THERE is not enough room to swing a cat on the site of the Highways Agency's A66 Surtees bridge replacement. But while this may be good news for the moggies of nearby Stockton-on-Tees, it presents something of a problem for Rob Sanger, Edmund Nuttall's agent for the £14.3 million project.
Rather than a tabby or a tomcat, Mr Sanger is swinging around slabs of br idge weighing up to 320 tonnes within a site hemmed in on all sides by road, river and rail bridge.
Nuttall has taken charge of a job that requires the demolition of the existing two-lane bridge by chopping it into lumps and lifting it out and replacement with a wider three-lane version.
Highways Agency project manager Arun Sahni explains why the bridge needs to be replaced: 'The existing bridge would have given the agency some asset management problems in the future and there was also a requirement to open it up to three lanes to allow the east on and off slips from the South Stockton link to operate.
'The structure has moved over and has locked in some st resses into the deck f rom set tlement. It is fair to say the original foundations were not well founded, or founded on top of the sandstone. These problems are purely down to the geological attributes of the site, the variation of the ground and the settlement parameters.
It just was not known when it was constructed that it would lead to this problem.'
To help carry out the work Nuttall has drafted in the largest mobile crane in the country. The Sarens Gottwald AK680-T 1,200-tonne crane has to be delivered to the site on the back of 40 wagons. It does not leave a lot of space for anything else when it is use.
'Space is at a prem ium on site so it is impor tant to sequence the works. When the crane comes on site for the major lifts virtually everything else stops, ' says Nuttall construction manager Simon Tanner.
Before the crane arrives on site the team first has to give it something to get its teeth into. Traffic levels of 55,000 vehicles a day have to be maintained so the team first surfaced the existing bridge's central reservation and removed the footpaths, creating space for two lanes in each direction on two-thirds of the bridge. This allowed them to separate and jack up the remaining third and demolish the abutment.
At this point the stresses in the deck could be relieved, letting the team get to work with a f loor saw, cutting the deck into four pieces.
The ground conditions that had caused settlement problems for the bridge also caused a headache for the Nut tall site team when it came to f inding somewhere to put the crane. The river bank was nowhere near solid enough to take the weight of Sarens' monster so a piled outrigger pad needed to be built on either side of the river.
'The natural river banks are made of relatively poor ground at higher levels so we installed steel H piles down onto the sandstone rockhead to accommodate the high outrigger loads with this crane, ' says Mr Sanger.
'There are six H piles in each pad and four pads on each side of the river. It then takes us about three days to rig the crane.'
The outrigger pads each feature four 5-tonne steel gr illages, each designed to take loads of up to 360 tonnes.
Once in place on these pads and with additional ballast added on the back of the crane, it can then operate under superlift, lifting out the sections of bridge.
With the first phase of demolition done the project moves into the construction phase. The first job is to gain access to the River Tees, to allow the river piers to be built up. This is achieved by construction of temporary jetties built from either side of the river: 'Because of the poor ground stability the jetties have come quite some distance back onto the land. We have an exclusion zone of 15 m there, so we have star ted n icely back onto the land , ' says Mr Sanger.
The jetties are created from 1,067 mm-diameter steel tubular piles sunk 30 m down onto the rockhead.
A structural steelwork grillage was put on top to allow the piling rig - a Hitachi KH300-3 with a 47.5 m mast that came from Nuttall sister company BAM piling - to get out to where the piers will be built in the middle of the river.
The Hitachi unit will be used to create 36 1,220 mm diameter, 30 m-long reinforced concrete bored piles in permanent steel casings, socketed into the rockhead.
It is from these piles that the piers are built up. First a bathtub-like precast pile cap is dropped over the piles. This is a non-structural sacrificial unit that allows reinforced concrete infill.
'From there you build the pier stem up. Initially it was intended that there would be precast concrete elements with an in situ core but we have decided to go for an in situ pour so it is conventional formwork on the stems, ' says Mr Tanner.
With the piers constructed the team can then move on to installing the new steelwork deck. As the bridge is in Cleveland , it was no huge surprise which firm Nuttall ended up working with.
'Cleveland Bridge is providing us with the main structural steelwork elements, which is a total of nine beams. There are six major lumps, each about 70 m long. We have to transport them overnight down the A66 f rom Darling ton, ' says Mr Tanner.
Given the cramped condition the team were keen to reduce the amount of addit ional work that needed to be done to the steel once it arrived on site.
'We are looking to do as much off site as possible, pairing up girders with cross-members, hanging the bearings from the girders and fitting the omniplanks, which are the permanent soffit for the bridge deck, ' says Mr Sanger.
Once the deck is in place the team will be able to put down the deck slab and surfacing before switching the traffic over to the new bridge to repeat the process with the remaining two-thirds of the existing bridge, creating a new, wider carriageway on a solid footing.
The job is due for completion and opening to traffic next summer.