Evolution Mechanism Of Seepage Failure And Prevention And Control Method Of Water And Mud Inrush In Tunnel Fault And Its Application

Water inrush disaster is one of the major geological disasters in the tunnel and underground engineering construction.When deep-buried tunnels pass through water-rich fault fracture zone,water and mud inrush disasters are easy to occur under the multiple actions of ground stress,excavation disturbance,and groundwater,which pose a serious threat to the safety construction of tunnel engineering,people’s life safety,and the protection of the ecological environment.Scientific cognition of water and mud inrush,the real simulation of the evolution process,and process control of disaster is the theoretical basis and key premise for realizing the main prevention and control of disasters.This paper focuses on the evolution mechanism and prevention and control decision method of water and mud inrush by the seepage failure of the filling medium in the fault fracture zone.Using case statistics,theoretical analysis,laboratory tests,and numerical simulations,the catastrophic mechanism of water and mud inrush in fault is clarified,the evolution process is reproduced,and the risk control strategy and avoidance measures of water and mud inrush disasters are proposed.These provide a scientific basis for effective prevention and control of major water and mud inrush disasters.The main research work and results are as follows:(1)The particle size distribution of more than 120 filling mediums in the fault fracture zone is analyzed statistically.Based on the fractal theory,the single and multiple fractal characteristics of the grain size distribution curve are established.The saturation moisture content test,specific gravity test,permeability test,and triaxial shear test of the filling medium under different fractal dimensions and initial dry densities are carried out.The relations of cohesion,internal friction angle and permeability coefficient with fractal dimension and initial dry density are established.Based on the filling characteristics and physical and mechanical properties of the filling medium,combined with the actual water and mud inrush cases,the filling medium types and water and mud inrush modes in the fault fracture zone are divided.(2)A three-dimensional visualization test system for the seepage failure of the filling medium in the fault fracture zone has been developed.The system is composed of a frame test bench,a visualization test chamber,a hydraulic control system,a water pressure loading system,and a data monitoring and acquisition system.The size of the sample designed is 200 mm in length,300 mm in height,and 60 mm in width.The system can apply high,uniform compressive loads in all three principal directions to perform tests under true triaxial static loading conditions.It has high loading accuracy,the maximum vertical ground stress is 3 MPa,and the loading accuracy is 0.001 MPa.During the loading process,the voltage stabilization system can intelligently compensate for the pressure loss.An intelligent constant pressure water device is developed to provide stable water pressure.Meanwhile,the automatic rotating lifting structure and transport structure were developed,which can realize automatic entry and exit of the visualization test chamber.(3)Based on the self-developed novel test system,laboratory tests of seepage failure under different in-situ stresses,fractal dimensions,and hydraulic loading gradients are carried out,the evolution law of multiple information such as permeability,porosity,shear strength,and viscosity in the process of water and mud inrush is revealed.The testing results show that when the hydraulic gradient increases to the abrupt change value,all parameters change significantly.The seepage state changes from Darcy flow to non-Darcy flow,and the constitutive of the gushing fluid changes from Newtonian fluid to Bingham fluid.Through the data fitting,the relationship among porosity,permeability coefficient and hydraulic gradient is established,and mathematical expressions of internal friction angle,cohesion and mass of particles are proposed.The relationship curve equation between fluid viscosity and mass water content is obtained.Based on multiple information fusion analyses,the seepage failure mechanism of "variable porosity-variable flow state-variable shear strength-variable permeability-variable viscosity"is revealed.(4)For the simulation analysis of the whole process of water and mud inrush in the fault fracture zone,based on the evolution law of shear strength and other parameters with the particle loss revealed by the laboratory test,an erosion weakening model that can represent the seepage failure mechanism is established to effectively describe the effect of groundwater seepage on the filling medium.The DEM-CFD fluid-solid coupling simulation of the seepage failure process of the filling medium is carried out,and the accuracy of the erosion weakening model is verified by comparing the testing results.Taking the case of water and mud inrush in the Paomashan Tunnel in Sichuan Province as the engineering background,using the established erosion weakening model,the simulation of the evolution process of the water and mud inrush disaster at the engineering scale is carried out.The evolution law of the displacement field during the tunnel excavation,the velocity field,and the total mass of flushed out particles during the water and mud inrush are revealed.(5)Aiming at the risk control of water and mud inrush in the fault fracture zone,the basic idea of disaster process control is proposed.The overall measures of "four steps" for active prevention and control of water and mud inrush disasters.The influence law of fault width,fault dip angle,tunnel excavation size,and water pressure on the water and mud inrush disasters is studied,and the key factors of disaster risk control are proposed.Focusing on the mud quality and displacement control,numerical simulation studies on the stability of the tunnel are carried out with different advanced grouting reinforcement schemes and hydrophobic pressure relief.Based on the heat map of tunnel displacement,risk avoidance measures of water and mud inrush in fault with four different combinations of fault dip angle,width,and excavation height are proposed,which provides support for determining a scientific,effective,safe,and economic disaster prevention and control decision scheme.The safe grouting thickness for the subsequent excavation of the Paomashan Tunnel is proposed and verified by numerical simulation,which provides a reference for avoiding the occurrence of water and mud inrush.