A contractor up in Smithfield called us last wet season. Their pavement had failed three weeks after handover. The subgrade looked fine during earthworks, but the lab numbers told a different story. Soaked CBR values had dropped to 1.5%. That is what Cairns does to a pavement if you skip the lab soak. We run the California Bearing Ratio test under controlled moisture and density, exactly how AS 1289.6.1.1 specifies it. No shortcuts. The Proctor compaction tests give us the target density, and our CBR rig applies the standard surcharge rings and plunger at 1 mm/min until we hit 2.5 and 5.0 mm penetration. Simple test, brutal consequences if you get it wrong.
In the Cairns coastal corridor, soaked CBR at 2.5 mm penetration dictates pavement thickness. We have seen a 1% drop in CBR double the required granular overlay.
Service characteristics in Cairns
Typical technical challenges in Cairns
Cairns sits on a mix of residual clay from the Barron River floodplain and decomposed granite from the Whitfield Range. Neither behaves predictably after saturation. Older subdivisions in Manunda and Bungalow were built on what looked like stiff clay in the dry, only to soften to CBR values below 3% once the water table rose. A pavement designer relying on a field DCP alone misses the swell component. The lab CBR captures that. It also gives the soaked strength at the design density, which lets you optimise the pavement section. Overdesign and you waste money on extra gravel and asphalt. Underdesign and the pavement ruts before the first cyclone season ends. We have seen both outcomes on projects that skipped the lab phase.
Our services
Our NATA-accredited lab on the north side of Cairns handles everything from a single CBR point to full pavement investigation suites. Turnaround is typically three working days for standard soaked CBR, faster for unsoaked if the project is in a bind.
Standard Soaked CBR
Four-day soak with swell monitoring, penetration to 7.5 mm, and full load-penetration curve. The baseline for any sealed road or industrial pavement in Far North Queensland.
Unsoaked CBR for Select Fill
Quick-turnaround test at optimum moisture content. Used for fill certification where the material sits above the permanent water table and the spec allows it.
CBR with Remoulding for Stabilised Subgrades
We compact the sample at the stabiliser dose you specify, cure it, then soak and penetrate. Ideal for lime- or cement-treated layers under airport pavements or container yards.
Q&A
What does a laboratory CBR test cost in Cairns?
A standard soaked CBR test typically runs between AU$180 and AU$350 per point, depending on whether it is standard or modified compactive effort and how many points you need. Volume pricing applies for full pavement investigation programs.
How long does a soaked CBR test take from sample drop-off to report?
Four working days minimum. The sample compacts and soaks for four days, then we run the penetration on day five. Unsoaked CBR can be reported in 24 hours if the schedule is tight.
Can you run CBR on material with gravel larger than 19 mm?
Yes, but the standard mould limits particle size to 19 mm. For material with oversize, we scalp it on the 19 mm sieve and apply a correction factor based on the percentage retained. For high-oversize materials, we recommend pairing the test with a gradation analysis to refine the interpretation.
What CBR value does Cairns Regional Council require for residential subgrades?
Most local specs call for a minimum soaked CBR of 5% at 95% standard compaction for residential subgrades, but that number can rise to 10 or 15% for commercial pavements and industrial hardstands. We always recommend checking the project-specific geotechnical brief before testing.