Study On Strainburst In Tunnel Using True-triaxial Test And Its Kinetic Energy Prediction

Rockburst is an engineering geologic hazard during the construction of underground tunnel at depth,which is frequently accompanied by rock fragments ejection.Due to its unpredictability and violent nature,rockburst may cause serious injuries and fatalities to workers,as well as damage to mining equipment.With underground construction migrating to deep ground,more and more rockburst are reported.Strainburst induced by tangential stress increasing after excavation is the most common rockburst.Thus,it is important to gain a good understanding of strainburst mechanism in general and to predict strainburst in specific.The main content and conclusion are drawn as following:(1)A new true-triaxial strainburst testing method based on stress state of rock mass in initial stress space is proposed.In this method,granite specimens with relatively large radial dimension,taken as a representative rock element,were tested in a non-principal stress space to simulate the strainburst development in relatively intact rocks near the excavation boundary within a certain depth.A special loading path,keep one free face,load on other faces,and continually increase the vertical stress,was used to simulate the tangential stress increasing process after excavation.Based on the new testing method,experimental study on strainburst was conducted using a new true-triaixal rockburst testing system.(2)An estimating kinetic energy method is put forward.The speed of the ejection fragments during strainburst can be obtain by analyzing the video recorded using high-speed cameras.Subsequently,the trajectory of the ejected fragments is captured,then the speed of ejected fragments can be obtained by dividing the distance by corresponding time interval.Finally,the kinetic energy can be calculated by multiplying the speed and mass of ejected fragments.(3)Influence of radial stress gradient on stranburst was studied.It is found that the radial stress gradient shows significant influence on strainburst.With the increasing of radial stress gradient,the failure mode of the tested specimen changes from tensile failure to shear failure,the strainburst transmits from tensile rockburst to shear rockburst;the rock strength increases and its increment decreases;the kinetic energy show three linear increasing trend,gradually rises first,then rapidly grows,finally slowly increases with extremely low increment.The result under high radial stress gradient condition manifest that high radial stress can to some extent prevent the rockburst,but it can also trigger the intensive rockburst.(4)Influence of tunnel axis stress on strianburst was investigated.It is found that the axis stress shows significant influence on strainburst.With the increasing of tunnel axis stress,the strength of the tested specimen first increases and subsequently decreases gradually after reaching a peak;the kinetic energy first increases,subsequently decreases,and finally drastically grows.The variation of kinetic energy under different tunnel axis stress can be explain by rock strength,constrain effect and Poisson effect caused by tunnel axis stress.(5)Influence of tangential stress increasing rate on strainburst was studied.It is found that the tangential stress increasing rate shows significant influence on strainburst.With the increasing of the loading rate,the failure mode of tested specimen transmits from static brittle failure without fragments ejection to strainburst;the rock strength shows a logarithmic increase trend;the kinetic energy shows an exponential increase trend.Furthermore,it can be found that the kinetic energy is related with not only the pre-peak accumulated strain energy but also the post-peak energy inputting,and is dependent on the post-peak energy releasing rate.(6)The strainburst ejection process and energy conversion were investigated.The strainburst ejection process is progressive,which can be divided into four stage including particle ejection,rock being split into rock plate,rock plate bending,and rock fragment ejection as soon as rock plate breaking.The energy conversion of the tested specimen under one free face true-triaixial loading condition can be divided into three phases including gradual elastic energy accumulation,fast energy dissipation,and rapid energy release.Otherwise,according the kinetic energy and its relation with different stress influencing factors obtained in one free face true-triaixal loading test,an empirical regression model,which can predict the kinetic energy of ejected fragments during rockburst test,was established.(7)Predicting method of in-situ strainburst based on discrete elment method is proposed.The used UDEC program is first verified by simulating the rock fragments ejection process of granite specimen during indoor rockburst test.Then a "two step" simulating method is proposed and applied to a rockburst case in a diversion tunnel of Jinping II hydropower station.Finally,the in situ rockburst under rock blocks with different side lengths in rockburst pit condition is carried out to determine the value range of kinetic energy of ejected fragment.Thus,the prediction of kinetic energy for in situ rockbust is realized.