Abstract: The ?800 mm model EPB shield machine was used to study the deformation and failure model of earth pressure balance (EPB) shield tunnel in sandy cobble ground. Meanwhile, three dimensional discrete element method (DEM) simulations were conducted to assess the face stability and the influence of buried depth, which couldn’t be observed in laboratory test. Results show that, after tunnel losing stability, the ground settlement surface is “round funnel” shaped with a gradual contraction from top to bottom in sandy cobble ground. The influenced zone is smaller than that of sand. The dynamic soil-cutting process greatly destroys the initial soil fabric, and the face stability is weakened, thus the limit support pressure increases. The limit support pressure increases linearly as buried depth increases, while the ratio of limit support pressure to initial support pressure decreases with increasing buried depth. The face failure mechanism can be divided into three modes according to buried depth. The whole dynamic construction process considered in this paper is close to real constructions, and can serve as guideline for guaranteeing EPB shield tunnel stability in sandy cobble ground.

Key words: sandy cobble ground, EPB shield, face failure, laboratory test, 3D DEM, dynamic soil-cutting process