| During the construction of large cross-section tunnels in water-rich fractured rocks,it is necessary to predict the water head height in front of the excavation face by the water outflow monitoring data from the probe borehole and study the effect of seepage on the stability of the excavation face.This paper takes Zhengpantai Tunnel in the new Chongli railway project as background,and the corresponding theoretical research and numerical simulation analysis are carried out.The main research findings are as follows:(1)Based on Bernoulli equation and considering the influence of permeability of surrounding rock,the simplified theoretical model and calculation formula are proposed,which can be used to predict the water head height in front of the excavation face by using the monitoring data of water outflow from the probe borehole.(2)Considering the condition of high water head,the numerical model of steady state flow of surrounding rock is established based on the axisymmetric assumption,and the influence of permeability coefficient of surrounding rock and length of probe borehole on the relationship between water outflow of probe borehole and water head height in front is calculated and analyzed.Then,according to the characteristics of numerical simulation results,the linear fitting is carried out,and the fitting formula of ‘water outflow of probe borehole-water head height in front’ using permeability coefficient of surrounding rock and length of probe borehole as parameters is obtained.Then the fitting formula is compared with the simplified theoretical calculation formula.(3)Considering the condition of high water head,the numerical model of axisymmetric transient flow of surrounding rock is established to calculate and analyze the time when the water outflow of probe borehole reaches steady state,and the influence of the permeability coefficient of surrounding rock and the length of probe hole on the time is analyzed.(4)In terms of the construction of tunnel in fractured rocks,considering different excavation methods,using the numerical calculation results to fit and determine the distribution function of water head in front of the tunnel,establishing the limit equilibrium model of excavation face with the influence of seepage,deriving the transcendental equation about the fracture angle of rock in front of the excavation face,and using Newton iteration method to solve it;using finite element software to verify it,and the influence of rock cohesion,rock internal friction angle,tunnel buried depth and water head height on the rupture angle and necessary stable thickness of rock is analyzed. |
