Soakaways
Soil infiltration rate from a BRE 365 pit test
- where
- volume between 75% and 25% depth, Vp75–25 (m³)
- pit length (m)
- pit width (m)
- effective depth of water (m)
- area of sides to 50% depth plus base, ap50 (m²)
- soil infiltration rate (m/s)
- time to fall from 75% to 25% (min)
Explain this equation
This works out how quickly the ground soaks water away, from a BRE Digest 365 trial pit test, so a soakaway can be sized.
L and W are the pit’s plan size and d the depth of water at the start. Vp is the water lost between 75% and 25% full, ap the area of sides up to half depth plus the base, t the time that fall took in minutes, and f the infiltration rate in m/s.
Water leaves through the wetted soil surface, so the rate is the volume lost divided by the area it soaked through and the time. Half the depth is used as the average wetted height over the test. The ×60 converts minutes to seconds.
Sense check: sandy gravel may give 10⁻⁴ m/s or faster; clay often 10⁻⁶ m/s or slower. A 1.5 × 0.6 m pit falling from 75% to 25% of 1 m in an hour is about 4 × 10⁻⁵ m/s.
Watch out: design with the slowest of the three test fills, not the first, fast one.
Infiltration rate of the soil from the falling-head trial pit test in BRE Digest 365, for sizing a soakaway.
Enter your values to see the result.
Good to know
Dig a trial pit to the depth of the proposed soakaway, fill it rapidly with water and time the fall from 75% to 25% of the effective depth. Do the test three times, ideally on the same or consecutive days, and design with the slowest (lowest) result. f = Vp75–25 / (ap50 × tp75–25), with ap50 the sides to half depth plus the base. Typical rates run from about 10⁻³ m/s in gravel to 10⁻⁶ m/s in silty clay; below 10⁻⁶ m/s soakaways are not normally practical.
Common mistake
Using the fastest of the three fills. The first fill soaks dry ground quickly; the later, slower results are the ones to design with.
Limits
Rectangular, vertical-sided pit. If the water does not fall to 25% the test must be extrapolated as BRE 365 describes or the site may be unsuitable.
Sources
- BRE Digest 365 Soakaway design (2016): f = Vp75–25 / (ap50 × tp75–25); S = A·R − as50·f·D; ts50 = 0.5·S / (as50·f) ≤ 24 h
- Unda, BRE 365 infiltration rate calculator (Vp = ½·L·W·deff; ap50 = (L + W)·deff + L·W)
- West Devon Borough Council, Infiltration testing (BRE 365 test method; f below 1×10⁻⁶ m/s unsuitable)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.