The alluvial soils along the Whanganui River demand a different approach than the volcanic ash layers you find up in St. Johns Hill. In Whanganui, a simple visual classification rarely tells the full story — the proportion of fines versus sands directly controls how a soil will behave under load and water. We run the full grain size analysis in Whanganui combining mechanical sieving for coarse fractions with a hydrometer test for the silt and clay fraction passing the 75 µm sieve. This dual method gives you a complete particle size distribution curve, from gravel down to colloidal clay, which is essential input for NZS 3404 and NZGS soil classification. For projects near the river terraces in Whanganui, we often pair this with an Atterberg limits test to confirm the plasticity characteristics of the fine portion, since Whanganui silts can exhibit surprising cohesion.
If your Whanganui site has more than 12% fines, the soil stops behaving like a free-draining granular material — and your drainage design must change.
