Mechanism analysis of tunnel collapse in a soft-hard interbedded surrounding rock mass: A case study of the Yangshan Tunnel in China

Abstract

Tunnels are unavoidably constructed in layered rock masses, and tunnel collapses are often associated with such layering. Two violent collapses at the Yangshan tunnel were analysed in this study to investigate the mechanism of tunnel collapse in a soft-hard interbedded surrounding rock mass. The in situ stress field was obtained by combining hydraulic fracturing tests and multiple regression analysis. The first collapse occurred in the area where extremely high in situ stress and high in situ stress transformed. The extremely high in situ stress state during construction had not received timely attention, and the support measure under the condition of a high-grade surrounding rock mass was still used. Insufficient support strength was the primary cause of the first collapse. The excavation process of the Yangshan tunnel was simulated by a numerical method. The surrounding rock mass of the second collapse, interbedded with soft mudstone and hard sandstone, was dominated by tensile stress. The soft mudstone could not resist the large tensile stress, and the displacement of the tunnel vault increased sharply. Microseismic monitoring was used to highlight the dangerous areas and the types of rock mass failure during the excavation. The microseismic monitoring results were consistent with the numerical simulation results, which were used to describe the mechanism of the second collapse of the Yangshan tunnel. The excavation of a soft-hard interbedded rock mass would lead to tensile stress in the tunnel vault. If the soft rock layer was located in the tunnel vault, tensile failure of the rock mass would occur due to its lower strength.