Subsoil and land drainage: fixing a wet section in Auckland
A lawn that stays soft through winter, a boggy corner that never dries, water sitting under a deck, a retaining wall weeping. These are groundwater problems, and they are a different discipline from the stormwater system that takes water off the roof.
Confusing the two is why so many attempted fixes do nothing.
Stormwater versus groundwater
Stormwater is surface water, collected from roofs and paving and piped away. Subsoil drainage deals with water already in the ground: perched water sitting on a clay layer, seepage moving downhill through the soil, or a high water table.
A downpipe connection will not fix a wet lawn, because the water is not arriving from above. Equally, a subsoil drain will not cope with roof runoff dumped into it.
If the ground is wet when it has not rained for a week, the problem is not your stormwater system.
What subsoil drainage actually is
The standard form is a trench cut to intercept the water movement, containing a perforated pipe bedded in clean drainage metal and wrapped in geotextile, laid to a fall so collected water is carried away to a legitimate discharge point.
Each of those components matters:
The perforated pipe collects and conveys, and it needs continuous fall to work at all
Clean drainage metal provides the void space for water to move through to the pipe
Geotextile keeps fine soil out of the metal, which is what stops the system silting up and dying quietly after two years
Position and depth determine whether the drain intercepts the water at all, which is the difference between a working system and a decorative trench
Auckland clay is the reason it is needed
Much of the region, including a great deal of the south, sits on heavy clay. Water moves through it slowly and tends to perch on top of it, sitting in whatever topsoil layer is above. That is why a section can be waterlogged while a soak pit a few metres away does nothing: the pit is trying to push water into ground that will not accept it.
This is also why soakage testing matters before choosing a disposal method. See soakage and on-site stormwater disposal.
Where the water goes
This is the question that decides whether the job is straightforward or complicated. Collected subsoil water needs a legitimate discharge, and the options are constrained: an approved connection to the stormwater system, a soakage system where ground conditions genuinely support it, or a designed overland route.
What it must not do is discharge onto a neighbouring property. Moving your water onto someone else land is how a drainage problem becomes a dispute.
Where it is used
Wet or boggy sections and lawns that will not dry out
Behind retaining walls, where drainage is a structural requirement rather than a comfort one
Around building platforms and foundations on wet sites
On benched and cut sites, where cutting into a slope intercepts water that used to flow past
On subdivisions, where lots need to be usable and buildable
Sports fields, yards and hardstand areas
On development sites this is part of the earthworks and civil package rather than a landscaping afterthought, and it is covered in earthworks.
Questions we get asked
Why is my section wet when it has not rained?
That points to groundwater rather than stormwater. Perched water on clay, seepage from higher ground, or a high water table are the usual causes.
Will a soak pit fix a wet lawn?
Usually not, and in clay it can achieve nothing at all. Soakage relies on the ground accepting water, which is exactly what is failing on most wet sections.
How deep does a subsoil drain go?
Deep enough to intercept the water it is meant to collect, which depends on the ground profile. Depth is a design decision rather than a standard figure.
Can I discharge it onto the neighbouring property?
No. Collected water needs a legitimate discharge point, and directing it onto a neighbouring property creates both a legal and a practical problem.
Why do subsoil drains stop working?
Most commonly because fine soil has migrated into the drainage metal. That is what the geotextile wrap is there to prevent, and it is why cheap installations fail quietly a few years later.
This page is general in nature. Discharge options and consent requirements vary by site. Confirm with Auckland Council.
Draeinail Construction has been putting drainage in the ground across Auckland and the Franklin district since 2006. Request a quote or call Darryn Muir on 021 271 0035.
