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Plumbing in Bombay

Cleaning up its act is a slow process for Bombay, but the giant sewage outfall project now under construction shows better living and environmental conditions can be achieved. Andrew Mylius reports.

Swim at your peril! Bombay's beaches are washed by slick, black slurry - the stinking efflux of the city's sewers.

Officially Bombay has 13.5M inhabitants but with immigrants flocking from rural areas to India's fastest growing metropolis, estimates put the figure higher. About 60% of human waste is not even collected: people defecate where they can - in fields bordering the city's northern suburbs, on the beach, and in the unloved spaces where squatter communities thrive.

Waste produced by Bombay's large middle class, meanwhile, enters an overwhelmed sewerage system. Vast volumes are untreated and run straight into the creeks that indent Bombay's peninsular coast.

The mucky shoreline is inhospitable, but there are wider environmental concerns: With little tidal flow to disperse the sewage, coastal waters have become oxygen starved and fisheries have been depleted. Pollution has impacted on people's livelihoods and decimated local wildlife.

And this is why a $296M outfall project now under construction cannot come too soon. Effluent from the largest two of Bombay's seven collection zones, Worli and Bandra, is to be piped offshore, 3.4km and 3.7km respectively, where deeper water and stronger currents will disperse it. With wider diffusion and better oxygen availability, organic breakdown and neutralisation can take place.

The outfalls project has a long and chequered history. Construction work on a massive land-based influent pumping station (IPS), carried out by local contractor Mohinda Singh and by joint venture Spie Batignolles/Capag for the marine outfalls, went ahead in 1980. But the cut and cover outfall pipeline designed by a US consultant proved desperately hard to build.

At the same time, according to additional municipal commissioner Rahul Asthana of client Brihanmumbai Municipal Corporation, poor project supervision by the Corporation itself resulted in lack of co-ordination between civil and mechanical contractors. Quality suffered and costs soared.

Badly behind schedule and way over budget, work both onshore and offshore ground to an acrimonious halt in 1987.

Total abandonment of the scheme was not an option available to the Corporation and its principal lender the World Bank. Too much money had already been committed and sanitation in Bombay was potentially a political hot potato. UK consultant Binnie & Partners (Binnie Black & Veatch) had drawn up a water supply and sewerage masterplan for Bombay in 1971, and was re-appointed with Indian firm Tata Consulting Engineers to review and rescue the project in 1988.

Land-based works have required major reappraisal, but less radical than the rethink of the outfalls. Cut and cover construction had proved difficult because of soft sea bottom conditions: the open trench was liable to fill with sediment. Heavy swell made sections of the 3.5m diameter concrete pipe difficult to place. And damage caused to the segments while joining them meant pipe laying was drastically slowed by retrieval and repair.

BBV/Tata calculated that, even offset against the cost of concrete elements already sunk, bored tunnels with a precast concrete segmental lining would be half the cost of any alternative method for constructing the outfalls. The solution was also technically attractive and, in terms of time, more predictable.

The Worli and Bandra tunnels are similar: they are gravity fed, each with a low lift pumping station at its head to create flow. At their far ends, 10 risers emerge on the seabed, each capped with a UFO-like concrete diffuser.

German contractor Dyckerhoff & Widman (Dywidag) has almost finished the first tunnel at Worli - it will be commissioned in May. Driving the second tunnel at Bandra started in September. Meanwhile Afcons, the marine works contractor, is now sinking risers for the Bandra outfall site.

On the outfalls, restarting from scratch was expedient, but the opportunity for a new approach was not fully appreciated until investigation of the abandoned land-based works got under way. At Worli a relatively modest pre-treatment plant and pumping station had been under construction and lent itself reasonably well to the new tunnelled outfall solution. Bandra works was a different story.

The collection point for the Bandra zone is towards the bottom of Mahim Bay, over 2km south of the tunnel head. The two sites - the influent and effluent pumping stations - are to be connected by a pressurised main. Four giant excavations - wet well, screen chamber and two pumping station shafts - had been blasted by Merhinda Singh and a substantial amount of the pressurised-main tunnel built. After construction stopped, the works filled with water. It was only after the works had been pumped out that BBV/Tata was able to see what had been done - and left undone.

BBV first identified a design problem: Engineering Science's design for the IPS placed the pumping stations above the level of the inflow tunnel. As a result the inflow tunnel, wet well and screen chamber would have been constantly full, allowing sediment to settle and consolidate. Over time the system would have clogged up.

Consequently, BBV decided to dispense with the screen chamber and lower the wet well and pumping shafts by 12m to a level deeper than that of the inflow. At the same time the 30 second capacity of the wet well was increased by excavating an extra 12m diameter, allowing the system to cope more effectively with fluctuations in flow.

Flooding of the works had opened joints in the tuff and basalt rock, raising concerns about its stability. Fear of a collapse was most acute when new contractor Hindustan Construction Corporation came to widening the two pump shafts. BBV resident engineer for the IPS Neil Adams says 'land in Bombay is like gold dust', and as a result the shafts are closely grouped on the compact site. There was just 7m between the pump shafts; excavation reduced the 'narrow neck' to 3m.

To guard against collapse HCC built a large reinforced concrete beam across the narrow neck and tied the wall below into it. Nonetheless, blasting was a delicate operation and progress was slow. HCC was further hampered when it came to breaking out a massively reinforced concrete raft placed by Merhinda Singh at the base of the wet well. Work at the IPS is 18 months behind schedule, and is unlikely, says Adams, to be complete before June 2001.

Work could be set back still further, though. Investigation of the tunnel system that collects and delivers sewage to the IPS suggests existing work requires extensive repair, if not replacement. The more than 2.5km cast insitu, horseshoe-shaped concrete tunnel shows dramatic variation in wall thickness - from 25mm to 300mm - and inconsistent concrete mixes. Backfilling on 600m 1.6m diameter precast pipe and 350m 1.2m diameter pipe was hit and miss. Sample cores show huge cavities.

Asthana says rehabilitation of the existing tunnels could cost anywhere between $7.75M and $16.7M; replacement would be $16M. To determine which strategy it will pursue, fixed price prequalification tenders have been invited for repair and replacement. BBV and HCC have meanwhile been filling the gaps left by Merhinda Singh in the pressurised main that crosses Mahim creek and circumnavigates Mahim Bay.

Adams says: 'In theory this was an element of work that should have been completed. Straight sections of the 3m diameter steel lined pipe were laid. But there were gaps at bends, there were crossings like storm drains to negotiate, and there was no connection at either end.' Unplaced steel sections were left where they lay when Merhinda Singh was pulled off site, and in the intervening decade they have been aggressively corroded. Up to half the 20mm wall thickness has been lost.

Adams comments: 'It has not rendered the elements useless,' - a concrete lining will be applied - 'but it is down to minimum thickness. That's absurd on a new pipeline and should never have happened.'

Last year a masonry cofferdam was built, a trench dug and the northern half of the creek crossing was laid. This year, the monsoon over, the same operation will be carried out to lay the southern side and join the two halves. He says approximately 50% work remains to be done.

With 60% funding from the World Bank, emphasis is being placed on quality control. The project is being built with an 80 year design life and will be capable of handling larger volumes of sewage as more of Bombay's citizens are plumbed into the system.

Meanwhile, though 30 years will have elapsed between drafting of the sewerage masterplan and operation of the Bandra outfall in 2001, the project is now setting standards for India. Water supply and sewerage are growing markets and it is likely there will be increased private sector participation in future construction and operation.

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