Last year we assessed a site in Helensvale for a new collector road. The natural subgrade was a silty sand with variable moisture across the profile. Running a CBR study for road design in Gold Coast gave us the soaked and unsoaked bearing ratios needed to match pavement thickness to the actual ground. Without that data the design would have been guesswork. We also paired it with a density test with sand cone to confirm compaction levels at the same test points. That combination removes uncertainty for the designer.
A soaked CBR of 5% on a Gold Coast clay subgrade can mean 400 mm of granular base versus only 200 mm for a 15% CBR material.
Methodology and scope
- We use the four-point method to define the moisture-density relationship.
- Results are reported at 2.5 mm and 5.0 mm penetration.
- Corrections for swelling are applied when the sample expands during soaking.
Local considerations
Much of Gold Coast sits on Quaternary alluvium and coastal sand deposits. These materials have low dry density and high moisture sensitivity. A road built without a proper CBR study for road design in Gold Coast risks subgrade failure within two wet seasons. The most common problem is differential settlement where the pavement cracks along old drainage lines. We have seen sections of the Smith Street Motorway require early rehabilitation because the design CBR was assumed too high. Testing removes that risk completely.
Applicable standards
AS 1289.6.1.1 (CBR test method), AS 1289.5.4.1 (Compaction control), Austroads Guide to Pavement Technology Part 2 (2019)
Associated technical services
Soaked CBR Testing
Four-day soak with swell monitoring. Delivers the design CBR used for flexible pavement thickness.
Unsoaked CBR Testing
Rapid assessment for preliminary designs or low-traffic roads where moisture exposure is minimal.
Field CBR
In-situ penetration test using the dynamic cone penetrometer (DCP) correlated to CBR. Useful for existing pavements.
CBR Correlation with DCP
We run DCP profiles across the site and calibrate against laboratory CBR values for a site-specific relationship.
Typical parameters
Frequently asked questions
How deep should I dig for CBR samples on a Gold Coast road project?
For new pavements we sample at subgrade level, typically 300 to 500 mm below the finished surface. If the subgrade is variable we take one sample per 500 m².
What is the difference between soaked and unsoaked CBR?
Soaked CBR simulates worst-case moisture after heavy rain. Unsoaked CBR reflects dry conditions. In Gold Coast we always recommend soaked CBR because the water table is high and rainfall exceeds 1,400 mm per year.
How much does a CBR study for road design cost in Gold Coast?
The typical range is between AU$270 and AU$550 per sample. This includes compaction, soaking, penetration testing and a certified report. Volume discounts apply for projects with more than ten samples.
Do you also perform compaction testing for the pavement layers?
Yes. We run field density tests using the sand-cone method or nuclear gauge. The lab CBR results are most useful when combined with field compaction records.
How long does the CBR test take?
The soaking period is four days. After that we run the penetration test and report within two business days. Total turnaround is six to seven working days from sample receipt.