Reconstruction of an under-strength rail bridge in Surrey is not due until the end of 2005. But Nuttall is already working on the outline design. Mark Hansford finds out why.
Reconstructing rail bridges is often a challenge. Getting access in the first place is tricky, getting possession of the railway a nightmare. And all this has to be considered before traditional construction problems like ground conditions, site constraints and unpredictable weather come into play.
So reconstructing a bridge that carries the two uplines of the main four track London to Guildford line over the River Wey (and a buried 132kV power cable supplying most of Guildford) but under the M25 motorway, and running adjacent to the Basingstoke canal through an area of natural beauty, sounds like a big one.
Fortunately, bridge and track owner Network Rail has recently woken up to the Early Contractor Involvement concept pioneered by the Highways Agency, and it has got its contractor Nuttall on to the tricky job well in advance.
Nuttall is leading the design and construction of the new bridge through a structures framework agreement with Network Rail Southern Region (see box).
Deck replacement is not expected to happen until Christmas 2005 when a nine day blockade is scheduled for trackwork further down the line. But planning of the work is already well under way and Nuttall has just submitted the Form A document, which sets out the outline design based on its proposed construction method.
This is a relatively new approach for Network Rail and should produce significantly better results. 'In the past we have been presented with the Form A and been told to go build it, ' says Nuttall design manager John Fergusson.
'But it is not until you look at things like how you're going to get the crane there that you can say whether it is unachievable.
Then you have to go back over old ground and redo the form.
Now we are getting involved a lot earlier on.'
The existing 1886 bridge is made of wrought iron riveted plate girders with an open wheel timber deck. It is beyond economic strengthening. For ease of maintenance Network Rail would prefer a ballasted deck and originally wanted a low-maintenance concrete design. 'But that's heavy and the combination of deck weight and ballast would have put too much pressure on the brickwork abutments, ' says Fergusson.
So steel it has to be, and Nuttall's first plan was to bring in deck units by road along a temporary track and deposit them on the footing of the M25 bridge adjacent to the railway. They could then be picked up and positioned by a Kirow crane.
Kirow cranes are mounted on rail wagons and have a low enough profile to pass under the M25 bridge. And one could swing to pick up the deck units without deploying its counterbalance. This would mean the works need not interfere with the running of the two downlines: at this stage it was thought the works could be carried out in a possession of the two uplines only.
The disadvantage of the Kirow crane is first the cost.
There are only three in the UK (two owned by Balfour Beatty and one by Grant Rail) and the hire charges are five times that of a conventional crane. Second is the limited load capacity. This ruled out the use of a Z beam structure with precast concrete deck - the structure would be too heavy for the crane.
So instead Nuttall opted for a steel plate deck with inverted T beams, broken up into four units with a maximum weight of 20t. Each unit would come with a precast concrete sill beam attached, as the crane would be unable to reach the other side.
Temporary scaffolding around the existing deck would allow the canal to be kept open throughout the works, another requirement.
'It seemed a reasonable solution, ' says Fergusson.
But then possession problems reared. 'This stretch of line is effectively a junction and so it is very difficult to get any possession at all, ' says Nuttall planning manager Graham Parker. 'Then we were told about a blockade of all four lines planned in Christmas 2005 and asked if we could use that opportunity.'
On the face of it, great news.
No more worries about cranes swinging over running lines. But the reality was different. 'All the Kirows were booked for switch and crossing work elsewhere in the possession, ' says Fergusson, 'which led us down the mobile crane route.'
The plan now - as submitted in the Form A - is to stick with the steel plate deck option but to set up a mobile crane on the canal bank.
'We will still design the deck to suit the Kirow to give us an option as the possession is not yet finalised, ' says Fergusson.
And just bringing in a mobile crane brings in new problems - remember the 132kV cable?
'We do have access issues, ' says Fergusson. 'The 132kV cable feeding Guildford runs in the canal towpath and the canal itself is retained by thin piled walls.'
The solution will be to locate the cable through trial holes to appease National Grid Transco and to use Screwfast piles to create a stable base for the four feet of the crane. These can then be removed afterwards, keeping the National Trust happy.
An existing footbridge - which must be retained - limits how close the crane can get to the bridge, meaning a 45m reach will be needed in places.
Nuttall aims to have the work done in the first two days of the nine-day possession, but has requested an extra six hours in day three to make sure there is time to put the new track back.
Reconstruction of the River Wey bridge will be a major challenge, but is just a small part of Nuttall's Southern Region construction partnership with Network Rail.
Since the partnership was formed in April 2001 Nuttall has carried out £38M worth of work on 200 separate projects, ranging in value from £4,000 to £4M.
Originally set to run until April 2003, it has been extended by two years already and is now set to expire in September 2006.
Work is carried out as part of an integrated team and is paid for on a schedule of rates; Nuttall takes schemes from project development through outline design to target cost, when they are benchmarked and authorised before it carries out the works.
This integration and early contractor involvement has already shown its worth and is encouraging Network Rail to commit to tough schemes like the River Wey.
'In the last 10 years Network Rail has done all the relatively easy ones, ' says Nuttall project director Alan Cox. 'Now, looking at the jobs we're starting to get, they are starting to pull them out of the 'too complicated' drawer.
And to do that you need confidence in the contractor.'
A major reason for success to date is Nuttall's insistence on carrying out work in-house - 98% has been done this way.
'Network Rail wanted a contractor that was going to do the work and not act as a programme manager. We sold it to them at the time that we operate that way, and we do, ' says Cox.
The biggest demonstration of the partnership's success to date remains the £3.5M replacement of 10 bridges in four days on a rail locked site between Clapham Junction and Waterloo at Easter this year.
In a similar manner to the River Wey, project planning started 18 months ahead of the job, which saw the first use of Kirow cranes on a large civils project.
But all the planning in the world could not have prevented the plant failure that almost led to disaster.
At the time there were just two Kirow cranes in the country and both were used on the job. With 12 hours to go the Balfour Beatty crane failed, leaving Nuttall staring at a 13 hour overrun on the approach to Britain's busiest station. 'It was one of the recognised risks, ' says Cox.
But by calling on the firm's inhouse staff the over-run was pulled back to four hours.