Geotechnical investigations for locating an underground powerhouse in calcareous rocks, Himachal Pradesh, India

摘要

The case pertains to a hydroelectric project (520MW) located in Kullu district of Himachal Pradesh. During the feasibility stage, surface powerhouse was proposed with an option of an underground alternative as well. In the detailed project report stage, when extensive investigations were undertaken for firming up the layout of the project, the underground location was further explored. However, weak carbonaceous shales were encountered due to which this underground alternative was not preferred. As the surface power house location was not attractive due to very deep open cutting as well as proximity to habitation, another alternative location about 2km upstream was studied through a new 350m long exploratory drift. Once again weak phyllite rock with thinly laminated slates was intercepted due to which second alternative location for underground powerhouse was not considered. The heterogeneity in geotechnical units which is so typical of Larji Group of rocks had posed a mammoth challenge in proposing a suitable site for locating underground powerhouse caverns viz., machine hall having dimension of 122.9 × 23.2 × 41.7m, transformer cavern having dimensions 96.45×18 × 25m and a downstream surge chamber. The study to locate a massive band of dolomite in order to accommodate the underground powerhouse complex and ensuring its continuity both ways has been very complicated and needed a thorough study of the geological conditions. Detailed geological mapping on 1:1000 scale was undertaken to locate a massive dolomite band of significant extent. Subsequently, another exploratory drift of 250m length was excavated to confirm the extension of dolomite band along the longer axis of machine hall. Finally a suitable location was found. During investigation stage, about 880.5m of exploratory drifting has been undertaken to study the three alternatives. Detailed 3-D geological logging on 1:50 scale of exploratory drifts has been done to evaluate the rock compositions, behavior, structural features such as joints, faults, shear/fracture zones etc. RMR rock classification system has been adopted to classify rock mass in terms of Good, Fair, Poor and Very Poor categories. Rock Mechanics and permeability tests were conducted to determine engineering properties of the rock mass. This paper describes a series of geotechnical investigations carried out at various alternatives to select most suitable location of power house complex within massive dolomite band. It also highlights the fact that detailed investigations and well considered decisions taken during the DPR stage help not only in timely completion of projects but also in keeping the project cost within estimates. This Power station has been commissioned in the year 2014.