True-triaxial Experimental Study Of Granite Failure Characteristics Induced By Rapid Unloading

With the development of large-scale projects such as China's West-East Gas Pipeline,South-to-North Water Diversion Project,Large-scale Water Conservancy and Hydropower Projects,considerable underground caverns and tunnels are built.The destruction of surrounding rock caused by the rapid unloading the minor press(such as full-face drilling and blasting or support failure suddenly in underground project)frequently occurs during underground excavation.However,the study of rock unloading mechanical properties and failure modes under true-triaxial conditions is still in the initial stage.Consequently,it is necessary to obtain a better understanding of the characteristics of rock failure under true-triaxial unloading conditions.Aiming at solving the problem,true-triaxial rapid unloading test on granite was carried out using the true triaxial rockburst test system of Guangxi University.The influence of different major principal stress,different intermediate principal stress,and different minor principal stress were investigated.The ultrasonic wave velocity of the rock samples was tested before and after the test.The acoustic emission was monitored during the test,and the damage process of the rock samples was recorded by the high-speed camera and the digital camera.In addition,the failure process,the failure form and the relationship of stress,AE hits,and AE accumulative absolute energy with strain were analyzed.Specifically,the failure mode of tested specimen was discussed.Some main conclusions are drawn as follow.(1)The major principal stress has a significant effect on the failure process,failure form and failure mode of the test specimen under the condition of true-triaxial rapid unloading the minor principal stress.With the increase of the major principal stress,the failure process of rock samples is more and more intense,and the dynamic characteristics are increasingly obvious;the failure modes of rock samples are more and more complicated,and the damage degree is more and more serious;the failure mode of the rock samples is damage,plate cracking and rockburst in turn.(2)The intermediate principal stress has an important effect on the failure process,failure form and failure mode of the specimens under the condition of true-triaxial rapid unloading the minor principal stress.Under the condition of?1=250MPa;?2 ? 70MPa;?3=lOMPa,with the increase of the intermediate principal stress,the severity of rock failure process is reduced and the dynamic characteristics are less and less obvious;the failure of the rock form is transition from complex partition damage to the single tensile fault and then to no obvious damage.The failure mode of the rock samples is rockburst,plate cracking and damage in turn.(3)The failure process,failure form and failure mode of the rock sample under the true-triaxial rapid unloading the minor principal stress condition are very sensitive to the change of the minor principal stress.Under the same major principal stress and intermediate principal stress,there exists a suitable minor principal stress ?3c that causes the failure of the rock specimen to be suppressed,and the failure mode is manifested as 'damage without obvious catapult failure.In other words,with the increase of the minor principal stress,the failure mode of the rock samples is plate cracking or rockburst,damage and rockburst in turn.