A soakaway is a below-ground drainage structure designed to receive surface water runoff and discharge it into the surrounding soil through infiltration. Soakaways are one of the simplest and most effective SuDS techniques, removing the need for a piped connection to a watercourse or sewer entirely.

Soakaways can take several forms:

Traditional rubble-filled pits: Excavated chambers backfilled with clean, coarse aggregate. Simple to construct but offer limited storage volume per cubic metre of excavation.
Geocellular soakaways: Modular plastic crate systems wrapped in geotextile, providing approximately 95% void space compared to around 30% for aggregate-filled chambers.
Concrete ring soakaways: Pre-cast concrete rings stacked and surrounded by permeable backfill, commonly used for individual properties.

The design of a soakaway requires site-specific soakage testing in accordance with BRE Digest 365. These tests measure the soil’s infiltration rate, which is then used to calculate the required soakaway volume for a given design storm. The key design parameters are:

Infiltration rate (from BRE 365 testing)
Contributing impermeable area
Design return period (typically up to 1 in 100 year plus climate change)
Minimum clearance to groundwater (usually 1 metre below the soakaway base)
Minimum distance from building foundations (5 metres as per Building Regulations)

Soakaways are particularly suited to individual plots, driveways, and small developments where ground conditions are favourable. For larger developments, Aegaea typically incorporates soakaways as part of a wider SuDS management train within our drainage design service.