Geological Challenges in the Construction of Underground Structures of Vyasi Hydroelectric Project G
Total Page：16
File Type：pdf, Size：1020Kb
Geological Challenges in the Construction of Underground Structures of Vyasi Hydroelectric Project G. P. Patel Managing Director, UJVN Ltd. Dehradun Sandeep Singhal Director (Projects) UJVN Ltd. Dehradun Harish Bahuguna Head, Environment and Geology Cell, UJVN Ltd. Dehradun Rajeev Kumar Agarwal General Manager (Vyasi and Lakhwar HEP), UJVN Ltd. Dakpathar, Dehradun Abstract Vyasi HE Project, located in Dehradun district of Uttarakhand, is a run of the river scheme on river Yamuna which envisages construction of concrete dam of 86m height located near Judo village (5 Km d/s of Lakhwar dam), a 7m dia and 2.7 Km long Head Race Tunnel (HRT), a 18m dia 63.5m high Surge Shaft, 2 nos. 4m dia. 209m long each Pressure Shafts and a surface Power House (IC 120 MW) to install two units of 60MW each. The project area is located in the Lesser Himalayas which are bounded in the south by Main Boundary Thrust (MBT) and in the north by Main Central Thrust (MCT). The Vyasi Project area exposes rocks of Jaunsar Group belonging to Pre-Cambrians of Kumaun - Garhwal Lesser Himalayas. The Jaunsar Group of rocks comprises slate, phyllites, quartzites and limestones of Mandhali, Chandpur and Nagthat Formations. The rocks of Jaunsar Group display a thrusted contact with the rocks of Subathu Group near project area. At the dam site purple and gray Quartzites of Chandpur Formation are exposed whereas the HRT pierces through Quartzites of Chandpur Formation (near Vyasi) and Lime stone and Slate bands, of Mandhali Formation (near Hathiyari). The surface powerhouse area is located over a terrace of Yamuna River, however, bed rocks of Mandhali Formation are expected at the foundation level. The underground structures that are under construction in the project include HRT, Surge Shaft, Penstock Chamber and two nos. of Pressure Shafts. Construction activities in most of these structures were started in late eighties but because of resource crunch they came to a sudden halt in 1992. The half constructed underground structures are standing for more than two decades without a final support. As a result the support measures need a review in critical reaches. Varied lithological conditions and complex structural disposition have posed challenges during the construction of different underground structures. Restarting the construction activities, after a long gap, is itself a challenging task ahead. During the earlier phase of construction, squeezing in HRT was noticed in the reach between RD 1600m and RD 2100m which was attributed to very poor and poor rock mass in that reach. Existence of a fault at RD±2050m may also have influenced the squeezing phenomenon. Minor failures from the overt portion of HRT have also taken place since the closure of construction activities. The restoration and augmentation of the existing support measures are being evaluated. The upper part of the surge shaft (excavated dia 20m) lies in the slumped rock mass, which, however, improves at lower levels. Pressure shafts have been excavated in fair to good rock mass and no major problems are anticipated during the course of their further excavation. .