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AGAINST THE CLOCK

UNDERGROUND CONSTRUCTION - Construction of phase one of the Delhi metro is going all out to meet its scheduled completion date in September 2005. Bernadette Redfern reports.

In the chaotic streets of Delhi, people, rickshaws, cars, cows, bicycles and construction traffic all fight for space, horns blaring and bells ringing. The sounds of concrete pumps, excavators, trucks, TBMS and ventilation fans whirring 24 hours a day for construction of the Delhi metro are hardly audible above the din of everyday life.

Delhi's masterplan is to build 244km of track around the city. Delhi Metro Rail Corporation (DMRC) was formed in May 1995 by the Delhi state and Indian central governments.

The first phase, costing £1.4bn, comprises 68.3km of line and 59 stations. Design contracts were awarded in 1998 to a consortium led by Japanese consultancy Pacific Consultants International. Eight main construction contracts have been awarded and to date 170 contractors and suppliers have been involved in the project.

But with a year to go to scheduled completion in September 2005, work on this phase is not yet halfway. Line one, 28km of elevated track from the west into central Delhi, finished in early 2002, but work has not yet started on the 29.3km of the elevated line three, on which construction is estimated at 20 months.

Efforts are concentrated on the 11km twin tunnelled section running north to south through central Delhi, where difficult ground conditions are hampering progress.

Three TBMs, cut and cover techniques and NATM are being used simultaneously in a frantic effort to ensure that all tunnels and stations are complete by June next year. On every site a clock counting down the days is displayed prominently.

Progress with the TBMs has been disappointing and cutting through hard quartzite rock has been much slower than expected.

'In soil [using an earth pressure balance TBM] progress has been very good, completing a maximum length of 30m, which is 25 rings, in 24 hours, ' says DMRC chief engineer Kamal Nayan. 'But in rock we are not doing so well, digging only 1m or 2m per day.

'The rock is highly fractured and full of softer schist which makes the face uneven. The machine is veering too much and hitting the rock too hard and we are constantly having to change the disc cutters, ' he says.

This has been a particular problem on the tunnel linking Delhi Main Station to Chawri Bazar. Conditions were so bad the TBM moving south from Delhi Main had to stop. Tunnels are instead being excavated using NATM, dug by hand northward from Chawri Bazar.

The tunnels are 10m wide and 9m high and to dig them manually requires a team of 100 men. However labour is cheap in India and is supplied on demand to the major contractors by agencies.

Excavation is done in three phases, starting with the 5m high crown section, in 1m drifts.

'Immediately after excavation the face is stabilised with primary support in the form of shotcrete sprayed over wire mesh and steel girders, ' Nayan says.

Another 2m thick slice is then dug out and support placed, before the final 2m to invert level is excavated. 'The three phases ensure minimal damage to the surface structures as well as the safety of people involved in the construction, ' says Nayan.

Delhi has many ramshackle buildings up to 150 years old with shallow or no foundations. A 50mm tolerance has been allowed for any deformation and an intensive monitoring operation ensures nothing falls down unless it is supposed to. 'We have carried out several controlled collapses and had to evacuate a few buildings, ' says Dayal.

With Chennai, Hyderabad, Bangalore and Headband waiting to see how Delhi fares before committing to their own systems, DMRC is keen to avoid the negative publicity directed at the last metro built in Calcutta (Kolkatta).

'Kolkatta metro was only 16km long but took 25 years to build and cost 12 times the estimate, ' says Dayal.'The whole city became fed up with it.We cannot let this happen in Delhi.'

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