Agricultural pipe commonly called ag pipe is the primary drainage component in a retaining wall drainage system. It collects water that drains down through the drainage aggregate behind the wall and carries it to a suitable outlet. Without ag pipe, water has no controlled path out from behind the wall, and hydrostatic pressure builds until the wall fails.
This guide covers everything you need to know about ag pipe for retaining wall drainage: what size to use, slotted vs solid, sock wrapping, fall requirements, outlet options, and how to install it correctly whether you're building with concrete sleepers or another retaining wall system paired with steel posts.
Agricultural pipe (ag pipe) is a flexible, corrugated plastic pipe with slots or perforations that allow water to enter the pipe from the surrounding soil or drainage aggregate. It's available in a range of diameters and in slotted (perforated) and solid (non perforated) versions.
In a retaining wall drainage system, slotted ag pipe sits in the drainage zone behind the wall to collect water. Solid ag pipe may be used to connect the slotted section to the outlet point, particularly where the pipe runs through soil rather than drainage aggregate.
The standard ag pipe size for most residential retaining walls is 100mm diameter. This provides adequate flow capacity for the drainage loads typical of residential retaining walls in Australian conditions.
Consider larger diameter pipe when:
For most residential retaining walls under 20 25m in length on a standard suburban block, 100mm ag pipe is the correct specification.
Slotted ag pipe has slots or perforations along its length that allow water to enter from the surrounding drainage aggregate. This is the type used in the drainage zone directly behind the retaining wall. Install with slots facing downward or to the sides never upward, where soil or aggregate can fall into the pipe and reduce its flow capacity over time.
Solid ag pipe has no perforations and is used to carry water from the slotted section to the outlet point. If the pipe runs through soil to reach a stormwater pit rather than staying within the drainage aggregate zone, use solid pipe for that section to prevent soil ingress along the way.
Ag pipe for retaining wall drainage should always be wrapped in a geotextile filter sock. The sock prevents fine soil particles from entering the pipe through the slots and blocking it over time a slow failure mode that can take years to become apparent, by which point the wall may already be under elevated hydrostatic load.
Most ag pipe sold for retaining wall applications comes pre wrapped with a sock. See our guide on geotextile fabric for retaining walls for how the sock and the separate fabric layer work together in the overall system.
Ag pipe must have a continuous fall from the high end to the outlet no flat sections, no low points. Water moves through the pipe by gravity alone. A pipe that's flat, or has any low point where water can pool, will not drain effectively regardless of pipe diameter or slot design.
| Fall specification | Gradient | Notes |
|---|---|---|
| Minimum acceptable fall | 1:100 (1cm per metre) | The lowest fall that will reliably drain no margin for construction tolerance error |
| Preferred fall | 1:50 (2cm per metre) or greater | Provides margin for minor settling or construction variance without creating a low point |
| Flat sections | Not acceptable | Even a short flat section creates a low point where silt accumulates and eventually blocks the pipe |
Check with your local council about stormwater connection requirements before connecting ag pipe to the stormwater system requirements vary by state and by council. For the post specification that pairs with your drainage design, see the steel post sizing guide.
100mm diameter slotted ag pipe is the standard specification for most residential retaining walls in Australia. This size provides adequate flow capacity for typical residential drainage loads. For long walls over 30 40m, high rainfall sites, or sites with a large upslope catchment area, confirm the pipe size with a drainage engineer before ordering undersizing the pipe for the catchment it serves is a common cause of drainage overload during heavy rain.
Yes, strongly recommended for every retaining wall application. The geotextile filter sock prevents fine soil particles from entering the pipe through the slots and gradually blocking it a failure mode that develops slowly over years and is difficult to detect until the drainage system is already compromised. Most ag pipe sold for retaining wall use comes pre wrapped with a sock, so this is rarely an extra step or extra cost.
The slots should face downward or to the sides never upward. Water enters the pipe from below and from the sides as it moves through the drainage aggregate. Slots facing upward allow soil and aggregate particles to fall directly into the pipe under gravity, which can block the pipe far faster than gradual silt migration through a sock.
A minimum fall of 1:100 (1cm per metre) is required, with 1:50 (2cm per metre) or greater preferred where the site allows it. The pipe must maintain a continuous fall along its full length with no flat sections and no low points even a short flat section creates a point where silt can accumulate and eventually block the pipe, regardless of how well the rest of the run is graded.
The ag pipe must connect to a suitable outlet a daylight outlet at the end of the wall, a stormwater pit, or a sump in free draining soils only. It cannot simply terminate in the ground with no discharge point. Confirm stormwater connection requirements with your local council before finalising your outlet design, as requirements differ between states and between individual councils within the same state.
No not in the drainage zone directly behind the wall. Solid pipe has no perforations, so it cannot collect water from the surrounding drainage aggregate. Solid pipe is only appropriate for the section of the drainage run that carries water away from the wall to the outlet, once it has already been collected by the slotted section. Using solid pipe throughout would leave the drainage aggregate with no way to shed water into the pipe.
Our drainage kits include ag pipe (with sock), geotextile fabric, and weep holes in one order. Geotextile fabric is also available separately.
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