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What happens when the unstoppable force of modernisation hits the immovable object of protected wildlife?

Damon Schünmann went to Norfolk to find out.

An old 17t weight limit bridge in Norfolk needs to be replaced with one capable of taking 40t. On the face of it, this should not be a problem. But in an age of environmental concern, such things are never straightforward.

The challenge facing designers at Guist near Norwich is how to saw up, remove and replace the existing concrete bridge without contaminating the river that flows through the single central span, or damaging the ecology of the area. And unluckily for them, half of Norfolk's protected wildlife seems to have taken up residence in the immediate vicinity of the structure.

The solution devised by subcontractor May Gurney Geotechnical was to install cased CFA piles through the existing bridge deck and into the riverbed below. A temporary platform could then be built beneath the deck which protects the river from pollution while the old bridge is being dismantled.

'The technical problem is that we're putting this piling rig on top of the existing bridge and [consultant] Mott MacDonald has to be satisfied the deck can support its weight, ' says piling projects manager Neal Willis.

The six, 550mm piles will form a new centre pier and extend down 19m from the deck so they found in chalk that lies beneath a layer the fill and alluvium overlying sand and gravel.

Main contractor May Gurney Civil Engineering & Building site agent Colin Beaumont says: 'We could have knocked out the old deck and used cofferdams but because of environmental concerns we've had to use steel piling tubes.'

The pile casings go 3m into the riverbed, protected by Flexcrete's Cemprotec E942, which Beaumont explains looks like concrete but does not pollute the water when wet. 'Everything is centred on avoiding disturbance of the riverbed.'

The rig crew are also putting in 23, 450mm piles for the abutments and wing-walls to depths of 16m, as well as 50m of sheet piling.

The scheme is the first to come out of the £400M Norfolk Strategic Partnership of Norfolk County Council, Mott MacDonald and May Gurney. This 10 year scheme involves upgrading and maintaining the region's highways, the bridge replacement being part of the HGV route upgrading.

Work on the £500,000 job began in August with construction of a pedestrian footbridge and diverting cables which ran through the old bridge. This contract also includes realigning a sharp twist in the river.The piling, which started in late September, was due to finish early last month.

Priority treatment

The species claiming ecological attention at Guist Bridge are white-clawed crayfish, bullheads, brook lampreys, the Desmoulin's whorl snail and the aquatic plant water crowfoot.

May Gurney site foreman Lee Nobbs explains that at just 3mm across, the snails are particularly difficult to spot.

As well as these there are bats, otters and marsh harriers in the area, all of which must be considered in making decisions that might affect their habitats.

May Gurney site agent Colin Beaumont says: 'The job is all about environmental issues and the lampreys and crayfish are the real concern.' Precautions are being taken to prevent the spread of crayfish plague that was introduced to the UK with the arrival of American signal crayfish.

A number of local and environmental groups were consulted before work began as the site is a special area of conservation (SAC), a site of special scientific interest (SSSI) and a county wildlife site (CWS).

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