Centrifugal model tests on face stability of shield tunnels in dense sand

When tunneling in sand using the shield machine, failure of the tunnel face occurs frequently due to the inadequate support pressure. Centrifugal model tests with different overburden-to-diameter ratios (i.e., C/D= 0.5, 1, 2) are performed to study the problem of tunnel face stability in dense sand. During the tunnel face failure, with the increase of the horizontal displacement of the tunnel face, it is found that the support pressure firstly decreases rapidly to the limit support pressure and then increases gradually to the residual support pressure. A “wedge-prism” failure zone occurs in front of the tunnel face after the face failure. For the relatively shallow buried tunnel (e.g. C/D=0.5), the failure zone has extended to the ground surface in the limit state. While for the relatively deep buried tunnel (e.g. C/D=1 and 2), the failure zone is still in the interior of the ground in the limit state. It is also found that the limit support pressure increases with the increase of relative depth C/Dand then remains almost the same. Finally, after comparing the limit support pressures obtained from the existing theoretical methods with those from the centrifugal model tests, the engineering applicability of the existing theoretical models is discussed. The results of this research may help to guarantee the face stability of the shield tunnels in sandy ground.