Centrifugal model tests on excavation above existing tunnels in sandy loess strata

Taking the excavation above the existing tunnels as the background, the centrifugal model tests are employed to study the change rule of pressures on the surrounding rock of the existing tunnel linings in sandy loess strata during excavation. The standards are proposed as follows: the deep and shallow tunnels are divided according to whether the loads that the linings and the surrounding rock jointly bear are distributed by the stiffness, and the shallow and ultra shallow tunnels are according to whether there exists a holding force. Then 1.25D~1.75D (D is the span of the existing tunnels) distinguished is obtained as the range of the critical buried depth between the deep and shallow tunnels, and 0.75D~1.25D as the range of the critical buried depth between the shallow and ultra shallow tunnels. Meanwhile, the greater the stiffness of the existing tunnel linings is, the smaller the range of the load-bearing arch is. That is to say, the boundary of the load-bearing arch is 1.5D for rigid supports, and the boundary of the load-bearing arch is 1.8D for flexible supports. The governing criteria for the adjacent influence partition based on the relative ratio of the pressures on the surrounding rock of linings are put forward. For the rigid supports, the critical buried depths of intensive-effect, weak-effect and no-effect areas are 1.5D and 2D, and for the flexible supports, the critical buried depths of intensive-effect, weak-effect and no-effect areas are 1.5D and 2.5D. The comparison between excavation steps of 0.5D and 0.3D indicates that the steps cause the difference of stress paths of the surrounding rock during excavation. So 0.3D or smaller step is suitable to be chosen. The general laws of the existing tunnels in sandy loess strata influenced by the excavation above them are released. The achievements may provide some reference and guidance for similar projects.