Deformation law and failure mechanism of surrounding rock of intersecting tunnel supported by NPR anchor cable

Urban traffic is gradually developing into underground, and the number of intersecting tunnels in underground interchanges is increasing. In order to explore the deformation law of intersecting tunnels under complex geological conditions, taking the intersecting tunnel section of the Shenzhen Nanlong underground interchange tunnel as the engineering background, the deformation mechanism of intersecting tunnels under excavation support and overload conditions was explored through physical model tests. Firstly, a three-dimensional model test system was designed independently, and a variety of rock samples and supporting materials were prepared by similarity theory calculations. Secondly, after the completion of the model, based on the excavation compensation theory, the intersecting tunnel was excavated and supported by the reduced-scale Negative Poisson's Ratio(NPR) anchor cable. Through the analysis of tunnel strain, displacement, and NPR anchor cable axial force, the stress redistribution characteristics of tunnel excavation were obtained. Finally, the overload test of the tunnel was carried out. Combined with the data and phenomenon analysis, the failure mechanism of the surrounding rock of the intersecting tunnel joint construction area was revealed. The results show that when the intersecting tunnel is excavated, the stress loss of the rock mass in the joint construction area due to tunnel excavation will slightly impact the tunnel excavated and supported. When the tunnel is loaded through, the failure of the intersecting tunnel is divided into three stages:the settlement stage of the vault stress concentration area, the failure stage of the joint construction area, and the failure stage of the tunnel portal. The specific performance is as follows:first, the vault of the upper tunnel joint construction area begins to sink, then the bottom drum of the lower tunnel appears, and finally, the stress spreads to the boundary, causing the collapse above the tunnel entrance. The test uses the NPR anchor cable to support the intersecting tunnel. The phenomenon and data show that the settlement of the tunnel joint construction area during the excavation stage affects the NPR support section. Due to the high-stress compensation characteristics of the NPR anchor cable, there is no obvious settlement in the support section. The test verifies the supporting effect of the NPR anchor cable in intersecting tunnels, which can provide a new idea for the similar engineering support.