LI Shaojun, ZHENG Minzong, QIU Shili, YAO Zhibin, XIAO Yaxun, ZHOU Jifang, PAN Pengzhi
The construction of deep high-stress tunnels can face various problems such as rockbursts, rib spalling, and tunnel collapse. The 2 400 m deep China Jinping Underground Laboratory (CJPL-II) is currently the world's largest buried laboratory. The construction of this group of tunnels included field monitoring and numerical analyses of the mechanical response of the rock around the tunnels, such as the deformation, stresses and microseismic events. The complex geological conditions are analyzed to predict tunnel disaster characteristics and the long-term in-situ mechanical response of the rock. The results show that the surrounding rock mass deformation is larger on the north side walls of laboratory 1# and laboratory 4# with a maximum deformation of 83.7 mm. The maximum rock bolt stress is 530 MPa. The rock mass deformation tended to become stable about three months after completion of the excavation. The excavation damage zone revealed by elastic waves and a borehole camera is generally 0.8~3.5 m. The results also show that the internal fractures in the surrounding rock mass evolve with the excavation with zonal disintegration. The high strength, good integrity rock has a small fracture zone while the low strength, poor integrity rock has a large fracture zone. The results also show that there is more micro-seismic activity in the completed rock mass tunnel and the area around the fault. The intensity of the microseismic activity in each laboratory tunnel during excavation was highest for 8# and decreased to 8#, 7#, 4#, 3#, 5#, 6#, 1#, 2# and 9# as the lowest. After excavation, the microseismic activity in each tunnel gradually decreased. The CASRock software analysis showed the high stresses and large relaxation depth of the southern arch shoulder and sidewall after excavation and unloading that created high-risk areas. The results provide direct support for disaster warning system development, stability assessments, dynamic designs, and long-term safe tunnel operation for safe construction of high-stress, deep tunnels with similar geological conditions.
