Engineering Geological Evaluation of Underground Works at Upper Tamakoshi Hydroelectric Project, Nepal

Abstract

Construction of underground structures in the Himalaya is a challenging issue due to the complex nature of the rock mass in the region. Proper engineering sological assessment of the rock mass conditions in which the structures are to be located is a compulsion in order to ensure that the structures are optimized, cost effective and safe. Due to the scale and complexity, this is even more important for the underground structures proposed for Upper Tamakoshi Hydroelectric Project.

Upper Tamakoshi Hydroelectric Project in Nepal is a run of the river scheme with daily peaking reservoir and is currently under construction. The total installed capacity of the project is 456 MW utilizing the flow of 66 m3 /s from the Tamakoshi River and gross head of 822m. The project is located in the metamorphic units of higher Himalayas. The major underground structures for this project consists of 7.9 km long Headrace tunnel, surge tank which is a combination of 400m long

vertical shaft and surge chambers, powerhouse cavern for housing six 79.5 MW pelton turbines and generating units and approximately 3 km long tailrace tunnel.

The headrace tunnel consists of approximately 4.8 km long high pressure tunnel. In the Himalayan region, constructing an unlined pressure tunnel is a challenge because due to the active tectonics, the rock mass is highly jointed, sheared, deformed and provide a favorable condition for excessive leakage. Leakage prediction in the pressure tunnel is a major part of this thesis. The assessment has been carried out by using two different approaches; Tokheim and Janbu approach and Panthi’s approach. Possible mitigation measures to control the leakage have been then proposed.

Further, the headrace tunnel for the project experiences a hydrostatic pressure of upto 4 Mpa. The possibility of hydraulic fracturing in pressure tunnel has also been analyzed. This has been performed by carrying out Numerical modeling using the Phase 2 Program.

Stability analysis of the headrace tunnel and the surge tank has been carried out using numerical models. The Phase2 program and the Hoek and Brown failure criterion have been used to determine the state of stress, strength factor and deformations in the openings. The input parameters needed to carry out the numerical modeling have been estimated based on the pre‐construction phase geological investigations carried out at the project area.

Finally, based on the analyses mentioned above, overall discussion on the stability of the proposed underground structures has been presented and recommendations have been made for further work.