AS 1289.6.1.1 drives every soaked CBR we run in the lab here in Perth. The standard matters here because of the Bassendean Sand and Guildford Clay. You get a clean sand that looks strong in the pit, but the CBR collapses to 3–5% after four days soaking. We see it all the time. Limestone fills from the coastal plain hit 80% or higher, but the variability is brutal if the crusher hasn't screened the fines. Before we compact the mould, we often run a quick grain size check to pick the oversize fraction. If the material is a clayey sand from the eastern suburbs, we pair the CBR with an Atterberg limits run because the plasticity index tells you how much swell to expect during the soak. Our NATA-accredited lab in Perth runs a 50 kN frame, and we log the load-penetration curve at 1.27 mm/min. No shortcuts. The plunger face gets cleaned between each specimen, and we trim the surcharge weights to match the design pavement depth. That level of detail is what separates a Perth pavement that holds up from one that rutts after the first wet winter.
A soaked CBR on Bassendean Sand can drop to 3% even when the unsoaked value reads 25%—that four-day soak reveals the real pavement life.
Approach and scope
Site-specific factors
The contrast between a Perth coastal plain limestone fill and a Swan Valley alluvial clay is stark. In the western suburbs like Cottesloe, a crushed limestone CBR can hit 80–90% unsoaked and still hold 60% after four days soaking—great for a residential access road. Ten kilometres east in Middle Swan, the same test on a clayey sand from the alluvium gives you 15% unsoaked and 4% soaked. That's the difference between a 200 mm pavement and a 350 mm one in the same council area. The risk isn't just the CBR number. It's the swell. A clay with a PI above 20% can swell 3–5 mm during the soak, and if the pavement designer ignores that, you get edge cracking within two winters. We've seen it on industrial hardstands in Kewdale where the fill was imported from a Guildford pit and nobody checked the soaked CBR before signing off. The fix isn't complicated: run the test early, pick the design CBR at the 90th percentile, and if the swell exceeds 1.5%, consider a lime-stabilised subgrade.
Service video
Relevant standards
AS 1289.6.1.1:2014 – Soil strength and consolidation tests – Determination of the California Bearing Ratio of a soil – Standard laboratory method for a compacted specimen, AS 1289.2.1.1:2005 – Soil moisture content tests – Determination of the moisture content of a soil – Oven drying method (standard method), AS 1289.5.2.1:2017 – Soil compaction and density tests – Determination of the dry density/moisture content relation of a soil using modified compactive effort, AS 1289.3.6.1:2009 – Particle size distribution – Sieving method
Related technical services
Soaked CBR (4-day standard)
Three-point compaction at 25, 10, and 5 blows per layer, four-day soak under surcharge, swell logging every 24 h, penetration to 12.5 mm. We report CBR at 2.5 and 5.0 mm, dry density curve, moisture content, and swell versus time. This is the standard for Main Roads WA and most Perth council specifications.
Unsoaked CBR (immediate penetration)
Same three-point compaction, but the specimen is tested immediately after compaction—no soak. Used for granular basecourse materials where the pavement is sealed quickly or for temporary haul roads. We still log the full penetration curve and apply the zero correction. Often paired with a Proctor test to confirm the compaction target.
Typical parameters
Top questions
What does a laboratory CBR test cost in Perth?
A standard three-point soaked CBR test in our Perth lab runs AU$200–AU$270 per sample, depending on whether you need the unsoaked companion or additional swell monitoring. The price includes specimen preparation, the four-day soak, penetration testing, and the signed report with the density-moisture curve.
How long does the CBR test take from sample drop-off to report?
Five working days minimum. Day one: moisture conditioning and compaction. Days two through five: four-day soak with daily swell readings. Day five: penetration testing and report. If the material is a clean sand from Perth's coastal plain, we can often turn the unsoaked portion around in two days, but the soaked result always needs the full soak period per AS 1289.6.1.1.
Can you test material with particles larger than 37.5 mm?
AS 1289.6.1.1 specifies a maximum particle size of 37.5 mm for the standard CBR mould. If your Perth fill contains oversize cobbles or rock fragments, we scalp the material on a 37.5 mm sieve and apply the oversize correction. For materials with significant oversize fractions—common in quarry-run limestone from the Perth Hills—we recommend running a parallel grain size analysis to quantify the scalping adjustment.