on the failure mechanism from both the macroscopic and microscopic perspectives. The macroscopic investigation was carried out by performing several model tests which incorporated a miniature shield, and the microscopic investigation was conducted by utilizing the advantage of the discrete element method (DEM). The face failure of tunnels with C/D = 2.0 (C = tunnel cover depth; D = tunnel diameter) propagates to ground surface in three phases due to soil arching. PSD has influence on the timing of face failure, the size of failure zone and the soil arching. The proportion of strong contacts and the maximum soil arching increase with larger content of middle particles, while they decrease with larger content of fine particles. The soil arching maximizes when the local failure occurs and weakens in the transition phase, it extinguishes when propagating to Z/D = 0.4 (Z = depth from ground surface) as the global failure occurs regardless of PSD.