Design and Construction of Trifurcate Junctions of Head Race Tunnel of Yamuna Hydel Scheme Stage II

Yamuna Hydel scheme stage II in district Dehradun of Uttar Pradesh, India is a run-of-the-river hydroelectric scheme envisaging utilization of a total drop of 186.0 m in river Tons, a major tributary of river Yamuna, which flows along a circuitous route forming S-curves, which have been cut across to make available the drop for generation of power by constructing a long water conveyance system of 7.5 m diameter tunnels. A study of the geological features along the alignment of the proposed tunnel had indicated severely adverse geological features in some length of the alignment which could result in delay of the completion of the work if the entire scheme was implemented in one stage. Hence the scheme has been decided to be implemented in two parts. Part I of the scheme utilizes a drop of 124 m by constructing a 60 m high concrete diversion dam at Ichari across river Tons, 6 km long 7.5 m diameter lined tunnel and an underground power house with an installed capacity of 240 MW. The outlet discharge from the draft tubes of Chibro Power House is led through another 6.0 km long tunnel system upto Khodri Power House to utilize, the balance drop of 62.0 m to generate 120MW power at a surface power station.

This paper describes in brief the salient features of water conductor system and in detail the location, design, three dimensional analysis by Finite Element Method and the construction sequence of the complex trifurcate junction structures which had to be constructed in the water conductor system to construct three 4.8 m diameter tunnels instead of a single 7.5 m diameter tunnel due to unprecedented adverse geological conditions encountered at site during tunnelling. The paper also discusses the system of subsequent monitoring by meticulous instrumentation, of the behaviour of proto-type trifurcate junction structures under actual operating conditions which in itself are unique and unparallelled in the world in this scheme.