Model test on support scheme for carbonaceous slate tunnel in high geostress zone at high depth

Abstract

The Muzhailing extra-long highway tunnel and corresponding inclined shafts in Lanzhou, Gansu Province, China passes through structurally complex carbonaceous slate that is under high ground stress. Rationally-designed and effective support is of high importance for achieving safe and efficient tunnel construction. The No. 2 inclined shaft of Muzhailing Tunnel was taken as the engineering background prototype, for which, a similar model test was conducted to evaluate the effect of highly pre-tightened constant resistance (NPR, Negative Poisson’s Ratio) anchor cable support provision to the geologically complex carbonaceous slate at different depths. Two schemes were proposed during testing: one scheme was without support and the second was with asymmetric support from highly pre-tightened constant resistance anchor cable. Digital speckle displacement analysis system and micro-ground-stress sensors were employed to measure the deformation and shear stress distribution of the tunnel. The results demonstrated that through the second support scheme, the deformation of the surrounding rock could be effectively ameliorated, while this support scheme was applied on the project site of the No.2 inclined shaft, to explore the rationality of the scheme through field engineering tests. On-site monitoring indicated that the deformation of the surrounding rock was within the reasonable design range and the problem of severe tunnel deformation was effectively controlled. The research methods and related conclusions can be used as a reference for the treatment of large deformation problems in deep-buried soft rock tunnels.