The huge piles currently supporting temporary works on the Surtees bridge, between Stockton and Middlesbrough, would probably do as permanent foundations for a lesser structure.
The steel tubes, 1067mm in diameter, have been driven to sandstone 30m down, to shoulder massive steel frame 'jetties' above, one each side of the river.
The 20-pile platforms, hefty enough to carry big piling rigs and medium sized cranes, are a central part of contractor Nuttall's method for rebuilding the bridge on exactly the same alignment as the old one. They allow work on replacement foundations, and then the superstructure for new piers in the river Tees.
The platforms help cope with difficult ground, and the river channel's awkward shape. They also eliminate the need for complex works in the river.
'The channel prole is a V-shape and the banks drop quite steeply to 8.5m deep' says Nuttall's project manager Graham Rice.
The river was almost a creek at this point until the early 1990s, he points out, after which the Tees tidal barrier was completed downstream.
Nowadays the water level is almost constant, rising perhaps 300mm after heavy rain.
The ground beneath the platforms is relatively soft as well. Most of the geology in the area is glacial, comprising firm boulder clay but just at the bridge location there appears to have been an old inlet in the river valley, according to studies by Nuttall sister company Ritchies.
The material lling it is softer, 'a fairly complicated mixture of silts, some stiff, some pretty liquid', says Rice. The silt is interspersed with peat and sand layers.
It is this soft ground which contributed to a gradual decay of the old A66 river crossing.
The concrete bridge has been ailing for some time, with distressed bearings overrun by about 100mm. Stress had also built up in the foundations and one of the abutments was subject rotational movement.
'It seems the embankment on one side had squeezed the ground and caused the approach to move towards the river' says Rice.
On top of that, concrete was spalling at the water line, probably due to corrosion effects from brackishness in the water.
Even so, until recently the dual two lane link carrying the important Stockton to Middlesbrough road was still just about serviceable. But the new South Stockport Link Road opened recently, including a major junction just west of the bridge. It needs feeder traffic lanes on the A66 for proper functioning and these will run across the bridge.
So the Highways Agency, with some prompting and nance from Stockton-on-Tees Borough Council, decided to replace the old structure with a wider modern version 145m long.
Renovating the old bridge was looked at, but would have been technically too demanding.
In addition, the change will allow other upgrading; the bridge design by the Agency's maintaining agent Halcrow/ Colas JV will have a 40t load capacity in line with modern requirements.
Better ship collision protection is also being added as the river has become permanently navigable since the downstream barrier was built and local cruise boats and other medium sized vessels can now use it.
Replacing the old five span bridge has needed careful planning, above all to keep the traffic running on the busy road at peak hours. 'Mostly we have to maintain two lanes each way with reducing trafc to one lane running at midday and having some closures at night' says Rob Sanger Nuttall's site agent.
The solution has been to demolish and rebuild one carriageway of the new viaduct at a time, transferring traffic first onto one remaining carriageway and then onto the first completed half.
Nuttall's core concept is to make extensive use of a very large crane, a 1200t Gottwald AK680 hired in from heavy lift specialist Sarens, which can lift out large sections of the old structure, up to 344t in weight.
'It takes 40 trucks to bring it here and three days to set up' says Rob Sanger, Nuttall's site agent on the project, 'so we need to plan things fairly carefully.' The big crane with its giant counterweights also lifts the deck sections for the new bridge. These are steel, fabricated by Cleveland Bridge at Darlington, 20km away, and brought in on multi-axle loaders.
Before the old bridge sections were lifted out, the contractor had to deal with potential locked in stresses of up to 200t caused by twisting. A steel frame was clamped onto the concrete deck to transfer loads to hydraulic jacks which could be slowly de-stressed before the deck was saw cut into liftable lengths.
With the old deck removed, there was space to install the two work jetties, with the tubular piles driven by vibration and hammer by subcontractor Aarsleff. They were pushed down to a 3m layer of dense sand and gravel overlying Bunter Sandstone about 30m down.
The same firm had already installed four separate clusters of nine driven H piles 30m deep on each side of the river bank; these were to support outriggers from the huge crane, which had to lift large loads and swing them over a big radius.
Once the jetty frames were welded together a second piling rm, Nuttall's Dutch sister company BAM Equipment, could move into place to install the main pier foundations. These are 1220mm diameter bored cased piles.
The piling subcontractor used a large 133t Hitachi rig with a 47m high mast to vibrate the permanent steel casings down to rock head before using a mucking out augur to clean the inside and finishing off with a 3m deep socket drilled into the sandstone.
Cages and concrete completed the piles which were nally cut back to 2.5m above the waterline. There are 22 piles for the full length of one pier.
'A shorter rig has been used for the second side because we have a railway bridge just downstream' explains Sanger.
A disused rail bridge sits just beyond the live rail bridge.
'Rather than try and create pilecaps within cofferdams we went for a precast solution with a cap just below water, ' says Rice. Concrete pile cap sections made in an onsite precast yard built on the worksite. These are each 3.5m long, and have holes in the base which allow them to be lifted in over the piers.
Five units make up half the pier length constructed at one time.
'They are positioned with a steel frame and bolted together with tunnel bolts, ' says Sanger. Waterproong between them was compressed as the bolts were tightened and then the units could be pushed downwards on the frame.
'Once in place a cage was dropped in and they were lled with concrete' says Rice.
Meanwhile arsleff arried out more driven piling with a row of steel H-piles, raked back slightly to form the abutment on each side. The rm also drove some 130 of its 300mm square reinforced concrete piles for the embankment on one side of the bridge, as a precaution against future settlement.
With the piers in place, the big crane was back on site just before Christmas for lifting in the steel beams for the new bridge.
The bridge has nine parallel steel girders with precast concrete Omniaplanks laid across them as permanent formwork for a concrete deck above. Four girders make up the first phase, arriving as pairs from Cleveland's works. A connection element is dropped into the centre to complete the full bridge length.
For the second phase, two pairs of girders will go in and one nal separate girder preassembled element also made up with the Omniaplanks in place. The final pieces of the new bridge deck are being delivered to site over the next two weeks, with the massive 163t girder assemblies transported on a special 7.6m wide, 70m long transporter vehicle.
Because some of the loads are so large some street lighting, road signs and barriers will have to be taken down and re-erected to allow the transporter to pass.
Concreting and road asphalting were completed for the rst section while the temporary jetties had their piles extracted, ready to go in on the other side for a repeat of the whole sequence.
The £14.4M contract will finish in October, a little later than an originally planned end of July date.
Site safety Nuttall decided on extensive prefabrication partly for safety purposes, to avoid excessive working over water and working at height on the new structure. In particular the new deck beams were fabricated to incorporate the lateral concrete Omniaplanks for the deck concreting. 'It is much easier to do at the factory than on site and did not add too much to the lift weight' says Rice.
Extensive scout protection works have also been carried out as part of the job installing geofabric on the river bed, to just beyond the railway bridge locations. Divers were used for the work guiding rip rap placement from the barge mounted excavator.