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Earth exchange

Wastewater; Cardiff East treatment works

Transforming a major eyesore into a new gateway to Cardiff was not Hyder's top priority when it started looking for sites for a major new wastewater treatment works. Hyder project director Jeff Chandler freely admits a greenfield site would have been preferable.

'But the local authority made it clear it would only support one possible site, an area of reclaimed land on the shore of the Bristol Channel. This had been created by the tipping of slag from the local steelworks - and lots of other waste as well.'

Ground conditions, inevitably, were horrendous (see box). Above ground was little better. In recent years the steelworks has recycled thousands of crushed cars, and the non-metallic fragments - seats, carpets, plastic trim - were also dumped on the site. As plans for the new waterfront Eastern Bay link road between Cardiff and the M4 took shape, it became obvious something had to be done, otherwise westbound motorists' first sight of Cardiff would be the smoking, stinking 'fragmentation tip'.

Part of the £180M Cardiff sewerage and wastewater treatment scheme, the works was designed to treat a future population of nearly 900,000. Almost 10km of new interceptor sewer will feed into the plant, and a 2.2km outfall will take treated water out to sea (NCE 5 February 1998). Two sets of sequential batch reactors will operate on four hour cycles, with sludge dried and disposed of off site.

Once committed to the unpromising site, Hyder was faced with two distinct but linked problems. First, large structures had to be founded on what turned out to be the most unpredictable area of slag fill. Any solution would also have to include answers to the environmental problems, especially the fragmentation tip.

Even if Hyder had been prepared to consider a piled solution, the near certainty of long term slag expansion meant piles would need to be sleeved where they passed through the made ground and significant voids left under structures and services.

Removing a weight of slag equal to the weight of the structures - the buoyant raft solution - was the other option. The drawback was that only around 5m of slag would need to be removed to achieve balance, leaving 5m of poorly compacted, unpredictably expansive material below the structures.

'The obvious answer was to remove all 10m of existing fill and import 4m to 5m of high quality granular material to make up the levels,' says Martin.

'But where do you get rid of 1M.m3 of contaminated slag - and how do you bring in 400,000m3 of fill without causing disruption on local roads?'

Paradoxically, the answer to the second question was, more slag. Good quality crushed blastfurnace slag from Llanwern steelworks was brought in by rail to a railhead just 800m away from site. 'We're very proud we found the slag, which is stable and inert,' comments Martin.

'The flue dust had to go off site, all 60,000m3, mostly to a licensed tip in Swindon. And we brought in 30,000m3 of rubble from the old Cardiff Arms Park, but overall the extra traffic on the roads was kept to a minimum.'

Working closely with the Environment Agency and local authorities Hyder drew up a plan to use the excavated slag to cap and landscape the 'frag tip'.

A vented HDPE membrane will cover the entire 12ha of frag tip by the time landscaping works finish in spring 2001. With the line for the planned link road virtually on a level with the treatment works a lot of attention was paid to the appearance of the main structures, even staging an architectural competition. And as a final ironic touch, long term durability of the huge sequential batch reactor tanks and other concrete structures is ensured by replacing 70% of the cement in the mix - with ground granulated blastfurnace slag.

The Cardiff Wastewater Partnership

Client: Dwr Cymru Welsh Water

Engineering design andproject management: Hyder Consulting

Architect: Stide Treglown Davies

Main contractor, treatment works: John Laing

Main contractor, tunnels: Byzak

Main contractor, sea outfall: Van Oord ACZ

Mechanical and process control equipment: Alstom

Open cut contractor: David Lewis

Strata strategy

A new 10km, £36M tunnelled interceptor sewer will feed up to 6m3 of wastewater into the new Cardiff East treatment works every second. Hyder principal engineer Dr Richard Coulton says two key decisions had to be taken at the start of the planning process.

'The first was which strata to tunnel through - the mudstone and sandstone bedrock or the overlying alluvium. Eventually we decided to go for the stronger, more predictable bedrock. Then we had to decide on exactly how much cover we needed under the River Rhymney.'

Unforeseen buried channels were the biggest risk here. A thorough site investigation convinced the project team that cover could be reduced to between two and three tunnel diameters, which meant the average depth of the entire tunnel could be 3.5m less than originally planned.

This produced significant savings, not just on construction - there are four major connections and 20 shafts up to 6.5m diameter - but on operating costs as well. On the 7km of smaller, 1.65m and 1.95m diameter sewer construction was by pipe jacking slurry machine, but for the final 2.5km of 2.4m diameter tunnel contractor Byzak brought in an earth pressure balance TBM.

Problems arose on the last 200m, where the tunnel is up to 50m below the slag tip. A higher strength design of tunnel segment was needed, which could have added substantially to project costs. Again, the partnering approach came up trumps. A special higher strength segment was devised, which could be cast in precaster Charcon's existing moulds at little extra cost.

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