A clogged-up 5 km stretch of the A30 is finally getting dualled – but not before testing Kier with difficult designs and complex traffic management.
Project: A30 Temple-Higher Carblake improvement scheme
Client: Cornwall Council with funding split 70:30 with Highways England
Contract value: £40m
Contract type: NEC Option C – Target cost
Main contractor: Kier Infrastructure
Start date: May 2015
Completion date: May 2017
Like all counties in the UK, Cornwall is dependent on the quality of its infrastructure network for its economic prosperity. Unfortunately, compared with some counties, its network is relatively poor.
Cornwall is served by just one main rail route to ferry goods and passengers in and out of the county, and fairs only slightly better in terms of road connectivity.
The A30 represents the region’s spine – an essential road link vital to Cornwall’s economy. However, as the region’s prosperity and popularity has risen, so has the volume of traffic.
The A30 can be clogged with vehicles during the busiest days of summer, with the approach on the outskirts of Bodmin representing a particular blackspot.
Having been dual carriageway for the 80 or so kilometres from Exeter, here the road suddenly narrows to single lane – with predictable results. If you drive into Cornwall regularly you will have no doubt been caught by this bottleneck on more than one occasion.
“The emphasis has been on getting the project completed in such a way that it limits any extra land-take and has as little effect on traffic”
Jamie Bee, Kier
But now a scheme for client Cornwall Council and which is part-funded by Highways England will replace this 5 km stretch of highway between Temple and Higher Carblake with a dual carriageway – cutting congestion, slashing travel times and boosting the local economy.
Kier Infrastructure is taking on the work, with project manager Jamie Bee well aware of the benefits it will give to the county, as well as the importance of limiting its impact on the travelling public.
“From day one the emphasis has been on getting the project completed in such a way that it limits any extra land-take and has as little effect on traffic flows during construction as possible,” he says.
Targeting serious accidents
The 5 km missing link runs through a host of protected sites and is constrained by geography, dwellings, farmland and businesses.
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The new design will feature three grade-separated junctions along that route. These junctions should help reduce the number of serious traffic accidents that occur along the section of road, which until now has been above the national average.
“That was of prime concern to Cornwall Council and Highways England,” Mr Bee explains. “The scheme had to make the road safer and the only way to do that and maintain access for the local traffic was to introduce the grade-separated junctions.”
These are located at Temple Tor at the eastern end of the scheme, Preeze Cross approximately halfway along, and at Cardinham towards the western end of the project.
Across the scheme some 75,000 cu m of material will be excavated, with around 100,000 cu m of fill expected to be placed. At Temple Tor the team is busy digging through the peat before it can start backfilling with suitable material.
Awkward peat gets reused
“There is quite a lot of peat in that area,” Mr Bee points out. “We are having to excavate it all out then backfill with 6A or 6C backfill. It is quite a long process.”
The excavated peat is being used at a restoration project in St Day deep in the mining heartlands of Cornwall.
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Because of that peat layer, the bridge at Temple Tor is the only one of the three on the scheme that features piled abutments (with the other two being in cuttings and sitting straight on bedrock). It will also be supported on a mid-span pier and is of a similar design to that of the new bridge at Cardinham.
Meanwhile the Preeze Cross bridge features a shorter span, with the road running through a cut on the south side and a piled retaining wall sitting within 2 m of an existing reservoir.
Ever-changing traffic management
As one of the only real road routes in and out of Cornwall, the A30 is incredibly important as an economic artery.
Keeping the traffic flowing along the 5 km Temple-Higher Carblake route is pivotal to the region, being the main corridor for freight, local car journeys and holiday-makers dashing to the coasts.
But with limited extra land-take available, the team has had to juggle with a design that sees the bulk of the scheme being built within its own footprint. This can make construction difficult and traffic management essential as the site team looks to maintain progress without disrupting traffic.
“We have to keep moving traffic management around and doing the work in sections,” Mr Bee says. “It makes life a little awkward and means there are only some sections where we can get a real flow. The main culvert at Pounds Conce, for example, we have had to construct in two halves. We built one half and then will divert traffic over it before we build the rest of it.”
The 600 mm-diameter CFA piles buried 15 m down into the granite bedrock at Temple Tor were installed by specialist ground engineering firm Keller, with those for the central pier going in during an overnight, traffic-controlled shift.
The abutments and piers are constructed from cast in-situ reinforced concrete, with Aggregate Industries in nearby Bodmin supplying the Highways England-standard high-strength concrete.
The 10 m-long 600 mm-thick abutment walls feature wing walls that are faced with Cornish granite and have proven complicated to erect. “There are various different retaining sections,” Mr Bee says. “It is quite a complex design and we didn’t have the time to recommend a change of that design to make them easier to build.”
The bridge decks feature a steel frame with precast concrete planks and will be prefabricated offsite before being lifted into position (see box).
See this project first hand
Kier’s A30 project will be opening to the public as part of Open Doors Week on 16 June at 1pm.
For more details on how to book your place, visit the Open Doors website.
There are several culverts along the project length, but the greatest attention has focused on one particular point near Pounds Conce farm toward the western end of the scheme.
Here a 48 m-long culvert measuring 2.8 m high by 2.8 m wide is being installed in sections across the full width of the carriageway. The team has installed a temporary sheet pile wall up against the existing road so that work can continue in the soft ground at that point.
“There is lots of soft ground and a high water table to deal with at Pounds Conce,” Mr Bee explains. “The sheet piles went in tight up against the road so that we could work in the area without fear of causing any subsidence.”
Across the scheme as a whole, a value-engineering exercise has circumvented the need for full-depth construction. This has seen cement-bound granular material at an overall layer thickness of 170 mm being specified for the project, with a further 280 mm total thickness of base, binder and wearing course making up the total pavement depth.
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Where the new road layout is being built over the existing carriageway, the team will inspect its structural quality before planing out between 50 to 100 mm and overlaying to final road surface level.
“We will have to perforate the existing road surfacing in certain areas where it is to be overlaid,” Mr Bee says. “There was quite a lot of discussion about it, but we have found that structurally the existing surfacing is very robust, and large parts of it have barely been touched because they have been in the hard shoulder or wide verge.”
With summer approaching and Cornwall’s busiest season set to test the scheme’s temporary traffic management, the site team is focused on delivering the scheme by its spring 2017 completion date, which will herald a huge improvement for the thousands who depend on the A30 for their livelihood.
Prefabricated bridges offer best solution
Two of the three bridges that are being erected on the project are very similar in design. The bridges at Temple Tor and Cardinham will both feature a central pier and abutments, while the slightly shorter and lighter bridge at Preeze Cross is single-span.
All three, however, will be prefabricated and then lifted into position from fabrication points alongside the project.
The beams of the steel frame deck will be lifted into place in braced pairs, with precast concrete deck panels already fitted, using a 220-tonne crawler crane located on one side of the carriageway.
These 150 mm-thick deck panels will then be overcast with a 250 mm layer before the thin wearing course layer is graded into the carriageway at either end of the overbridge.
The two ends and the middle section will be poured separately, with temporary work supporting the deck around the central pier until the final pours are complete.
“Initially the bridge beams don’t rest on the central piers, so there is quite a lot of temporary works around there to support the deck,” Mr Bee says. “There’s 110 tonnes of steel in Cardinham and Temple Tor bridges with 90 tonnes at Preeze Cross.”