Creating a solution that is not only more sustainable but also cheaper and faster to deliver sounds too good to be true? Claire Smith takes a look at one Yorkshire road that proves it is possible to have it all.
Residents of Doncaster and holiday makers heading for Robin Hood Airport will be able to take advantage of faster journeys and less congestion when a new motorway link road opens next year. Improved infrastructure will be a benefit but the environment and council coffers are also major beneficiaries as a result of geosynthetics use to cut costs, fast track the work and improve sustainability.
The Finningley and Rossington Regeneration Route Scheme (FARRRS) will connect the A638 at Parrot’s Corner to the south of Doncaster directly to the M18 at junction three. The 4km long route will also link up with Robin Hood Airport, a proposed inland port (iPort) and new 1,200 housing development, as well as easing local traffic congestion.
Initially the construction approach was fairly conventional but client Doncaster Metropolitan Borough Council (DMBC) and contractor Carillion wanted to value engineer the scheme and called on Tensar to help.
“The council and Carillion were looking to lower costs and reduce the construction period,” says Tensar technical manager for pavements Craig Andrews.
Work on the £57M scheme started in October 2013 and, when Tensar got involved, the preparation and earthworks were already underway.
“The original scheme was to be built with a pavement design to IN73/06 and an aggregate foundation subbase to HD26/06 using estimated ground conditions,” says Andrews. “This considers the foundations and asphalt pavement separately. We looked at the pavement as a whole to offer a better solution.”
Andrews looked at the original concept and noticed that some areas the design was deficient in terms of bearing capacity while other parts of the route were over engineered. Tensar proposed to use geogrids to add efficiency to the design.
“Originally there were six or seven different design sections but we were able to rationalise this to two – one for the dual carriageway and one for the single carriageway,” says Andrews.
The original design was to withstand 50M equivalent standard axel loads based on a CBR of 3 to 5% but the roads for the planned iPort were based on 80M axel loads, so Tensar offered a design to increase capacity to match the designs.
“DMBC wanted to reduce time and cost and Carillion’s focus was to improve efficiency on site with less compaction,” explains Andrews. “The original design called for both the aggregate in the sub base and asphalt in the road base to be compacted in two layers so we redesigned to include Triax geogrids to enable both to be compacted in one layer.”
The addition of the Triax mechanically stabilised the sub base to increase the stiffness. “By considering the pavement and foundations together we could increase the contribution of the sub base to the performance and reduce the contribution – the thickness – of the asphalt,” says Andrews.
This resulted in a 26% reduction in the thickness of the pavement. “We could have made the single carriageway thinner than we have but there was a requirement for a minimum of 450mm of non-frost susceptible materials.”
Just before GE spoke to Andrews, DMBC asked if it was possible to further reduce costs on the single carriageway route. Andrews has just submitted a proposal to reduce the thickness of the asphalt but increase the thickness of the aggregate sub base, which has the potential to save a further £50,000 on top of the £500,000 and 7,000t of materials already saved through the redesign.
Andrews says that more local authorities should look at the pavement structure as a whole in order to leverage similar savings. He believes that the redesign at Doncaster achieved not only time and financial savings, but also considerable environmental savings too.
According to Andrews, projects like FARRRS will be key to meeting the Construction 2025 targets of a 33% reduction in construction costs, 50% reductions in greenhouse gas emissions and construction periods and also a 50% improvement in exports.