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Cross-market term

Soakaway and infiltration structures

A void in the ground that receives surface water and lets it disperse into the surrounding soil, rather than discharging it to a sewer or a watercourse. Sized by how fast that soil accepts water and by how much water arrives.
  • 4Markets
  • NoSame thing everywhere
  • 3Calculators

What it is called, by market

One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.

  • United Kingdom

    Close, not identical

    soakaway

    also soakage pit, infiltration crate, rubble soakaway

    Covers both the traditional stone-filled pit and the modern modular crate system, which hold very different volumes of water per volume of excavation.

  • Australia

    Close, not identical

    soakwell

    also soak well, absorption trench, leach drain

    Soakwell normally means a precast concrete or polymer chamber rather than a filled pit, and is a standard stocked product in a way the British equivalent is not.

  • United Statesyours

    Same word, different thing

    drywell

    also dry well, seepage pit, infiltration trench, French drain

    French drain is the trap: in American usage it frequently means a perforated pipe in gravel that CONVEYS water elsewhere, which is a different job from dispersing it into the ground.

  • Canada

    Close, not identical

    drywell

    also soakaway pit, infiltration gallery, sump

    As the United States, with frost depth a governing constraint that milder markets do not have to design around.

The governing standard, by market

  • United Kingdom

    BRE Digest 365 / Approved Document H3

    Soakaway design, including the percolation test method and the storage volume calculation.

  • United States

    ASTM D3385 / local stormwater manuals

    Infiltration rate of soils using a double-ring infiltrometer, and jurisdiction-specific sizing rules.

Calculators for this

Each works in either measurement system, and the terminology on the page follows whichever market you have selected.

French Drain Gravel Calculator →PICP Permeable Pavement Infiltration Storage Calculator →Foundation Sheet Drain Drainage Rate Calculator →

Frequently asked questions

Why does a soakaway need a percolation test rather than a rule of thumb?
Because the soil is the whole design and it varies enormously over short distances. A soakaway has to store the water that arrives during a storm and release it into the ground before the next one, so its size is set by two numbers: the volume arriving, which comes from the roof or paved area and the design rainfall, and the rate the ground accepts it, which comes from a test. That rate can differ by a factor of a hundred between a sandy gravel and a clay, and it can differ substantially between two corners of the same garden. The test — BRE Digest 365's method fills a pit three times and times the fall from 75% to 25% full — exists because that number cannot be guessed. A soakaway sized without it is either several times bigger than needed or useless, and the useless case is the one that shows up as a flooded lawn.
How much water does a rubble-filled pit actually hold?
Around a quarter to a third of its own volume, which is the fact that surprises people pricing one. Water is stored in the VOIDS between the stones, not in the stones, so a pit filled with clean single-size aggregate stores roughly 30% of the excavation — meaning a soakaway needing three cubic metres of storage needs about ten cubic metres of hole. Modern modular crates exist precisely because of that arithmetic: they are around 95% void, so the same storage fits in a little over three cubic metres of excavation, and on a constrained site that difference decides whether a soakaway is possible at all. The trade is cost per unit and the need for a silt trap upstream, because a crate cannot be dug out and washed the way a stone pit theoretically can.
Where can a soakaway NOT go?
Close to a building, close to a boundary, in ground that is already wet, and above certain things. British guidance keeps them a minimum distance from any structure — commonly five metres — because the whole purpose of the thing is to put water into the ground, and putting it next to a foundation is putting it under the foundation. They cannot work where the water table rises into them seasonally, which is why a test done in August and a soakaway used in February can disagree completely. They are restricted over contaminated ground, in groundwater protection zones, and where infiltration could destabilise a slope. The commonest real-world failure is none of these: it is a soakaway discharging into ground that was disturbed and backfilled during the build, which behaves nothing like the ground that was tested.