Grain Size Analysis (Sieve + Hydrometer) in Pukekohe: Particle Distribution for Geotechnical Design

Pukekohe’s development accelerated through the 1960s as market gardens gave way to residential subdivisions, and with that shift came a quiet geological challenge that still defines local earthworks today. The volcanic loams overlying weathered Waitemata Group siltstone create a layered profile where coarse pumiceous topsoil masks fine-grained subsoil, and assuming uniformity across a building platform has led to more than one costly drainage failure south of the town centre. Our laboratory team runs the combined sieve-and-hydrometer analysis precisely because that transition zone—where the sand fraction drops and the silt/clay fraction climbs—controls both permeability and stiffness in ways that standard visual logging misses. For deeper investigation we often pair the particle distribution curve with CPT testing to correlate fines content with cone resistance across the full soil column.

A particle distribution curve is not just a classification exercise—it is the first reliable predictor of drainage behaviour, frost susceptibility, and compaction response in Pukekohe’s volcanic-derived soils.

Methodology and scope

A mistake we see repeatedly on Franklin district sites is accepting a single sieve stack result as the complete particle size distribution when more than 12% of the sample passes the 75 µm sieve. The hydrometer portion is not optional at that point—it is the only routine method that quantifies the silt-versus-clay split below 75 µm, and that split determines whether a fill material will drain freely or hold water under repeated traffic loading. Our procedure follows the NZGS guideline for fine-grained soils, using sodium hexametaphosphate dispersion and ASTM D7928 hydrometer readings at 15 s, 30 s, 1 min, 2 min, 4 min, 15 min, 30 min, 60 min, 240 min, and 1440 min. We report the full curve from the 37.5 mm sieve down to 2 µm clay size, with coefficients of uniformity and curvature calculated automatically. The raw data stays traceable to the balance calibration log, which matters when earthworks specifications are disputed on site.

Local considerations

A medium-density residential project off Beatty Road hit a layer of reworked tuff that looked like a silty sand in the cut face but held over 40% clay fraction when the hydrometer curve came back from our lab. The contractor had already placed 600 mm of granular fill over the exposed formation, assuming it would drain to the perimeter subsoil drains. Within three months of wet winter weather the platform was ponding water because the clay-rich tuff formed an aquitard that trapped moisture beneath the imported fill. The full grain size analysis—sieve plus hydrometer—would have flagged the clay fraction before fill placement, and the drainage design would have included a geotextile blanket and deeper cut-off drains. In Pukekohe’s winter-saturated ground, guessing the fines content from field texture alone is a risk that converts a few hundred dollars of lab testing into tens of thousands in remedial drainage.

Technical parameters

Parameter	Typical value
Sieve range (dry)	75 mm – 75 µm (full stack per NZS 4407)
Hydrometer range	75 µm – 2 µm (ASTM 152H, sedimentação)
Dispersion agent	Sodium hexametaphosphate (4% solution)
Minimum sample mass	200 g for fine-grained; 500 g for sandy soils
Coefficients reported	Cu, Cc, D10, D30, D50, D60, D85
Hydrometer readings	10 intervals from 15 s to 1440 min
Reporting standard	NZGS soil description guideline + ASTM D2487

Associated technical services

Full combined sieve and hydrometer analysis

Complete particle distribution from 37.5 mm to 2 µm, with Cu, Cc, and USCS classification reported per NZGS guidelines. Suitable for earthworks compliance and drainage design.

Soil classification to NZGS and USCS

Dual classification output using both the NZGS field description system and ASTM D2487 USCS, so the results are compatible with local council requirements and international design standards.

Fines content verification for fill specifications

Targeted hydrometer analysis on the minus-75 µm fraction to confirm compliance with TNZ M/4 or project-specific fines limits before material is placed and compacted.

D85 and filter compatibility assessment

Extraction of D85, D50 and D15 from the full curve for filter design, drainage aggregate specification, and geotextile selection in subsurface drainage systems.

Applicable standards

NZS 4407:2015 Methods of sampling and testing road aggregates, ASTM D7928-21 Standard test method for particle-size distribution of fine-grained soils using the sedimentation (hydrometer) analysis, NZGS Guideline for the field classification and description of soils, ASTM D2487-17 Standard practice for classification of soils for engineering purposes (Unified Soil Classification System)

Frequently asked questions

When is a hydrometer analysis required in addition to sieve testing in Pukekohe?

A hydrometer analysis is required whenever more than 12% of the sample passes the 75 µm sieve. Pukekohe’s volcanic soils frequently contain a significant silt and clay fraction that cannot be quantified by sieving alone. The hydrometer determines the fine particle distribution down to 2 µm, which is essential for USCS classification and for predicting drainage and compaction behaviour.

How much does a combined sieve and hydrometer test cost in Pukekohe?

A combined sieve and hydrometer analysis in our Pukekohe laboratory typically ranges from NZ$170 to NZ$280, depending on sample condition, the number of sieving stages required, and whether additional pretreatment such as organic matter removal is necessary.

What sample mass do you need for a grain size analysis?

For fine-grained soils we require a minimum of 200 grams of material passing the 2 mm sieve, while sandy soils need at least 500 grams of the bulk sample. The sample must be representative of the stratum in question and should be taken from a disturbed bag sample or split from a larger bulk sample in accordance with NZGS sampling guidelines.