Barrett Byrd Associates

Summer season is in full swing on the M6 Toll project as improving weather allows contractors to crack on with a demanding one million tonne surfacing programme. Jon Masters reports.

Large quantities of asphalt are being laid on the new M6 Toll road and asking a lot of the resources of the two main surfacing contractors Tarmac and Lafarge. Both suppliers have a roughly equal and testing task in terms of tonnage of asphalt to be laid for design and build contractor CAMBBA (Carillion, Alfred McAlpine, Balfour Beatty and Amec).

The surfacing contractors are now making use of every opportunity to make sure that any delay to the first tolls being taken is not down to them. Tarmac and its bitumen supplier Nynas Bitumen, for instance, are currently managing to get more than 3,000t of asphalt to the M6 Toll site each day from a network of quarries and asphalt plants in the Midlands.

This enabled Tarmac to lay around 50,000t of blacktop during May. This is no mean total for a single month on one job, but greater quantities have been programmed for June and July.

“The weather is now going in our favour and since April CAMBBA’s construction programme has been offering large areas of main carriageway for laying base, binder and surface courses,” says Tarmac project manager Richard Beal.

Tarmac has four machine gangs working on the M6 Toll and one hand lay gang filling in small areas such as splitter islands and footways. All base binder and surface courses of the site’s sections one and two, running around 19km west to east from the M6 at Cannock to Wall Island, is Tarmac’s responsibility. Lafarge is doing the same for the 23km of sections three and four, which includes some 5km of M42 widening and finishes at the M42/M6 junction.

The first two sections are being constructed through relatively built up areas in comparison to the largely greenfield section three and M42 section. As a result, Tarmac has more side roads to surface than Lafarge, which has considerably more main carriageway surfacing to carry out including a large plane-off and re-lay job on the M42. But overall, each contractor has a roughly equal 500,000t total to lay and a similar value target cost contract.
“Lafarge is currently ahead of us in terms of tonnage laid as CAMBBA was ready for surfacing works to start on sections three and four first,” says Mr Beal. “We are all monitoring how we are all doing and there has been a certain amount of pooling of resources.”

CAMBBA’s design team, an Arup Atkins Joint Venture (AAJV), has presented Tarmac and Lafarge with a challenging overall specification that is also testing surfacing operations, says Mr Beale.

There are more than 30 different individual specifications for various parts of the M6 Toll, its side roads and footways. The whole job is getting a black top in the form of a thin asphalt surface course. Large asphalt aprons are being laid at the six toll plazas and even lay-bys, traditionally surfaced with concrete, are being surfaced with asphalt due to advances in materials resistant to fuel spillage, such as Tarmac’s Mastershield which incorporates Nynas’ Nyguard binder.

Main carriageway construction consists of 250mm of CBM (Cement Bound Material) overlain by either 250mm of Continuously Reinforced Concrete Pavement (CRCP) or the same thickness of an asphalt High Modular Base Binder (HMBB) incorporating a relatively stiff 35 pen bitumen. In both cases, the surface course is provided by a 35mm layer of proprietary thin surfacing, either Tarmac’s Masterpave or Lafarge’s Axoflex material.

“We are working to a very strict specification and quality assurance system,” Mr Beal says. “AAJV is regularly auditing our checking of base and surface course materials and the thickness, surface texture, compaction, line and level and ride quality achieved in the laying process.

“The M6 Toll must be handed over to MEL, and its side roads signed over to several different local authorities so tolerances are being adhered to strictly. Any surface course below 30mm thick comes out and any layer thicker than that specified is not allowed as it will increase maintenance volume.”

Tarmac is supplying asphalt from plants at four of its Midland quarries at Caldon Low, Wreadon, Ballidon and Mancetter. All are limestone quarries providing stone for base courses, with the exception of Mancetter, which is one of five Midland hardstone quarries Tarmac is using to source stone of PSV (Polished Stone Value) high enough for use in thin surfacing mixes.

High PSV materials are a concern for the big asphalt suppliers due to the current great demand for thin surfacings. Lafarge is using its Mountsorrel quarry north of Leicester for base binder granite with back up from its Weeford and Banbury pits. Haford in south Wales is the primary source of its surface course stone, which is being transported by rail to Lafarge’s Mountsorrel asphalt plant.

CAMBBA’s approval has been needed for every asphalt mix to be produced at each plant, including laying trials on temporary surfaces for all but the surface course materials.

“Once the approvals were in place, we had options for flexibility of supply at the quarries, but this adds to the need for careful resource planning that comes naturally with a job of this size,” Mr Beal adds. “For QA purposes also, we have had to keep track of where each batch came from and exactly where and when it was laid.”

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Tarmac has taken responsibility for surfacing the M6 Toll’s entire length of V-profiled central reserve drainage channel with asphalt after developing special plant to do the job. Asphalt was specified for the 1.5m wide and 150mm deep channel, but CAMBBA and its AAJV design team did not specify how it should be formed. To make the idea work, Tarmac has adapted a Dynapac paver to lay the 20mm binder course material to one side and produce the required profile. Amec’s workshop has engineered a roller to have 1.5m wide drums with a diameter 150mm greater at the centre than at their edges – and stabilising outriggers to stop the plant tilting beyond the comfortable.

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