Study on local primary support method for tunnels based on the zoning results of excavation disturbed zone

To reduce tunnel construction costs, primary support methods are often optimized from the perspective of the support structure, while neglecting the influence of the surrounding rock, as the supported object. Firstly, a model test of primary support methods for tunnels was conducted to investigate the pressure, displacement, and failure characteristics of the surrounding rock under unsupported and full-face supported conditions. A preliminary concept for a local primary support method, which is based on the degree of disturbance cause by excavation in the surrounding rock, was proposed. Subsequently, using the Finite-Discrete Element Method, a quantitative index, the Crack Propagation Index (CPI), was introduced to characterize the degree of crack propagation, and a zoning standard for the Excavation Disturbed Zone (EDZ) based on the CPI was established, while a precise method for zoning the EDZ was proposed. The zoning method for the EDZ was applied to tunnels with different rock strength, revealing the influence of rock strength on the EDZ and validating the accuracy of the proposed zoning method. Finally, based on the zoning results of the EDZ, a local primary support method was developed. Shotcrete combined with anchor bolts was applied to the surrounding rock in the excavation fracture zone, while only shotcrete was used in the crack propagation and tiny crack zones. Compared to unsupported conditions, the local primary support increased the ultimate bearing capacity of the surrounding rock by 38.3%, ensuring rock stability and enhancing the economic efficiency of tunnel construction.