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All the surface as well as underground hydropower stations has Electrical Overhead Travelling (EOT) cranes to facilitate erection and maintenance of the generating equipment. In the powerhouses built over ground, such cranes run on column and beam frames, provided for supporting the roof structure. However, in the case of underground powerhouses, built in rock caverns, advantage can be taken by utilizing the in situ rock walls to support the crane rails. This not only saves the construction of separate column and beam facility, but also allows early erection of the crane. The most commonly used methods for supporting the crane rails in underground powerhouses are building rock bolted, reinforced concrete beams along the powerhouse long walls or by providing a bench/rock ledge of suitable width along the walls. Bench/rock ledge type of design has been adopted in the under construction Shongtong Karcham HEP, Rekong Peo, District Kinnaur, Himachal Pradesh which is a run off the river scheme for harnessing hydropower potential of Satluj River between Powari and Ralli villages located in Kinnaur District of Himachal Pradesh. The barrage site is located 1.5 km upstream of the confluence of Tangling Khad with Satluj River near Powari village. The diverted water shall be conveyed through underground desilting chambers, 10.5 m finished dia Head Race Tunnel for a distance of 8095.39m long, 33-meter dia. underground surge shaft and underground power house, to generate 450 MW (3 x 150MW) of hydro power and to let out the TRT in the d/s at Ralli. The Project components are mostly placed in underground and are located on the left bank of the Satluj River. The Project envisages construction of ±24m high diversion barrage across Satluj River to generate 450 MW (3 x 150MW) of hydro power by utilizing a design head of 128m. The subsurface power house (131.15m (L) x 23m (W) x 44.29m (H) has been excavated in N43°E direction parallel to the principal stress axis. The detailed geological mapping of the rock ledge in the power house cavern where Electrical Overhead Travelling (EOT) cranes is to be placed, was carried out on 1:100 scale. The 3m height vertical wall of the rock ledge from base El. ±1827m to top El ±1830m has been excavated in N43°E direction from RD 40m to RD 131.15m and is driven across the foliation plane. During the geological wall mapping, it was found that the rock type exposed is mainly Quartzo-felspathic augen gneiss with thin bands of biotite schist belonging to Kharo Formation of Vaikrita Group. The general foliation of the exposed rock is N30°-45°W/20-25° due N40-65°E. Besides foliation joint, two other prominent joint sets and some random joints have also been encountered. The 5 to 20cm thick shear seams have been encountered on the both the wall having persistence of more than 3m. The rock mass assessment of the rock ledge was carried out and it was found that most of the rock mass in the left wall falls in Class III i.e, ‘Fair’ category except from RD 85-92m, where it is Class IV, whereas in the right wall it was found the rock mass falls in Class II to Class III, except from RD 110-113, 115-116, 119-122, where it is in Class IV. In both walls logged, 85.2% of the rock mass falls in Class III, while 10.4% in Class IV & 4.4% in Class II. The recommendations and suggestions were given in form of shotcrete and wiremesh as per rock class, low pressure grouting, three rows of rock bolting (the upper 2 rows are tension bolts and the lower row is of compression bolts) and to provide steel reinforcement in the base concrete in order to take care of any differential deformations due to varying rock conditions. Optical deformation monitoring system using 10 m long multipoint borehole extensometers on both the walls at EL ±1830 was also suggested to monitor deformations continuously. Keywords—Satluj River; Tangling khad, Run off river; Kharo Formation; Vaikrita Group; RQD; EOT; rock ledge

Rock mass characterization of rock ledge in power house cavern at Shongtong Karcham Project, Himachal Pradesh