Soakaway — StormWater Solutions

Soakaway is a sub-surface infiltration structure — typically a pit, trench or modular crate system — that disposes of surface water run-off by allowing it to soak into the surrounding ground. Soakaways are widely used in the UK where soil permeability is adequate and where discharge to a sewer or watercourse is impractical.

When a soakaway is appropriate

Soakaways are only viable where four conditions are met. The ground must have sufficient permeability, normally tested by an infiltration test to BRE Digest 365. The water table must be at least 1 m below the base of the soakaway. The structure must be set back from foundations (5 m minimum to buildings per Building Regs H3) and from highways and trees. The site must not be located in a Source Protection Zone where pollutants could reach groundwater.

Types of soakaway

Three forms are common in the UK, each suited to different scales and ground conditions:

• Traditional rubble-filled pit — granular fill (40–80 mm clean stone) wrapped in geotextile. Simple, cheap, used for domestic roof drainage.
• Modular crate soakaway — high-void plastic crates wrapped in geotextile, providing 95% void ratio versus 30% for granular fill. Compact footprint for commercial and residential SuDS.
• Pre-cast concrete or HDPE chamber — used for large-scale infiltration where access for inspection is required.

Sizing using BRE Digest 365

BRE Digest 365 (1991, revised 2016) is the UK standard for soakaway design. The procedure has three steps. First, determine the soil infiltration rate (f) from at least three percolation tests at the proposed depth. Second, calculate the required storage volume for the 1-in-10-year storm (domestic) or 1-in-30-year (commercial), with a 1-in-100-year check for flood risk. Third, verify that the soakaway empties within 24 hours after the storm.

The required surface area for infiltration is calculated from S = V / (f × t), where V is storage volume, f is the soil infiltration rate (m/s) and t is the half-empty time (50% of total storage volume drained).

Related ViaCon solutions

ViaCon supplies corrugated steel infiltration tanks and complete SuDS systems where soakaway-style discharge is required. See our ViaCon Storm Solutions, stormwater solutions and geotechnical solutions. Related glossary entries: SuDS, permeability and French drain.

Frequently asked questions about soakaway

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