3D Seepage Analysis And Hydraulic Head Prediction For Probe Borehole Ahead Of Tunnel Excavation Face In Water-rich Fractured Rocks

During the construction of tunnel under the geological conditions of water-rich fractured rocks,the high pore water pressure near the tunnel excavation face often leads to a large amount of water inflow and slag gushing,which seriously affects the construction period and operation.At present,there are relatively few researches on the seepage field distribution of the probe borehole ahead of the excavation face of the tunnel under the geological conditions of water-rich fractured rocks,and the results of the water head height in front of the excavation face obtained by the existing theoretical calculation method often differ greatly from the actual situation.In order to solve the problems mentioned above,this paper adopts numerical simulation method,combined with related theories to verify.A three-dimensional steady-state distribution finite element model of advanced probe borehole is established by using numerical simulation software,and based on the numerical simulation results of the three-dimensional finite element model,the distribution characteristics of seepage field and the expression for predicting the water head height in front of the excavation face of the tunnel under the geological conditions of water-rich fractured rocks are given.Then,the influence factors and rules of the relative error of the simplified finite element model of the axisymmetric steady-state distribution of the advanced probe borehole in calculating the pore water pressure and the water inflow of the probe borehole are analyzed respectively.The applicable condition of the simplified finite element model is given.Finally,the research results are applied to practical engineering.The main research results are as follows:(1)By analyzing the calculation results of the three-dimensional finite element model,the seepage field distribution characteristics of the probe borehole in front of the tunnel excavation face of water-rich fractured rocks are obtained.(2)By comparing and analyzing the water inflow of the probe borehole calculated by the three-dimensional finite element mode under different working conditions,the law of the influence of the effective length of the probe borehole and the permeability coefficient on the relationship between the water head height and the water inflow of the probe borehole is obtained,and based on the characteristics of the calculation results of the model,the expression for predicting the water head height is obtained.(3)By comparing the calculation results of the axisymmetric simplified finite element model and the three-dimensional finite element model,it is found that the relative error of the pore water pressure calculated by the axisymmetric simplified finite element model is related to the effective length of the probe borehole,but not to the water head height,and the relative error of the water inflow of the probe borehole calculated by the axisymmetric simplified finite element model is related to the permeability coefficient and the effective length of the probe borehole,but has little relationship with the water head height.(4)The applicable condition of the axisymmetric simplified finite element model to calculate the pore water pressure distribution at the location of the probe borehole is given.And the application of the fitting formula for predicting the water head height based on the characteristic of the calculation results of the axisymmetric simplified finite element model is given.