Experimental Study Of Rockburst Under Gradient And Gas-liquid Loading And Criterion To Predict Rockburst

Due to the complex occurrence environment of deep rock mass,there are often difficulties of “high in-situ stress,high ground temperature,high hydraulic pressure and dynamic disturbance” in the underground engineering.After the excavation of deep rock mass,the stress concentration of surrounding rocks often leads to the non-uniform release of the stored elastic energy,thus causing sudden rockburst disasters.In terms of the research on rockburst mechanisms and its prediction at present,although many scholars have considered the influence of the stress path of surrounding rocks on strainburst,the influence of different stress gradients on strainburst and its disaster mechanisms in the stress field are generally ignored,lacking systematic cognition.However,different stress gradients inevitably lead to the changes in the ratio and direction of the principal stress,as well as in the crack evolution of surrounding rocks,further affecting the accuracy of rockburst failure mechanisms and prediction.Therefore,the study of rockburst model tests and rockburst criteria under different stress gradients has both theoretical value and application prospect for understanding the mechanisms of strainburst and improving the prevention and control of rockburst disasters.For addressing the aforementioned problems,in this paper,based on the theoretical analysis of the mechanical factors that cause the different failure modes of surrounding rocks,laboratory model tests were carried out under 4 stress gradients in view of the stress environment of high ground stress rock mass and stress path of surrounding rocks during excavation unloading.The aim is to research the rockburst failure mechanisms and the energy evolution rule under different stress gradient loading.On the basis of test results,the mesoscopic simulation model of particle flow was built,and the whole process of the model stress-damage-failure and energy evolution was studied.Moreover,rockburst mechanisms under different stress gradient loading were revealed from the mesoscopic perspective,and combined with the engineering rockburst,the rockburst prediction criteria were established,which took the stress distribution into account.The main research contents and results are as follows:(1)Based on the examination of in-situ strainburst,it can be categorized into three types according to the fracture modes of surrounding rocks: tensile rockburst,tensile-shear rockburst,and shear rockburst.Through studying on the factors influencing strainburst and the analysis of surrounding rock stress state and failure modes according to elastic and fracture mechanics,it is deduced that the stress gradient of surrounding rocks is related to such factors as location,lateral pressure coefficient and etc.,which influence rockburst failure by changing the ratio and direction of the principal stress.(2)In combination with the stiffness theory and the stress characteristics of surrounding rocks in practical projects,the gradient and pneumatic-hydraulic composite loading test equipment was adopted in this paper,with its rationality expounded.Through 4 groups of rockburst model tests under different stress gradients,the model stress and failure phenomena were analyzed.The study shows that the stress and failure process of the model rockburst are consistent with those of the actual rockburst.Besides,the failure load and rockburst intensity are both affected by the stress gradient.With the increase in its loading,the failure load decreases,while the rockburst intensity increases.(3)By analyzing the macroscopic and microscopic fracture surface morphology of typical rockburst debris,it is found that the plate-shaped debris is formed by tensile failure and the block-shaped debris is formed by shear failure.Through the acoustic emission(AE)characteristic parameter method based on GMM,the evolution of model fracture under different stress gradient loading was analyzed.The results show that as stress gradient loading increases,the proportion of shear failure rises gradually,and the model rockburst mode transits from tensile failure to tensile-shear failure and finally to shear failure.(4)According to the kinetic energy of rockburst debris,unloading surface strain,and AE energy from rockburst,the average ejection kinetic energy of debris per unit mass shows an upward trend with the increase of stress gradient loading.Moreover,through the analysis of unloading surface strain and AE energy,as the stress gradient loading rises,the rockburst energy is released on the unloading surface of the model spatially and before the rockburst temporarily.(5)The energy accumulating-dissipating-releasing process of rockburst was analyzed based on the AE intensity characteristic values of the model.The results show that with the increase of stress gradient loading,the energy accumulating period becomes longer,while the energy dissipating and releasing periods become shorter.According to the characteristics of AE b-value and energy probability density distribution,the energy precursor law when the model rockburst occurs was explored.It is obtained that the larger the stress gradient loading,the sharper the AE b-value decreases and the faster the rate during the model failure process.Besides,with the increase of the loading stress gradient,the critical index(r)presents a dropping tendency.(6)Rockburst models under different stress gradient loading were established by PFC2 D.On the basis of verifying the model adaptability,the meso-fracture and energy evolution of the model were analyzed.From the perspective of mesoscopic crack evolution,the spatial and temporal distribution of crack development is divided into different stages.Apart from that,with the increase of the stress gradient,tensile crack propagation varies from along a certain direction to without directions,while shear cracks propagate from showing asymmetry to symmetry.(7)In view of the model rockburst rules under the stress gradient,the concept of stress concentration degree was put forward.Based on a large number of engineering rockburst,the empirical fitting of two-parameter rockburst intensity,which includes stress concentration degree and stress value of surrounding rocks,was carried out.Besides,the rockburst prediction criteria which considers the intensity-stress ratio of stress concentration degree were established,and engineering verification was conducted.The prediction accuracy of the empirical relationship established in this paper can reach more than 90%,which offers a good adaptability to the rockburst prediction in actual engineering.