Study On Stability Of Large Cross-section Tunnel Face In Shallow Fractured Section

With the increasing mileage of highway tunnels in China,extra-long,large cross-section,high geostress and deep buried tunnels also appear.Although various technologies and construction methods are emerging,engineering accidents such as tunnel face collapse,water inrush and mud inrush still exist,especially in the construction of Large Cross-section Tunnels in shallow fractured sections.Therefore,it is necessary to study the stability of large cross-section tunnel face in shallow fractured section,which is of great significance to guide the safe and smooth completion of the tunnel.Based on the Baishui Tunnel of Shiqian Expressway in Chongqing,the main influencing parameters of surrounding rock mechanics are obtained by sensitivity analysis of surrounding rock mechanics parameters and GA-BP neural network inversion.The formulas for calculating the stability safety factor of tunnel face and the outburst prevention thickness of front rock plate are derived by limit equilibrium method,and different supporting strips are simulated by numerical simulation.The stability of tunnel face is discussed.This paper mainly completes the following aspects:(1)Through collecting and sorting out the geological investigation report and the site laboratory report of Baishui tunnel,and combining with the current "Highway Tunnel Design Code",the range of mechanical parameters of surrounding rock of Baishui tunnel is obtained.The sensitivity degree of mechanical parameters of surrounding rock of Baishui tunnel is calculated by single factor sensitivity analysis and multi-factor sensitivity analysis,respectively,and the influence degree of mechanical parameters of surrounding rock of Baishui tunnel is determined.The main influencing factors of vault settlement of water tunnel are mechanical parameters of surrounding rock and secondary influencing mechanical parameters of surrounding rock.According to the determined main influencing factors of surrounding rock mechanics parameters and orthogonal experimental design method,the samples of neural network are established,and the mature GA-BP neural network is obtained through training.According to the measured values of Baishui tunnel field monitoring,the mature GA-BP neural network is inverted.The displacement and deformation of Baishui Tunnel are calculated by forward analysis,and the error between deformation and measured value is analyzed.The feasibility of intelligent back analysis method of displacement based on GA-BP neural network is verified.(2)According to the existing failure mode of tunnel face-three-dimensional wedge model,based on the unified strength theory and the relaxed earth pressure theory of Taishaji,the relaxed earth pressure theory of Taishaji under the unified strength theory is obtained,and then the formula of stability safety factor of tunnel face is deduced by the limit equilibrium method,and the calculation parameters are analyzed;the quasi-static method is used to deduce the relaxed earth pressure theory of Taishaji tunnel face.The formulas of safety factor of tunnel face stability under blasting vibration are derived,and the influence parameters of blasting vibration are analyzed.(3)Considering mud inrush and water inrush in front of tunnel face,according to the three-dimensional failure model of rock Plate-Conical platform model and based on limit equilibrium method,the calculation formula of rock plate outburst prevention thickness in front of tunnel face is deduced,and the relevant parameters are analyzed;the rock plate thickness in front of lower tunnel face is compared and analyzed by limit equilibrium method and limit analysis upper limit method.The results of degree calculation verify the rationality of this method.(4)The influence of different thickness of advance support and different length of step on the stability of tunnel face is analyzed by numerical simulation;the stability of tunnel face under the concepts of NATM and Xinyi method is compared and explored,and the parameters of glass fiber bolt support under the concepts of Xinyi method are analyzed;based on these two concepts,the capacity of tunnel face under different thickness of rock plate is compared.The maximum cavern pressure is applied and compared with the theoretical calculation results.