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MGF struts its stuff in the basement

PILING

Specialist ground support contractor MGF has had to employ almost its entire range of products to support a dif. cult excavation behind a Grade I listed magistrates' court in central London.

Dan Gilkes reports

DESPITE newspaper predictions of doom and gloom in the housing market, the demand for residential property in London continues to confound pundits.

The biggest problem is not attracting potential buyers but finding the land on which to develop the property in the first place. And because this is London, this usually involves redeveloping an existing building or starting from scratch on a brownfield site.

At Vincent Square near Victoria, Fairpoint Properties' latest project, the contract calls for a combination of both techniques. The street-facing buildings on Rochester Row are being retained and incorporated into a new build that will take place out of the public eye on Vincent Square to the rear.

The retained buildings are a Grade I listed magistrates' court and the former Rochester Row Police Station, the facades of which will form the northern face of the development of more than 50 luxu ry apar tments.

To the rear, the buildings facing into Vincent Square had to be demolished and the rubble cleared away before contractor J Reddington could start on the reinforced concrete frame of last January.

Reddington's biggest concern has been holding up, and not causing damage to, the fragile listed buildings while excavating to a depth of two storeys for a future residents' car park.

The basement area is between 23 m and 30 m wide, roughly 74 m long, and in places reaches a depth of 8 m, creat ing a total void of almost 15,000 cu m.

The f irm called in piling specialist Rock and Alluvium, which installed a contiguous piled wall along three sides of the site, including the Rochester Row side up against the listed buildings. This provides a retaining wall for the surrounding buildings and the land to the Vincent Square side. Concrete piles have also been driven into the ground at the base of the site, which will be tied in to the basement f loor slab and supporting columns within the car parking area.

To support the project through excavation and reconstruction Reddington called upon specialist ground retention firm MGF (Trench Construction Services) to come up with a strut bracing system that would provide the required rigidity.

The system has to support the two buildings and the busy road that runs just 1.5 m from the site boundary along the top of Vincent Square.

In addition a hotel adjacent to the site had obvious concerns regarding dust and noise.

MGF's in-house technical services department offers a full design and methodology service, with experienced civil engineers putting together bracing components with a mind to the practicalities of the installation on the actual site. As MGF fabricates its own components it can produce dedicated designs for each project rather than simply bolting together parts from existing stock.

For the site at Vincent Square, MGF opted for multilevel propping, with modular 120 Te hydraulic cross struts and 100 Te mechanical struts spanning up to 23 m across the void. These struts are bolted to the concrete capping beam that sits atop the contiguous piled wall, forming a solid structure for Reddington to work within.

The struts, which can handle loads of up to 120 tonnes each in the case of the hydraulic braces, are suppor ted by 54 Te raking struts and knee braces at the lower level. The spans of the props were among the largest undertaken by the firm, calling for the fabrication of extended components.

One problem that had to be overcome involved the level of the capping beam, which is 1,200 mm higher on the Rochester Row side of the site than on the Vincent Square side. As a result the cross struts have to sit at a slight angle, requiring special adaptors, and are therefore longer than the 23 m span.

MGF has built in a considerable safety buffer, so that in the unlikely event of a strut failing or being hit by a machine on the site, the remaining components could easily take the weight.

'A site usually has raking shores, props, struts or braces, ' says MGF's technical director Steve Hesketh.

'But this project uses almost every product we've got.' The contract called for a lot of engineering work prior to installation and MGF has been creative in its approach and use of components. The 120tonne hydraulic struts have been specifically developed for the site and the firm is using its super tank brace as a waler in places to spread the load across the contiguous piled wall.

Mick McManamon, Reddington's site foreman, was particularly impressed with the ease of installation of the system.

He says: 'The way they're manufactured makes installation foolproof. Anyone can install them.' With the piled wall in place, Reddington excavated down to a depth of 3 m using two 20-tonne excavators before installing the main strut system. The company then used 6-tonne and 13-tonne machines to work among the struts to dig down a further 3 to 5 m.

As with so many u rban developments, par t icularly in busy cit ies like London, space is very much at a premium. Any materials needed for the job have to be stored on the site. In addition there are now two tower cranes within the contiguous wall, sitting between the supporting cross struts, so there is very little extra space available beneath the support system.

This restriction on space also resulted in MGF having to adhere to very tight delivery schedules, arriving outside the site's hoardings at an allotted time for the bracing components to be lifted off the trucks and into place. This was not too much of a problem, however, as MGF has its own haulage f leet.

John Reddington praises MGF's flexibility.

'You couldn't get bet ter blokes to work with, ' he says.

As work progresses on the site Redding ton will construct a two-way radial ramp at one end of the basement for cars to gain ent ry and exit from the car park. The ground f loor will be part of the reinforced concrete frame, but the lower ground f loor will be built using a post-tensioned concrete slab to reduce the f loor depth and increase headroom in the underground areas.

The company will construct the five-storey reinforced concrete frame before handing over to Morrison Construction for the remainder of the project in February 2006. Morrison will then have a further year to complete the project for Fairpoint.