Too many building contractors in Auckland assume the sandy fill at their site will drain well. Then the first heavy rain arrives and the retaining wall moves. That happens because the fines fraction was never quantified. Grain size analysis using sieves and a hydrometer shows exactly how much silt or clay is mixed with the sand. Without that data you cannot predict permeability or compaction behavior. We run the full test in our Auckland laboratory to give you a breakdown from gravel down to the finest colloids. It is the first step before any cimentaciones superficiales design or backfill specification.
A soil with 12% passing the No. 200 sieve behaves completely different to one with 35% fines. Grain size analysis shows you the line.
Methodology and scope
Auckland sits on the Auckland Volcanic Field with over 50 eruption centres. The soils here range from weathered basalt clays to pumice sands and alluvial silts. That variety demands a test that covers the full particle spectrum. Our sieve analysis handles material down to 75 microns and the hydrometer extends the curve into the clay fraction. We report the coefficients of uniformity and curvature plus the percentages of gravel, sand, silt and clay. For road subgrades we often combine it with a CBR vial to link grading to bearing capacity. If the project involves deep foundations we also cross-check the data with a clasificacion suelos index test. The result is a unified classification that guides every subsequent geotechnical decision in Auckland.
Technical reference image — Auckland
Local considerations
The volcanic soils of Auckland can be deceptive. A sample that looks like clean sand in the field may contain 18% silt after wet sieving. That changes the drainage class from free to moderate. The risk is that retaining walls and pavements designed for free-draining soil will saturate and lose strength. Our grain size analysis catches that discrepancy early. The hydrometer step is essential here because the fines are often non-plastic and invisible to the naked eye. Skipping it leads to drainage failures and cracked slabs within two years.
Grading curve, Cu, Cc, USDA or NZGS textural class
Associated technical services
01
Dry Sieve Analysis (Gravel & Sand Fractions)
For coarse-grained soils where the fines content is low. We wash the sample, dry it, and pass it through a stack of sieves from 75 mm down to 75 µm. The result is a grading curve and the percentage retained on each sieve. Suitable for road base, select fill, and granular subgrades.
02
Wet Sieve & Hydrometer Analysis (Full Grading)
For soils with significant silt or clay content. The sample is dispersed with sodium hexametaphosphate, wet-sieved on the No. 200 sieve, and the fines are tested in the hydrometer sedimentation column. We produce a continuous curve from gravel to sub-micron clay. Essential for Auckland's volcanic clays and alluvial silts.
Applicable standards
NZS 4402 (Standard Test Methods for Particle-Size Distribution of Soils Using Sieve Analysis), NZS 4402 (Standard Test Method for Particle-Size Analysis of Soils — Hydrometer Method), NZS 4402:1986 (Methods of Testing Soils for Civil Engineering Purposes)
Frequently asked questions
What is the difference between sieve analysis and hydrometer analysis?
Sieve analysis separates particles by shaking them through a stack of wire mesh sieves down to 75 microns. Hydrometer analysis uses Stokes' law to measure the settling velocity of particles in a water column from 75 microns down to about 1 micron. The two methods together give a continuous grading curve for any soil type.
How much does a grain size analysis cost in Auckland?
The typical cost for a combined sieve and hydrometer analysis in Auckland ranges from NZ$160 to NZ$340 per sample, depending on whether the soil requires wet preparation and the number of sieve fractions requested. Bulk project discounts are available for multiple samples.
Do I need both sieve and hydrometer for my project?
If the soil is a clean sand or gravel with less than 5% fines, dry sieve alone is usually sufficient. For any soil that contains visible silt, clay, or weathered material — which is common across Auckland's volcanic and alluvial deposits — the hydrometer step is necessary to classify the fines properly. We can advise based on a visual assessment of the sample.
How long does the test take and what sample size is required?
Dry sieve analysis takes 1 to 2 working days. Wet sieve plus hydrometer takes 3 to 5 working days because the sedimentation column needs 24 hours for the fine fraction to settle. Sample size depends on the maximum particle size: for gravelly soils we need about 5 kg; for sands and silts, 500 g to 1 kg is sufficient.
Location and service area
We serve projects across Auckland and its metropolitan area.
