基于收敛-约束法的软岩隧道初支时机估算——以义永公路枫坑隧道为例

围岩纵向变形曲线能直观、有效地反映隧道开挖过程中洞壁围岩变形受掌子面前端“空间效应”的影响,为支护结构施作的最佳时机提供理论依据。以某软岩大断面隧道为例,基于Unlu和Gercek推导围岩纵向变形曲线方程（位移释放系数）,在综合考虑泊松比和弹性模量以及粘聚力、内摩擦角、爆破参数等提出优化改进;运用FLAC3D分析改进围岩纵向变形曲线方程的合理性和有效性。结果表明:（1）围岩纵向变形曲线方程与弹性模量以及粘聚力、内摩擦角呈非线性正比关系,与爆破参数呈非线性反比关系;（2）对比现场监测数据与理论计算数据发现Unlu和Gercek推导围岩纵向变形曲线方程在x>=0段偏差较大,提出增加“扩大收敛函数”提高其精度,相关系数由原来的R-square=0.8左右,提高到R-square=0.95左右;（3）通过与数值模拟数据对比,改进后的围岩纵向变形曲线方程能更好的与其相吻合,证实了改进后的围岩纵向变形曲线方程更具有合理性和实用性;（4）提出围岩位移增量出现陡增点时的位移释放系数值为施加支护的最佳时机,得出Ⅲ级围岩在长台阶法施工施作时,距掌子面x=2.24m左右处开始施作支护为最佳,Ⅳ级围岩在采用CRD工法施作时,距掌子面x=1.47m左右处开始施作支护为最佳。

Prediction of initial support time of soft rock tunnel based on convergence constraint method

The longitudinal deformation curve of surrounding rock can intuitively and effectively reflect that the deformation of surrounding rock of tunnel wall is affected by the "space effect" at the front end of tunnel face during tunnel excavation, and provide a theoretical basis for the best time of supporting structure construction. Taking a large cross-section tunnel in soft rock as an example, the longitudinal deformation curve equation (displacement release coefficient) of surrounding rock is derived based on unlu and gercek. The optimization and improvement are proposed after comprehensively considering Poisson''s ratio, elastic modulus, cohesion, internal friction angle and blasting parameters; The rationality and effectiveness of using FLAC3D to analyze and improve the longitudinal deformation curve equation of surrounding rock.The results show that: (1) The longitudinal deformation curve equation of surrounding rock has a nonlinear proportional relationship with elastic modulus, cohesion and internal friction angle, and a nonlinear inverse relationship with blasting parameters; (2) Comparing the field monitoring data with the theoretical calculation data, it is found that the deviation of the longitudinal deformation curve equation of surrounding rock deduced by Unlu and Gercek is large in the section x>=0. It is proposed to increase the "expanded convergence function" to improve its accuracy, and the determination coefficient is increased from the original R-square = 0.8 to R-square = 0.95;(3) Compared with the numerical simulation data, the improved vertical deformation curve equation of surrounding rock can be better consistent with it, which proves that the improved vertical deformation curve equation of surrounding rock is more reasonable and practical; (4) It is proposed that the displacement release coefficient value when the displacement increment of surrounding rock appears the sharp increase point is the best time to apply support. It is concluded that when the class III surrounding rock is constructed by the long bench method, it is the best to start supporting at about x=2.24m from the tunnel face, and when the class IV surrounding rock is constructed by the CRD method, it is the best to start supporting at about x=1.47m from the tunnel face.