Improving properties of unsealed roads

A joint IPWEA and AustStab taskforce recently explored the use of stabilisation to improve the durability and safety of unsealed roads in black soil regions. The most recent trials in Goondiwindi had some positive outcomesIn 2007, Australian stabilisation industry association AustStab was allocated funds from the federal Department of Transport and Regional Services (DOTARS) (as it was known at the time) for a research project investigating recycling unsealed roads using stabilisation methods.

An Institute of Public Works Engineering Australasia (IPWEA) and AustStab taskforce was set up to investigate the use of stabilisation in improving the durability and safety of roads in black soil regions in New South Wales and Queensland.

Black soil is loosely used to describe the fine-grained uniform soils of moderate to high plasticity, which typically have a grey to black surface. The black colour is mainly a result of iron and manganese, and organic matter to a lesser extent. These kinds of soil types can also vary significantly from one location to another.

Another main aim of the study was to reduce the amount of dust generated from unsealed roads, reduce the frequency of maintenance works and lessen the related costs to local government bodies. Another focus was to provide safe all-weather access on unsealed roads and improve the structural strength of the pavements.

Early trials resulted in successful cost reductions of up to $1200 per kilometre annually for some regions, based on whole of life costing and changes to regular minor maintenance schedules.

IPWEA NSW – Road Directorate and AustStab wanted to look at additional trial sites in black soil regions, particularly the unsealed roads that are affected in the wet season and are often inaccessible for long periods of time as a result.

NSW was ideal for trials. Its road network comprises roughly 160,000 kilometres of road, of which 50 per cent is unsealed.

Four unsealed pavement trials had already been undertaken in the region’s south-west, as part of a previous Federal Government-funded study. These roads were stabilised with lime, cementitious blends or dry chemical polymer binders, depending on the existing pavement materials. The trial areas were brown field sites made up of existing worn granular pavements at the end of their pavement life.

The design of each site varied slightly and typically consisted of a 150-millimetre area of existing materials stabilised using a powder binder as deemed fit for purpose.

Before stabilisation, substantial testing was undertaken in accordance with Australian Standards. These tests for plasticity index (PI) identified the expansive clays and aid binder selection, grading to determine the suitability of the material for stabilising and unconfined compressive strength (USC) to assist in determining the suitability of the selected binder application rate.

The current project is aiming to achieve a plethora of outcomes similar to the previous trials. These include an overall reduction of the whole of life cost for unsealed pavements in black soil regions of western NSW, and south-west Queensland.

Part of the goal is to also establish a basic preliminary testing regime in this area to help guide councils on stabilising expansive clays (black soils) in the rehabilitation of unsealed granular pavements, as well as reducing the whole of life maintenance costs for the local government bodies.

The trials also aim to reduce the amount of time that unsealed pavements need to be closed due to seasonal flooding or extended periods of wet weather.

Queensland’s Goondiwindi Regional Council was the first council to work with the task force on the project. The costs of the project were shared between council, IPWEA, AustStab and AustStab members who would provide plant, labour, materials and supervision at greatly reduced rates.

Burumbah Road, approximately 35 kilometres west of Goondiwindi, was chosen as the testing site. The road had a gravel overlay, sourced from a local council pit, on top of the natural black soil material. However, the granular material had worn away in parts, varied in thickness and was due for a new overlay. The project team decided on stabilising to two depths – 150 and 300 millimetres – as part of the trial.

Laboratory testing showed that the mixture of granular material and black soil, in a ratio of one part granular to two parts black soil, had a PI of 20. From previous experience, the team found that if the PI is greater than 12 then lime is normally the preferred binder. It was decided to use two and four per cent hydrated lime, which resulted in the PI changed to 9.8 and 7 respectively in the laboratory tests. The CBR of the treated material was 58 per cent (four per cent lime) and 22 per cent (two per cent). The team used two and four per cent quicklime for both depths on site.

Four 100-metre sections of lime stabilised pavement were constructed with a control section of 200 metres of unstabilised standard granular material over the black soil. The control section was simply mixed by the stabiliser at a suitable moisture content to achieve compaction.

All sections were sampled for grading, PI and CBR. Results are variable due to variability in the cover of granular material and the depth of stabilisation. More analysis of these results is required before reporting.

The site will be monitored for up to five years with emphasis placed on performance, safety and maintenance requirements (if any).

An initial inspection was carried out in late December by Luke Tanner (Goondiwindi Council) and Graham Hennessy (AustStab). The trial was performing well but it was noted that no significant rainfall had been experienced in the two months since construction.

The hope is that the black soil, with the addition of lime, will perform as well as the materials trialled in the initial DOTARS study and result in reduced annual maintenance costs for regional councils.

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