How long does a percolation test take?

What takes the time?

A rainwater soakaway test involves preparing the trial pit, recording its geometry and ground conditions, filling with water and observing the fall. Repeat fills provide evidence of how the ground performs after wetting. Excavating the pit is only part of the visit.

The soil, test depth and water level affect the fall. Conditions can differ across a garden and between layers of ground, so a quick result at a neighbouring property does not give a finish time for your test.

We agree the locations, depths, water supply, repeat testing and reinstatement before booking. Send us your designer’s specification or planning request so the quotation covers the work you need.

What happens if the water is still draining at the end of the day?

Where the site arrangements allow it, we leave the Percologger monitoring the water level and return for the next fill when the test is ready. A test can cross midnight without losing the distinction between one day’s readings and the next.

This is where automated monitoring is particularly useful. A measurement before leaving and another on returning show the overall change. Readings taken between those visits provide a fuller record of how it happened, including when the water passed the target levels.

We plan access and the protection of the trial pit before leaving equipment. A water-filled excavation needs suitable protection; monitoring equipment is not a substitute for a safe site.

Live progress depends on the site connection. The report is based on the available recorded evidence, checked for gaps, changes in level and any interruption that affects interpretation.

Manual measurements and automated readings

A properly carried out manual test can provide valid infiltration evidence. The practical challenge is keeping observations close enough together throughout a long fall. Our Percologger helps us maintain that record between visits.

More readings reduce the gaps between observations. They do not make a measurement exact or prove that a soakaway will work. The value is a clearer record for checking the result.

What will the result tell my designer?

The infiltration coefficient, f, is reported in metres per second. It describes the rate supported by the test at that location and depth. Your drainage designer uses it with the proposed drainage area, storage, rainfall and other site information.

The report brings together the trial-pit location and dimensions, ground observations, repeated fill-and-drain records and the calculation. A crossing time found between recorded points is an interpolation; a projected crossing beyond the recorded fall is an extrapolation. Those are different forms of evidence and should be explained.

A long trial-pit run is not, by itself, the designed soakaway’s half-emptying check. That depends on the proposed drainage design. You can commission our ground test for the coefficient alone and pass the report to your own designer.

Our price is fixed for the agreed testing scope, however long that test takes. Return fills are planned as part of the quotation. Your report follows the next day after the final testing visit, once testing is complete and we leave site.

What should I send before booking?

• The site postcode and a plan or photograph showing the proposed test area.
• Your designer’s required pit positions, depths and test specification, if available.
• Access width, buried-service information and any restrictions on excavation or water supply.
• Anything affecting overnight access or protection of the test area.

Tell us whether this is for a rainwater soakaway or a sewage treatment plant. Foul-water drainage fields need a different assessment: see drainage field testing.

Questions about longer tests