A contractor in Cairns North was prepping a foundation pad last wet season. The soil looked clean enough until the lab ran a full particle size distribution. Fines content was 34 percent — not fill you can compact without drainage. That single grain size analysis changed the footing spec from a spread footing to a deeper pier-and-beam system. SPT drilling had given blow counts, but without the gradation curve nobody knew the soil would hold water. In the Cairns region, where decomposed granite and alluvial silts mix unpredictably, sieve plus hydrometer testing is not an optional extra. It defines the construction approach before concrete goes in.
A hydrometer reading at 24 hours tells you more about Cairns soil than a dozen borelogs without fines data.
Service characteristics in Cairns
Typical technical challenges in Cairns
Cairns sits on a coastal plain where the Barron River delta deposited metres of soft silty clay. Groundwater is often less than 1.5 m below the surface. Any project that underestimates the fines content in that profile risks differential settlement, poor compaction and drainage failure. Without a full hydrometer curve, you cannot calculate the Hazen coefficient for permeability. You cannot design a filter that will not clog. And you cannot correctly classify the soil for AS 4678 earthworks. A sieve-only report misses the fraction that controls behaviour. That gap has caused retaining wall failures in Edge Hill and pavement heave in Earlville. The cost of the test is negligible compared to a drainage rework on a completed slab.
Our services
Our Cairns lab provides the complete particle size distribution package, tailored to local geology.
Full Sieve + Hydrometer Package
Wash sieving through 75 µm, oven drying, sieve stack from 75 mm down to 75 µm, plus hydrometer sedimentation. One report, one curve, no gaps.
Classification & Compliance Report
Unified Soil Classification System (USCS) naming, grading coefficients (Cu, Cc), fines content and AS 1726 compliance statement.
Filter & Drainage Gradation Check
We compare your drain material gradation against the soil curve using Cistin-Ziems or similar criteria. Prevents clogging before it happens.
Q&A
When does a project in Cairns need a hydrometer test?
Any time the wash-sieve shows more than 12 percent passing the 75-micron sieve. AS 1726 requires hydrometer sedimentation for that fraction. Cairns alluvial soils often hit 30–60 percent fines.
What does the grain size analysis cost?
A complete sieve plus hydrometer package typically runs between AU$140 and AU$250, depending on sample condition and rush requirements. We quote firm before starting.
How long does the hydrometer part take?
The sedimentation readings run over 24 hours minimum, then data reduction and report drafting add one working day. Standard turnaround is three business days from sample receipt.
Do you test gravels for filter design?
Yes. We run the full coarse stack up to 75 mm. For drain aggregates we provide the D15 and D85 values your engineer needs to check piping and permeability criteria against the base soil.
What sample mass do you need?
For a fine-grained soil we need at least 200 g dry mass. For a sand-gravel mix, 500 g minimum. We can advise on sampling containers when you book the job.