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When groundwater flooded a stormwater storage tank under construction in Sheffield, it was time for a rethink. Bernadette Redfern reports.

Yorkshire Water knew that providing 13,175m 3 of stormwater storage in the centre of Sheffield City Centre was not going to be easy.

Access to the cramped site was difficult, and joint venture contractor EarthTech Morrison found unexpectedly high groundwater on the site. But despite these problems, the £7.5M project is still on time and coming in £300,000 under budget.

Extra stormwater storage is being installed in four locations in the city as part of a £35M project aimed at cutting combined sewer overfl ow discharges into the River Sheaf.

'We usually put projects greater than £2M out to tender but for this 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.' Sheffi d City 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.

Subcontractor Barhale is in the middle of installing a 10,000m 3 concrete storage tank, the main component of the scheme. Another 3,175m 3 of storage is being spread between three circular offline storage shafts further downstream.

The decision to build a tank rather than another shaft was taken after boreholes revealed 'massive blocks of hard sandstone underlying mudstone, and high artesian water pressures 7-8m down', says Earth ech Morrison project manager John Eaton.

'If we had gone for a shaft it would have had to be 30m deep, and we were told that we would need specialist breaking work to get past [the difficult ground]. It was too risky.' Ward adds: 'The alternative was to dig a shallower horizontal box. Usually in an urban location like this you wouldn't have the space.' Luckily, the council offered up the park.

EarthTech Morrison was by no means home and dry, however. 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, then start on the main walls and the 'dwarf walls', which act as separators within the tank. Ground anchors were cast into the walls for stability.

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 grouted, but groundwater 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 reprogramme and move the anchors so that the concrete team could carry on working, ' she says.

Simply moving the anchors was not enough to address the run away grout problem. 'We decided to sleeve the anchors, ' says Chaplin.

The steel bars were slipped into 'socks' which held the grout 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.

The base slab is reinforced using the Bamtec carpet reinforcement system. Bars come in prefi xed mats. 'It cuts down manual handling and fixing time. We got an amazing tonnage of steel laid every day that we would never have got with bars, ' Chaplin says.

The carpet was laid in two passes. Longitudinal bars were overlaid with lateral bars, creating a regular mesh, before 250mm long spacers were placed and the process repeated for the topside of the 400mm slab. Concrete is C35. 'It worked so well on the base that we did the roof with it too, ' says Chaplin.

Main works on the £7.2M scheme were due to finish at the end of this month.

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