Study Of The Failure Mechanisms Of The Surrounding Rock Of Deep Tunnels Using The DEM

The advancement of geotechnical engineering has been accompanied by an increase in working depths,which has been correlated to the increasingly frequent occurrences of a variety of underground engineering accidents.These accidents are a hazard to both the monetary security of the operation and the safety of the personnel of the operation,therefore the study of the damage and failure mechanisms of deloading during deep excavation is extremely important.The engineering background of this study is the Jinping-II Hydropower Station water tunnel,and utilizes PFC^2D,a discrete element method（DEM）based numerical simulation software,to study the failure mechanisms of the rock surrounding the tunnel that is under high ground stress conditions on the macroscale and mesoscale.Firstly,codes for numerical experiments were written in PFC^2D and mechanical parameters were calibrated using trial by error.Afterwards,the parameters that were obtained using the self-written code were used in a uniaxial compression test to compare the resultant mechanical properties and experimentally determined mechanical properties.If the mechanical properties of the numerical and physical experiment are equivalent,then that would validate the self-written code.In the process of parameter calibration,the commonly used trial by error method of parameter calibration was found to be lacking in terms of both time efficiency and accuracy.Therefore,this study has elected to design a parameter calibration method based on the orthogonal array test,multifactor variance analysis,and regression analysis for parallel bonded models（PBM）.The parameter calibration method can simultaneously calibrate parameters such as the elastic modulus,Poisson’s ratio,uniaxial compressive strength,cracking strength,tensile-compressive strength ratio and friction.The results of numerical simulations using the calibrated parameters are comparable to that of physical experiments,therefore this verifies the parameter calibration method.The verified granite mechanical parameters were used to model a granite specimen with a pre-excavated cavity to study the failure mechanisms of the specimen under varying confining pressures.The results of the numerical simulation indicates that the pre-excavated cavity weakens the strength of the granite specimen,and that the magnitude of the confining pressures will affect the location of the formation of the initial cracks,crack propagation direction,and the failure mode.Finally,the granite model was expanded to field scale modelling of the Jinping-II hydropower station water tunnel,wherein the confining pressures were varied to study the damage and failure mechanisms of circular and right angle tunnels.The results of the field scale model indicates that the failure modes of the rocks surrounding the tunnels would be tensile fracture and far-field combination failure,V damage and ring damage,wherein the failure mechanism is mainly affected by the side pressure coefficient.Furthermore,another code was written to show the failure mechanisms at various areas in the model,where the failure mode of the region was determined by the state of the parallel bonds.The results of the study show can be used as a reference in deep tunnel engineering.