Bridges form a roundabout solution
In Shakespeare’s home county main contractor Morrison Construction is partway through a deal that will help improve traffic flow, reduce noise and boost safety at a vital highway interchange. Paul Thompson
Scheme: M40 Junction 15 (Longbridge) Bypass
Main contractor:Morrison Construction
Contract type: Early Contractor Involvement (ECI)
Contract value: £60 million
Concrete sub-contractor: JOS Concrete
Surfacing sub-contractor:Bardon Contracting
Plant Hire and specialist lift contractor:Ainscough
For travellers and commuters using the road network around the Warwickshire towns of Stratford-upon-Avon, Warwick and Leamington Spa negotiating Junction 15 where the A46, A429 meets the M40 has always been difficult.
Queuing traffic on the M40 exit slips as well as the two A-roads as they approach the junction have forced Highways Agency engineers to look at the layout of the roundabout in the past, a new improvement is afoot.
“The scheme is part of the overall plan to reduce congestion on the A46/M69 east/west route across the Midlands,” says Highways Agency project manager Stephen Coe, “this is one of the main stumbling blocks along that route. At the moment there is something like 130,000 vehicles use the M40 each day and more than 60,000 use the A46. This is forecast to rise to more than 70,000 by 2023.”
Initially the scheme was twinned with a similar project to improve the A46 junction closer to Warwick at Tollbar. The two schemes, with an overall price tag of £200 million were due to be packaged together. But the Tollbar end has been delayed and is unlikely to start before main contractor at Junction 15, Morrison Construction; part of the Galliford Try Group, has completed its £60m contract.
“The plan was to carry out work at the Tollbar end first,” says Morrison Construction’s project manager Steve Harris, “but that has been switched. There are a few more issues with the Tollbar scheme; this one is a little more straightforward with more greenfield land.”
But with a new 2.7 km long dual carriageway, 100 m long four span overbridge, 20 m span bridge on reinforced earth abutments, river diversions, structural demolitions and a massive 1m tonnes of earth to be moved the Junction 15 improvements is hardly a simple task.
By using a system of borrow pits placed strategically around the scheme the site team has been able to limit the amount of imported fill needed to provide the reinforced earth abutments and 9 m high embankments - a move which saved around £5m.
“The scheme initially went forward with no borrow pits and the fill imported, but we were able to get planning permission for them,” says Danny Brown, Morrison’s structures project manager on the site.
Three borrow pits either side of the M40 will feed the work, the largest of which will be the 350,000 cu m one that will come from a pit to the north of the motorway. Generally the material, once the top soil has been stripped from the pit and stored, is a highly serviceable Mercia Mudstone and Keuper Marl with an 85 per cent efficiency rating for suitable fill. The material won from pits on the south side of the M40 has a slightly higher moisture content compared to that from the north side and has had to be left to drain once it has been excavated. However, that from the northern borrow pits can be placed directly.
“It is very strong material and it is pretty uniform, but there does seem to be a distinct difference between material won on the north side of the M40 and that won from the south,” says Mr Harris. “Geotechnically, they are both suitable, but we have had to work the material from the south side a little more because it is wetter.”
And it is moisture that has meant the project is currently running three months behind its October 2010 completion date. With a start date of early March 2008 the project team ran straight into one of the wettest summers on record with work severely disrupted during July and August.
“It was very wet and time was lost, but we are hoping to speed up the programme and claw those months back,” says Mr Harris.
Certainly the project team has not been wasting any time in installing some of the 25 structures that are being built across the site. The reinforced earth abutments are on their way up and the main four span bridge which will carry the new line of the A46 across the M40, farm access tracks and the Horse Brook.
With a total span of 100 m, the reinforced concrete bridge sits on 10 m deep, 1050 mm diameter bored piles at the abutments and 750 mm diameter at the centre of the motorway span. It will carry the new A46 dual carriageway with a narrow central reservation and concrete step barriers. It also features a 4 m wide, 1.5 m thick pile cap with walls of a similar thickness and is 5.5 m high from the base of the cap to the underside of the precast concrete deck beams.
Cast using a blast furnace slag-rich Cem III C50 design mix concrete by Birmingham based specialist JOS Concrete Structures, the abutments will carry the 96, 70 tonne precast concrete beams that are lifted into place using an 800 tonne mobile crane. Thanks to the fact that the crane could be located off-line and reach out over the M40 carriageways, the team has been able to install 12 beams during an evening closure, averaging just three nights for each carriageway. Arriving direct from suppliers ABM Precast and Tarmac Precast, the huge beams are lifted straight from the trucks and placed in position butting up against one another.
The diaphragm and bridge deck itself will be cast at a later date. There are some hefty 32 mm diameter, 5 m long reinforcement bars in the centre of the bridge that help lock the deck around its support. This makes the two double spans each side of the centre point act as simple supported beams, enabling the expansion and contraction to be taken up by joints at either end of the overall span. The butting beams will allow the team to cast the deck in one single pour of around 300 cu m. A similar technique will be used on the new A46 underpass bridge to the north end of the scheme, where 78 precast beams are being placed across a 20 m span, 100 m long bridge, bearing on reinforced soil abutments.
The next phase of the scheme will involve widening the existing roundabout at the motorway junction to four lanes. This will involve a little tinkering with the existing layout on the bridges over the motorway - but no major structural work - and is another challenge on a project that still boasts a long ‘to-do’ list, although Mr Harris is confident that this list will soon start to be whittled down.
“We are doing lots of 24 hour working at the moment to accelerate the programme,” he says.
For motorists using junction 15 and the A46 that time can’t come soon enough.
Alongside the new A46 underpass bridge the project team is using reinforced soil techniques to build the abutments and walls. Along their line the weaker ground is removed before a starter layer of 6C granular rock goes in. A further layer of 6N graded granular material acts as blinding before a small concrete strip footing is placed to formation level of the hexagonal concrete panels that will be tied back into the embankment through a galvanized steel reinforcement strap.
These precast concrete panels are started with half panels on a hit-and-miss basis and are propped on the non-embankment side. The complete panels are then placed between those already located and the whole structure is built up in layers as the fill is compacted behind them, tying the reinforcing straps into the fill and creating a monolithic structure.
Up to the surface
As part of its commitment to reduce impact on local residents, a Hitex 14 mm Thin Surfacing system is being used on the scheme. This will offer significant noise reduction compared to more traditional hot rolled asphalt wearing courses. Surfacing contractor Bardon Contracting, part of the Aggregate Industries group, will use the blend of polymer-modified binder with filler and graded fine and course aggregates of gritstone, granite or dolerite in a 40 mm thick layer of thin surface course. This will sit on top of a 60 mm layer of dense binder course, a further 260 mm layer of dense base course and a 450 mm depth of type 1 sub-base. The sub-base thickness varies depending on the traffic loading and the California Bearing Ratio of the formation. In common with all Highways Agency schemes where the volume of traffic over the projects design life is likely to be more than 80 msa (millions of standard axles), the top 150 mm of sub-base is cement stabilised.