A road widening project along Rockhampton’s Musgrave Street required a reliable CBR value before pavement design could proceed. The local soils, a mix of alluvial clay and weathered basalt, often shift between low and moderate bearing capacity depending on compaction and moisture content. Without a proper laboratory CBR test, the risk of overestimating the subgrade strength would have led to premature rutting and cracking. These conditions are not unique to that project — they appear across the city’s expanding industrial zones and residential estates. The laboratory CBR test provides a direct measure of the material’s soaked and unsoaked California Bearing Ratio, which directly feeds into pavement thickness design per Austroads guidelines. For Rockhampton, where seasonal wetting can soften the upper subgrade significantly, the soaked CBR result is often the governing parameter.
A 10% drop in soaked CBR can translate to a 25 mm increase in required pavement thickness under heavy-load scenarios.
Scope of work
- Maximum dry density and optimum moisture content (AS 1289.5.1.1)
- Swelling percentage after soak
- CBR at 2.5 mm and 5.0 mm penetration
- Correction factor for curved load-penetration plots
Area-specific notes
Rockhampton’s urban growth has pushed developments into areas underlain by reactive clays and loose sandy fills from old creek alignments. During the 1990s, several industrial slabs on the eastern corridor experienced differential heave because the design CBR was based on unsoaked values that did not reflect wet-season conditions. The laboratory CBR test in Rockhampton is the only reliable way to capture this seasonal vulnerability under controlled conditions. Ignoring the soaked CBR in these clays can lead to subgrade failure within two to three wet cycles, forcing expensive slab replacement or overlay corrections.
Watch how it works
Standards used
AS 1289.6.1.1:2014 – Determination of California Bearing Ratio, AS 1289.5.1.1:2017 – Compaction control test (Standard/Modified), Austroads Guide to Pavement Technology Part 2: Pavement Structural Design
Linked services
Standard CBR Test (Soaked & Unsoaked)
Full AS 1289.6.1.1 compliant test including sample compaction, 96-hour soak, swelling measurement, and penetration curve analysis. Suitable for residential roads and light industrial pavements.
CBR + Dynamic Cone Penetrometer Correlation
Combines the laboratory CBR test with field DCP testing to generate site-specific correlations for rapid quality control during earthworks. Ideal for subdivisions and large-area pavement projects where multiple test points are needed.
Typical parameters
FAQ
What is the difference between soaked and unsoaked CBR, and which one should I use for Rockhampton projects?
Soaked CBR simulates worst-case moisture conditions after heavy rain, which is critical for Rockhampton's reactive clays and seasonal wet-dry cycles. Unsoaked CBR represents as-compacted conditions. For pavements in this region, soaked CBR is the conservative design value recommended by Austroads.
How much does a laboratory CBR test in Rockhampton typically cost?
The typical cost for a single laboratory CBR test in Rockhampton ranges from AU$200 to AU$340, depending on the compaction standard (Standard vs Modified), whether soaked or unsoaked, and the number of samples tested. Volume discounts may apply for multi-sample projects.
Can I use a CBR test for pavement thickness design on a residential driveway?
Yes, but for light-duty driveways a laboratory CBR test may be overkill. A field DCP test combined with local experience often suffices. For any driveway supporting heavy vehicles or with poor subgrade drainage, the laboratory CBR test provides a defensible design value and is recommended by most local councils in Central Queensland.