Challenges in River Diversion in Subansiri Lower Hydro Electric Project (2000 MW)

摘要

Subansiri Lower Hydro Electric Project (2000 MW) is located in the North Eastern part of Indiarnon Subansiri River, a tributary to Brahmaputra River, forming a boundary between Assam andrnArunachal Pradesh states. The project location is shown in Figure 1.rnThe owner of the project is National Hydroelectric Power Corporation (NHPC) and the majorrncivil engineering work was awarded in two packages named as SSL-1 and SSL-2. BGS-SGSSOMArnJV was awarded the SSL-1 consisting of 5 diversion tunnels of 9.5m diameter and 600mrnaverage length each, Inlet and Outlet structures, two cofferdams, main concrete gravity damrnand appurtenants. The project layout is shown in Figure 2rnThe project has many unique features with challenges in manifold. This is the largestrnhydroelectric project in India and South-East Asia and SSL-1 package is the largest value civilrnengineering contract in the hydel sector in India. The river diversion with 1000 cumec or belowrnand design diversion of 4500 cumec will be the largest flow diversion in India. The brief workingrnseason of 7 months a year, the youngest series of rock in Shivalik Sandstone and its uncertainrnbehaviour are some of the natural challenges encountered in the project. River diversion is onernof the major challenges amongst many others. Initial design of cofferdams to divert the riverrnenvisaged wash out type rock and earth fill structure with clay core. The cofferdams would bernwashed out by overtopping during onset of monsoon and to be re-build at the beginning of nextrnconstruction season. This would have further restricted the working period. Various alternativesrnwere studied including roller compacted concrete or concrete cofferdam, concrete faced earthrnand rockfill dams were also considered. The final design of cofferdams envisages rockfill withrnclay core and concrete faced on compacted fill to make it a semi permanent structure. The riverrndiversion is planned by construction of two diversion dykes when the flow is below 1000 cumecrnfollowed by dewatering between two dykes, permeation grouting on two dykes, main cofferdamrnconstruction, dam foundation excavation and concreting.rnThis paper will briefly discuss the technical issues related to detailed construction planning andrnactual execution for river diversion with a very limited period available after the monsoon.