Study On Rockburst Failure Mechanism And Stability Of Cavern In High Geostress Tunnel

Rockburst is a sudden and strong dynamic damage phenomenon during the excavation of underground cavern in high geostress areas.It can be seen from many existing examples of rockburst in engineering that it has many adverse effects on the progress,quality and safety of the project.This article takes the Tongzi Tunnel of Lanzhou-Haikou National Expressway as the project background.Field investigations,indoor mechanical tests,and numerical simulations of loading,unloading rockbursts,and cavern excavation based on the particle flow code were carried out.The failure mechanism and rockburst characteristics of the surrounding rock of the cavern are studied.The damage characteristics of surrounding rock of different shapes of cavern are analyzed.It provides theoretical and technical support for cavern excavation in high ground stress areas.The main research results are as follows.(1)Based on the summary and analysis of research results of definition,classification and grading of rockburst in academia,the basic characteristics and manifestations of rockburst are proposed,and the effects of various influencing factors are discussed.It is believed that various rockburst theories describe the mechanism of rockburst from a certain angle.It is necessary to combine the various theories in order to fully explain the rockburst.(2)The uniaxial compressive strength,deformation modulus,Poisson's ratio and uniaxial tensile strength parameters of the limestone were obtained by rock chamber mechanical tests.Through the orthogonal design numerical experiments,the influence relationship between the mesoscopic parameters and the macroscopic parameters of the flat-joint model is studied,which provides a basis for the rapid calibration of the mesoscopic parameters,and the calibration values of the mesoscopic parameters of the limestone are obtained.(3)According to the state of surrounding rock stress during tunnel excavation,it can be divided into two stages:instantaneous unloading and stress concentration.According to this,the rockburst is also divided into loaded rockburst(rockburst induced by stress concentration or stress disturbance after transient unloading phase)and unloaded rockburst(rockburst induced during unloading effect).The higher the second principal stress is,the higher the surrounding rock strength is,the higher the critical stress of rockburst occurs,and the rockburst is more intense.The test shows that the stress relief zone of the rock mass is within a certain depth range of the unloading surface.It is explained that the radial deformation of the rock mass leads to the decrease of the rock mass stress and the stress transfer to the interior of the surrounding rock.(4)The tunnel excavation results of four shapes under various sets of in-situ stress levels indicate that the tunnel excavation is the most stable,followed by the inclined well and the main tunnel,and the surrounding rock damage of the gate-shaped tunnel is the most serious.When the maximum ground stress is consistent with the tunnel direction,the surrounding rock damage is minimal after excavation.When the maximum principal stress is the tangential and radial directions,the degree of damage of the surrounding rock of the circular cavern is similar,and the deformation of the surrounding rock of the gate-shaped cavern and the inclined shaft are more serious when the maximum principal stress is radial,and the main tunnel is the opposite.