Pare Hydro Electric Project (110 MW), Arunachal Pradesh, India owned by North Eastern Electric Power Corporation, a Government of India Undertaking is in an advanced stage of completion with likely commissioning in Dec 2017. Soft, immature, poorly cemented sand stone belonging to Kimin Formation of Upper Siwalik Group of Pliocene-Pleistocene Age as dam foundation has posed a major challenge in design and construction of 63 m high concrete gravity dam. The soft rock in dam foundation with presence of a few wide fissures carries the risk of erodibility in the long run after impoundment of reservoir. Therefore, adoption of good seepage management measures in dam foundation to minimize the risk of foundation erodibility has become a necessity. Groutability test carried out prior to construction indicated a very poor permeability of bedrock in general with occasional grout intake in certain areas. Therefore, conventional curtain grout as seepage arresting measure beneath the dam foundation cannot be depended upon and this needs to be supplemented with other seepage management measures like positive cut-off wall, drainage curtain, etc. Based on seepage analysis, two cut-off walls, one at upstream edge and the other at downstream edge of dam are designed and constructed. Plastic concrete with low compressive strength is used in the walls. The main intention of plastic concrete is to provide desired compressive strength, low permeability and elastic modulus close to properties of adjacent soft foundation strata. Robust drainage system is provided to capture water from as many erratic bearing joints of the foundation as possible so as to reduce foundation uplift force and seepage force beneath the foundation. Additional drainage gallery in downstream connected with three rows of dewatering curtain is provided as part of drainage system. Contact/consolidation grouting with microfine cement is carried out before pouring of dam concrete in order to improve mechanical properties of foundation rock. Grout intake is not significant and it matches with earlier groutability test result. Curtain grout with deep holes shall be tried despite low grout intake estimation becuase of poor groutibility of rock. Based on Lugeon value of rock after completion of primary holes, decision on secondary and tertiary holes shall be taken. Post construction monitoring of Pare Dam is very crucial to assess performance of the structure vis-a-vis seepage and erodibility threat. Dam instrument outputs, observation of ground water in the vicinity, behaviour of already mapped and monitored seepage points in the adjoining areas shall help in evaluating performance of the dam. Successful construction and subsequent functioning of Pare dam shall be an important milestone in the path of dam construction in challenging rock condition.
Concrete Dam, Drainage, Seepage, Foundation Treatment, Grout Curtain, Cutoff
