Nathpa Jhakri Hydroelectric Project (Lot 2.2)
INDIA

Nathpa Jhakri Hydroelectric Project (Lot 2.2)

Owner/Client:

NATHPA-JHAKRI H.P. J.V./Impregilo S.p.A.

Cost of works:

n.a.

Period of Activity:

1997 - 1998

The Nathpa Jhakri Hydropower Project is one of the largest of its kind in the world located within the Himalayan Mountains in Himachal Pradesh. The project was beset with numerous problems and challenges one being the very poor ground conditions encountered in the headrace tunnel.

— Project Description:

The assignment refers to the 10km-long portion of the headrace tunnel (Ø 12M) out of the over 27km in total, which crosses the rocks in the Daj Khad shear zone. This shear zone includes quartz-mica schist, sericite schist, and a few gneiss bands. Rocks are folded into an anticline with a N-S axial trend. The axial portion of this fold, which runs along the Daj Khad, is sheared over an area of 700m. The shear zone comprises blocky and fractured rock with kaolinised and sericitised materials. The conditions for tunnel excavation in this shear zone were expected to be difficult and risky due to the variability of the geologic structure, the poor geomechanical characteristics of the rock mass, high tectonic stress, and high water head, but the final design solution was found to be inadequate during construction. The actually encountered conditions: exceptionally and extremely poor rock mass conditions for 600m of the alignment under an average overburden of 200m, resulting in highly squeezing behaviour, and high water head resulted in great water-inflow conditions.

— Description of Services:

Detailed design of the excavation solution for the crossing of the shear zone, including the definition of a monitoring plan. Implementation of the construction solution on site – construction follow-up. The solution developed by Geodata, based on a substantial amount of experience in design and excavation supervision of tunnels through difficult ground, was the system of Drainage-Reinforcement-Excavation-Support Solution, that was code-named the “DRESS” solution. The DRESS solution comprises the following elements: • Systematic, active drainage all around and ahead of the face. • Rock reinforcement of the face and the peripheral ground to obtain a self-supporting arch. The type, pattern, and installation time for the reinforcement are defined for the actual requirement; all these aspects are verified through monitoring and the design is modified, if necessary, following a pre-defined procedure.

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