Geotechnical Investigations in Adverse Geological Occurrences in HRT of Tala H.E. at Kalikhola, Bhutan Dixit, Mahabir Babbar, S.K. Kumar, Bibhas 1 Senior Research Officer Joint Director Chief Engineer e-mail: [email protected]e-mail: [email protected]e-mail: [email protected]Central Soil and Materials Research Station, Hauz Khas New Delhi 1 Central water Commission, R.K. Puram, New Delhi ABSTRACT Tala Hydroelectric Project is a run of the river scheme in South West Bhutan in Eastern Himalayas, located 3 km downstream of the existing 336 MW Chukha Hydroelectric Project on river Wangchu. The project consists of 92 m high concrete gravity dam. The construction of HRT near the upstream face of Kalikhola was very problematic due to very poor rock conditions. The area was full of shear seams and pockets of water bodies. From RD 1008 m onwards to RD 1088 m there were large over breaks and squeezing rock conditions. Forepoling was used to construct the tunnel. The designs of forpoles required initial and secant modulus values of the affect area. In the present study, in order to evaluate strength deformation characteristics of the material undisturbed samples have been collected from the affected locations and were subjected to Gradation analysis, Atterberg limits, Insitu density, Specific gravity tests, Triaxial shear under unconsolidated undrained condition and Unconfined compressive strength tests. Initial Modulus values were determined as per Kondner (1963) and Duncan & Chang (1970) hyperbolic model and secant modulus were determined from stress strain curve of unconsolidated undrained triaxial tests. It is found that initial and secant modulus values increases with increase of confining pressure. Secant modulus values decreases with increase in deviator stress. Indian Geotechnical Conference – 2010, GEOtrendz December 16–18, 2010 IGS Mumbai Chapter & IIT Bombay 1. INTRODUCTION Tala Hydroelectric Project is a run of the river scheme in South West Bhutan in Eastern Himalayas, located 3 km downstream of the existing 336 MW Chukha hydroelectric project on river Wangchu. The project consists of 92 m high concrete gravity dam, 3 desilting chambers each of 250 m x 13.9 m x 18.5 m size for removal of suspended sediments of 0.2 mm and above size coming with the river water diverted through the intake structure, a modified horse shoe tunnel of 6.8 m diameter and 23 km in length to carry the water to underground powerhouse (206 m x 20 m x 44.5 m) for utilizing a gross fall of 861.5 m. A simple horse shoe type tail race tunnel of 3.1 km length and 7.75 m diameter discharges the water back into river Wangchu. The installed capacity of powerhouse is 1042 MW (6 x 170). The construction of HRT near the upstream face of Kalikhola was problematic area due to very poor rock conditions. The area was full of shear seams and pockets of water bodies. During excavation of HRT from Kalikhola U/s face, the rock mass conditions started deteriorating from RD 662 m onwards with unstable crown and face, large over breaks, flowing and squeezing rock mass conditions. Tunnel finally collapsed at RD 709 m on 16 th July 2002 infilling 70 m already excavated HRT with about 3500 m 3 of fallen muck. It was, thereafter, decided to divert HRT from RD 607 m at 45 0 from the original alignment and 110m inside the hill abandoning 102 m of already excavated HRT. Similar adverse geological occurrence (AGO) was encountered from RD 1008.25m in Kalikhola U/s face during March 03. The HRT reach of 337 m was excavated in extremely weak rock mass conditions. The soil in the affected reaches was squeezing out during excavation. Under similar conditions the tunnel collapsed at Rd 662. Therefore, tunnel alignment was changed to 45 0 from original alignment. The forepoling was used to construct the tunnel. The designs of forpoles required initial and secant modulus values of the affect area.
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Geotechnical Investigations in Adverse Geological Occurrences in
HRT of Tala H.E. at Kalikhola, Bhutan
Dixit, Mahabir Babbar, S.K. Kumar, Bibhas1
Senior Research Officer Joint Director Chief Engineer