Rock fragmentation mechanism by TBM cutter and TBM tunnelling under stressed grounds

With the advance of technology, TBMs are becoming more versatile and TBM tunnelling has become a common tunnelling method. As TBM tunnelling is a special excavation method depending on the interaction between machine and rock mass under a certain geological circumstance, the rock mass properties and geological circumstance are directly related to the rock breakage process by TBM cutters and TBM excavation efficiency. Recently, more and more long tunnels at great depth are constructed by TBM method. In situ stress, as one of the important circumstance factors, has been paid more attention for TBM tunnelling at great depth. However, the influence of in situ stress on rock fragmentation by TBM cutter and TBM performance has not been explored thoroughly. Moreover, there is a gap for TBM performance prediction under stressed grounds. The objectives of this research are to study the rock fragmentation mechanism by TBM cutters with different confining stress, and to explore the influence of in situ stress on TBM performance, finally to develop a TBM penetration rate prediction model involving in situ stress. By indentation test with biaxial confining stress, the rock fragmentation processes with the different confining stress levels were studied for granite and marble based on the characteristics of force-penetration depth curve and the acoustic emission (AE) parameters (i.e. AE energy rate, hit counts, amplitude and location of AE events). The test results showed that the thrust force for crack initiation and the size of crushed zone increase with increasing confining stress. Thus, the confining stress restrains rock fragmentation by TBM cutter. The cracks gradually propagate parallel to the plane of the biaxial confining stress with increasing confining stress. While a critical confining stress level is observed from the tests of marble sample. As the confining stress is higher than the critical level, the stress-induced failure occurs in marble under the TBM cutter, and the thrust force for crack initiation and size of crushed zone decrease dramatically with the increase of confining stress. The rock fragmentation by TBM cutters is facilitated by the confining stress. To determine the stress level for stress-induced failure occurring on TBM tunnel face, the true triaxial rock burst test in laboratory is performed with the typical marble in the Jinping project. By simulating the stress state ahead of tunnel face after tunnel excavation, the stress-induced failure processes at the tunnel face are explored by the rock burst test. Combining the field investigation and the previous researches on the stress-induced failure, the different stress levels for stress-induced spalling and rock burst are obtained for TBM tunnels of the Jinping project. And then, a classification for the stress condition is proposed based on the influence of in situ stress on rock fragmentation and TBM performance. Subsequently, the field TBM penetration tests are performed to analyze the rock mass boreability under different stress conditions in three TBM tunnels. The TBM penetration curve, the results of muck sieve tests and comparison with the previous penetration tests showed that in situ stress has significant influence on rock breakage process by TBM and rock mass boreability. The validity of the classification for the stress condition is also verified by the penetration tests. Based on the data obtained from the penetration tests, the popular TBM penetration rate prediction models are performed. By comparing the prediction results, it is found that the rock mass characteristics (RMC) model is reliable and suitable to be modified to a new prediction model involving the parameter of in situ stress. Subsequently, extensive site data collection is conducted for the Jinping project to obtain rock mass properties, in situ stress conditions, TBM performance data and machine parameters. A database is then established for modifying the RMC model. Based on the previous conclusions obtained from this study, the modification for the RMC model is proposed dependent on the classification for the stress condition. The modified RMC model is obtained through nonlinear statistical analysis. It has potential application in the TBM tunnelling under stressed grounds.