Horizontal trap-door investigation on face failure zone of shield tunneling in sands

A novel horizontal trap-door test system was devised in this study to analyze the face stability of shield tunnels in sands. The test system can be used to investigate both the longitudinal and cross sections of the face failure simultaneously at one single apparatus and was employed to perform face stability tests on small-scaled tunnel models at single gravity. The lateral support pressures and failure zones were studied with varying sand materials and earth covers. The results demonstrate that the tunnel face moves back, the lateral active earth pressure on the tunnel face decreases rapidly to a residual value, and the lateral pressure distribution can be categorized into three stages during the failure process: 1) initial state; 2) pressure dissipation stage; and 3) pressure zone diminution stage. Furthermore, face failure firstly develops from a stable condition to the local failure state, and then continues to develop to the global failure state that can be divided into two sub-zones with different failure mechanisms: rotational failure zone (lower zone) and gravitational failure zone (upper zone). Further discussion shows that under the effects of soil arching, the shape of the gravitational failure zone can adopt arch shaped (most frequent) and column shaped (in shallow tunnels). Limit support pressure for face stability usually appears at δ/D=0.2%–0.5% (ratio of face displacement to tunnel diameter).