Abstract:

Due to the severe disturbance between closely spaced tunnels, the rock pillar is in the unfavorable state of single or double-direction compression, and the construction safety is seriously affected. Therefore, the prestressed and grouted pillar-reinforcing bolt is widely used to support the rock pillar actively. The mechanical models and analytical methods are established to clarify the function modes and control mechanism of the pillar-reinforcing bolt. The theoretical approach is verified by field test, the influencing parameters are analyzed, and the corresponding engineering applications are carried out. The results show that the pillar-reinforcing bolt can provide strong horizontal restraint on the rock pillar, improve the mechanical properties of the rock mass, and has the extrusion reinforcement effect. The pillar-reinforcing bolt also utilizes friction resistance to improve the stress state of the shallow surrounding rock and anchor the deep rock, which has the load transfer effect. The axial force and shear stress of the pillar-reinforcing bolt decrease nonlinearly from both outsides to the insides, and the function range is limited within the shallow layer of the rock pillar. This trend is verified by field measured data. The parameter influences on the mechanical behavior of the pillar-reinforcing bolt are systematically analyzed, and the critical bolt length and reasonable bolt diameter are proposed accordingly. Based on the engineering practice of the closely spaced tunnels in the Great Wall Station of Beijing Zhangjiakou high-speed railway, the design of pillar-reinforcing bolt is actively enhanced. After the design change, the average values of surrounding rock pressure, crown settlement, and horizontal convergence are reduced by 12.2%, 14.1%, and 10.2%, respectively. The safety state of closely spaced tunnels is significantly safeguarded.

Key words: closely spaced tunnels, rock tunnels, rock pillar, pillar-reinforcing bolt, anchoring mechanism