GRW/Drainage/TBT-GRW-015

Soakaway Construction

Groundworks & Earthworks › Drainage › Soakaway Construction

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Soakaways are underground chambers or trenches that allow surface water to infiltrate into the surrounding ground.
Types include rubble-filled pits, precast concrete ring soakaways, and modular geocellular crate systems.
Excavations for soakaways can be deep, often 2 to 4 metres, requiring full excavation support controls.
The base of the soakaway must reach permeable ground below any clay or impermeable surface layers.
Precast concrete rings weigh several hundred kilograms each and require crane lifting into the excavation.
Workers entering soakaway excavations to lay geotextile or connect pipes face collapse and access risks.
Groundwater encountered during excavation must be managed to allow construction of the soakaway base.
Backfilling around soakaways must use the specified granular material to maintain infiltration performance.
Percolation test results determine the soakaway size and design; incorrect sizing causes flooding.
CDM 2015 and excavation safety requirements apply to all soakaway construction regardless of depth.

Deep excavation	Soakaway excavations are often deeper than expected to reach permeable ground, creating significant collapse risk.
Heavy components	Precast rings dropped or mishandled during crane placement crush workers in the excavation below.
Flooding failure	Incorrectly constructed soakaways fail during storms, flooding properties and causing costly damage claims.

Don't

Support or batter excavation sides before anyone enters the soakaway dig.
Use a crane with a lift plan for placing precast rings into the excavation.
Provide safe access and egress from the excavation at all times during work.
Manage groundwater with pumping to keep the excavation base dry during construction.
Use the specified granular backfill material to maintain infiltration around the soakaway.
Wrap geocellular crates in geotextile to prevent silt clogging the drainage voids.
Verify the excavation reaches the permeable ground layer identified in the design.
Establish exclusion zones during crane lifting of heavy precast components.
Inspect the excavation support daily and after any rainfall events.
Record the construction details including depths and backfill materials for handover.

DON'T enter an unsupported soakaway excavation regardless of the soil conditions.
DON'T lower precast rings into the excavation without a crane and lift plan.
DON'T backfill with clay or fine material that blocks infiltration into the ground.
DON'T assume the required depth matches the design; permeable layers may be deeper.
DON'T stand in the excavation while precast components are being lowered in.
DON'T leave deep soakaway excavations open and unbarriered when unattended.
DON'T connect surface water to the soakaway before the system is fully constructed.
DON'T compact backfill directly against geocellular crate systems.
DON'T skip the groundwater management step if water is present in the excavation.
DON'T ignore the percolation test results when sizing and constructing the soakaway.

See also: Below Ground Drainage Installation | Excavation Safety Awareness

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