Stability analysis of the middle soil pillar for asymmetric parallel tunnels by using model testing and numerical simulations

Abstract

The stability of the middle soil pillar (MSP) is considered to be a crucial factor in ensuring the safety during the construction of asymmetric parallel tunnels. In this paper, both geomechanical model tests and numerical analysis were carried out to investigate the mechanical characteristics of the MSP between asymmetrically closely-spaced parallel-tunnels. First, large-scale three-dimensional (3-D) geomechanical model tests were conducted to study the stress release laws and mechanical behaviors of the MSP during the excavation and overloading conditions. Then, a numerical model was implemented based on the physical model for verifying the MSP’s stress response obtained from the model tests. Failure degree index (FDI) was put forward to evaluate the degree of disturbance to the MSP and to analyze its damage extent during tunnels excavation. Furthermore, the effect of construction parameters (i.e., support strength and excavation sequence) on the damage behaviors of the MSP were discussed by extended numerical analysis. The model tests results show that the stress was mainly released from the upper and lower ends of the middle pillar to lateral direction in sand ground. Both the horizontal stress release and horizontal stress release rate of the MSP increased along with the external applied loads. Meanwhile, the overloading process only increased the stress of surrounding soil, while the load-bearing pattern of the MSP was basically unchanged. In addition, the numerical results indicate that due to the differences in the cross-section dimensions and construction parameters (e.g., support strength and excavation sequence) of the closely-spaced parallel-tunnels, the final damage pattern of the MSP was asymmetrical.