Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

mountains moving The Himalayas are being bolted and glued together to enable construction of a giant hydroelectric scheme on the Satluj River, northern India. Andrew Mylius reports.

Apples in their hundreds and thousands, truckloads of them, are exported from mountainous Himachel Pradesh down winding, rutted Himalayan roads. Apples are the north Indian state's economic sap; the only commodity that can be produced, large scale, in its inhospitable environment.

Himachel Pradesh is one of India's poorest regions. Tortuous terrain and extreme weather make it impossible to develop a competitive, manufacturing- based economy.

But these same conditions are now being tamed to generate wealth for the state. Coursing through ravines and steep valleys, water is being harnessed to generate electricity.

The 1,500MW Nathpa-Jhakri hydroelectric project is, at 1.5bn, the state's largest energy scheme to date. Development and running of the plant is being handled by the Nathpa-Jhakri Power Corporation, a special company joint-owned by the

central Indian and Himachel Pradeshi governments. The project's largest lender, the World Bank expects to see a 12% economic return in its investment.

Work started in 1992 and is scheduled for completion in 2001. However, the project has been beset by huge challenges; it is 10% over budget and 30 months behind schedule. World Bank supervisor Anthony Sparkes explains: 'The Himalayas are very young mountains; the rock is extremely unreliable.'

Contractors have had to contend with collapsing valley walls, constant danger from rockfalls in the scheme's 31.4km tunnel, and landslides that can block supply to the remote site for days. Nick Smortchevsky, assistant project manager for dam and desilting chamber works, being carried out by Continental-Foundation JV, sums up the impact of the environment on progress: 'You have to allow for slower productivity, slower response times, and for difficult logistics.'

Construction has been divided into three contracts, each undertaken by an Indian-international joint venture. Indian firm Continental with Foundation of Canada has completed a tunnel diverting the river's 350.m3/s flow around the dam and intake site. The JV is now building a 60.5m high concrete gravity dam and four cathedral-like desilting chambers.

Contract two, drilling and blasting of the main tunnel and a 301m surge shaft, is carried out by Hindustan Construction Corporation, India and Impreglio, Italy. And construction of cavernous turbine and transformer halls at the foot of the project, contract three, is being undertaken by Indian Jaiprakash Industries with Korean firm Hyundai.

All three contracts have experienced geotechnical difficulties. The first setback occurred in 1992 at the head of the valley when 1.2M.m3 of deeply fractured gneiss and augneiss rock - crashed from the right hand bank into the river. Continental-Foundation was mobilising to start work on the diversion tunnel, and found the proposed site a mass of rubble. Redesigning and building the diversion, which at 730m is 320m longer than that originally planned, cost 17 months. The unexpected collapse also alerted the JV to the potential for further falls.

A survey showed a 20m deep area of 'chronic weakness' in the left bank rock face, directly above the dam site. According to NJPC supervising engineer for the dam works Subhash Mahajan, two options were available: remove the weak zone or stabilise it.

'There was a danger that removal of the wall would cause the whole hillside to collapse,' says Mahajan. 'If so, the dam would be rendered unbuildable.' Stabilisation was the only option if the scheme was to be realised.

Calculations showed the unstable area would need to be tied to at least 12m depth of underlying solid rock. An extra 10m safety margin was designed in and, with Indian specialist Usha Ismal, Continental-Foundation embarked on an uncontracted programme of cable anchor installation.

Fixing of 468 anchors is now nearly complete. Each 42m long, 150mm diameter cable is made up of 12 seven strand high tensile steel ropes from the Indian Steel Authority. After epoxy bonding the anchors are post-tensioned to 200t, clamping a wall of reinforced concrete to the rock face. Meanwhile, on the opposite bank at the site of 1992's rockfall, smaller-scale anchoring and shotcreting has been carried out. According to Mahajan, anchoring is unusual in India, and has never been as used so extensively.

Unforeseen work has not been limited to the dam site. Some 78 shear zones, from 0.1m to 2m wide have been found along the length of the project. In the desilting chambers Continental-Foundation has so far encountered three major shear zones.

The four chambers are already colossal - and only a third of their volume has been excavated. Egg-shaped in section, each is 525m long, 16.3m wide and will be 27.5m deep. Project manager for the desilting chambers John Barry says that care is called for as drilling and blasting goes ahead, in parallel, in all four chambers.

'You can't carry out full-face excavation because the risk of collapse is too high,' he observes. 'Instead you have to drill a number of pilot holes, remove the loose material and expand by degrees.' As material is removed small shears are stitched. However, larger faults are anchored with 7.5m rock bolts in the walls and 6m bolts in the crown. Distribution is 1-2m2.

Barry says that, because of their huge surface area 'casting the chambers in concrete would be challenging, if not impossible'. To produce a strong but cost effective finish, Continental Foundation is using between 0.5m-0.6m thick steel fibre reinforced shotcrete from Dramix, Belgium. Not commonly used in hydro projects, research showed that the steel fibres deliver tensile strength that will enable the lining to withstand pressure changes within the chambers and from the surrounding ground.

In the main 10.15m diameter tunnel, meanwhile, shears and fractured rock conditions were complicated by ingress of up to 100 litres/second 66degreesC water. Drainage holes, 24m deep, were drilled in advance of excavation, while 20 dumpers carried ice to cool workers at the face.

Weak strata have been excavated and filled with concrete. Roof support has been provided by driving 12m fore-poles ahead of the face. NJPC executive engineer Virender Pathak says the 114mm diameter poles are spaced at 0.5m around the tunnel's circumference and grouted in position. Where conditions have been most acute, in a roughly 3km length of tunnel, work advanced at only 0.8m/day.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.