True Triaxial Test Study On Multiple Rockburst In The Same Cross Section Of Deep Tunnel

Rockburst is an engineering geological disaster in which the elastic strain energy inside the surrounding rock is suddenly released and the rock rupture,spall,and even eject due to the excavation of underground engineering.As the underground engineering gradually migrates to the deep part,more and more rockburst disasters are encountered in underground engineering during the construction,and even multiple rockbursts occurred in the same cross section of the tunnel.Multiple rockbursts in the same cross section of the deep tunnel pose a serious threat to the safety of the project,delay the construction period and cause huge economic losses.For its highly complex mechanism,rockburst is difficult to predict.Therefore,the rockburst study is still a hot issue in rock mechanics and engineering.So far,scholars have carried out coniserable research on rockburst mechanism and have achieved many beneficial results,which greatly promoted the research progress of rockburst.However,the traditional rockburst study can not effectively reflect the characteristics of multiple rockbursts occurrences in the same cross section of the tunnel.Therefore,the mechanism of multiple rockbursts is unclear.In order to deeply explore the occurrence mechanism of multiple rockbursts,the true triaxial test of multiple rockbursts was conducted indoors by using the true triaxial rockburst test system.The ejection failure process,debris characteristics and acoustic emission signals of multiple rockbursts were analysed in detial.The main conclusions are as follows:(1)Multiple rockburst failure processes in the same cross section of the tunnel was successfully reproduced under laboratory conditions.It is known from the ejection failure process of multiple rockburst simulation tests that the multiple rockbursts occurred in the test have the characteristics of intermittent and multiple occurrences.The generated debris and the finally formed rockburst are similar to those in the actual project,which indicate rationality of the obtained results.The indoor multiple rockburst test provides scientific data support for further rockburst research.(2)The research result indicates that the initial rockburst occurrence process of intact surrounding rock is a gradual destruction process which can be roughly divided into five stages:no macroscopic damage,particle ejection,splitting into a plate,plate flexing drum,and plate folding projectile.However,subsequent multiple rockbursts may not go through the stage of small-particle ejection,directly into the stage of splitting and plate-breaking,and even directly rockburst again,which is more sudden and serious,and may bring severe threat to the project.(3)The study found that the surrounding rock is in a metastable state after the initial rockburst,and its stability is sensitive to the change of the maximum principal stress.The smaller increase of maximum principal stress will cause the instability of the surrounding rock again and even multiple rockburst.There are a lot of cracks in the surrounding rock where the initial rockburst occurred,and the uneven rockburst pit is more likely to cause stress concentration.At this time,the surrounding rock is in a metastable state,and the maximum principal stress is further increased or affected by external disturbance.The surrounding rock will be subjected tosplitting and plate-folding ejection,and even rockburst directly.(4)The analysis of the test results shows that the energy released by multiple rockburst is mainly from the interior of the rock,and the subsequent input energy acts as a trigger,and multiple rockbursts are more likely to occur in the depths and vicinity of the rockburst,and in the opposite surrounding rock.Therefore,in the actual project,after the rockburst occurs in the tunnel,the surrounding rock at the rock blast pit,adjacent and opposite should be strengthened to prevent multiple rockburst and ensure the smooth progress of underground engineering.(5)The results of acoustic emission analysis show that with the application of subsequent loads,when the multiple rockburst occurs,the energy release rate is higher and the burst is more voilent.The initial rockburst in the surrounding rock will induce cracks inside the surrounding rock,deteriorating the bearing capacity of the surrounding rock.If the maximum principal stress increases or the external disturbance induces is encountered,rockburst will occur again in the surrounding rock,the surrounding rock properties will be degraded again,which will promote the formation of more secondary rockbursts.In this situation,multiple damages will occur in surrounding rock in a short time and bring more serious threats to project safety.