Who this guide is for
- Owners of plots with both a wet area and a surface water arrangement
- Self-builders adding a soakaway to ground that already has land drainage
- Renovators dealing with a garden that has wetted up again
- Anyone coordinating a landscape contractor and a drainage designer
- Readers who have met each arrangement separately and never together
Opposite intentions, similar appearance
Both are trenches of permeable material with a pipe and a filter separation, and from above they can be indistinguishable. What separates them is direction. A land drain is the easier path water already in the ground moves towards; an infiltration structure holds collected surface water while the surrounding soil takes it.
The entries also put the two into different design conversations. Land drainage is laid out by the fall of the ground and the water within it; an infiltration structure is positioned by what the ground will accept and by how far it must sit from things that react to being wetted.
Why both end up on one plot
Neither arrangement is unusual, and each answers a real problem. A plot that holds water in its soil wants land drainage; a plot adding hard surface with no permitted discharge wants somewhere for the run-off to go.
The difficulty is that these are usually separate briefs, given at separate times to separate people. The land drainage entry treats this as the reason the two have to be settled together, because nothing else in the process brings them into the same conversation.
What each assumes about the ground
Land drainage assumes the soil will release water towards the trench, and its influence reaches only as far as that release extends, which is what the spacing of runs is really about. It also assumes an outfall sitting below the level the water is being asked to fall to.
Infiltration assumes the soil will accept water at a rate established by testing at the position and level of the structure, and that the seasonal high groundwater level sits clear of it. Introducing water nearby is exactly what can invalidate the second assumption.
- Land drainage: soil releases water; an outfall sits below the target level
- Infiltration: soil accepts water; the seasonal high groundwater sits clear of the face
- Both: a filter separation holds back fines so the permeable material stays open
- Both: a buried arrangement whose condition is inferred rather than observed
How the conflict shows itself
Rarely as anything dramatic. Ground that becomes wet again long after the land drainage was laid is usually reporting a silted surround, a collapsed outfall or a drain that was always in the wrong place, and telling those apart needs the layout record more than it needs an excavator.
An infiltration structure that stops accepting can be reporting a blinded face, a seasonal water table, or ground already carrying water introduced nearby. In both cases the symptom appears somewhere other than at the arrangement, which is why the record of what is where matters more than any single observation.
Where the two arrangements meet a wall
Retained ground drainage is a third arrangement that touches both. A base collector behind a wall frequently ends in a land drain run, and the entry warns that which of them the outfall level was set for matters, because a wall drain standing full of water is doing the opposite of its job.
Water collected behind a wall is also frequently taken into the ground nearby rather than to a piped system, which puts an infiltration arrangement immediately behind a structure whose design may have assumed drained ground. Whether that is workable here is a geotechnical question rather than a layout one.
The capping that changes what a drain receives
A detail small enough to be decided on site can convert one arrangement into a version of the other. A land drain given a permeable capping starts receiving run-off as well as ground water, bringing surface silt into a system whose filter was arranged for soil fines.
The entry calls that a deliberate choice with consequences rather than a finish. Whether the capping is sealed or left open is part of the design and belongs in the record, because nothing at the surface afterwards tells you which was chosen.
What lowering and adding each do to the ground
Lowering ground water changes what the surrounding soil does. Shrinkable clays and organic soils respond to drying, and whether that matters near a structure is a question for a geotechnical professional rather than for a drainage layout.
Adding water does the reverse and reaches further than people expect. The infiltration entry notes that shrinkable clays, made ground, sloping sites and existing below-ground structures all react, which is why the position of such a structure is settled with the structural and geotechnical advisers.
Settling them together
The land drainage entry states the requirement in one sentence: positions, depths and outfalls have to be settled together rather than by separate designers. That is the whole of the remedy, and it is available only before the work happens.
Where both arrangements already exist, what they are doing to each other is a determination for a drainage designer working with geotechnical advice and from whatever layout record survives. What an owner can bring is the history: what was laid, when, by whom, and what changed afterwards.
Ground water and infiltration coordination checklist
- 1Establish what drainage already exists in the ground before adding any more
- 2Ask whether any existing run is intercepting water or lowering it across an area
- 3Ask where each existing outfall is and who controls it
- 4Establish what infiltration testing supports any proposed structure
- 5Establish the seasonal range of groundwater on the plot
- 6Check whether a proposed infiltration structure sits upslope of a land drain
- 7Check whether a wall base drain discharges into a land drain and whose level governs
- 8Ask whether any land drain has been given a permeable capping
- 9Ask who is coordinating the landscape works and the drainage design
- 10Ask what separation from foundations, walls and boundaries has been assumed
- 11Ask what effect lowering or adding water would have on nearby structures and planting
- 12Record both arrangements on one drawing and keep it with the property
Common mistakes to avoid
- Commissioning land drainage and surface water drainage as unrelated jobs
- Placing an infiltration structure without knowing what already drains the ground
- Reading a wet garden as proof that more land drainage is the answer
- Assuming an outfall serves both arrangements equally well
- Capping a land drain permeably and feeding it surface silt
- Treating separation from foundations and walls as a layout convenience
- Leaving the two arrangements off the same drawing, so nobody sees the conflict
When to involve a professional
- Altering ground water conditions can affect neighbouring land and structures
- How ground accepts or releases water is established by investigation on that site
- Positions, depths and outfalls for both arrangements are settled together
- Separation from foundations and retaining structures is a geotechnical question
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
Can a soakaway and land drainage exist on the same plot?
The entries treat them as commonly combined while doing opposite things, with land drainage taking water out of the soil and an infiltration structure deliberately putting it in. What they require is that positions, depths and outfalls are settled together rather than separately.
My land drainage worked and now the garden is wet again. Why?
The entry lists the usual candidates: a silted permeable surround, a collapsed or drowned outfall, or a drain that was always in the wrong place. Telling those apart needs the layout record more than it needs an excavator.
Does a land drain pipe need cleaning?
The collecting is done by the permeable surround rather than by the pipe, and that surround cannot be cleaned. A silted pipe can sometimes be rodded where access exists, which is why catchpits and rodding eyes are the whole of the maintenance provision.
Who should coordinate the two arrangements?
The entries place the coordination with the drainage designer, working with geotechnical advice, rather than with whichever trade was asked first. Where both arrangements already exist, that coordination begins with establishing what is actually in the ground.
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