When the ground can absorb the water, and when it cannot

Sizing infiltration storage and soakaways, and recognising the soils that will not drain.

The engineering question

Surface water has to go somewhere. Which soils can take it, and how do you size the trench that stores it?

Worked example

The figures below were produced by running the Soakaway Calculator with the same inputs — during the build, from its authoritative engine.

Results
Storage required	3.5625 m³
Required base area	2.9688 m²
Soakage rate	50 mm/h
Square pit side (square option)	1.723 m
Assessment	Need ≈ 3.6 m³ storage; a 1.2 m-deep pit with ≈ 1.7 m × 1.7 m base (≈ 3 m²) suits the 50.0 mm/h soakage rate from the 20 s/mm test. Backfill with washed aggregate and geotextile.

The percolation test time inverts to a soakage rate in mm per hour.
Storage is the design rainfall depth times the area and the runoff coefficient.
Base area is that storage divided by the depth, giving the pit footprint.

Sanity check

Clay soils give very long percolation times. If the test result is slow, a soakaway is the wrong answer regardless of the storage calculation.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

BS EN 752-4 — soakaways and small developments
Local authority soakaway test requirements