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Giving it the full treatment

WATER - An integrated team and extensive use of prefabrication lie behind the rapid delivery of a new water treatment plant in Chingford. Phil Bishop reports

WHEN Thames Water identified a potential shortfall in its water treatment capabilities in 2002 it did not have much t ime to act.

Within th ree years it would be unable to meet demand. It quickly ruled out upgrading the capacity of existing facilities in favour of a new plant on a greenf ield site in Chingford , Essex. It was a small site ? just 1.5 ha ? but perfectly located, nestling between the Sir William Girling reservoir and the King George V reservoir on land already owned by the company.

By April 2005 the new Chingford South water treatment works was supplying clean water to half a million people in north-west London. At peak output, it supplies 58 megalitres of water per day (Mld), tak ing 40 Mld from the adjacent reservoirs and up to 18 Mld as required f rom fou r local boreholes. By Thames Water's standards it is not a huge plant ? some have a capacity of 600 to 1,200 Mld ? but it is still a significant size and is the first it has built since one in Walton-on-Thames in 1992.

Outline design of the new treatment works was completed in March 2003, planning approval was secured in November 2003 (it is on a designated site of special scientific interest) and const ruct ion began on site in Apr il 2004.

A 13-month construction programme was planned. It was completed in 12 months ? quite an achievement for a complex £42 million project.

This was possible largely down to two key factors: the integrated team approach, heavily involving the contractor in the design process; and the use of offsite prefabrication to compensate for lack of space. The latter was as much a factor as the need for speed.

The site is triangular and bordered on one side by the A110 Lea Valley Road and an existing pumping station, on another by the River Lea diversion channel, and on the third by the Sir William Girling reservoir.

Roughly 70 per cent of the site area was to be built on, and a further 20 per cent or so of the surrounding area dug up for installation of services. An additional constraint was the number of water mains under the site that had to be protected du r ing const ruct ion.

As much as 40 per cent by value of the structural/civil component was constructed offsite, including 1,000 tonnes of rebar and 450 cu m of precast concrete. Even site offices were kept off site, on the other side of the A110, connected by a purpose-built footbridge.

Thames Water undertook the outline design with one of its framework consultants, MWH, then in March 2003 brought on board main contractor MJ Gleeson and process contractor Purac to work up detailed design. Other members of the supply chain were also brought on board. These companies were already among Thames Water's prequalified Asset Management Programme partners so there was no need to go out to tender. Th is saved four or five months, says project manager Duncan Stewart.

A value engineering process involving constructability reviews and critical path analysis identified that construction could be made more efficient if the location of the last two of the seven treatment processes was changed.

The eight granular activated carbon tanks and the massive 3,000 cu m contact tank were moved at the suggestion of Gleeson in such a way that the main treatment works building would not need to be as big as or iginally planned. This also reduced the amount of excavation required.

Another outcome of the value engineering process was the decision to use prefabricated steel tanks for the main process units instead of cast in situ concrete. Steel tanks are more expensive and have a shorter design life (25 years instead of 60) but save considerably on time. The 14 of them ar r ived on the back of lorries over the cou rse of fou r weekends and were craned into place. The tanks were designed to be as big as possible yet still be movable by lorry. Each weighs 25 tonnes and is 4.5 m wide.

Mr Stewart estimates that using steel tanks instead of concrete saved as much as six months and also solved the problem of lack of space.

Coming up with these ideas, though, was just the start of more work identifying the knock-on effects to other parts of the project, including the water treatment process, and addressing these.

As well as the 70 m by 40 m main process building, four smaller buildings had to be constructed, one each for the low lift pumping station (at the start of the process), the ozone generator and contactor, the high lift pumping station/electrical incomer, and the Lamella plant.

These buildings use precast concrete panels measuring 6 m by 3 m for the lower part of the walls, with steel cladding for the upper sections and roof. This quickly gave a weatherproof envelope for the process contractors to work within. Wet trades were only required for blockwork facings after process installations were completed.

On the main building the prefabricated concrete panels came with the blockwork already attached. It was an extra cost, but it saved time.

Prefabricated concrete panels were also used for the roof of the contact tank, measuring approximately 40 m by 20 m. That saved time decking the whole area with formwork, says Bryan Hiscock, Gleeson's site agent.

'By hitting it hard at the beginning, ' says Paul Lloyd-Henry, Gleeson's cont racts manager, a five-week lead on the programme was quickly established. 'We worked hard to keep it, for fear of losing it later on.' Within just the first six months, 335 tonnes of structural steelwork, 1,550 tones of rebar and 9,000 cum of concrete were installed to get M&E work under way. In addition, some 15,000 cu m of spoil was removed.

Inevitably for a water treatment works, the process and M&E contract was a significant part of the total works. Gleeson's £30 million contract includes £8 million for Purac.

Although the Chingford South water treatment works has been supplying clean water to north Londoners for several months, Gleeson's work on site is not quite finished. Unlike a supermarket, which will not start serving its customers until every last detail is in place, Thames Water's priority was simply to get the water treatment process and supply under way. Since water began f lowing work has included adding pseudo-Alpine scree to the roof of one of the buildings to create a brown roof habitat, designed to attract the local black redstarts, a rare species. And there are still roadways and landscaping to complete.

The project team agree that although aspects of the project added front-end cost, there was no overall cost increase to the project by accelerating the work. What was spent on the steel tanks, for example, was saved by completing the project sooner and spending less t ime on site.

Asked about lessons learned for future Thames Water projects, Mr Stewart says: 'It's all sitespecific but there are elements here we can cut and paste. It has raised the profile of the amount of prefabrication you can incorporate into a works.' 'What we've achieved here is phenomenal, ' says Mr Lloyd-Henry. 'I'm not convinced you could repeat it every time, but it does demonstrate what is possible.'

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