Granular pavement design is a crucial aspect of road construction, particularly in western Queensland where the harsh climatic conditions coupled with “black soils” can take a toll on the road surface (resulting in undulations and/or structural pavement failure).
What are black soils?
Soft, deep, fertile black and brown soils make up a large part of Queensland’s geology. These cracking Clays are commonly termed “Black Soils” or can also be defined as “Grey Brown and Red Clays” and “Black Earths”.
Black soils are generally clayey, deep and impermeable. These soils swell and become sticky when wet and shrink when dried (hence, during the dry season, these soil develop wide cracks which can be referred to as a kind of ‘self ploughing’).
While self-ploughing can be good for growing seeds, it has a detrimental impact on road infrastructure if not managed properly.
Black soils occupy approximately one third of Queensland’s land mass and they occur in three generic variants:
- Residual black soils (generally around the Central West of Queensland)
- Alluvial black soils (generally around the Warrego and Flinders Highways)
- Residual black soil (generally around the Darling Downs, Central Highlands and Northern Tablelands)
If you’re driving along a country highway and notice that the power poles and fence posts are slightly crooked, leaning at strange angles, you’re driving through black soil country (refer to Figure 1 for an overview of black soil or “cracking clays” locations).
Figure 1 – Locations of various soil types throughout Queensland (Queensland Main Roads, 2000)
What are design considerations?
There are various factors that need to be considered while designing granular pavements for western Queensland, with the pavement structure generally considered in the layers described in Figure 2. While not an exhaustive list, a brief description of design inputs shown in Table 1.
Figure 2 – Typical layers of a western Queensland road pavement design
Table 1 – Summary of design elements to be considered
| Element | Requirements |
|---|---|
Once the design is completed with stabilisation or replacement of expansive soil material, the new durable and stable pavement structure should be built by a competent contractor. An example of a project designed by JJ Ryan Consulting for Isaac Regional Council is shown in Figure 3.
Figure 3 – Reconstructed pavement on Golden Mile Road, Dysart (Durack Civil, 2020)
References:
Durack Civil (2020), Golden Mile Road Upgrade (Retrieved 2 January 2023 from https://durackcivil.com.au/projects/golden-mile-road-upgrade/)
Queensland Department of Main Roads (2000), Technical Notes, Western Queensland Best Practice Guidelines, Sols of Western Queensland (WQ32)
