3D stability analysis of SiahBishe underground powerhouse cavern

Category Engineering Geology
Group GSI.IR
Location 3rd Iranian rock mechanics conference
Holding Date 10 March 2008
      In this study SiahBishe powerhouse cavern and excavations related to it, including busbar shafts, transformer cavern, manifolds and tailrace tunnels have been modeled by FLAC3D. Roof and floors excavation sequences in powerhouse cavern and related excavations as well as support system including rock bolts, tendons and shotcrete have been simulated. Powerhouse cavern is located in stratified sedimentary rocks which are mostly quartzitic sandstones and reddish-brown and blackish siltstone of shaly appearance (for convenience usually called "Red shales"). Last sections (approximately 30 meter) of powerhouse cavern are located in volcanic rocks. Shear zones including crushed and altered rocks which cross the caverns space are one of the important geological characteristics of the site. These shear zones to some extent follow the bedding plane's direction of stratified sedimentary rocks. Regarding the closely spaced bedding planes, sections including the sedimentary rocks and shear zones have been simulated as transversely isotropic materials. The dominant bedding plane has been chosen as isotropy plane. The sections located in volcanic rocks are simulated by elastic model .The results obtained from installed extensometers in the roof and sidewalls of powerhouse cavern show good agreement with the displacements obtained from numerical results. Comparing the induced stress in elements surrounding the excavated spaces with their strength, a safety factor is determined for rock mass surrounding two caverns. Results obtained from numerical modeling show two unstable zones in upstream sidewall in first monitoring section (chainage 26 m) and roof of the second monitoring section (chainage 48.7 m)as well as a zone of overstress extends across the entire pillar between two caverns in the third monitoring section (chainage 67.1 m) which can be a potential threat for busbar system and the whole cavern which are of utmost importance when assessing the powerhouse cavern's long-term stability and support requirements