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Going down a storm

A rethink of the construction methods being used to build a stormwater storage tank in Sheffield was needed when groundwater flooded the site, discovers Bernadette Redfern.

Yorkshire Water knew that providing 13,175m 3 of stormwater storage in Sheffield City Centre was not going to be easy. Access to the cramped site was difficult, and joint venture contractor EarthTechMorrison found unexpectedly high ground water flooding the site.

Despite the setbacks the £7.5M project is still on time and £300,000 under budget.

Extra stormwater storage is being installed at four different locations within the city and will cut combined sewer overflow discharges into the River Sheaf.

'We usually put projects greater than £2M out to tender but for this project we decided to stick with our framework contractor, ' says Yorkshire Water solutions manager, Peter Ward.

'It was a leap of faith but it has paid off.' The Council felt comfortable enough with the relationship to allow Yorkshire Water to dig up 2,100m 2 of the city's finest green area, Millhouses Park. The hole will contain a 10,000m 3 concrete storage tank and subcontractor Barhale is midway through the installation. Another 3,175m 3 of storage is being spread between three circular offline storage shafts downstream of the tank.

The decision to construct a tank rather than another shaft was taken after boreholes revealed 'massive blocks of hard sandstone underlying mudstone, and high artesian water pressures 7m-8m down', says EarthTech Morrison project manager John Eaton. 'If we had gone for a shaft it would have had to be 30m deep, and were told that we would need specialist breaking work to get past it. It was too risky.' 'The alternative was to dig a shallower horizontal box. Usually in an urban location like this you wouldn't have the space, ' says Ward. Luckily the council offered up the park.

EarthTech Morrison was by no means home and dry, though. As the excavation neared its deepest point, water began to flood the box. 'The lads on site told us that they had never seen anything like it in 25 years, ' Eaton says. The aim was to dig the box, pour a concrete base, and then start on the main box walls and the 'dwarf walls', which act as separators within the tank. Ground anchors were installed into the walls, keeping the tank floor clear and smooth.

But things did not go quite as planned.

'We were using 6m long Macalloy bars to anchor the tank in place. Once these were placed, the 80mm [diameter bored] holes were filled in with grout, but ground water was washing the grout away, ' says site supervisor Alice Chaplin.

'The concrete contractor was following the ground anchor people, so delays installing the anchors were throwing the concreting programme out. We had to re-programme and move the anchors so the concrete team could carry on working, ' she says.

Simply moving the anchors was not enough to address the runaway grout problem, though.

The steel bars were slipped into socks which both held the grout fast and expanded hard against the boreholes as the grout was pumped in. 'It was an 80mm hole but they were 100mm socks, so they really locked in to the jagged edges of the mudstone, ' Chaplin explains.

Where sleeved anchors failed under test loading 4m long stainless steel rock bolts were installed and grouted into place.

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