Face failure of EPB shield tunnels in dry dense sand: a model test and DEM study

This paper aims at addressing the face failure of earth pressure balance (EPB) shield tunnels in dry dense sand by model tests and discrete element method (DEM) models. The model tests incorporated a miniature EPB shield which could fully reproduce the real tunnel construction of excavation and support. DEM models simulating the model tests were developed to capture the underlying face failure mechanism. Results show that both the limit support pressure obtained at chamber board and tunnel face increase with increasing C/D (C is tunnel buried depth, D is tunnel diameter). The ratio of the former to the latter approximates 0.60 due to the soil retaining of cutterhead panel, and it is independent of C/D. The local face failure initializes around tunnel face and develops directly to the global failure outcropping the ground surface in one phase with C/D≤1.0, while the local failure develops to the global failure in three phases with C/D=2.0 due to the soil arching evolution. The soil arching gets weaker when it propagates upward, and the horizontal stress concentration in the longitudinal direction is stronger than the transverse direction due to the difference of arch foot.