Invercargill’s flat topography and high water table create a soil profile where fine-grained silts and clays dominate, often interbedded with gravel lenses from the Oreti River floodplain. The city sits on deep alluvial deposits that can vary dramatically over short distances—one borehole might hit clean gravel while another, just metres away, encounters fat clay. This variability makes grain size analysis indispensable for any geotechnical investigation. A sieve stack alone won’t tell the full story when 30 percent of your material passes the 75-micron screen. That’s where the hydrometer step comes in, quantifying the silt-versus-clay fraction that governs drainage, frost susceptibility, and long-term settlement. In our experience across Southland, skipping the full combined analysis is the fastest way to misclassify a soil—and misclassifying it leads to undersized drainage or overestimated bearing capacity. We run every sample through both the mechanical sieve series and the sedimentation hydrometer method, reporting the full particle-size distribution curve under Grain Size Analysis and the NZGS soil classification system so engineers receive a complete picture before committing to foundation depths or pavement subgrade treatments.
A hydrometer curve isn't just lab decoration—it tells you whether that grey Invercargill silt will drain freely or hold water like a sponge, and the difference changes your whole subgrade strategy.
Methodology applied in Invercargill
Local geotechnical conditions in Invercargill
One thing you learn working on Invercargill sites is that the top metre of soil often looks nothing like what sits at two metres. The surface can be a silty loam with reasonable drainage, but just below it there’s frequently a dense, over-consolidated glacial till with a clay matrix that holds moisture for months. If the grain size analysis stops at the sieve portion, that clay fraction gets lumped into ‘fines’ and the real behaviour of the soil remains hidden. We’ve seen retaining wall designs fail because the backfill material, classified only by sieve, turned out to have a plasticity index above 20 once the hydrometer data was paired with Atterberg Limits. In Southland’s seismic setting, fine-grained soils with more than 35 percent silt and clay also require careful evaluation of cyclic softening potential. A full grading curve gives the first screening for liquefaction susceptibility—soils with more than 15 percent fines may behave very differently from clean sands, and the NZGS-MBIE guidance module relies on accurate grain size data to make that call.
Our services
Our Invercargill laboratory runs the full particle-size distribution workflow under one roof, from sample preparation to final NZGS classification. We offer two service levels depending on project requirements.
Standard Combined Analysis (Sieve + Hydrometer)
Covers the complete mechanical sieve series plus 24-hour hydrometer sedimentation. Delivered with a particle-size distribution chart, NZGS group symbol, and commentary on drainage and compaction implications for subdivision earthworks, pavement subgrades, and foundation assessments across Invercargill.
Wash Sieve Analysis (Coarse Fraction Only)
For gravelly materials where fines content is expected to be below 5 percent—common in Oreti River extraction pits and clean aggregate stockpiles. Includes a full sieve stack after washing, with gradation parameters reported against NZTA M/4 specification envelopes.
Quick answers
Why does Invercargill soil need a hydrometer test instead of just a sieve analysis?
Much of Invercargill sits on alluvial silts and clays deposited by the Oreti River and its tributaries. These soils often contain more than 20 percent material finer than 75 microns. A sieve analysis alone cannot distinguish between silt and clay, yet that distinction controls drainage, frost heave potential, and seismic site classification under the NZGS-MBIE guidelines. The hydrometer step measures the sedimentation rate of the fines fraction, giving the silt-clay split that engineers need for accurate geotechnical design.
How long does a combined sieve and hydrometer analysis take in the lab?
The full procedure typically runs over two to three working days. The sample is oven-dried overnight, then the sieve portion takes approximately four hours including washing through the 75-micron screen. The hydrometer sedimentation series requires readings at 0.5, 1, 2, 4, 8, 15, 30, 60 minutes, and then at 4 and 24 hours. We batch multiple samples simultaneously, so turnaround for a set of three to five specimens is usually three working days from receipt.
What standard do you use for soil classification from the grain size data?
We classify soils using the NZGS Soil Classification System, which is the standard adopted across New Zealand geotechnical practice. The system uses the particle-size distribution together with Atterberg limits to assign a group symbol (such as CL, ML, or GM) and a full description. This aligns with NZS 4402 methods and the requirements of NZS 3404 and NZS 4203 where soil properties influence structural design parameters.
What does a grain size analysis cost for a typical Invercargill residential project?
For a standard combined sieve-and-hydrometer analysis, the cost ranges from NZ$180 to NZ$320 per sample, depending on whether the soil is predominantly fine or contains significant gravel that requires a larger initial mass. A wash sieve analysis for clean gravels falls at the lower end of that range. We recommend budgeting for at least two to three samples on a typical residential section to capture the variability across the site.
Can you test samples we collect ourselves, or do you need to do the sampling?
We accept samples collected by contractors or consultants provided they are sealed in airtight bags, labelled with the date, depth, and location, and delivered to our Invercargill lab within 48 hours of extraction to preserve natural moisture content. That said, for critical projects—especially where the fine fraction is expected to be high—we recommend our team handle the sampling to ensure the material is representative and undisturbed where needed.