Application and research of NPR bolt for shallow-buried large-span metro tunnel

During the excavation of shallow-buried large-span metro tunnel, the surrounding rock is easily affected by the excavation disturbance, using the common passive support system which often leads to the problem of increasing the deformation of the surrounding rock. In this paper, a shallow-buried large-span metro tunnel station under construction in China is used as the engineering background, We here firstly introduce the supporting application effect of the new negative Poisson’s ratio (NPR) bolt based on the research and development of micro-materials, and we optimized the bolt element according to the test results. The results show that the use of NPR bolt significantly reduces the tensile stress zone and effectively avoids the problems of support structure damage and lining cracking and deformation caused by the passive support method. The active support system is more effective than the passive support system in suppressing the surrounding rock deformation, with the surface settlement, vault displacement, and horizontal convergence accounting for 47%, 29%, and 50% of the passive support in the same period, respectively. Meanwhile, based on simulations of the deformation of the surrounding rock and the structure stress, as well as the field monitoring results, the characteristic curves of the deformation of the surrounding rock and the support stress under the active support system with the excavation of the pilot tunnel are obtained, and the mechanism of the active support system of the NPR bolt in the shallow-buried large-span metro tunnel is revealed. The results of this paper can provide a practical reference for the construction and support of similar shallow-buried large-span metro tunnel.