Study On Surrounding Rock Pressure And Stability Of Unequal Span Bifurcated Tunnel

At present,there is only the calculation method of the surrounding rock pressure of the tunnel with equal span and small clear distance in the relevant tunnel specifications,but the calculation method of the surrounding rock pressure of the tunnel with unequal span is not given.In this paper,based on the actual Zhuhai dahengqinshan No.1 Tunnel project,there are unequal spans between ramp and main tunnel.Based on the upper bound method of limit analysis,the surrounding rock pressure of unequal span and small clear distance tunnel is studied,and the pressure arch characteristics under different construction steps are analyzed.Finally,combined with numerical simulation,the construction stability of bifurcation and reverse approach in tunnel is studied.The main research results are as follows:（1）Based on the nonlinear M-C criterion,a method for calculating the pressure of the surrounding rock with unequal span and shallow buried and small net distance is proposed.On the premise of equal span,the calculation results are compared with the standard method.The influence of each factor on surrounding rock pressure is analyzed.The variation law of surrounding rock pressure under different failure criteria（linear M-C criterion and H-B criterion）is discussed.（2）This paper systematically summarizes the theory of pressure arch,comprehensively analyzes the judgment standards of various types of pressure arch,and obtains a unified judgment method for the inner and outer boundary of pressure arch.Based on the above contents,the dynamic changes of pressure arch under different construction schemes under two-dimensional conditions are studied;meanwhile,the internal and external boundary determination method is extended to three-dimensional conditions,and the spatial dynamic changes of three-dimensional pressure arch at tunnel bifurcation are discussed.（3）Taking Zhuhai dahengqinshan No.1 Tunnel as the supporting project,aiming at its bifurcation section to large-span section,three-dimensional simulation model of reverse construction of large-span section and reverse construction simulation model of bifurcation section are established respectively to analyze the dynamic change process of surrounding rock stress,displacement and plastic zone,and study the safety and stability of tunnel construction.