Model tests on face stability of shallow shield tunnels in sandy cobble strata

In order to study the response of shallow sandy cobble strata to shield excavation and face stability, an earth pressure balance shield model is designed to conduct the indoor model tests. The failure mode and detailed process of strata are observed in these tests. The regulation of surface settlement and feature of settlement through are analyzed based on experimental data. The results show that: (1) The soil in different spatial position have different sensitivities to face displacement, but have similar failure process. (2) With the increase of the face displacement, the ground settlement-face displacement curves exhibit the regulation of ground settlement, which can be divided into four stages as insensitive stage, slow-linear settlement stage, accelerate stage and fast-linear settlement stage. (3) The directly overall instability occurs after elastic deformation and elastic-plastic failure in the shallow tunnel, and local instability has not appeared. (4) The settlement trough of sandy cobble strata is not exactly symmetrical with the tunnel axis. (5) The influenced region of the shallow tunnel in sandy cobble stratum is smaller than that of the pure sandy one in transverse, while it is greater in the longitudinal direction.