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Hidden depths

Water and drainage - Early contractor involvement has helped construction of Scotland's £120M Milngavie water treatment plant. Adrian Greeman reports.

It is not often you have to sink a brand new five storey building underground, but that is exactly what contractor MJ Gleeson is doing on the new Milngavie water treatment plant in Glasgow.

Located just west of the city, the water treatment building is the central element of Scottish Water's Loch Katrine water supply upgrade project.

The northern end of the project is located at the southern tip of the Trossachs National Park. This scenic rolling landscape is an environmentally sensitive recreational area.

For this reason most of the other structures being built at the site will go below ground too, including two huge new 80Ml concrete water storage tanks over 40km of pipeline, plus a new inlet works and pumping station to bring water in from two existing Victorian reservoirs.

These reservoirs are fed via aqueduct from Loch Katrine some 30km to the north.

'It was an incredible piece of engineering, ' says MJ Gleeson senior contract manager Mark Allan admiringly 'with a fall of only one foot every mile (200mm/km) it was quite an achievement even today.' Little can be seen of the existing reservoirs other than a small 'strainer' building where the supply was simply dosed with disinfectant and passed into the city's distribution network.

This clean loch water is good enough to go with whisky and needed very little treatment.

But a new treatment works was needed to meet the demands of the European Drinking Water directive which seeks to eliminate bacteria like the parasite cryptosporidium.

Scottish Water chose Gleeson to build the new facility in two parts, working in partnership with the client to assess options.

'I think we got the work because of our ability to fit into that working ethos, ' says Allan 'and also because we brought with us a number of collaborators that we had worked with before.' These cluded hames Water for process engineering and MWH as design consultant.

The core team also drew in cost advice from EC Harris, the Scottish Water engineering department's technical information and environmental consultant ERM. It also sought additional advice sought from research body WRC.

Around 200 variations of design on some 17 possible sites were considered within the reservoir zone and none of them produced a complete set of ticks for all of the criteria.

But eventually the team decided that a pumping station and treatment works close to the reservoir was the best solution.

It was to be as invisible as could be managed and rather than spread the treatment functions over a wide area, they are concentrated into a single multi-level building.

'The idea was to keep the footprint as low as possible, ' says Allan.

What is more, the resulting complex stack of filter beds, holding tanks and flow paths would be mainly underground.

Tasteful stone cladding would finish the main building above ground. The two large processed water tanks for the supply buffer will be embedded into the landscape and banked over.

Despite the effort to reduce the scheme's impact it was rejected by a planning inspector in 2002. Among a host of reasons were local fears about the chlorine gas tanks that would have been on the site, obtrusiveness, and amenity.

Rather than go back to the drawing board, Scottish Water kept the core team together to revise the scheme. It switched from using chlorine gas as a disinfectant to sodium hypochlorite, negating the need for gas storage tanks.

And by playing with the geometry of the building, the team managed to reduce its footprint by another 20%.

Extensive underplanting of vegetation around the reservoirs and within the local forestry zone was also included to reduce visual impact.

In total, 12 filter tanks have been designed into the building with some complex recirculation required for the bed cleaning.

'One of the beds is always in washing which involves reverse flow of the water with solids then extracted from the wash water, which goes back into the system for refiltration. Some 14% to 16% of the water goes in the backflow' explains Allan.

Gleeson continued on phase one, working on a cost reimbursement basis until 2004 when the new scheme finally won planning approval. The extra time was highly useful, says Allan. It allowed the contractor to make substantial input to the design.

'It all helped save money - partly by buildability - but much more by looking very carefully at design stage to see what can be left out without compromising the scheme.

It is not done by finding a cheaper concrete supplier. If anything, ' he says, 'additional concrete is used in places to simplify design and win bigger savings through speed of construction.' Following this approval the client/contractor team devised and agreed a target cost construction programme falling into three main areas.

For the water inlets, there are two small 200m long tunnels, which together with a 27m deep intake shaft were subcontracted to AMCO Donelon. The 2.1m drives were hand dug, using temporary segments as support before a cast in situ lining was inserted 'It was as well they chose the hand method because the ground is a mix of quite hard rock, fractured areas and clays and a machine could have become stuck, ' says Allan.

Meanwhile large scale excavation was underway. This stage of the project nearly proved disastrous when heavy rains, washed out the site in the first winter. 'We found a thin layer of very fine clay was getting into the runoff and clouding it - it took nine days to settle in a jar test.' Rather than pollute local water supplies Allan had to flood the site, and set up a temporary water treatment works for the perimeter drainage, dosing the wash water with a coagulant.

He lost 11 weeks he says, although much of that has been recovered now by rescheduling critical paths and juggling with resources.

The giant cavities for the water tanks meanwhile have been the scene of 'production line' concreting, using a Peri formwork system of steel faced shutters for the walls and slabs.

'They (Peri) had an engineer on site who helped design that - it uses only four through-ties which speeds up work and gives a better finish'.

Columns followed, again using proprietary forms, six at a time and currently the first of the tanks is being back filled and landscaped. Allan hopes to finish the second clearwater tank by the filter building in time for back filling this year, completing all the earth moving by winter.

Main tasks then will be the M&E and fitting out.

'And we have a pipeline float out to do, ' he adds. 'That involves welding plastic pipe on shore and then sinking it with concrete weights.' That will provide the final connection between the treatment building and the supply network.

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