This paper introduces the studies of rock mechanics behaviors of marble for the Jinping II project. Fracturing can occur in marble under relative low stress level as pre-peak response to loading. As a result, drilling at great depth below a certain depth, such as 1000m from surface, can induce damage to the rock specimen taking from drilling cores. A damage-free sampling technique has been proposed and applied for solving this problem. Lab test indicates that marble presents a complex brittle-ductile-plastic transition behavior as post-peak responses to the increase of confinement. Such behavior can be numerically duplicated with Hoek-Brown constitutive law built in FLAC3D.
Jinping Second Stage Hydropower Station (JP-II) is located at Yalongjing River in Liangshan Yi Autonomous Region, Sichuan province. The sluice gate is situated at Maomaotan in the west of Jinping Mountain and the plant at Dahuigou in the east. The four hydropower tunnel, each of approximately 16.7 km in length running parallel to each other and crossing the Jinping Mountain, an alpine karst zone, will connect the sluice gate and the plant units. The four hydropower tunnels are designated 1 to 4, respectively. Tunnels 1 and 3 will be driven by Drill & Blast method (D&B) and tunnels 2 and 4 will be mainly constructed by TBMs and partially by Drill and Blast method. The average overburden is 1500 ~ 2000 m thick, the maximum overburden is 2525m. The TBM bore diameter has been defined as 12.4m, while the D&B diameter is 13m. The two transportation tunnels lay parallel to hydropower tunnels which construction works started in November 2003 and breakthrough on 8 August 2008. Fig.1 shows the layout of the tunnels, sluice gate, shaft and plant. The transportation tunnels will not only serve as traffic and transport access to both JP-I and JP-II 2 hydropower projects, but also be the exploratory adit, test adit and construction adit to the hydropower tunnel of JP-II. The construction works of hydropower tunnel 2 and 4 started in December 2006, and TBM tunnel, hydropower tunnel 1 and 3, started in October 2008.
The tunnels run mostly in marble, except some 5km in length as interlaid slate and sandstone in beneath of Jinping Mountain. The strata are heavily folded with a dip usually over 60 degrees and a strike in the range between North and N30E. The ground response of marble to excavation is an engineering concern at this project. This paper summarizes the efforts made and results achieved in the study of rock mechanic behaviors of intact marble.
Generally marble has not been considered brittle rock. For instance, a value of 9 is usually assigned to the Hoek-Brown parameter mi. However brittle behavior has been widely revealed with various ground responses of fracturing, spalling, and severe rockburst while advancing the tunnels in marble at Jinping project.