A common mistake we see on Gold Coast projects is assuming that a single sieve curve tells the whole story about a soil. In this coastal environment, many sands contain measurable fines that a dry sieve alone misses entirely. Without a combined sieve and hydrometer analysis, contractors often underestimate the plasticity and compressibility of the finer fraction, leading to unexpected settlement under structures. We have seen this firsthand on residential slab foundations in the northern suburbs and on road embankments near the Nerang River. A full grain size distribution, down to 0.001 mm, should be the baseline for any earthworks or foundation design in this region, not an afterthought.
A full grain size distribution down to 0.001 mm should be the baseline for earthworks and foundation design in the Gold Coast region.
Methodology and scope
- Wet sieving through 2.36 mm, 425 µm, and 75 µm sieves to remove clay coatings
- Sedimentation with ASTM 152H hydrometer calibrated at 20°C
- Correction for temperature and dispersant (sodium hexametaphosphate)
- Curve plotting on semi-log paper with D10, D30, D60 and Cu, Cc calculations
Local considerations
The subtropical climate of Gold Coast, with its summer rainfall averaging over 1200 mm annually, forces fines migration and alters the in-situ grain size distribution over time. A sample taken in the dry season may show a coarser curve than the same soil after a wet season. That discrepancy can mislead drainage design and slope stability calculations. On canal-front properties along the Broadwater, the high water table combined with fine sands and silts creates a classic liquefaction-prone profile. Running a hydrometer analysis on those soils is not optional; it is part of the ensayo de licuefacción screening we perform for every waterfront development. Ignoring the silt fraction in those settings means underestimating pore pressure buildup during a seismic event.
Applicable standards
AS 1289.3.6.1:2009 – Determination of the particle size distribution of a soil – Standard method of analysis by sieving, AS 1289.3.6.3:2003 – Determination of the particle size distribution of a soil – Standard method of fine analysis using a hydrometer, AS 1289.3.6.1 – Standard test methods for particle-size distribution (gradation) of soils using sieve analysis
Associated technical services
Combined Wet Sieve and Hydrometer (Full Curve)
For soils with more than 12% passing the 75 µm sieve. Includes wet sieving, sedimentation with hydrometer, and full curve plotting. Delivers D10–D60, Cu, Cc, and USDA textural classification. Ideal for foundation design and earthworks specifications.
Dry Sieve Only (Coarse-Grained Soils)
For clean sands and gravels with fines content below 5%. Uses mechanical shaker and nested sieves from 75 mm down to 75 µm. Quick turnaround for pavement subgrade and drainage layer assessments.
Hydrometer Only (Fine-Grained Soils)
For clays and silts where the entire material passes the 2.36 mm sieve. Direct hydrometer sedimentation with temperature and dispersant correction. Used in conjunction with Atterberg limits for full fine-soil characterisation.
Typical parameters
Frequently asked questions
What is the difference between sieve analysis and hydrometer analysis?
Sieve analysis separates particles down to 75 µm using a stack of woven wire sieves. Hydrometer analysis measures the sedimentation rate of particles finer than 75 µm, down to about 0.001 mm, based on Stokes' Law. The two methods are combined when the soil contains significant fines, because dry sieving alone cannot differentiate between silt and clay sizes.
How much does grain size analysis cost in Gold Coast?
The typical range for a combined sieve and hydrometer test in Gold Coast is between AU$160 and AU$300 per sample, depending on the number of sieves used and whether wet pre-treatment is required. Dry sieve only is generally at the lower end, while full hydrometer with multiple dispersant checks sits near the upper end. Volume discounts apply for projects with more than ten samples.
Why is hydrometer analysis important for Gold Coast soils?
Gold Coast's coastal and alluvial soils often contain 15% to 30% fines that a sieve alone cannot resolve. The hydrometer reveals the percentage of clay and silt, which directly influences plasticity, compressibility, and drainage behaviour. For canal-front sites and low-lying suburbs, this information is critical for liquefaction assessment and foundation design.