ABSTRACT :
An underground powerhouse chamber at chibro was constructed about two decades ago in dolomitic-lime stone of lower Himalayan region. Long-term instrumentation was planned during construction to check the safety and adequacy of the support system and to monitor the post construction behaviour of the excavation. The present paper essentially ?eals with the analysis of ten year data. The observed roof support pressure has been compared with the support pressures estimated from different empirical theories. The time-dependent effect has been noticed significantly where the rock mass is saturated due to seepage problem and near thick plastic shear zone.
Construction of underground excavations for hydroelectric projects in the lower Himalayan region is a challenging task due to support problems under complex hydrogeological conditions and tectonic influences. Further, these problems could be attributed to the non-homogeneous and anisotropic nature of rockmass and their time dependent behaviour. In this context, the Chibro underground powerhouse complex has set a major precedent by being the first venture of its type in the lesser Himalaya. The 240 MW Chibro underground power station exploits the drop of about 124m along the first loop of Tons river, a tributary of the Yamuna between Ichari and Chibro, which is part-I work of Yamuna Hydroelectric Scheme Stage-II. This was the first venture of its type in the lesser Himalaya and was necessitated because the location of a surface power station would have involved large scale excavation of steep slopes. Finally, the powerhouse complex was sited in a band of limestone which has horizontal width of 193 to 217m. The complex comprises a network of excavation for the machines, transformer, turbine inlet valves and control room and also provides operating galleries and hydraulic connections to the Part-II. This latter stage involves a 120 MW Khodri power station utilising the remaining drop of 64m along the second loop. Fig.1 shows a general layout of the powerhouse complex.
