The construction of the underground Rogun hydropower station in Tajikistan was interrupted due to the termination of central financing with the disintegration of the USSR in the beginning of the 1990s. Recent plans to update and complete the project require careful evaluation of the trends in rock mass behavior in the vicinity of the large caverns that make up the machine and transformer halls and the gate chamber, which is undercut by numerous penstocks, draft tubes, drainage galleries and bus ducts. The most challenging task is to account for the rock mass/cavern behavior and interaction during the almost 15 years of stalled excavation activity and to formulate prognoses for the period during which the construction will be completed. This evaluation is being carried out numerically and backed up with sporadic in situ measurement data.
The Rogun hydropower station was designed as the next development stage of the RiverVakhsh inTajikistan to be constructed after completion of the 2700MW Nurek project with the highest (300 m) rockfill dam in the world. The project called for six generating units of 3600MW total installed capacity in a fully underground outfit with a dam of 335 m, even higher than the Nurek dam (Yufin et al., 1999). This was to have been the largest underground hydropower station in the former USSR, sixth, if completed recently, hydropower station on the River Vakhsh. The complex of underground structures at the Rogun site, with a total length of some 40 km and excavation volume of 3×106 m3, is located in rock mass consisting of sandstones and aleurolites. Two major caverns for the machine hall (MH) and the transformer hall (TH) are situated at a depth of 350–400m from the ground surface and are 21×70×220m3 (MH) and 19×37×182m3 (TH) in size. The distance between the axes of these caverns is 63m and the pillar contains drainage galleries and bus ducts. At an angle to these two caverns there is a shorter penstock assembly/gate chamber (AC) with a span of 12 m, height 37m and length 50 m. All three caverns are situated in a single tectonic block and separated by active faults from the surrounding rock mass. Sandstones and aleurolites in this region are rather strong rocks with uniaxial compressive strength of 100–120MPa and 60–80MPa respectively.
The excavation of the machine hall started in September 1986 and was carried out in benches 6–11m high along the total width of the cavern. By the beginning of 1990, theMHcavern had been excavated to a depth of 36m and the TH cavern to a depth of 16 m.
To evaluate the feasibility of completing cavern construction, a series of numerical analyses were performed in 2005–2006 under contract to the RUSAL Company.