Abstract: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.