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TRIPLE WHAMMY

A waterside housing development in Yorkshire requires complex logistics to pile for three types of bridges, often in difficult ground.Damon Sch³nmann reports.

Clarence Docks in Leeds is the target of a serious makeover.

The latest development, by Crosby Homes, is taking shape next to the Royal Armouries Museum, built in 1996, which houses the UK's national collection of arms and armour. The Crosby scheme will create housing blocks and a leisure complex that should help regenerate the area.

The docks lie on a waterway junction fed by the River Aire, so main contractor C Spencer is building three bridges under a £1.5M contract. Designed by Bennett Associates, the bridges will be mainly used by pedestrians but will also take 3t maintenance vehicles over foundations designed by the contractor.

Two are bascule bridges: a 16m single lift and a 19m double lift. The longest of the three is an 80m fixed bridge and here Van Elle, which won the £37,000 piling contract, is installing minipiles for a series of abutments and a pier.

Spencer will build the fixed bridge out of fi ve deck sections on the long, narrow island in the centre of the river. It will be pivoted so that workers can turn it into place through about 90¦.

Spencer site agent Dave Cooper says: 'The diffi cult aspect, from our point of view, is putting bridges in a development that's mostly complete in the areas we are working. That's dictated our methodology of building the fixed bridge on the island and spinning it into position.' On the day of GE's visit, Van Elle was installing piles on the island with equipment that had to be brought across the river on pontoons, because the area is outside the working radius of the site crane on the west bank.

The pontoons, which can be flooded to change buoyancy and working platform levels, will also be used to transport the double lift bridge into position. Another advantage is that being sectional they can be built to varying sizes to suit differing cargoes.

Van Elle business development manager Philip Woodcock says: 'The interesting thing is the three types of bridge. The piles have different loads as the bridges are opening and shutting so they will have to resist uplift as well as static vertical loads.' The 300mm grouted sectional auger mini piles are designed to take 325kN compression loads, 100kN in tension and 19kN laterally. They will have six T16 bars to 6m depth and are designed with a safety factor of three.

Although it is not the most technically challenging of piling jobs, Woodcock says: 'The piles are founding on sandstone through sands and gravels which is pretty easy but the rig needed to be small because of the access problems on the island. Sometimes the engineers look at what machines they can get on site and then work out what piles they need.' Although the ground is pretty well known, conditions are not always exactly as expected. Cooper says: 'We were anticipating 8m long piles but we've managed to get them in at 6m as the [site investigation] boreholes weren't exactly where we are piling.' Of course unexpected ground can be a double-edged sword and work was briefl y held up by a 300mm sandstone slab until a hardened drill-bit was used to breach it.

The fi xed bridge needs eight piles under the island pier, six on one bank and four on the other. The bascule structures need eight under each of the four abutments.

Health and safety observance on the site is a textbook affair. Cooper explains: 'We got warned early on in the job that there were 17 safety officers in the Fearns Wharf regional headquarters for British Waterways [which overlooks the project], so we've got to be whiter than white.' Van Elle's work on the fixed bridge began at the end of January and was due to fi nish two weeks later. Work on the two lift bridges was scheduled to begin at the end of February.

But just before GE went to press Van Elle director Matthew Large said: 'We've completed the central island piles of the fi xed bridge and two on the rotunda side [east bank] but we've hit underground obstructions and there are four more to do, so it looks like we'll be using an Odex system here.' This uses a down the hole hammer and an eccentric rotary bit for an increased bore diameter to help ease a permanent casing into the hole.

Large said: 'There are a lot of obstructions in the ground because it's been developed but the contractor and client still want to spin the bridge in [next month].

'We're still hoping to hit that deadline. The site investigation was done about 30m away. I thought the piles on the island would be harder as it was built with sandstone blocks and backfilled but we overcame that.'

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