How do you widen a motorway in one of the country's worst congestion black spots while keeping the traffic flowing? Balfour Beatty and Stent jointly developed a novel retaining wall system to get around the problem.
Paul Wheeler reports
THE M25 between the M3 and M4 is one of the UK's worst congestion black spots and even lays claim to being one of Europe's busiest roads, with around 200,000 vehicles using it daily.
But by Christmas next year, things should have improved considerably.
Balfour Beatty is now nearly a quarter of the way into a two-year contract to stitch an extra lane on each side of the 11 km-long section of motorway.
This will take the M25 to five lanes in each direction between junctions 12 and 14 and six lanes from junctions 14 to 15.
The Highways Agency took the view that doing nothing about the problem section was 'not a viable proposition'but faced a tough dilemma: how could the motorway be widened safely without significantly adding to the delays and congestion?
It is an issue that Balfour Beatty has taken to heart.
'The key challenge of the project is protecting the safety of the travelling public and the workforce, while maintaining the throughput of vehicles, ' says project director Peter Anderson.
Widening is taking place within the Highways Agency's existing land take.The majority of the route is on embankments and the Highways Agency's preference is to build out new granular-fill embankments, made from recycled construction waste, crushed and imported from London sites.
But as there is not always sufficient space, a critical part of Balfour Beatty's strategy is the use of a novel piled retaining wall system over a length of 3.5 km.
'We needed a quick and flexible retaining wall system that was easy to construct, ' explains Mr Anderson.
In an attempt to find one, Balfour Beatty ran a series of workshops early on in the planning stages, attended by its sister foundation contractor, Stent.Out of this emerged a retaining wall system that makes use of cast in-situ piles and capping beam, topped by prefabricated wall panels.
The piles provide two main functions, explains John Spence, contracts manager for Stent.They support the retaining wall panels but they also reinforce the soil against slope failures.
Logistical constraints, such as how close the piling equipment can work to live traffic, and the need for long-reach piling rigs are critical to the project's success.
'First off, we looked at continuous flight auger piling, but these rigs have limited reach and we would have had to build out a platform from the embankment to reach the pile positions, which are up to 8 m out from the top of the existing embankments, ' says Mr Spence.
As an alternative, Stent looked at fitting extended-reach arms to its auger boring and ended up selecting not its latest dedicated hydraulic rigs but the oldest conventional crane-hung rigs in its fleet.
It turns out that, when equipped with specially made long frames, these fit the bill perfectly.As Mr Spence says, there will always be a place for 'an old Watson digging in London Clay'Mr Anderson adds that the project is 'a feather in the cap for these old boys' Normally Stent would use a 40-tonne crane but the extra lever arm resulting from the long-reach extensions makes it necessary to mount the rigs on cranes up to double that capacity.
'The first concern is the stability of the crane.After that we have to investigate the stability of the embankment due to having such a big crane working on top of it, ' says Mr Spence.
In fact, this is not a problem, mainly because the cranes work from the motorway's hard shoulder, which is designed for high loads.
Usual practice with bored piles is for the main contractor to cut back the top 300-500 mm of the pile head to expose the rebar and then remove what is often poor-quality concrete at the pile top. (This results from concrete in the pile mixing with surface debris, particularly when the casing is withdrawn and the concrete level in the pile shaft slumps as it fills the space previously occupied by the casing. ) But this technique was simply not practical given the tight programme.
Stent has overcome this by a simple but novel refinement to the bored piling process, which enables it to cast clean, good-quality pile heads as part of the main pile installation process.
The trick is using a cast-in-situ guide wall and circular cardboard former to create a small 'upstand', which leaves the pile head standing 50 mm proud of the guide wall.
It is crucial to ensure the concrete does not slump below the bottom of the guide wall as the casing is removed.
'It is a matter of being careful and making sure the concrete levels are topped up, ' says Mr Spence.
Pile construction itself is otherwise straightforward. Piles are 750-900 mm in diameter and between 6 m and 11 m deep, depending on the height of the embankment and the soil conditions.
The process is to vibrate in casing through the guide wall and then bore through fills, sands and gravels into London Clay. Pile shafts are reinforced to full length, the concrete poured in and its level maintained as the casing is pulled. Stent then cleans off the top of the guide wall, places the circular cardboard shutter around the protruding rebar and tops up the concrete to create the 50 mm pile head upstand.
The resulting piles need virtually no breakdown, saving considerably on the programme.As Mr Anderson puts it, 'all I want is a finished pile cap'and that is what he is getting.
Balfour Beatty then takes over, slotting the flat precast panels into a recess in the guide wall, tying in the steel work with the piles and casting a pile cap before backfilling behind the panels.
Once the piles are in place, Mr Anderson describes the wall construction as 'an adult version of Lego, ' adding hastily, 'not that it needs to be' Stent has five rigs on site and typically bores 90-100 m a day, using around 250 cu m of concrete.
The aim is to turn the pile production into a reliable mechanical process, says Mr Anderson.This enables him to control logistics, such as delivering large quantities of concrete to the rig at the right time through the M25 traffic.
THE HIGHWAYS Agency awarded Balfour Beatty the M25 junctions 12-15 widening contract in May 2003, and work on site started in January this year.With a construction period of 102 weeks, the project is carefully timed to run over only one Christmas period.
In addition to lane widening, other work includes remodelling junctions 13 and 14 and a short motorway spur to Heathrow Terminal 5 from junction 14.Heathrow Airport Limited, an operating company of BAA, is the client for this part of the work.
Existing bridge parapets will also be replaced and noise barriers installed along the route.The drainage is being upgraded and the entire carriageway will be overlaid with a low-noise surfacing.
Balfour Beatty has divided widening work into two chunks. Section one runs from junctions 12-14 and section two junctions 14 and 15.The T5 spur work runs concurrently throughout the project.
Within each section the process is to complete the entire south-bound carriageway, move across to the north-bound and finally replace the central reservation. Balfour Beatty is trialling a new traffic management approach of maintaining an unused 'buffer lane' in the contraflow at all times.