Study On Mechanical Characteristics And Stability Of Surrounding Rock In Deep Buried Tunnel With High Geostress

Deep-buried tunnels with high-geostress have a deep buried depth and the surrounding rock is in a high initial geostress state.Besides,the rock mass around the cavern is generally a high-strength brittle hard rock.So the rock mass will character brittle failure when unloading occurs by the mining disturbance,which will cause the rock mass failure by ejection,expansion,collapse and even blastion.In this paper,the physical and mechanical tests of high geostress gneiss are carried out to reveal the mechanical properties and failure modes of high geostress gneiss.Based on the tsets’ results,the surrounding rock’s elastic-plastic analysis of deep-buried tunnels with high geostress is carried out.By using theoretical analysis,numerical simulation and field test,the surrounding rock loose circle,dome sinking and convergence value are determined for the study of surrounding rock stability of deep buried tunnels with high geostress.The specific conclusions are as follows:(1)By measuring the density and wave velocity,the density and initial wave velocity of the high geostress gneiss are large,and the wave velocity reduction ratio is high after the specimen is destroyed.This is because that under the high geostress condition,there are huge geostress in rocks,and the gneiss specimens are relatively compact and the internal structure is relatively complete.In uniaxial compression test,the residual stress decreases greatly compared with the peak stress of gneiss specimen.This is due to the simultaneous release of geostress during uniaxial compression failure,which is equivalent to the secondary failure of rock.Therefore,under the condition of high geostress,the fracture degree of gneiss specimens is relatively high,and the specimens show typical brittleness.(2)In conventional triaxial compression tests,peak stress,residual stress,peak strain and residual strain will increase with the increase of confining pressure.When confining pressure is less than 15 MPa,the growth rate of peak strain and residual strain is faster When confining pressure is greater than 15 MPa,the growth rate of peak strain and residual strain is slower When the fitting functions between peak stress,residual stress and confining pressure are linear function,the fitting functions between peak strain,residual strain and confining pressure are logarithmic functions,the correlation coefficient is the largest.(3)The strength criterion widely used in rock is selected to fit the peak stress of conventional triaxial test of gneiss under high geostress.It is concluded that the peak stress of gneiss calculated by M-C criterion and the unified strength theory is close to the test results,the average relative error is the smallest,and the standard deviation is small.Therefore,the applicability of the M-C criterion and the unified strength theory to the theoretical calculation of peak stress of gneiss under high geostress conditions is the best.In the elastic-plastic analysis of surrounding rock under high geostress considering the middle principal stress,economy and the applicability of strength criterion,the unified strength theory is selected for elastic-plastic analysis of surrounding rock of deep tunnel with high geostress.Using the unified strength theory,the calculation formulas of the plastic zone radius of the surrounding rock,the loose circle radius,and the surrounding rock displacement are deduced.(4)The unified strength theory is written by Fortran language to import MIDAS/GTS numerical simulation software for numerical calculation.The numerical simulation is used to determine the loose circle,dome sinking and convergence value of surrounding rock,and the stability analysis of tunnel surrounding rock is carried out.Based on the ultimate tensile strain of the rock mass,the range of the surrounding rock loose circle is determined.The dome sinking and convergence value after the initial support is completed are recorded,and the stability of the surrounding rock of the tunnel is studied.(5)Based on a tunnel project of Baohan Expressway,the theoretical calculation and numerical simulation of the loose circle,the dome ainking and the convergence value are determined.The theoretical calculation,numerical simulation and field test results are compared and analyzed.The theoretical calculation and numerical simulation are close to the field test results,which shows that the unified strength theory has good applicability to the surrounding rock stability in deep buried tunnel with high geostress.Combing the theoretical calculation,numerical simulation and field test results shows that the dome and sidewall loose circle belong to the large loose circle.the dome sinkinge and convergence value are small.Therefore,under the existing supporting conditions,the supporting structure is stable,the surrounding rock deformation is small,and the surrounding rock is relatively stable.