In Blenheim, we often see projects where the initial bore log suggests one soil type, but the actual behaviour during earthworks tells a different story entirely. This is particularly common across the Wairau Plains, where the massive alluvial fan deposits laid down by the Wairau and Omaka rivers create complex interbedded sequences. A standard visual classification simply cannot capture the full story when you are dealing with silty gravels that have a significant clay fraction, or sandy silts that look uniform but contain a damaging proportion of reactive fines. The combined sieve and hydrometer analysis is what cuts through that uncertainty, quantifying the entire particle size distribution from coarse gravel right down to the colloidal clay fraction. Our laboratory on the South Island processes samples from Blenheim projects weekly, and we routinely see how this data transforms a contractor's understanding of material workability and compaction potential. For deep foundation design near the Taylor River, where liquefiable sands are a known hazard, we pair the grain size curve with an in-situ SPT program to directly correlate penetration resistance with fines content and assess cyclic mobility risk using NZGS Module 4 guidelines.
A single grain size curve from a combined sieve and hydrometer test tells you more about a soil's engineering behaviour than a dozen visual classifications ever will.
