脆岩超大断面隧道双层初支支护时机研究

Study on supporting time of double-layer primary support for super-large section tunnels in brittle rock

通过改进现有的破坏接近度理论,建立脆性岩体力学状态的安全性评价指标(BRSI),实现对脆性岩体弹性状态、应变硬化状态、脆性破坏和残余状态的定量表达。基于脆性围岩力学状态与BRSI指标之间的定量关系,提出一种脆岩超大断面隧道分步施工中双层初支最佳支护时机的确定方法。通过三维数值模拟,将所提方法在下北山隧道工程中进行分析与验证。结果表明:(1)低围压状态下,凝灰岩达到极限强度后立即发生脆性破坏,随着围压的增加,凝灰岩表现出逐渐软化的特征;(2)隧道施工中,围岩BRSI值的发展经历初始发展、快速发展和逐步稳定3个阶段;(3)中导坑岩柱的开挖使洞壁围岩的应力状态和破坏模式发生转变,导致中导坑的最佳支护时机比边导坑滞后2~10 m;(4)BRSI指标能够准确、直观地表征脆性围岩的力学状态,所提支护时机确定方法合理、可靠。

By improving the existing failure approach theory,a safety evaluation index(BRSI)for assessing the mechanical state of brittle rock masses is established,enabling a quantitative expression of the elastic state,strain hardening state,brittle failure and residual state of brittle rock masses.Based on the quantitative relationship between the mechanical state of brittle surrounding rock and the BRSI index,a novel approach is proposed to determine the optimal timing for double-layer primary support during the sequential excavation of super-large section tunnels in brittle rock masses.The proposed method is analyzed and validated through three-dimensional numerical simulations conducted in the Xiabeishan Tunnel project.The results demonstrate that:(1)Under a low confining pressure,tuff undergoes brittle failure immediately upon reaching its ultimate strength.With the increase of the confining pressure,however,tuff shows the characteristics of gradual softening.(2)During tunnel construction,the development of BRSI value of the surrounding rock undergoes three stages as initial development,rapid development and gradual stabilization.(3)The excavation of the rock pillar of the middle pilot tunnel alters the stress state and failure mode of the tunnel wall¢s surrounding rock,resulting in the optimal support timing for the middle pilot tunnel lagging behind the side pilot tunnels by 2–10 m.(4)The mechanical state of brittle surrounding rock can be accurately and intuitively characterized by BRSI index,and the proposed method for determining support timing is reasonable and reliable.