Research On Water Gushing And Mud Bursting Mechanism Of Deep Buried Tunnel Passing Through The Weathered Granite Fault Zone

A series of theoretical and numerical methods,including field investigation,physical test,acoustic test and numerical simulation,are adopted in this paper to fully illustrate the water gushing and mud bursting mechanism of deep buried tunnel passing through the weathered granite fault fracture based on a real case study in Longjinxi,Fujian Province.In this study,the physical and mechanical properties,water combination reaction and softening properties of the granite falt zone as well as the factors affecting the water gushing and mud bursting in tunnel are analyzed.In addition,the affecting factors are also studied on their influence on the pore water pressure,seepage field,stress and strain in the surrounding rock.And then,the research proceeded into the evolution regularities in the collapse and crack propagation process in the surrounding rock of the deep thick fault zone.With the condieration of the real condition of the case in Fujian,the water gushing and mud bursting mechanism of deep buried tunnel passing through the weathered granite fault fracture is analyzed and the main contents and results are as follows:(1)Based on the site engineering geological mapping,regional geological data and geological survey report,the engineering geological conditions and hydrogeology conditions are acquired,and environmental information including the landform,geological structure,the distribution of strata,the classification of underground water,the characteristics of fault distribution,the characteristics of rock weathering and the preliminary classification of surrounding rocks is obtained.It is found out that weathered grooves with depth more than one hundred meters existed in the tensile characteristics of the research area and the strike-slip fault of the granite.The grooves are favorable water structures and provide a large amount of sediments for water and mud outburst.(2)Through the analysis of the structural characteristics of faults,the weathering features of granites and the water combination reaction and softening properties,several experimental methods as physical analysis and microstructure test(the analysis of X-ray diffraction and scanning electron microscope analysis test),physical property test(moisture content,density,void ratio/moisture content and air void),hydraulic test(water absorption and softening coefficient),mechanical test(index of shear strength and compressibility as well as uniaxial compression test of various water-bearing states)and acoustic test of rock mass(velocity ratios,integrity indexes)are adopted to obtain the mineralogical compositions,the microstructure and morphology,the basic physical and mechanical properties as well as the failure characteristics of the granite weathered residues,residual soil,in fractured zone and the strong-slightly weathered granites on both sides of the faults.The statistics of testing data are analyzed and detailedly compared with the physical and mechanical features of weathered granite in the nearby area.The results show that the failure modes of strong-slightly weathered granite are in accordance with Mohr-Coulomb’s theory.The physical and mechanical properties of weathering residues in deep-buried granite fault zone are similar to those in surface weathering crust.The granite residual soil is rich in clay minerals,and most of it belongs to soil with high liquid limit.The water combination reaction and softening mechanism is the same as that of surface granite residual soil,but it does not contain free iron oxide and its disintegration pattern is significantly different.The grain grading characteristics show that the composition of the residual particles in the deep burial granite fault zone is complicated,and there are many kinds of failure under seepage.The experimental results provide the basis for analyzing the surrounding rock deformation,fluid-solid coupling and seepage development when the deep buried tunnel passing through weathered granite faulte zone.It also provides the fundamental parameters for numerical simulations(i.e.finite difference method and discrete element method)and provide a basis for revealing the mechanism of water gushing and mud bursting in tunnels.(3)Based on the principles of fluid-solid coupling,the numerical analysis model for water gushing and mud bursting in faulte zone is established by the software FLAC3 D after analyzing the influence of various factors on the surge of tunnel,and the various cases are discussed.The results show that higher the pore water pressure and the worse the rock type,the greater the risk of the sudden surge.Meanwhile,the impact of the dip angle on water gushing and mud bursting is more complex,the smaller the dip angle,the larger the impacting region and the earlier the impacting,but its impact shows obvious spatial effect when the dip angle increases.The analysis of the combined fault shows that it is more likely to cause a sudden event when the intersection is below the tunnel.(4)Based on the particle flow discrete element method,the tunnel model buried in deep faulte zone was established by the software of PFC2 D.Processes of the tunnel excavation,large deformation and the slagging were simulated.The collapse and the crack development of vault and surrounding cavern were studied in the excavation process of the tunnel.The formation process of the collapse arch and the water collecting cavity were observed.Results show that when the deep tunnel was excavated,the deformation of the loose rock in the granite fault zone and crack there developed rapidly.This would easily lead to landslide,mud outburst and water gushing unless timely reinforcement.If the reinforcement range and strength was limited in the first large-scale accident,the water cavity developed dynamicly and the groundwater recharged balanceas at the same time.due to the sudden change of the pore water pressure,the vault collapsed again,resulting the secondary water gushing and mud bursting.(5)With the combination of engineering cases and theoretical analysis and numerical simulation results,the inoculation mechanism,evolution mechanism in different stages and the disaster pattern of mud outburst and water gushing of the deep burial tunnel through the weathered granite faulte zone were analyzed.The water gushing and mud bursting accidents in the engineering examples were divided into four kinds of disaster-failure modes: palm surface anti-burst layer destruction mode,vaulted cavity water burst collapse mode,penetration deformation damage mode and pressured piping destroied mode.And corresponding mechanism of each mode was analyzed and main prevention and control measures were proposed.