Research On Fault Stick-slip Mechanism Based On DEM

The fault is a geological structure that can affect the safety of engineering construction,especially an active fault.The famous San Francisco earthquake,with a magnitude of 7.8,was triggered by the compression and dislocation of the active San Andreas fault.However,fault slip doesn’t just cause earthquakes.In mining,fault slip will cause pillar collapse and ground pressure near mining face;in shale gas mining,it will cause casing deformation,which will seriously affect the mining process;in the process of tunnel excavation,it will cause tunnel deformation or even collapse.Fault slip seriously affects engineering safety.The unstable slip(stick-slip)mechanism of fault slip is recognized as the inherent mechanism of these engineering disasters,so it is of great significance to study the fault stick-slip mechanism.Fault stick-slip will produce rapid energy release and cause great harm.Due to the equipment limitations of the current laboratory tests,the research process of stick-slip only focuses on exploring the law of the dynamic crack propagation process.The research on the quasi-static nucleation process in the stick-slip process is not detailed enough and cannot accurately capture this stage.This stage is widely regarded as a precursor phenomenon and the key to successful earthquake prediction.In this simulation,the stick-slip mechanism of fault-slip instability was explored.The discrete element software UDEC and the double-shear fault model were used to explore the effects of the normal stress level and horizontal loading speed on the stick-slip stress drop,period,slip behavior and energy,and got certain rules.The conclusions drawn from this study are as follows:(1)During the stick-slip process,with the increase of the normal stress,the drop value of the shear stress increases,the period is prolonged,and the sliding velocity is suppressed and shows a decaying trend.(2)In the stick-slip process,with the increase of the horizontal loading speed,the shear stress drop decreases significantly.There is a power exponential function relationship between the stick-slip period and the horizontal loading speed.Correspondingly,there is a power exponential function relationship between the stress drop and the period under the influence of horizontal velocity.(3)The overall stick-slip is jointly promoted by the stick-slip behavior of the contact points on the fault plane.The distance from the contact point where stick-slip occurs first to the next contact point without stick-slip on the fault plane is the nucleation length of stick-slip,and the nucleation length is 1.01 cm in the simulation.In the process of stick-slip nucleation,the nucleation zone does not run through the entire fault plane,but local nucleation,that is,stick-slip occurs at some contact points,which is also the key to the laboratory’s difficulty in capturing the static nucleation stage.(4)The energy change of the stick-slip process is affected by the normal stress and the horizontal loading speed.The specific performance is as follows:with the increase of the normal stress and the horizontal loading speed,the energy release of the stick-slip process shows an overall increasing trend.The work done by the external force is not only dissipated in the form of energy release,but also continuously converted into friction work and the accumulation of material strain energy.No plastic work is produced in this process,that is,no irreversible deformation is produced in the stick-slip process.(5)Considering the heterogeneity of the material,it can be seen that the main factor affecting the stick-slip behavior is the heterogeneity of the shear stiffness of the fault plane,that is,the inconsistency of the shear deformation of the contact point of the fault plane under the same normal stress level.which directly affects the shear stress of the block and the stress drop value in the stick-slip.Applying the stress drop value to the real fault scale,it is obtained that when the fault scale exceeds 10~4 m,earthquake events with magnitude>5 can be generated.