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Up the junction

Highways A34/M4

Removing one of the worst bottlenecks on the UK highway network is proving a major challenge for contractor Costain.

Phil Shirley reports.

Carrying out road improvements on the A34 has never been easy. Remember Swampy?

Construction of the Newbury bypass will forever be recalled as the scheme that changed the way engineers think about the environment.

What ignited the treetop and tunnel protests was a road scheme with the goal of improving traffic flow on the A34, the main route between the Midlands and the south coast ports of Portsmouth and Southampton.

And it did - with the opening of the bypass in November 1998 the entire route, from M40 to M3, is dual carriageway. But there remained one major bottleneck: its junction with the M4, the only at grade roundabout.

Unsurprisingly, this was a major problem. On an average day, the A34 in the vicinity of Junction 13 carries 60,000 vehicles, of which about one third are A34 through traffic. Peak hour traffic queues frequently extend back on to the M4 and for a considerable distance along the A34, resulting in rat-running through local villages.

So the objective of the £38.5M A34/M4 junction improvement scheme is fundamentally simple:

to provide unimpeded flow for the north-south through traffic by taking the roundabout offline.

Making this happen is design and build contractor Costain. It is building 3km of completely new dual carriageway, linking to the A34 Newbury bypass at its southern end and the A34 at Priors Court Road to the north.

The carriageway will run under the M4 to the west of the Junction 13 roundabout, through an underpass. North and south of the M4, sliproads connect between the A34 and Junction 13.

Work began last May and is on schedule to finish later this summer, six months ahead of the two year contract period.

Ten bridges are required to make the new interchange work in a grade separated fashion. But the most significant is the 125m long M4 underpass, requiring 157 piles and 66 precast concrete bridge beams.

The first of three phases was completed within the first 20 weeks of the project, with M4 traffic using the bridge for the first time in September. Using top-down construction, the bridge was constructed in a specially widened central reserve created by slewing the carriageways, widening them by 8m on the eastbound and 2m on the westbound edges.

The carriageways have now been slewed to the north, to allow the westbound 'on' sliproad to be constructed. With completion in mid-February, they will be slewed to the south, to allow the eastbound 'off' slip to be completed by mid March.

Thin surfacing

Thin surfacings are becoming increasingly common on road schemes, with their ability to reduce noise and spray. But durability has always been a concern.

Indeed, the Highways Agency's Design manual for roads and bridges recognises that such surfacings will have a reduced life in comparison with hot rolled asphalt (HRA) and that it is only 50% as strong.

But Costain believes the surfacing it is using on the A34/M4 could change that.

In total it will use 115,000t of asphalt to lay 29,000m will be a thin surfacing; Foster Yeoman's Stratagem system with a twist.

Into the mix will go 1,200t of BP Bitumen's Olexobit 100, a high performance elastomeric synthetic rubber modified bitumen that is particularly effective in thin surfacing applications.

Olexobit 100 promises greater resistance to cracking at low temperatures and reduced deformation at high temperatures.

Cohesive strength and fatigue life is improved and, vitally for contractors, provides improved laying and compaction characteristics.

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