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Geotextiles: Stabilising influence

a556 pic8

Using geogrids to mechanically stabilise the founding aggregate layers on the A556 improvement scheme in Cheshire has potentially saved up to £2M

Congested and accident-prone, the vital A556 route from north Cheshire to south Manchester ranked high on the list of UK roads in need of attention.

The route links the city with the M6 and M56 motorways and Manchester Airport, with the busy M6 to M56 section carrying around 51,000 vehicles, including some 5,000 lorries, daily.

Here the road combines single and dual carriageway, with a number of junctions to local routes. As a result, it suffers frequent congestion, and over the past few years there have been a number of serious accidents.

Aiming to improve safety, relieve congesting and reduce traffic noise and pollution, Highways England’s A556 Knutsford to Bowden Improvement Scheme involves building a new, 7.5km dual carriageway to the west of the A556.

The new link runs between Junction 19 of the M6 near Knutsford and Junction 7 of the M56 near

Altrincham and will cost between £165M and £221M to build.

Construction is being carried out by principal contractor Costain, scheme designer is Capita, with

Atkins acting as Highways England’s independent quality verification team.

Ground conditions along the route are highly variable and challenging, with very dense sand and gravels, very soft silty clay and, in places, substantial thicknesses of made ground, says Costain senior engineer Anthony Mackenzie.

“At the Yarwood Roundabout, for example, at the route’s northern end [near to where the new road will tie into the M56], excavations revealed the ground was much weaker than we first thought,” he says.

“The silty clay is very soft here and it gets softer with depth – we think that some of the made ground may actually be spoil from the construction of the M56. Its average CBR was found to be between 1.5% and 2% but the design for the new road requires a minimum CBR of

2.5%.”

Designed to give a capacity greater than 80M equivalent standard axle loads (ESALs), the road pavement comprises a 250mm thick unbound granular capping layer with two, 150mm thick layers of cement-bound granular material (CBGM) above and, finally, 160mm to 180mm of asphalt.

Costain had three options to deal with the soft soils: lime stabilisation, “dig and replace” or install

geosynthetics under the capping layer.

“We discounted lime stabilisation of the made ground, and digging and replacing large quantities of material was potentially very expensive and time consuming,” says Mackenzie.

“We needed an alternative solution that would provide sufficient support to construction traffic, that would also form part of the permanent road structure; one that was cost-effective and that did not hold up the programme.”

Costain approached Tensar International to see if it could come up with a way of meeting the design requirements using geogrids and mechanical stabilisation.

“We had to develop a solution that would stiffen the unbound capping layer to provide adequate support but that would not change the pavement design,” explains Tensar highways manager Craig Andrews.

“Highways England accepts geosynthetics in road construction, but Costain wanted to ensure we did not depart from the original design, which would have led to delays while the alternative was approved under a departure from standard,” he says.

The firm’s proposal was to install a layer of its Triax geogrid directly under the capping layer. Under load, this geogrid interlocks with the aggregate particles to create a mechanically stabilised layer.

“The mechanically stabilised layer has three main functions,” explains Andrews.

“First, it provides support to construction traffic, limiting subgrade deformation; second, it provides sufficient support to the cement-bound layers and the asphalt surfacing above, limiting deformation during construction; and finally, it enhances the performance of the pavement structure during the operational life.

“Even though the mechanically stabilised layer was not considered as part of the pavement design, we still needed to run a full pavement analysis to confirm it would provide adequate in-service design capacity and that it was the correct thickness.

Analysis, and a subsequent trafficking trial, showed the founding capping layer would meet the interim performance requirement of having a mean bearing capacity of 50MPa, when measured with a lightweight deflectometer.”

Andrews recommended installing a trial section before moving ahead with its proposed solution. “We felt it was important to demonstrate to Costain, Capita, Atkins and Highways England that our proposal would work and to identify any problems before the CBGM was installed,” he explains.

Unfortunately, things did not go to plan. “The trial took place on a 40m section in November last year,” Andrews says. “It had been a very wet autumn and the winter was continuing in the same vein; the ground was heavily saturated and, because this section is in a cutting, surface run-off was being channelled towards the road. This made it difficult to prepare the ground surface and to install drainage.”

Results of the trial were disappointing, he admits. “The section did not perform very well because of the already saturated sub-formation layer.

“Also, the grid and the aggregate had not fully engaged to create the mechanically stabilised layer and the heavily saturated stone capping layer had not been trafficked sufficiently to achieve the mean bearing capacity of 50MPa. Additionally, the high moisture levels gave variable test results.”

However, Costain recognised the trial as an opportunity to understand the problems encountered due to the challenging weather and to learn from the results. So, rather than abandoning the idea completely, a second test was planned.

“Fortunately, the weather had improved by the time of the second trial,” Andrews says. “We also recommended more accurate cutting and proof rolling of the ground and that an effective cross-fall be created to allow surface run-off.

“Costain excavated temporary drainage ditches and subsequently installed the permanent narrow filter drains for the capping layer.

“Finally, Costain tried to ensure the stone for the capping layer was as dry as possible and that it was rolled and trafficked, to engage the grid and dissipate any water.”

“The second trial section performed far better and has continued to improve over time, as more construction traffic has passed over it. The Tensar system ensured bearing capacity at the top of the stone layer was between 60MPa and 70MPa.”

With performance exceeding expectations, Costain and Capita were happy to sign off the Tensar solution and Atkins agreed to approve the progress of the project.

In March of this year, the team adopted the same approach when it encountered similarly poor ground conditions further south along the route. In both areas, about 300m of road is supported by Triax, with a total of 30,000m2 laid.

Earthworks contractor Walters installed the Triax and the lower aggregate layer. Pavement contractor Aggregate Industries followed on, laying the two CBGM layers ready for the final asphalt layer.

Highways England’s Interim Advice Note 73/06 requires a demonstration area to show the design of a performance foundation is adequate.

However, Costain also tested the bearing capacity of the first layer of cement-bound material to confirm interim performance, which has continued to exceed requirements.

This included traffic tests using a fully loaded, four-axle lorry passing over the section 130 times, equating to 1,000 ESALs.

“The top of the first CBGM layer far exceeded the requirements for Foundation Class II (100MPa), in some cases reaching between 400MPa and 700MPa,” Mackenzie says. “So, not only is the Triax solution performing well, we have also found it to be a faster, more simple and cost-effective alternative to lime stabilisation or dig and replace.

“Additionally, the Triax was easy to lay, so there was very little effect on the construction programme, with the added bonus that the method was far safer than the other options on the table.”

Andrews says that in a comparison with the alternatives, the Triax approach was a clear winner.

“We estimate that by using Triax, it has saved close to £2M in costs, when dealing with the difficult sections, compared with digging out 1m to 3m of unsuitable material and replacing it with acceptable material. This also meant a potential 10,000 lorry movements have been avoided.”

With the most challenging sections dealt with, the A556 improvement scheme is on target for completion next year.

The new road is due to open in spring 2017, when the focus will switch to the existing A556, which will be narrowed to single carriageway road with improved provision for pedestrians, cyclists and horse riders.

This “de-trunking” is set for completion by the winter of 2017.

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