• Author / Uploaded
• Amal Kumar De

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EXERCISE-2 TEHRI HPP PROJECT

INTAKE STRUCTURES OF TEHRI HPP & PSP 1. General Tehri HPP comprises of a 260.5 m high earth & rock filled dam at the confluence of Bhagirathi & Bhilangana rivers in the hill state, Uttrakhand of India. It consists of a underground Power House with installed capacity of 1000MW( 4 x250MW) on the left bank of the river Bhagirathi, 4 Nos Intake structures for receiving water from Tehri reservoir and 4 nos Head Race Tunnel (HRT) (8.5m dia) for conveying water from 4 nos Intake structures to Power House. The power intakes have a semicircular configuration and each of them is 38.5m high and consist of rigidly connected thin wall elements forming an integral structure(5 radially located piers up to 1.7m thick and 5 horizontal cross bars and massive elements, Top slab with a minimum thickness of 2m), platform at EL 745M and water intake adjustments adjoining the rock slope. HPP and PSP Water Intakes are located on the left bank of the Bhagirathi River to the south of confluence with the Bhilangana River. Due to complex geological conditions of the rock mass around the intake structure certain movements have been observed in the top slabs of the intake structure after the first filling of the reservoir in the year 2006.The movements are being closely monitored and geodatic measurements are being taken every year. 2. Geology of Intake structures and slopes above them Geologically ,the area is divided into three large structural sections from the base of Bhagirathi and Bhilangana rivers, a rock mass formed by non-uniformly inter bedding phyllites, argillaceous phyllites and quartzitic phyllites with different degree of alteration with a number of faults and some sections higher are marked by the same type of rock mass with better geotechnical properties. The slope within the range of EL 710-840m is recognized to be having rather complicated engineering-geological condition with faults displaced and in zones of these faults crossing the rocks are characterized by crushing and fracturing and by ferruginization and hydrothermal alteration as well. Colluvial-landslide deposits are confined to the outcrops of such rocks. Besides a considerable deepening of zones of near-surface distressing and weathering was revealed along major infirmities.

HPP Intakes: The slope in the zone of water intakes of HRT-1 and HRT-2 at EL 710-745 is formed of rather strong phyllites and quartzitic phyllites which are confirmed also by relatively high steepness of natural slopes. At the construction stage additional excavations and concreting of excavations at the shortest possible time before further distressing of the foundation rocks had been carried out. Lithologically the foundation is composed mainly of plastic rocks where there are no large shear fractures but intensive corrugation of schistosity, higher jointing and small blocky structures. All rocks here are assessed to be of high strength with thin shear zones and construction stage did not pose problems. PSP Intakes: The foundation of the HRT-3 water intake and some stripping in front of it, revealed the presence of potentially unstable blocks formed by intersection of the fault with longitudinal shear zones which was treated adequately by selective excavations and concreting. The foundation of the HRT-4 water intake exposed the occurrence of phyllites and quartzitic phillites of group 2B broken by longitudinal shear fracture. Additional geological-geophysical investigations demonstrated that in the foundation of the water intake and around it, longitudinal shear fracture accompanied by zones of schistosity and mylonitized zones were recognized and Water Intake foundation was brought into the zone ‘A’ of intensive technogenous distressing and weathering. In HRT-4, to improve the stability conditions design provides for a pile field around the intake inlet structure and reinforced concrete slab joins all piles into a cap beam grillage along with grouting. This was followed by provision of 20 pre-stressed anchors in two rows and anchors located across the site edge across bedding planes in the rock mass .Construction of intake area has been done in lifts of 0.75 to 3m and shear keys have been provided in each block to receive next lift of concrete. 3. Problem It was reported that vertical and horizontal displacements of the top slab at the top of intake structures have taken place. The maximum values were in 2008-09 but it is reported to have stabilized in later years. Problem is to arrest any kind of displacement in intake structure. ---0---