David Crowther takes a look at developments in pavement reinforcement and how simple techniques can enhance the design life of pavement construction.
10/11/2008
Traffic volumes on Britain’s roads have never been higher and, despite recessionary pressures on the country’s economy, significant increases in traffic levels over the coming years is a virtual certainty.
These high levels of traffic loading, particularly from heavy commercial vehicles, put enormous strain on the road network’s overall capacity. The imposed stresses from hundreds of millions of standard axle movements also test the physical capability of its pavements to breaking point.
Pavements, whether bound or unbound, are exposed to repeated, highly focussed and damaging loads. The intensity of loading can cause premature aging and failure of the road construction.
Reinforcing products, included within the construction layers can mitigate these damaging effects with beneficial results including:
- Strengthening of the pavement
- An increased working life
- Reduction in deflections
- Reduction of rutting and fatigue cracking
- Reduction in the amount of construction materials required
Selecting the most appropriate product solution is based on a number of interdependent factors:
- Anticipated loading
- New construction or repair/reinstatement
- Design life
- Bearing capacity of the underlying ground
Over the past decade, the geotechnical industry has developed a range of geogrid and geotextile-based reinforcement systems optimised for virtually every permutation of site condition and loading requirement.
It is important to recognise however, that a wide range of ground conditions requires a wide range of reinforcement products, and one style of reinforcement will not be optimal for all ground types.
Bound pavements consist of a number of layers of unbound and asphalt-bound aggregates, combined to produce a composite construction. Being flexible, they are resilient, and offer a good ride quality. However, they can be prone to fatigue failures, reflective cracking, rutting and ‘shoving’ failures. The introduction of reinforcement systems can strengthen the pavement construction, improving its performance, and reducing the “whole life costs” of the road.
When flexible, asphalt-bound pavements are exposed to loads over time, rutting, pothole formation and fatigue cracking can occur. With increasing numbers of heavy vehicle movement and high axle loads, the potential for pavement failure also increases. Simply resurfacing a problem pavement will only temporarily improve the situation as cracks from the underlying layers soon propagate into the new overlay, causing premature aging.
By reinforcing the bound layers of the pavement construction, the road is strengthened, extending the fatigue life and reducing rutting and shoving. Overall this saves the client money by reducing the whole life cost of the road including maintenance commitments.
Table 1, below, illustrates a range of bound-pavement reinforcement products, their application and features/benefits.
Table 1
|
Product
|
Features/Benefits
|
Location
|
|
Maccaferri Road Mesh
|
Woven steel mesh with transverse reinforcing bars, placed within the upper bound layers of the road construction. Absorbs stresses within the pavement and proven to extend the service life of the road.
|
Upper bound layers
|
|
Polypropylene biaxial geogrid delivering strength at low strain.
|
Granular unbound layers
|
|
|
High modulus multifunction geogrid to separate, filter and reinforce in one product. Strains only 1.5% under working loads.
|
Sub-grade / soil interface
|
|
|
Non-woven needle punched polypropylene geotextiles for separation and filtration of soils.
|
Sub-grade / soil interface
|
In this situation, roads use only unbound materials in their construction. These pavements may be used to provide temporary access for construction traffic especially over poor or wet ground or for permanent low-use pavements where bound pavements are unnecessary; e.g. forestry or mining roads, haul roads for construction materials, plant access tracks, parking areas. This type of pavement construction is prone to damage by focused loads and excess water.
Experience has shown that, instead of removing soft sub-base material to facilitate the road construction, it is often possible to leave it in place and build a geogrid reinforced structure on top of it. By offering high tensile strength at low strains, geosynthetics can limit differential settlement and can stop the road construction squeezing into the poor material beneath it, with consequent reduction in deformation and contamination.
Reinforcement of these unbound construction layers provides the benefits of:
- Reduced thickness of construction layers
- Allows construction over soft ground
- Limits differential settlement
Table 2. Unbound-pavement reinforcement systems
|
Product
|
Benefits
|
Location
|
|
Polypropylene biaxial geogrid delivering strength at low strain.
|
Granular upper layers
|
|
|
High modulus multifunction biaxial geogrid to separate, filter and reinforce in one product. Strains only 1.5% under working loads.
|
Sub-grade / soil interface
|
|
|
MacTex W1 or MacTex W2 Geotextiles
|
Polypropylene or polyester woven geotextiles in a variety of strengths from 15kN/m to 800kN/m, to offer separation and reinforcement in one product.
|
Sub-grade / soil interface - full range of strengths
|
|
Non-woven needle punched polypropylene geotextiles for separation of soils.
|
Sub-grade / soil interface
|
Case Study 1. Reinforced “Floating Roads” in the Scottish Highlands
Much of the rural road network of Sutherland in the Highlands of Scotland is single track and built on very weak ground - mainly peat. This poses no problem for light car traffic but this “floating road” construction is unsuitable for use by heavy vehicles serving the local timber extraction industry.
To reinforce the roadways and bring them up to useable standard without resorting to the importation of huge quantities of stone road-base materials, Highland Council worked with Maccaferri on three trial roads serving the communities of Syre, Kinbrace and Helmsdale.
Bituminous overlays incorporating Maccaferri Roadmesh pavement reinforcement mesh were applied to each in a long term trial to assess their viability. This reinforcement mesh causes the pavement construction to work as a cohesive mass, absorbing the horizontal tensile stresses and spreading the imposed traffic loadings over a wider footprint, reducing its damaging effect and helping the more heavily trafficked pavement to effectively “float” on the weak underlying ground.
The Sutherland trials are being undertaken as a partnership agreement between Forest Enterprise and the Highland Council. They started in 2001 and will run for 25 years and are also being monitored by The Roadex Project, a multi national technical co-operation which brings together Northern European countries to share knowledge on forest-type road construction techniques.
Norway, Sweden, Iceland, Finland, Greenland, and Scotland are all party to the project and each shares information with the others, with the objective of improving and maintaining local infrastructure.
In earlier work in 1995, a section of the nearby B873 in Scotland was reconstructed using Roadmesh. Immediately after installation, a multi wheeled tipper truck fell off the pavement and dug into the soft verge with dramatic results. Because the road was effectively floating on the surrounding peat-based sub-soil, the truck was unable to extricate its-self and had to be dragged back on to the pavement by a heavy lift recovery vehicle. This road is still performing as intended over 12 years on.
Case Study 2. A4144 Abingdon Rd Oxfordshire
Abingdon Road links Oxford City Centre to the Southern By-Pass. It is heavily trafficked by up to 20,000 vehicles per day and coupled with poor ground conditions, the road could no longer meet the demands of present and future traffic. A substantial section needed repair, which had to be carried out without closure of this main arterial road.
Abingdon Road lies over drained flood plains, close to the River Thames. The natural ground beneath the road construction consists of alluvial silts and soft clays with extremely low CBR values. The northern section of the road is built over a marshy flood plain. Sections of the road had been widened and improved over a considerable time span resulting in an existing carriageway of highly variable makeup.
A decision was taken to reconstruct a 2km stretch of the roadway to provide a 20 year design life. A design incorporating Maccaferri pavement reinforcing materials was selected because deep replacement was not possible owing to the presence of services at shallow depth and a Roman Road causeway under a preservation order.
The Council’s engineering consultant, Babtie Group, was responsible for the design and specification of the new road pavement, with term contractor Isis Accord carrying out the construction.
Babtie sought design advice from Maccaferri who proposed a combination of products to offer the required design strength for the ‘thin pavement’ construction.
The proposal included Enkagrid TRC 30, a multifunction geogrid, laid on the weak formation to provide reinforcement and separation at the base of the new construction. In addition, Maccaferri Roadmesh, placed deep within the HDM bituminous layers (260mm below surface) would then reinforce the thin pavement construction to meet the design requirements.
In the summer of 2003 a trial phase of 350m was completed successfully. Further phases were carried out, mainly during school holiday periods, leading to completion of the full 2km length in November 2005.
How Roadmesh works
Roadmesh is a double twist wire reinforcing mesh which is sandwiched between bituminous pavement layers. Although originally conceived as an interface layer which could reduce crack reflection, further research identified that the unique geometry and high tensile-strength at low-strain properties of the steel mesh introduces a fundamental tensile stress bearing benefit to the stiffer asphalt layers.
Consequently, the pavement construction works as a cohesive mass, spreading the imposed traffic loadings over a wider footprint and reducing its damaging effect.
Installing reinforcement mesh usually involves the milling away existing surfacing to 110-130mm depth in preparation for the application of the mesh. The mesh is rolled out over the exposed asphalt layer and mechanically fixed, the method being dependent on condition of the exposed surface.
A tack-coat is applied followed by an appropriate depth of base-course, finished with a new wearing course, all laid and compacted in the conventional way.
Conclusion
These examples have illustrated that significant increase in service life can be expected for pavement overlays over reflected or fatigue induced cracks and in overlays over roads that experience heavy traffic or poor foundation conditions.
Furthermore, the potential to reduce the quantity of road construction materials required saves on their export and import from site, embraces sustainability and reduces pollution caused by heavy vehicle movements. Cost savings through the reuse of site won material in combination with geogrids and other reinforcement systems can be substantial on a project.
