Grain Size Analysis (Sieve + Hydrometer) in Canberra

Canberra's geology catches people out. You hit a clean quartz sand at one site in Belconnen, then move 2 km south and the bore returns stiff silty clay derived from weathered Canberra Formation volcanics. Grain size analysis tells you what you're really dealing with. We run the full hydrometer + sieve suite because a sieve-only curve misses the fines that control permeability and shrink-swell behaviour. In our experience, the gap between a quick field classification and the actual PSD often explains why a footing design felt too conservative—or why a drainage layer clogged prematurely. Combining a test pit log with a complete particle size distribution gives you a defensible site model before you commit to earthworks volumes.

A sieve-only curve is half the answer—Canberra's silty residual profiles demand the hydrometer data to get drainage and consolidation parameters right.

Method and coverage

AS 1726:2017 sets the framework for geotechnical site investigation across Australia, and it ties directly to AS 4678 for earth retaining structures. The hydrometer analysis becomes essential when fines content exceeds about 12%, which happens frequently in the silty colluvium mapped across the Molonglo Valley. We run the mechanical sieve stack from 75 mm down to 75 µm, then switch to a calibrated 152H hydrometer for the silt and clay fraction. The combined curve feeds directly into USCS classification and lets the design engineer pick realistic permeability coefficients. Where road subgrades sit on deeply weathered S-type granites, we often pair grain size data with Atterberg limits to nail down the plasticity behaviour of the decomposed rock fines. Another scenario we see regularly is filter compatibility: a coarse sand wrapped in geotextile looks fine on paper, but without a precise D85/D15 ratio from the PSD, the spec is guessing. We also cross-check the hydrometer data against triaxial specimens when the grading suggests potentially collapsible fabric, particularly in alluvial terraces along the Molonglo River.

Regional considerations

The most expensive mistake we see in the ACT is a contractor assuming a uniform sand based on a single bulk sample, then finding metre-thick silt lenses during bulk excavation. That discovery halfway through the cut changes the battered slope angle and brings the earthworks program to a halt while the superintendent renegotiates the spec. Another pattern: a structural engineer designs a pad footing on a stiff clay with D10 around 0.002 mm, but no one ran the hydrometer curve, so the actual clay fraction is 40% and the site turns reactive. We've watched retaining walls tilt within two wet seasons because the backfill grading was too uniform and the drainage column silted up. Grain size analysis is cheap insurance. When you add the hydrometer data, you stop designing for the average and start designing for the material that's actually in the ground.

Technical parameters

Parameter	Typical value
Sieve stack range	75 mm down to 75 µm (ASTM E11 sieves)
Hydrometer method	152H, ASTM D7928 / AS 1289.3.6.3 guidelines
Sample mass for fines	50 g dry mass, dispersant pre-treatment
Reported coefficients	Cu, Cc, D10, D30, D60, D85, D15
Minimum sample mass (coarse)	5 kg for material with +20 mm fraction
Lab accreditation	NATA-accredited to ISO/IEC 17025
Reporting standard	AS 1726:2017 borelog format with PSD chart

Complementary services

Combined Sieve + Hydrometer Suite

The standard package for foundation investigations: mechanical sieving from 75 mm to 75 µm plus a full 24-hour hydrometer sedimentation run. Delivers the complete PSD curve, USCS classification, and key coefficients.

Washed Sieve Analysis

A single wash-through determination of the minus-75 µm fraction, useful for quality control on concrete aggregates and filter sands where the fines percentage is the critical parameter.

Hydrometer-Only Fines Analysis

For sites where the coarse fraction is already well characterised and the priority is the silt/clay split. We use dispersant pre-treatment and temperature-controlled sedimentation to AS 1289.3.6.3.

Filter Compatibility Assessment

Using the full PSD we calculate D15, D50, and D85 for both the base soil and the filter material, checking against the Terzaghi retention and permeability criteria referenced in AS 4678.

Q&A

What does a combined sieve and hydrometer test cost in Canberra?

A combined sieve-plus-hydrometer analysis typically runs between AU$180 and AU$280 per sample, depending on the maximum particle size and whether the sample needs drying, splitting, or special dispersant preparation. We quote firm once we know the material type and the number of samples.

How long does the hydrometer part of the test take?

The sedimentation phase requires readings at specific intervals over a minimum of 24 hours, plus an overnight pre-soak with dispersant. Most reports are ready within two to three working days from sample receipt.

Can you run grain size analysis on samples from our own drilling crew?

Yes, as long as the samples are sealed in airtight bags and labelled with depth, borehole ID, and date. We prefer disturbed bulk samples of at least 500 g for fine soils and 5 kg where gravels are present.

Why does Canberra's geology make the hydrometer step so important?

Much of the urban area sits on deeply weathered Ordovician and Silurian rocks that produce a silty, micaceous residual soil. The fines content directly affects drainage, compaction behaviour, and reactivity, so skipping the hydrometer often leads to an overestimate of permeability and an underestimate of volume change potential.