Study On Creep Characteristics Of Argillaceous Siltstone And Long-term Stability Of Tunnels Crossing The River

Rock creep is an important cause of large deformation and even destabilization damage in geotechnical and tunneling projects.At present,creep mechanics experiments and theoretical studies have made great progress,but the understanding of the mechanism of accelerated creep damage of rocks is still unclear,and most of the studies are exploring the disturbance damage caused by the tunnel excavation process,and few studies have investigated the effect of creep on the long-term stability of tunnels.In order to ensure the safety and stability of the tunnel during longterm use,this paper analyzes the stability of the tunnel considering the time effect by using a combination of experimental research,theoretical analysis and multi-field coupled numerical simulation with the engineering background of the tunnel interval through the river of Changsha Metro Line 6.Uniaxial graded loading creep test was conducted on the muddy siltstone specimens sampled from the borehole to determine the long-term strength of the muddy siltstone and analyze the creep characteristics of the muddy siltstone under different stress levels.Further uniaxial graded loading creep tests with different loading time steps were carried out to study the effects of different loading time steps on the creep properties of muddy siltstone.The test results showed that the shorter the loading time steps,the higher the loading stress required to damage the specimens.Combining rheological theory and damage mechanics theory,a nonlinear creep damage intrinsic model was constructed,and the creep parameters were identified and compared with other model fitting results.The influence relationship between the parameters and creep curves is analyzed to provide a theoretical basis for the parameter assignment in its application.The damage evolution equations for different stages of the rock under ideal conditions are derived.A numerical simulation study of uniaxial graded loading creep test of muddy siltstone was carried out with the finite element analysis software COMSOL Multiphysics,combined with the one-dimensional intrinsic structure model and the corresponding parameter identification results.The creep intrinsic model of the rock in three-dimensional state was further derived and validated,and on this basis,triaxial graded loading creep simulations of the muddy siltstone were carried out to analyze the effects of different surrounding pressures on the long-term strength of the rock.The "YJW-20000 S microcomputer-controlled electro-hydraulic servo three-way five-face loading test system" was jointly developed to analyze the stability of the tunnel surrounding rock during the long-term interaction between excavation and creep for large-size specimens,and the surface stress,strain-time curves and annulus curves of the inner wall of the borehole were obtained in the excavation and creep stages.The surface stress,strain-time curves,and circumferential strain-time curves of the borehole wall were obtained.Combined with AE and DIC techniques,the acoustic emission characteristics and displacement evolution of the surrounding rock under different working conditions were revealed.The test results show that the stress and strain changes of the surrounding rock are influenced by the excavation disturbance,and the more obvious the disturbance is,the longer it takes for the measurement points to enter the stable creep phase,and the acoustic emission events are intensive during the excavation period,and the cumulative count curve appears to increase abruptly.The established creep damage model was embedded into COMSOL Multiphysics software,and the stability calculation of the underpass tunnel under creep-fluid-solids coupling was carried out to obtain the change of stress,displacement,and settlement curves of the key monitoring points of the tunnel with time.The simulation results show that the settlement deformation becomes increasingly obvious and the settlement rate decreases when the creep time keeps increasing.The field-measured monitoring data were processed and compared with the simulation results to verify that the established creep-fluid-solid coupled tunnel model is reasonable and feasible.Further,the numerical calculation models under different working conditions were established to analyze the deformation and creep strain changes at the tunnel monitoring points under the influence of different factors,and it was concluded that the most influential factor is the tunnel burial depth.