Engineering Characteristics Of Wudangshan Group Schist And Its Influence On Tunnel Stability

The northwest of Hubei Province is located in the middle mountainous area of China.In this area,there are high mountains,rivers and valleys with complex topography and geological conditions.There are many large-scale geological structures with near East-West trend,and the main lithology is shallow metamorphic rock.The schist in this area is affected by multi-stage geological structure,and the rock mass has the characteristics of low strength,loose structure,poor integrity,non-uniformity and significant anisotropy,which makes the surrounding rock structure easy to be damaged during the excavation process of the highway tunnels and makes great difficulties to the normal construction of the tunnel.Therefore,the research on the engineering characteristics of metamorphic schist has great significance for the construction of tunnel in schist area.Based on the 14 tunnels in the Baoxia-Yishui section of Shiyan-Wuxi Highway,this paper studies the micro,physical,water-physical and mechanical properties of Wudangshan group schist by means of geological survey,field test,laboratory test,theoretical analysis and numerical simulation.The damage statistical constitutive model of schist is established.Combined with engineering practice,the influence of engineering characteristics of schist on tunnel stability is analyzed.The main research contents and results are as follows:(1)The engineering background,engineering overview,topography,stratum lithology,geological structure,hydrogeological conditions of 14 tunnels in the study area are investigated on site.The field data are collected and sorted out,and the existing investigation reports are analyzed,providing engineering and geological basis for the study of the engineering characteristics of Wudangshan group schist and its impact on the stability of the tunnel.The in-situ stress field in this area is dominated by horizontal stress,and the initial in-situ stress magnitude is extremely high.(2)The mineral composition,microstructure of Wudangshan group schist are studied by polarizing microscope test.The differences of shape and arrangement characteristics of mineral particles in vertical and parallel foliation planes are analyzed.Through X-Ray Diffraction(XRD)test,the mineral composition and content of various schists are analyzed.Scanning Electron Microscopy(SEM)is used to analyze the microscopic features of the surface morphology of mica schist in vertical and parallel foliation planes.The PCAS software is used to analyze the SEM images quantitatively.It is determined that the proportion of fracture area in the direction of parallel foliation plane is 1.24%,and that in the direction of vertical foliation plane is 21.37%.(3)The wave velocity anisotropy characteristics of Wudangshan group schist is studied by carrying out wave velocity test.Density test,water absorption test,softening test and disintegration resistance test are carried out to obtain the basic physical and water-physical parameters of different schists,such as density,water absorption rate,softening coefficient and disintegration resistance index.(4)Through uniaxial and triaxial compression tests,the strength,deformation parameters and anisotropy characteristics of schists with different foliation plane orientations under the influence of water and confining pressure are studied.The uniaxial and triaxial compressive strength and elastic modulus of the schist decrease first and then increase with the increase of the angle β between the loading direction and the foliation plane,and the minimum value appears at β=30°.After being fully saturated with water,the compressive strength and elastic modulus of schist are reduced to different degrees,and the decline was the greatest when β=30°.The uniaxial compressive strength and elastic modulus of albite schist are higher than that of mica schist under the same orientation of foliation plane and water content.(5)The mechanical properties of foliation plane and the anisotropic characteristics of shear strength and failure mode of schist under the influence of foliation plane are studied by direct shear test in laboratory and the shear strength,internal friction angle and cohesion of schists with different foliation plane orientations are obtained.The peak shear strength,residual shear strength,cohesion,internal friction angle and failure mode of the schist show obvious anisotropic characteristics.The shear strength and shear strength parameters at β=90° are the smallest,indicating that the schist foliation plane is the weak plane in the rock.After being fully saturated with water,the shear strength and shear strength parameters of schist decrease in different degrees.The peak shear strength,internal friction angle and cohesion of albite schist are higher than those of mica schist under the same orientation of foliation plane,normal stress and water content.(6)Since schist is a transversely isotropic body,the constitutive equation in the elastic stage and elastic parameters with different foliation plane orientations of transversely isotropic rock under triaxial stress are derived.Based on Weibull probability density function and strain equivalence theory,a damage statistical constitutive model of transversely isotropic rock is established.The established constitutive model is verified according to the triaxial compression test results,which confirms the rationality of the model.(7)The influence of surrounding rock anisotropy characteristics,in-situ stress and softening coefficient on deformation and failure of tunnel surrounding rock is studied by numerical simulation method.The effect of weakening anchor bolt and strengthening the strength and thickness of initial support on the surrounding rock deformation,plastic zone and stress of the anchor bolt is studied,and the support structure is further optimized according to the numerical simulation results.Based on the orthogonal test method,the influence degree of each supporting structure on surrounding rock deformation is determined.(8)Based on on-site monitoring and measurement,the variation law of vault settlement and horizontal convergence of surrounding rock with time is analyzed.Based on the least square support vector machine,a time series prediction model of tunnel surrounding rock deformation is established.Particle swarm optimization and genetic algorithm are used to optimize the parameters of support vector machine,respectively.The vault settlement and horizontal convergence of surrounding rock are predicted and analyzed to determine the best prediction model.