The Tehri Dam

Sudeshna Mukherjee  
Assistant Editor
Civil Engineering and Construction Review

 

 

Tehri Dam is a 260.5 m high earth and rock-fill embankment dam. Its length is 575 m, crest width 20 m, and base width 1,128 m. The dam creates a reservoir of 4.0 cubic kilometres with a surface area of 52 km2. The installed hydro capacity is 1,000 MW along with an additional 1,000 MW of pumped storage hydroelectricity. The lower reservoir for the pumped-storage plant is created by the Koteshwar Dam downstream.

The Tehri Dam and the Tehri Pumped Storage Hydroelectric Power Plant are part of the Tehri Hydropower Complex, which also includes the 400 MW Koteshwar Dam. Power is distributed to Uttar Pradesh, Uttarakhand, Punjab, Delhi, Haryana, Jammu & Kashmir, Chandigarh, Rajasthan and Himachal Pradesh. The complex will afford irrigation to an area of 270,000 hectares, irrigation stabilization to an area of 600,000 hectares, and a supply of 270 million imperial gallons (1.2×106 m3) of drinking water per day to the industrialized areas of Delhi, Uttar Pradesh and Uttarakhand.

Project Formulation and Planning

The project was conceptualised, detailed feasibility report was made based on contemporary technology and expertise available. Necessary secondary and primary data about topography, geology, hydrology, seismology and environment were used in technical formulation of the project. This is as per the norms prevalent in the country, however, far short of international standards.

There were no major changes in the project design. Minor changes were effected for safety considerations only. These changes were in no way responsible for delays. The basic designs were made based on the information available and contemporary technology. Both got refined with time and one should take its advantage. This was done at Tehri Dam Project in view of long time available. Geological information available at the time of design gets refined during project execution. Rock treatments were done to enhance its quality and extra reinforcement, grouting and concreting were done in view of the geology revealed in the course of construction.

The project was constructed from 1978 to 2006. A Technical Advisory Committee was overseeing the project construction and helping in evolving solutions to the problems faced by the construction engineers. The land acquisition and R&R were being looked after by a separate Directorate, wholly devoted to the task. Land acquisition and R&R are very difficult issues because they are linked to the local population and their interest where State Govt. has a major role. The local pressures, political influence, personal egos and individual interest very much influence the process, adding to the complexity. R&R was the sole cause for cost and time over run and this process is still going on. In any future project this must be investigated thoroughly, and appropriate steps be initiated to ward of its ill effect on the progress of any future project. The initial provisions in 1994 were about 13% of the ` 2,851 Crores project cost, which rose to about 15% as in March 2005 figure of  ` 6,896 Crores, not a big rise, but its influence on time over run was very significant.

The project never suffered on account of finance after the Ministry of Power assured funds under joint venture with THDC and the state govt. There are instances of funds not being utilized within a specified period. There was no major flaw in project formulation and planning. Had there been no agitations, the project would have been completed in time as planned in 1994.

 

Benefits Of The Project

The main benefits of the project are:

  • Addition to the installed capacity in Northern Region: 1000 MW (2400 MW on completion of entire Complex).
  • Annual energy availability peaking: 3568 Million Units (6200 MU on completion of entire Complex).
  • Additional irrigation in 2.7 lakh hectares area besides stabilization in existing 6.04 lakh hectares area.
  • 162 Million Gallons of water per day (300 Cusecs) for drinking water supply to Delhi to cater to a population of Rs. 40 lakh.
  • 108 Million Gallons of water per day (200 Cusecs) for drinking water supply to the towns and villages of Uttar Pradesh.
  • Integrated development of Garhwal Region, including construction of a new hill stations viz. New Tehri Town (NTT) with provision of all possible facility, improved communication, education, health, tourist traffic, setting up of non- polluting industries, development of horticulture, fisheries, forestation of the region etc. much to the advantage of the people of the region.

 

 

Brief Description Of Project Components

The project complex has two main components, the Dam and the Powerhouse. They are interdependent and without the dam being completed neither the powerhouse can work nor can the water for irrigation be released. In the following paragraphs brief description of these components, with figures is being given.

Dam

It is an earth and rock fill dam 260.5 m above the deepest foundation level. The dam is 1.25 km wide at the rivers bed level and 25 m at the abutments. The dam body is composed of clay core, shell material and rip rap. The clay core makes the dam body impervious and the shell material adds weight to the dam body. The ripraps are laid on the body of the dam body to ward off against erosion caused by the water of the lake and the ripples caused. The dam body has inspection gallery with numerous instruments installed to monitor the behaviour of the dam during operation.                 

Powerhouse

Powerhouse has several caverns and tunnels. The machine hall is the largest of all. Here the power generating machines are installed. There is a transformer cavern which is smaller as compared to the machine hall. Four head race tunnels (HRT) bring water from the dam through an intricately designed intake structures and surge shaft. The HRTs terminate into penstocks, which ultimately feed water under head to turbines. The turbines are coupled to alternators, which rotate at a given spaced to generate electricity at a rated voltage and currents.

 

Standing Committee Report

Tehri Project has a chequered history, and has been subject of controversies and agitations on various issues, viz., safety of Dam, environmental issues, R&R etc. Project work had suffered on account of agitation and Dharnas. A chronological sequence of events, since project approval, affecting schedule of implementation is given below:

 

Safety Measures

In the year 1949, Nautyal and Pathak of GSI identified the Tehri Dam site for the first time. Various international Experts for found it suitable construction of high dam. Mr.N.N.Yakovlov, UN Expert, visited the project in Feb. 1967 and recommended an earth and rock fill dam for Tehri. Mr. Cook, famous Consulting Engineer of US, who visited the site in 1972, had concluded that the site geology, topography and available material combined to make the 800 ft. high dam feasible.

Tehri Dam Design: Earth and rock fill type dam for Tehri Dam, is the safest man – made structure in earthquake prone zones. The S-shaped valley, along which the Tehri Dam is located, provides additional stability and safety against seismic effects. Tehri Dam design further includes several defensive features to ensure its safety under severe earthquake, which can occur in the area.

 

 

Consideration Of Safety Aspects by High Level
Committee: The Government of India in 1990 constituted a high-level committee, under the Chairmanship of the Director General. Geological Survey of India, with Members from Central Water Commission, Director, National Geophysical Research Institute, Head of the Earthquake Engineering Department, University of Roorkee and Dr.V.K.Gaur, an eminent Scientist and also the then Secretary (Ocean Development), Govt. of India, as its other members. This Committee after examining the matter in all its aspects came to a unanimous conclusion that even for a worst case scenario of assuming earthquake of magnitude +8 occurring at a depth of 15 km, right under the dam, the dam would be safe vis-a-vis the maximum earthquake potential. All dangers arising out of the seismicity have been taken note of and taken care of in the planning of the Tehri Dam project. The Committee also noted that there would be no additional threat posed by the reservoir induced seismicity to the dam and the civilian structures in the vicinity.

A rockfill dam, as the term implies, is built by packing in rock material to high density. Thus, unlike a concrete or masonry dam, it is not a rigid structure and it cannot be displaced as one single homogeneous mass. Rather, under the effect of the horizontal forces, like earthquake and the weight of the reservoir’s water body itself, there is only a relative displacement of the various layers of the dam. So, under such a shearing stress, as it is called, the dam adjusts itself, by loosening here or tightening there the compacted rubble.

Under a shearing stress, besides such settlement, the stability of slopes of the dam is another critical factor. As one layer tends to slip over the other, the slopes tend to become unstable and result in what is called the slip surface. Dam designers attempt to prevent such occurrences by making the slopes flat or gentle enough.

Data on the great dams in seismic zones shows that the Tehri dam has one of the gentlest slopes. In tests done by the Russion Experts the dam was first subjected to real destabilising forces as experienced during the Gazli earthquake and two ‘most unfavourable’ simulated ones.

They found that the factors of safety are much higher than the specifications in the Indian code. Even at worst, the experts found that there is an initial slip surface confined only to the upper surface of the slopes after which it stabilizes and the factor of safety increases. Indeed, the Soviet study points out that the seismotectonic situation, which produced very strong ground motion, is hardly comparable to Tehri where the ground motions are likely to much milder.

References

  1. Draft Final Report: Tehri Hydro Power Development Project by Dr. A.K.Dube [1996] http://www.rlarrdc.org.in/images/Tehri%20Dam%20Project%20Report.pdf
  2. http://www.thdc.co.in/content/tehri-dam-hppstage-i