Stability and reliability of a shallow tunnel in soft surrounding rock: construction technology and prediction

Shallow-buried tunnels are often constructed under strata of gravel or unconsolidated sediments; one distinguishing property is that the tunnel crown is near to the ground surface, surrounded by a weak mass of rocks mixed with unconsolidated material. So, it is of high importance that improved excavation methods integrated with modern reinforcement technologies are utilized to ensure stability and reliability of the tunnel. In this paper, the Xizicheng Shallow-Buried Tunnel is presented as an example. First, a geological radar experiment is presented, and safety factors are calculated to analyze and assess the stability of the tunnel; second, the construction methods and reinforcement technologies of grade IV and grade V surrounding rocks are proposed, respectively. Based on these steps, a finite element numerical model is built; the displacement of surrounding rock and horizontal stress, vertical stress, and vertical displacement of tunnel support are studied, with the mechanical laws and characteristics of surrounding rock and tunnel supports being analyzed at the same time. During the tunnel excavation period, the displacement and deformation of grade IV and grade V surrounding rocks is measured. The results show that the vault subsidence and clearance convergence are all small, confirming that the skill measures and methods can ensure safety and stability of the shallow tunnel in soft surrounding rock.