A common mistake we see on Sunshine Coast projects is assuming sandy soils will always drain fast enough for on-site wastewater or stormwater infiltration. That assumption often backfires when a perched water table or thin clay lens appears just below the surface. Without a properly conducted infiltration test using either the Porchet or double-ring infiltrometer method, the design flow rate remains a guess. Our field team has tested across coastal and hinterland sites here, and the variability between a well-drained dune sand and a silty sand over B horizon can be dramatic. Getting the measured infiltration rate right avoids costly redesigns and compliance failures under the Plumbing and Drainage Act.
A single infiltration test on a dry day can overestimate the design rate by 300% if the perched water table is ignored.