Grain Size Analysis for Upper Hutt Ground Conditions

NZS 4402 sets the framework for laboratory soil testing in New Zealand, and in Upper Hutt its application is non-negotiable. The Hutt River terraces and the weathered greywacke of the Remutaka foothills produce particle size distributions that vary sharply over short distances: one borehole hits clean sandy gravel, the next hits silty clay with trace organics. A combined sieving and hydrometer analysis resolves the full curve from coarse gravel down to colloidal clay, giving the design team reliable D10, D30 and D60 values for filtration, drainage and compaction control. When the Terrace Gravel formation or alluvial silts from the Mangaroa floodplain are the foundation material, we avoid assumptions and run the full hydrometer procedure on every sample with fines fraction exceeding 12 percent. Before opening a trial pit in the Trentham area, we often cross-check the expected profile with an in-situ permeability test to link gradation to field hydraulic conductivity.

A two-metre sample from Upper Hutt alluvium can shift from sandy gravel to silty sand within 500 millimetres. Combined sieve and hydrometer analysis catches the transition that field logging misses.

Methodology applied in Upper Hutt

A practical observation from years testing Upper Hutt materials: the fines content in the alluvium north of the city centre is consistently higher than what the bore logs suggest. Contractors report good gravel at two metres, but the sieve stack tells a different story once the material is washed over the 75-micron screen. We routinely see 15 to 25 percent fines in what the field log describes as sandy gravel, which changes the compaction target and the permeability by an order of magnitude. Our laboratory runs the full NZS 4402:1986 sequence: mechanical sieving from 37.5 mm down to 63 µm, followed by sedimentation hydrometer analysis for the silt and clay fraction. The combined curve is plotted on a standard NZGS particle-size chart, and we flag any gap-graded or uniformly graded material that could create internal instability in drainage blankets. For structural fill imported from the Tawa or Belmont quarries, we pair the gradation result with a sand cone density field check to confirm that the placed material meets the specification envelope.

Grain Size Analysis for Upper Hutt Ground Conditions

Parameter	Typical value
Sieve range	37.5 mm to 63 µm (full stack)
Hydrometer method	NZS 4402 sedimentation
Minimum sample mass	500 g for fine soils, up to 15 kg for gravels
Dispersion agent	Sodium hexametaphosphate solution
Fines threshold for hydrometer	>12% passing 63 µm
Reporting parameters	D10, D30, D60, Cu, Cc, % gravel, % sand, % fines
Test duration	24 to 72 hours (hydrometer)
Typical application	Earthworks spec, drainage design, liquefaction screening

Critical ground factors in Upper Hutt

A cut-to-waste excavation in the Totara Park hillside hit a lens of completely decomposed greywacke that looked like competent rock in the trench wall. The contractor treated it as engineered fill and compacted it, but the grain size analysis showed 38 percent fines and a plasticity index of 14. That material failed the drainage blanket specification and had to be removed and replaced with imported granular fill. The rework cost three weeks of programme and added about $40,000 in trucking. Upper Hutt soils catch people out because the Terrace Gravel can be clean and free-draining at one location and matrix-supported with silty fines fifty metres away. Performing the hydrometer analysis on every sample with visible fines is the only way to confirm the material meets the permeability and compaction requirements of the earthworks specification before it gets placed and compacted.

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Applicable standards: NZS 4402:1986 — Soils testing methods, NZS 4404:2010 — Land development and subdivision infrastructure, NZGS Guideline — Laboratory testing of soils

Our services

Our Upper Hutt laboratory covers the full particle-size workflow, from field sample preparation through to combined-curve reporting for NZS-compliant projects.

Combined Sieve and Hydrometer

Full particle size distribution from 37.5 mm gravel down to 2 µm clay fraction, reported with NZGS classification and standard shape coefficients.

Wash Sieving and Fines Determination

Quantify the percentage passing 63 µm on washed samples before dry sieving, critical for Upper Hutt alluvial materials with variable silt content.

Gradation Envelope Compliance

Check imported fill, drainage aggregate and filter sand against specified upper and lower bound curves per NZS 4404 zone requirements.

Liquefaction Screening by Gradation

Apply the NZGS-MBIE liquefaction screening criteria using gradation curves to flag potentially liquefiable silty sand layers before committing to cyclic triaxial testing.

Frequently asked questions

What does a combined sieve and hydrometer test cost in Upper Hutt?

A full combined analysis including wash sieving and sedimentation hydrometer typically runs between NZ$160 and NZ$340 per sample, depending on the number of sieves in the stack and whether the sample requires pre-treatment for organics. Volume pricing applies for earthworks quality control programmes with five or more samples.

How long does the hydrometer analysis take?

The sedimentation hydrometer procedure per NZS 4402 requires readings at specific time intervals over a minimum 24-hour period, extending to 72 hours for material with significant clay content. We report combined curves within three to five working days of sample receipt.

When is a hydrometer required instead of just a sieve analysis?

We run the hydrometer whenever the wash sieve shows more than 12 percent passing the 63-micron screen, because the silt and clay fraction controls permeability, plasticity and compaction behaviour. For drainage design and liquefaction screening in Upper Hutt alluvium, skipping the hydrometer is not an acceptable shortcut.