Siah Bishe Powerhouse Cavern and other related excavations have been modeled by 3D numerical programs. The main concern in this large span underground space was closely bedded rock formations which are mostly quartzitic sandstones and reddish-brown and blackish siltstone of shaly appearance with low RQD index. This had raised doubts about its long term stability. Sheared and altered zones which cross the cavern axis are the most problematic sections of the project. Excavation sequences in powerhouse cavern as well as support systems have been also simulated. Regarding the closely spaced bedding planes, sections including the sedimentary rocks and shear zones have been simulated as transversely isotropic materials. The results obtained from installed extensometers in sidewalls of powerhouse cavern show good agreement with the displacements obtained from numerical results. Comparing the induced stresses in elements surrounding the excavated opening and their strength, a safety factor is determined for rock mass surrounding the powerhouse cavern. Results obtained from evaluation of safety factor show some unstable zones around the cavern in the first & the second monitoring sections (chainage 26m & 48.7m) due to the existence of the main sheared zone. An unstable zone also extends between transformer and powerhouse caverns in the third monitoring section (chainage 67.1m).
The Siah Bishe pumped storage project is located in Alborz mountain range, about 125 km north of Tehran. The powerhouse cavern is located in closely bedded rocks which concerns the engineers about stability of the cavern. Sheared and altered zones which cross the cavern axis, are the most problematic sections of the project. Modeling the behavior of these closely bedded rocks and sheared zones seems very important for long-term stability analyses of the cavern.
In current study, Siah Bishe powerhouse cavern and related excavations including transformer cavern and main galleries and tunnels joining to the powerhouse cavern have been modeled by 3D finite difference numerical model.
Due to the closely spaced bedded sedimentary rocks around the cavern, the transversely isotropic elastic model was chosen to simulate the powerhouse cavern and related excavations. A safety factor is determined and used to analyze the stability of powerhouse cavern in different sections.
Engineering design of caverns and other underground spaces in closely bedded and jointed rock masses must take into account the relative size of cavern span and discontinuity spacing.
In closely spaced discontinuities, in comparison with the size of span, the excavation space can be treated as a homogeneous but transversely isotropic material where properties in direction parallel to the joints are different from those perpendicular to them (Vicenzi et al. 2001).
The rock mass around the cavern is basically consisted of closely bedded sedimentary rocks, including quartzitic sandstone and reddish brown siltstone of shaly appearance and also an altered sheared zone. The furthest end of the cavern (last 30 meters) is located in volcanic rocks which are simply simulated by isotropic elastic models.
An equivalent transversely isotropic model is used for closely bedded sedimentary rock mass and the sheared zone.
