Microseismic Monitoring of Energy Changes in Deep Tunnels during the TBM Tunneling of the Jinping II Hydropower Station

The dynamic failure behaviour of tunneling rock in the cold region where freezing-thawing frequently occurs is unclear. This study aimed to test and understand the damage characteristics of tunneling sandstone samples in the cold region via triaxial unloading test and acoustic emission (AE) technique. The sandstone samples were first subject to different cycles of freezing-thawing. Their stress-strain curves, deformation modulus, and the AE characteristics were then measured under triaxial unloading conditions and through the AE test. The results showed that the freezing-thawing treatment with less than 60 freezing-thawing cycles caused rather less damage compared to the triaxial unloading condition. For the samples subject to more severe freezing-thawing treatment, more cracks were produced. These cracks were not closed under small confining pressure during the triaxial test, causing weaker mechanical properties of samples. We also found that the freezing-thawing treatment had a significant deterioration on the mechanical properties of the sandstone samples when the number of freezing-thawing cycles exceeded a certain threshold (between 60 and 80 in this study). As the AE characteristics matched well with the key stages of the measured axial stress-strain curves and the deformation modulus that varied with the decreasing confining pressure, the AE characteristics can be potentially used to quantify the released energy of rock cracking and identify the critical damage phases during the tunneling engineering process.

Section: Discussionmentioning

confidence: 99%

Mechanical and Acoustic Emission Characteristics of Sandstone through Triaxial Unloading Test after Cyclic Freezing‐Thawing Treatment

Shen

Zhu

2020

“…e microseisms determine the failure characteristics and mechanism of rockburst [19][20][21][22][23][24]. To date, the interactive mechanism between microseismic activity and supporting structure remains less investigated, and the microseismic activity is rarely used to guide support strategy.…”

Section: Introductionmentioning

confidence: 99%

Effect of the Initial Support of the Tunnel on the Characteristics of Rockburst: Case Study and Mechanism Analysis

Fan²,

et al. 2022

Rockburst is still a stubborn disease in the field of engineering geology. The present research pays more attention to the influence of geological conditions on rockburst and less to the influence of type and stiffness of engineering support on rockburst. We explore the influence of support stiffness from weak to strong on rockburst and reveal the characteristics of fracture and microseismicity during rockburst through microseismic monitoring and numerical simulation. The main results and conclusions can be drawn: (1) Strong stiffness support makes the surrounding rock accumulates higher energy before rockburst. The evolution of microseismicity and its indexes can be used as precursors of potentially strong rockburst. (2) Strong stiffness support is easy to concentrate high stress under the action of surrounding rock pressure, and it is easy to fail under the disturbance of external load. This will produce a “sudden unloading effect” on the surrounding rock mass and may lead to a more serious rockburst. Numerical simulation verifies the existence of that effect and are consistent with the actual signs of failure. Our research is helpful to clarify the rockburst problem in the field of engineering geology, specifically to reveal the mechanism of rockburst and the early warning criteria of rockburst hazard under the action of supporting structure, which can provide practical data and theoretical support for scientific and reasonable prevention and control of rockburst risk in tunnel and underground engineering.

“…Jiang et al studied the rockburst phenomenon using the numerical simulation method and developed a new local energy release rate (LERR) index, which can satisfactorily predict the depth of the outburst pit and the intensity of the rockburst, providing a new method to evaluate the rockburst orientation in deep underground projects [14]. Zhuang et al conducted realtime monitoring of microvibration events and rockburst during the TBM excavation of Jinping II diversion tunnel 1 # and found that the top guide tunnel excavation method can effectively reduce the risk of rockburst during TBM tunnel excavation [15]. It was found that the unloading rate has an important influence on the degree and failure models of rockburst during the excavation [16][17][18].…”

Section: Introductionmentioning

confidence: 99%

Comparative Study of the Excavation Damage and Rockburst of the Deeply Buried Jinping II Diversion Tunnels Using a TBM and the Drilling‐Blasting Method

Yang

Zhou

et al. 2020

The rock mass failure induced by high in-situ stresses during the excavation of deep diversion tunnels is one of the key problems in the construction of the Jinping II Hydropower Station. Based on the results of acoustic wave tests and rockburst statistical analysis conducted, this study focuses on the excavation damaged zone (EDZ) and rockburst events in the Jinping II diversion tunnels excavated using the tunnel boring machine (TBM) method and the drilling-blasting method. The unloading failure mechanism and the rockburst induced by the two different excavation methods were compared and analyzed. The results indicate that, due to the different stress adjustment processes, the degree of damage to the surrounding rock mass excavated using the drilling-blasting method was more serious than that using the TBM method. The EDZ induced by the TBM was usually distributed evenly along the edge of the excavation surface. While, the drilling-blasting method was more likely to cause stress concentration, resulting in a deeper EDZ in local areas. However, the TBM excavation method can cause other problems in high in-situ stress areas, such as strong rockbursts. The drilling-blasting method is more prone to structural controlled failure of the surrounding rock mass, while the TBM method would induce high stress concentration near the edge of excavation and more widely distributed of stress adjustment induced failure. As a result, the scale and frequency of the rockburst events generated by the TBM were significantly greater than those caused by the drilling-blasting method during the excavation of Jinping II diversion tunnels. The TBM method should be used carefully for tunnel excavation in high in-situ stress areas with burial depths of greater than 2000 m. If it is necessary to use the TBM method after a comprehensive selection, it is suggested that equipment adaptability improvement, advanced prediction, and prediction technology be used.