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Skirting around the issue: Cobham Services

The first ever temporary diversion of the M25 is being removed by contractors building its newest service station.

For the past 12 months or so, anyone driving clockwise around the M25 between junctions 9 and 10 has used a 600m long temporary stretch of carriageway.

This short curved diversion skirts around the motorway’s normal alignment, close to where a Balfour Beatty Civil Engineering team has been constructing a massive new underbridge.

Combined, the diversion and structure have been the good idea that was needed to allow construction of the soon to open M25 Cobham Services area. Access across the M25 is essential because all of the services’ shops and amenities are being built on one side of the motorway, outside the orbital.

“We were looking at a thrust bore tunnel at one stage, but the costs became excessive”

An overbridge was ruled out by planning constraints; so, with a specimen design showing only the general road layout, developer Swayfields’ highway designer Arup and project manager Project & Building Consultancy (PBC) had to find a way of making the scheme possible without closing motorway lanes.

“We were looking at a thrust bore tunnel at one stage, but the costs became excessive, more than the whole current project value,” says PBC senior project manager Graham Rose.

“So that was dropped and other options were considered such as building an underbridge in multiple sections with a number of traffic switches. But we were not getting anywhere until we sat down with Arup and dreamed up the idea of a complete carriageway diversion with top down construction of an underbridge,” he adds.

Design standards

The first ever temporary diversion of the M25 only became a realistic possibility once PBC and Arup had checked that the temporary clockwise carriageway could be built to design standards, curving away and back onto the existing alignment between two overbridges 900m apart. With this confirmed, the diversion would allow anticlockwise traffic to be switched onto the existing clockwise carriageway while the first half of the underbridge was built. Anti-clockwise traffic was then transferred back to its original alignment for the second phase of construction.

Even with a viable idea the project might still not have happened, says Rose, as Swayfields went into administration part way through contract tendering.

Fortunately administrator PriceWaterhouseCoopers kept the project afloat as a valuable asset. Even so, delays to the award of the £20M design and build contract for the access roads and underbridge, and to the award of another £20M contract to Bam Nuttall for construction of the services buildings and fuel station, pushed the project’s projected end date past the start of this year’s London Olympics.

The main challenge to progress has come from the M25 itself

The Highways Agency had set an embargo on any work on the M25 during the Olympics so design and build contractor Balfour Beatty started on site early in April 2011 so it could be finished before the Games. “The Highways Agency monitored how our programme was progressing very closely during the first six months or so,” says Balfour Beatty project manager Steve Tomkins.

“The Agency could have put a halt to the project at the half way point, preventing us from starting the second half of the underbridge, had it looked like we were not going to have the motorway’s alignment restored and the temporary speed limit removed in time.”

The main challenge to progress has come from the M25 itself, given that the planning consent only allows access to the site from the motorway. Balfour Beatty and its suppliers have been unable to use surrounding roads for any deliveries or site traffic. This puts planning at the mercy of the M25 and its notorious congestion.

Effect of congestion

“We plan around this and have been keeping a careful eye on the traffic, particularly at times of big concrete pours. If anticipated, the morning and evening congestion actually has a fairly minor effect,” says Tomkins. Using 8m3 concrete wagons instead of 6m3 trucks has helped a lot, reducing the number of vehicles on turnaround.

Balfour Beatty has also received assistance in the form of a disused farmer’s access underpass fortunately located immediately south of the site of the new structure. Initially this allowed the team to move around half of the 30,000m3 of fill needed for the road diversion embankment from the anticlockwise side of the motorway.

“Crucially this meant we could maintain a cut/fill balance on site, using material won from existing batters dug back for the services’ slip roads; and we could keep all muckshift traffic off the M25, which helped the programme no end,” says Tomkins.

Reuse of material

The project team has also reused all material won from demolition of several hundred metres of existing concrete barrier and reinforced concrete carriageway, much of which has been moved around and recycled several times - from one piling platform to another. It has ultimately been used as structural fill. The concrete barrier has been replaced with permanent precast Delta Block units keyed into a channel cut into the motorway’s central reserve. “

This is a first for England. It benefitted our programme because we could put more Delta Block down at once than normally done with extruded concrete barrier which, to be efficient, needs crews to be allowed long continuous lengths of barrier to go at.

“The Delta Block gave us more flexibility in terms of programming,” says Tomkins.

Latterly, Balfour Beatty has been using the farm underpass to provide pedestrian access from the site compound to where the second half of the underbridge has been built between anticlockwise traffic and the clockwise diversion. The underbridge - “a lump of a structure”, in Tomkins’ words - is now virtually complete (see box), leaving the tasks of finishing the access roads and landscaping.

This last includes digging up the M25 diversion as soon as the clockwise carriageway has been reverted to its original alignment and using the material in noise bunds around the site. This also has to be finished rapidly so Balfour Beatty can remove its site access and speed restriction from the M25 before the end of July.

Cobham Services is due to open in October.

Bridge design


The M25 Cobham Services underbridge is an integral steel composite bridge at a 34° skew to the motorway, 72m long in total and 22m wide between contiguous piled wall abutments.

These are topped by 1m deep and 2m wide cast insitu capping beams, which support similarly sized diaphragm beams and the deck structure of 300t of steelwork and a 300mm deep deck slab. The deck steelwork - 90t lighter than originally designed following refinement by Parsons Brinckerhoff - consists of 20 beams, each 1m deep.

These were fabricated and erected by Mabey Bridge in prepainted and braced pairs, five pairs for each half of the bridge.

Not until the deck slab concrete of the second half of the structure had been poured could the underbridge be considered fully integral, however.

In the interim, the deck beams were propped from below using Mega Shores on a ground level about 6m above final formation These props allowed Balfour Beatty to load the completed deck half before the final diaphragm beam concrete pour.

Since the second deck slab was poured in April, the contractor has reduced the ground level beneath down to finished formation over 8m below. A 1m deep reinforced concrete base slab the full length and width of the bridge will be completed with two 1,000m3 concrete pours in June.

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