| At present,in the relevant research on the stability analysis of tunnel excavation face construction,a lot of researches have been carried out on shield tunnel and mine tunnel excavated by full section method,but few researches on the stability of large section mountain tunnel construction by three-step method.In terms of theoretical analysis,in order to study the stabilization effect of step rock mass on the excavation face,some scholars assume that the part of the excavation face below the step is in a stable state,and only the part above the step is analyzed for the stability of the excavation face.This method is not comprehensive enough to consider the step rock mass,ignoring the step length,and only considering the effect of the step height,so it is unable to comprehensively analyze the stabilization effect of the step rock mass on the excavation face.In terms of numerical analysis,there are few researches on parameter optimization of three-step method for deep water-rich rock mass tunnel at present,and most of the numerical simulation test design adopts single-factor rotation method,which requires more tests and cannot estimate the test error.Aimed at the deficiency of the research,this article relies on is plate tunnel project,in terms of theory,the concept of model of shield tunnel excavation face supporting force,step length,height and excavation surface is derived the relation between the equivalent supporting force,by considering pore water pressure of the upper limit of limit analysis method to calculate the support force of the guarantee the stability of excavation face power to determine the minimum construction parameters,Numerical simulation was used to verify the theoretical results and analyze the effects of shear strength parameters,groundwater environment and tunnel height on the minimum construction parameters.In numerical analysis,with the most dangerous section of the engineering test section in Zhengpantai tunnel as a background,considering a three-tier heading-and-bench construction method,three level orthogonal tests of stress-seepage coupled numerical modeling for stability analysis and parametric optimization are conducted with the height of the heading,the length of the upper bench,the length of the lower bench,and the advancement step length as the parameter.The main research results are as follows.(1)The minimum construction parameters of the three-step method for deep buried tunnels with different shear strength parameters of surrounding rock,different head height of excavation face and different tunnel height are obtained under the construction conditions of water plugging and drainage restriction.By comparing the minimum construction parameters under different working conditions,the influences of rock mass shear strength parameters,groundwater environment and tunnel height on the stability of the excavation face are studied: 1)In the deep tunnel constructed under water plugging and drainage restriction conditions,the self-stable cohesive force increases with the increase of internal friction Angle.2)When the cohesion of surrounding rock is less than 20 k Pa and the internal friction Angle is less than 30°,other reinforcement measures should be taken to help stabilize the excavation face.3)When the shear strength parameter is larger,the stability of the excavation face is more sensitive to the change of head height and tunnel height than when the shear strength parameter is lower.(2)By analyzing the numerical simulation results and comparing with the relevant codes,the optimal construction parameters of the stable excavation face are obtained.The calculations reveal that 1)the optimal head height of the excavation face should be controlled in the vicinity of 20 m from the tunnel vault for the rock class IV stretch,that2)the height of heading is the most influential parameter on the crown settlement,the wall displacement and the intrusive displacement of the face,and the length of the lower bench is the most influential parameter on the scope of the plastic zone,and that 3)the optimal parameters for the rock class IV stretch are 0.278 times of the excavation span for the height of heading,roughly 4m,and 0.556 times of the excavation span for both the upper bench and the lower bench lengths,roughly 8m,and 0.208 times of the excavation span for the length of advancement step,roughly 3m. |
