GEOLOGICAL AND GEOTECHNICAL APPRAISAL DURING CONSTRUCTION OF UPSTREAM SURGE CHAMBERS AND SHAFTS - A CASE STUDY OF TEHRI PUMPED STORAGE PLANT (1000MW)

Inadequate geological and geotechnical investigation may lead to over-budgeting, schedule overrun, and serious loss of safety. Geological surveys which include proper assessment and investigation of geological strata are essential for any kind of projects especially for large and most expensive structures like dams and hydropower plants. In the case of Tehri Pumped Storage Plant which is in EPC mode possess lot of challenges during construction stage. As an integral part of Tehri Hydropower Complex (THPC) located in the Uttarakhand State of Northern India; an underground 4x250 MW Tehri Pumped Storage Plant (TPSP) parallel and close to the existing 1000 MW Tehri Hydro Power (HPP). Tehri PSP is located on the left bank of Bhagirathi River in the district of Tehri about 1.5 km downstream on its confluence with river Bhilangana. The major project components are Machine Hall, Upstream Chambers and Shafts, Butterfly Valve Chamber (BVC), Penstock Assembly Chamber (PAC), Down Stream Surge Shafts, a pair of TRT’s and outlet structures are in construction stage. The arrangement consists of surge shafts with Surge Chambers at the top of each shaft and is inpoor geology with rock cover of 175.00m to 230.00m. The chambers were relocated and resized from the originally planned span due to poor geological conditions which called for a transition in an excavation with milk cane shape varies froma 15.60m chamber invert to 25.50m diameter circular surge shaft. This paper represents the geological and geotechnical appraisal encountered during excavation of both the upstream surge chambers (37.50m long X 15.00m wide X 15.00m high) and shafts with remedial measures using NATM construction methodology as well as proper excavation sequence. Strengthening of rock mass by adopting proper design measures from day-to-day monitoring instrument data of deformation as well as suitable construction methodology with highlighting the importance of orientation of structure with respect to prominent joint sets. Reinforced self-compact concrete lining using jump form shutter and grouting plays a very important role to complete the tasks in a specified time period, which is also briefly discussed in this article,