Middle Rock Pillar Stability Criteria for a Bifurcated Small Clear-Distance Tunnel

Middle rock pillars (MRPs) play a crucial role in the stability of bifurcated small clear-distance tunnels. Assessing the stability of the MRP is a key challenge in design and construction. This study focuses on the bifurcated small clear-distance section of the Xiamen Haicang Shugang evacuation channel underground interchange tunnels. The stability criteria for the MRP during both the early design and later construction stages were analyzed by using the strength reduction method (SRM) via numerical simulations. In the design stage, the SRM was applied to determine the stability limit state of the MRP. Relationships between rock mass density, cohesion, and elastic modulus were identified, and these parameters were combined with basic cohesion values for an initial stability assessment. During the construction stage, the full excavation process was analyzed by examining the distribution and changes in the plastic zone of the rock mass. Two key construction stages, a 10 m excavation on the main line upper step and a 10 m excavation on the ramp upper step, were identified as points where the plastic zone of the MRP began to form on the sidewall and the center, respectively. Multiple linear regression was used to determine the displacement, stress, and plasticity criteria for MRP stability. A comprehensive criteria formula incorporating the width–span ratio, tunnel vault settlement, and horizontal clearance convergence was developed, providing technical guidance and a scientific basis for similar projects.