Improving the performance of flexible pavements – Rajit Rajaram – UK Highways Manager, Tensar International

Pavement optimisation uses Tensar geogrids to create mechanically stabilise aggregate layers to deliver high performing flexible road pavements quickly and economically, while minimising environmental impact.

The demand for reliable road networks that meet increasing traffic levels and heavier vehicles, combined with a need to reduce environmental impact and to deliver more, for less, has placed additional pressure on highways owners and operators, who want to build safe, economical and low maintenance roads with long operational lives.

Construction techniques for flexible pavements have not changed significantly over decades. The materials and methods have remained essentially the same – modifications to binders and improved compaction methods have not brought about the step change in performance required to meet these changing demands.

As a result, road authorities are encouraging consultants, contractors and supply chains to work collaboratively to harness innovation that will deliver quality outcomes faster and more cost effectively. Industry is expected to make best use of available technology to create value-engineered solutions to build new roads and to bring existing ones up to standard.

How mechanical stabilisation works

Tensar stabilisation geogrids have been used on thousands of road projects around the world for the past three or four decades, particularly for construction over weak and saturated ground (although they deliver benefits to projects on any type of ground).

An aggregate layer incorporating a geogrid performs as a composite, due to the interlocking mechanism and particle confinement that develops between the granular particles and the geogrid under loading. This ‘mechanically stabilised layer’ increases bearing capacity and can help mitigate total and differential settlement.

Pavement optimisation

When it comes to flexible pavements – anything from a car park to a major highway – mechanically stabilised layers in the pavement structure can delay the onset of failure, by controlling lateral and vertical displacement of aggregates from traffic loads. This reduces asphalt rutting and cracking, helping to prevent moisture and contaminants entering and weakening the pavement structure.

Tensar calls this ‘pavement optimisation’ because it enables engineers to deliver a design that meets a project’s priorities in the most economical way, striking a balance between reducing pavement thickness with increased trafficking performance.

Pavement optimisation gives designers three choices:

  • To retain pavement life and optimise construction costs
  • To enhance pavement life and reduce construction costs and whole life costs
  • To optimise pavement life and whole life costs.

These options are simply illustrated in the following diagram:

The benefits of pavement optimisation

Broadly speaking, there are four key benefits of pavement optimisation:

  • Reduced pavement costs: Construction costs are typically cut by 20%, by using fewer materials and accelerating construction programmes, while maintaining trafficking performance
  • Increased pavement life: Traffic capacity can be increased by up to six times that of traditionally-built pavements
  • Lower whole-life costs: Increased trafficking performance can reduce maintenance and repairs, delivering whole-life cost savings
  • Reduced carbon footprint: Road construction materials have high embodied energy in terms of CO2e emissions. Reducing the amount of materials used in the road construction can reduce a project’s carbon footprint.

Backed by research

Research by the US Corps of Engineers (USCoE) has shown aggregate layers stabilised with Tensar geogrid deliver significant performance benefits to flexible road pavements, reducing surface rutting by up to 44% compared with non-stabilised aggregate.

USCoE researchers confirmed that “incorporation of a multi-axial geogrid in a flexible pavement base course provides a significant structural benefit”. Results also demonstrated that incorporating geogrid into a road pavement could cut construction time and costs by up to 19% and reduce carbon emissions by a quarter, compared with a thicker, non-stabilised pavement.

Delivering a thinner, better performing pavement for the Great Yorkshire Way

Doncaster Metropolitan Borough Council asked Tensar to develop an alternative road pavement design for the Finningley and Rossington Regeneration Route Scheme (FARRRS – now the Great Yorkshire Way), to help cut construction costs, while increasing the road’s service life.

The Great Yorkshire way runs south of Doncaster between junction three of the M18 and Robin Hood Airport, with links to Rossington and a major distribution hub, iPort.

Conventional pavement design would have involved lime/cement stabilisation of the ground beneath the route, which had an average CBR of between 3-5%. By adopting the Pavement Optimisation approach, incorporating stabilising geogrid in the road pavement structure, the Council was able to reduce overall pavement thickness by a quarter, saving £500,000 on construction costs, while improving the road service life by an average of 60%, resulting in cost savings throughout the life of the road.

Tensar’s UK Highways Team recently hosted a Tensar Academy Webinar on: “Improving the performance of flexible pavement structures through Pavement Optimisation”

This webinar is free to view and available on demand. Please check it out by visiting:

https://tensarcorp.wistia.com/medias/prel8fhlhk

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