Effect of particle shape on the progressive failure of shield tunnel face in granular soils by coupled FDM-DEM method

The face failure of tunnels has been extensively studied. However, the effect of particle shape on the failure process still remains unclear. In this paper, the progressive face failure of a shield tunnel in sand is analyzed with coupled discrete element method (DEM) and the finite difference method (FDM) in three-dimension. Soils at the tunnel face, where large deformation occurs and continuum mechanics description does not apply, are modeled with DEM. And the FDM is used for the rest areas where deformation and displacement are relatively small. In order to obtain appropriate parameters for soils as reference, a series of triaxial tests on both loose and dense soils are conducted. The face failure is generated by moving the tunnel forward and backward, which, respectively, simulates the collapse failure corresponding to the tunneling speed higher than soil excavating speed and the blow-out failure corresponding to the converse case. In particular, the effect of particle shape on the failure process is investigated in detail by adopting tetrahedral particles, elongated particles and spherical particles with rolling resistance coefficient of 0.0, 0.1, 0.2 and 0.4. The soil movement, ground surface movement, supporting force of the tunnel face, and the distribution of microscopic contacts are analyzed during the progressive failure of the tunnel face, which demonstrate a significant effect of particle shape.