The percolation rate is the single number that decides system type, bed area and most of the price. It is measured on your site, in your soil, and it cannot be inferred from the neighbours.
Perc test and soil evaluation: what it takes, how deep
A percolation test times how quickly water drops in a pre-soaked hole, which gives the rate the bed has to be designed around. On its own that number is not enough, because a fast rate over shallow bedrock or a high water table is misleading: the water leaves the hole but has nowhere safe to go.
So test pits are dug alongside, deep enough to read the soil horizons, identify mottling that shows where the water table seasonally rises, and find rock if it is there. Those three findings together are what a designer needs for the permit drawings.
Where the results rule out a conventional bed, they also point at the alternative, usually a raised bed with imported fill or an aerobic treatment unit. Either way you learn it before committing to a build, not during one.
Test pits open
How it works
How a soil evaluation runs
Choose the locations
Pits sited where the bed could actually go, including a reserve area, rather than wherever the machine can reach.
Dig and log
Horizons recorded by depth and texture, mottling noted, and depth to rock or water table measured where present.
Pre-soak and time
Holes pre-soaked so the reading reflects saturated conditions, then the drop timed over repeated intervals.
Report the rate
A written result with the pit logs, in the form the permit application needs, plus what it means for system type.
Why the season matters
A August test and an April test can disagree
Soil that drains beautifully in a dry August can sit saturated in April, and a bed designed on the August number will fail every spring, showing the usual failure signs within a few years. That is why pit logs record mottling: the colour change left by seasonal saturation tells you where the water table reaches even when it is not there on the day.
It is also why we will say when a result is weaker than it looks and should be re-checked in a wetter month. A cheap test in the wrong month is the most expensive thing on a septic project, because the bed built on it is the part that cannot be repaired.
Buyers should note that a perc test is not part of a standard property inspection. If a lot is being bought to build on, this is the test that determines whether it can be, and it belongs alongside the septic inspection.
Sited where the bed goesPits
Horizons and mottlingLogged
Permit-readyReport
Installation services
Other installation work
From a single tank swap to a full build on a difficult site.
For buyers, builders and anyone facing a replacement.
Loosely, soil that takes water neither too fast nor too slowly. Very fast soil does not treat effluent before it reaches groundwater; very slow soil will not accept it at all. Both extremes force a different system type.
Most of a day on site including pre-soak, sometimes spread across two where the soak needs longer. The pits themselves are quick; the timing is what cannot be rushed.
Often not, and where it is decades old you should not want to. Neighbouring development, regrading and drainage changes all move the answer, and the bed is the part you cannot fix later.
It means a conventional bed will not work. Raised beds, imported fill and treatment units exist precisely for those sites. See aerobic treatment unit.
They are backfilled and the surface made good, though the ground over a backfilled pit settles for a season. On a lot about to be built on it makes no practical difference.
No. A geotechnical test looks at bearing capacity; this looks at how the soil accepts and treats water. Different depths, different readings, different report.
