What Type of Pipe Is Used for Retaining Wall Drainage?

The pipe used behind a retaining wall is a perforated agricultural pipe, commonly called ag pipe or an ag drain. In Australian residential construction the usual specification is 100 mm diameter corrugated agricultural pipe, laid at the base of the wall in clean angular aggregate, running to a continuous fall, and discharging to a stormwater connection. In clay soils the pipe is normally supplied with a geotextile sock, and on sites where the drain has to be cleaned out later a slotted PVC pipe is used instead.

The Short Answer: Perforated Agricultural Pipe

Agricultural pipe is a flexible corrugated polyethylene pipe with slots or perforations moulded through the corrugation valleys. Water in the surrounding aggregate enters through those perforations, collects inside the pipe, and runs along it to an outlet. It is used behind retaining walls because it is flexible enough to follow the line of the wall, cheap enough to run the full length of it, and open enough to take water in along its entire run rather than at a few points.

The alternative that appears on engineered and commercial jobs is slotted PVC, which is a rigid pipe with saw cuts or drilled perforations along its length. It costs more and takes longer to lay, but it holds its grade, it does not deform under compacted backfill, and it can be rodded or jetted if it silts up. On a wall retaining anything of consequence, that maintainability is often worth more than the saving on the pipe.

Retaining Wall Drainage Pipe Types Compared

Solid unperforated pipe has a place in the same system, but not as the collector. It is used downstream of the drain to carry collected water to the discharge point without letting it back into the ground on the way. Using solid pipe where perforated pipe belongs, or perforated pipe where solid pipe belongs, is a common and expensive mix up.

Socked or Unsocked Ag Pipe in Melbourne Clay

Most of Melbourne sits on reactive clay soils classified Class M, H1, H2 or E under AS 2870. Clay produces fine particles that migrate with moving water, and those particles are what block a drainage system. In that context the sock is not optional.

The important detail is that a sock does not replace the aggregate. A socked pipe laid straight into clay backfill will blind off, because the fabric ends up in direct contact with the material it is meant to be filtering. The sock protects the pipe perforations, the surrounding clean aggregate provides the flow path, and a separate geotextile wrap around the aggregate zone keeps the clay out of the aggregate. Each layer has a job, and skipping one transfers its job to a layer not designed for it.

What Diameter of Pipe Is Used

100 mm is the standard diameter behind residential retaining walls and is adequate for most domestic wall heights and catchments. 150 mm is used where the wall is long, where the catchment feeding it is large, where the wall is retaining a slope that concentrates surface flow, or where the fall available is shallow and the extra capacity compensates for the low grade.

Diameter is rarely the constraint in practice. A 100 mm pipe carries a substantial flow, and drainage systems behind walls almost never fail because the pipe was too small. They fail because water could not get into the pipe, or could not get out of it.

The Pipe Is Only Part of the Drainage System

Fall is the step most often missed. A drainage pipe laid dead flat, or worse laid over an uneven trench base so that it dips, holds standing water along its length. Around 1 in 100 is the grade commonly specified on residential details, which is a drop of about 10 mm over each metre of wall. Over a long wall that adds up, and the invert level at the outlet end has to be set before the trench is dug rather than discovered afterwards.

Mistakes That Stop a Drainage Pipe Working

• Laying unsocked pipe directly against clay backfill, so the perforations block within a season or two
• Using the excavated clay as backfill instead of importing clean aggregate
• Omitting the geotextile wrap around the aggregate zone
• Using pea gravel or crusher dust, which do not hold open voids the way angular aggregate does
• Laying the pipe flat or letting it dip between supports
• Discharging at the base of the wall, which returns the water to the soil the drain just removed it from
• Discharging onto adjoining land, which creates a liability problem as well as a drainage one
• Crushing the pipe during compaction because no cover was maintained over it
• Providing no access point at either end, so the line can never be inspected or cleared

What the Australian Standards Require

AS 4678:2002 Earth retaining structures requires drainage provisions for earth retaining structures, and treats drainage as part of the structural design rather than a landscaping accessory. The reason is straightforward: the design of the wall assumes a level of water pressure behind it, and that assumption only holds if the drainage works. A wall designed on a drained condition and then built undrained is loaded well beyond what it was designed for.

AS 2870 Residential slabs and footings governs the site classification that describes how reactive the soil is. That classification matters for retaining wall drainage because it indicates how much the soil moves with moisture change and how poorly it drains. AS 3500.3 covers stormwater drainage and is the relevant reference for where collected water is legally allowed to go.

When the Drainage Has to Be Engineered

A low garden wall on free draining ground is a landscaping job. Beyond that, drainage becomes part of an engineered design, and the situations that trigger it are the same ones that trigger the requirement for a structural engineer generally. The threshold question is covered in the guide on what height retaining wall requires a structural engineer, and the approvals position is set out in can I build a retaining wall without a permit in Victoria.

Drainage should be specified by an engineer where the wall is over one metre high, where it sits on or near a boundary, where the retained soil supports a structure such as a slab, garage or shed, where a driveway or parking area sits above the wall, or where an existing wall is already showing cracking, bulging or lean. Principal Built Engineering provides retaining wall engineering across Melbourne, covering site assessment, geotechnical review, design, drainage specification and certification for permit purposes. The wider drainage picture, including surface treatment and retrofitting to an existing wall, is covered in the retaining wall drainage guide.

Frequently Asked Questions

What type of pipe is used for retaining wall drainage?

A perforated agricultural pipe, usually 100 mm diameter corrugated ag pipe with a geotextile sock, laid at the base of the wall in clean angular aggregate. On engineered walls, commercial sites and long runs, slotted PVC is often used instead because it holds its grade under backfill and can be rodded or jetted if it silts up.

Should ag pipe be socked or unsocked behind a retaining wall?

Socked, in almost all Melbourne situations. The clay soils across most of the metropolitan area produce fine particles that migrate with water and block unsocked perforations. The sock filters those fines out. It does not remove the need for clean aggregate around the pipe or a geotextile wrap around the aggregate zone, because a sock in direct contact with clay will blind over.

What size ag pipe is used behind a retaining wall?

100 mm is standard for residential walls. 150 mm is used on long walls, on large catchments, on sloping sites that concentrate flow toward the wall, or where the available fall is shallow. Pipe capacity is rarely the limiting factor, because drainage systems behind walls usually fail at the inlet or the outlet rather than in the pipe.

Can you use normal stormwater pipe for retaining wall drainage?

Solid stormwater pipe cannot be used as the collector, because water has no way to enter it. It is used downstream of the drain to carry collected water to the discharge point. Slotted or perforated PVC, which is a different product, can be used as the collector and is often preferred where the line may need clearing later.

Where does the retaining wall drainage pipe discharge to?

To a legal stormwater connection, or to a dispersal point well clear of the wall, its footings and any adjoining property. Discharging at the base of the wall returns the water to the soil the drain has just removed it from, and discharging onto a neighbouring property creates a liability issue as well as a drainage one. The discharge point should be settled before the trench is dug, because it sets the invert level the whole run has to fall toward.

How do you know if a retaining wall drainage pipe has blocked?

Usually you cannot tell by looking at the pipe, which is why the failures are so common. The signs show up in the wall instead: bulging or bowing in the middle of the wall, horizontal cracking, a forward lean, efflorescence or persistent damp on the wall face, or water pooling at the base after rain instead of dispersing. Any of those warrants an assessment by a structural engineer before the movement progresses.

Getting the Drainage Right the First Time

The drainage behind a retaining wall is buried the day the wall is built and is not seen again unless something goes wrong. That makes it the part of the job least worth economising on. Principal Built Engineering assesses existing retaining walls showing signs of distress and designs new walls with the drainage specified for the site rather than copied from a generic detail. To discuss a wall, get in touch, or see the full range of Melbourne structural engineering services.