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Digging under Delhi


Construction of India's first metro project in Calcutta took 22 years.

The second, Delhi metro, should be finished in six. Damian Arnold reports from the subcontinent.

Looking down main street of Delhi's bazaar can be a giddy experience as a sea of faces moves towards you in the blazing sun.

Twenty metres below the crowds, tunnel boring machines will soon be chewing their way through challenging ground conditions as metro construction reaches the heart of the Indian capital.

Tunnelling below the old 'Walled City' is one of the main technical challenges thrown at the 90 strong design team, marshalled by a core of 10 experienced tunnel engineers from UK based consultant Mott MacDonald.

As joint venture design and build contractor International Metro Civil Contractors begins work on the southern half of Line 1 of the three line network, its designer Mott MacDonald is racing to meet a deadline just six months away. The team of engineers is working late into the evening six days a week to complete the detailed design of 6.6km of tunnels and six station boxes which will be a mixture of cut and cover, hard and soft bore and New Austrian Tunnelling Method (NATM).

'It's a punishing schedule and a tough learning curve for the local engineers, ' says Mott project director Derek Winsor whose team is largely made up of engineers from Indian consultant Dalal, purchased by Mott last year.

'Most of them have never been involved in a metro project before but they are fast building up relevant experience, ' he adds.

Toughest task has been finding a design solution for Chawri Bazaar station which is located directly under the city's main bazaar. With the help of the soils engineering laboratory at Mott's headquarters in Croydon on the outskirts of London, the team has designed a station box which will be excavated through random areas of hard rock and soft alluvium.

Construction will take place directly underneath historic buildings up to 500 years old, some of them of questionable structural integrity. And the entire operation has to take place without disrupting Delhi's thronged market streets.

'There are some very sensitive buildings, ' says Winsor, 'but they will all be monitored and, where necessary, preventative measures undertaken, involving ground treatment, underpinning and internal strengthening.'

A tall order then, but one accepted by Mott, which has chosen to excavate the station box using NATM to minimise disruption overhead. The established if sometimes controversial observational method will be used to select appropriate solutions for stabilising the box.

Site investigation has shown a top strata of shallow alluvium above 3.5bn year old quartz rock inter layered with softer decomposed rock.

'This is typical of the ground conditions in the northern half of the route, ' says Winsor. 'Ground support will be a combination of temporary diaphragm walls within the alluvium and rock bolts lower down.'

At the southern end of the project the alluvium is deeper and diaphragm walls will extend below the level of the station base slab. 'There the diaphragm walls also form a permanent integral part of station construction. This is typical for Patel Chowk and Connaught Place stations, ' says Winsor.

Uncertain ground conditions can spell delays and cost overruns, as on the US mega Boston Artery project in Massachusetts where costs have ballooned out of control to over US-14bn.

But client Delhi Metro Rail Corporation (DMRC), along with its project management joint venture of Parsons Brinckerhoff and Japanese firm Pacific Consultants International, has built a reputation of being 'very demanding'.

It expects fast delivery from international firms, after the debacle of the subcontinent's first metro in Calcutta which used domestic contractors and consultants. The result was a 'very low tech' project, beset by union problems which saw workers down tools in the late 1970s for 10 years, stretching the project completion to 1996.

Diaphragm wall construction for the cut and cover tunnelling started in November 2001 in preparation for the main construction phase to begin this May. Three tunnel boring machines - two earth pressure balance machines (EPBMs) and one rock machine, are being brought in from as far afield as Thailand and Germany.

While the Boston project has become a lawyer's paradise as disgruntled residents claim for millions of dollars in compensation, Delhi's judiciary has been very sympathetic to the scheme.

'Many projects have been held up because of land acquisition court cases, ' said DMRC chief project manager Mangu Singh. But with the city's 14M population literally choking in smog and only 1% of the 11.7M transit trips a day made by rail, 'the courts have been very sympathetic. They seem to have made a clear decision not to stand in our way.'

Line out of congestion The plan for the Delhi metro stemmed from a government transport study of the city completed in 1990.

The client's project managers Parsons Brinckerhoff/Pacific International Consultants completed outline designs for Line 1 before a design and build tender was awarded last year. The contract, estimated to be worth -1bn, was split into two sections. The northern half, MC1A, is a joint venture including UK consultant Maunsell and contractor Skanska. The southern MC1B was awarded to the construction consortium International Metro Civil Contractors, led by Dyckerhoff & Widmann of Germany, with Shimizu Corporation of Japan, Samsung of Korea and local firms Larsen & Toubro and Ircon International. Mott MacDonald was appointed to advise on design of the 6.6km of tunnels in March 2001.

Lines 2 and 3 of Delhi Metro are expected to run overground to complete the 62.5km network by the end of 2005. A further 33.5km will then be commissioned.

A soft loan from the Japanese Bank of International Co-operation will cover an estimated 55% of the -2.25bn cost, the rest coming from government sources.

Once complete the project is expected to cut the number of buses in Delhi by 2,600 and increase the speed of the rest from 10.5km/h to 14km/h, saving 2M man hours a day in reduced journey times. Some -100M a year should be saved on fuel, with pollution cut by as much as 50%.

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