Retaining Wall Drainage Guide: Agi Pipe, Gravel and Geo Fabric

Does a retaining wall need drainage?

Yes, and it is not a finishing detail. Water is the single biggest reason retaining walls fail. Soil that cannot drain builds hydrostatic pressure behind the wall, and that pressure is on top of the earth pressure the wall was designed to hold. The Concrete Masonry Association of Australia puts it plainly in its guide to concrete masonry gravity retaining walls: a properly functioning drainage system is effective in removing hydraulic pressure. That is why drainage is a structural component.

The ABCB Housing Provisions identify retaining walls as a typical location where a subsoil drainage system may be needed to manage subsurface water, and where one is installed they require the drain to be graded with a uniform fall of not less than 1:300 and to discharge into an external silt pit or sump.

This page is about what to buy and how much of it. If you are already on site and building, our drainage management guide covers the same components as step six of the installation sequence, and the installation cross section diagram shows how it all sits together.

The four things that go behind the wall

1. A free draining aggregate zone against the back of the wall, so water can get down rather than building up.
2. A geotextile between the retained soil and that aggregate, so fine soil does not wash into it and clog it.
3. A slotted subsoil pipe at the bottom of the aggregate zone, graded to carry the water away.
4. An outlet, somewhere lawful for the water to go. Without this the other three are a bucket.

Agi pipe: socked or unsocked

Socked pipe has a filter fabric sleeve bonded around the outside. Its job is to keep fines out of the pipe itself so the pipe does not silt up and stop working.

Published Australian guidance for these walls specifies a filter layer at the pipe. The Concrete Masonry Association guide specifies a 100 mm slotted PVC agricultural pipe with a geofabric sock, or an equivalent system. A NATSPEC based council specification for masonry walls specifies a 100 mm diameter slotted corrugated plastic pipe with a seamless tubular filter fabric.

When unsocked is reasonable

Unsocked pipe relies on the surrounding system to do the filtering instead: clean, washed aggregate with the drainage zone properly wrapped in geo fabric, so the fabric separates soil from aggregate and the aggregate separates soil from pipe. Done properly that is the equivalent system the guidance allows for.

When to pay for socked

Use socked pipe if there is any doubt about the aggregate being clean, about the fabric wrap being continuous, or about fines reaching the pipe from below or from an unwrapped end. The price difference between socked and unsocked is small. The cost of getting it wrong is a silted drain sitting under a wall you cannot dismantle to fix. On that trade, socked usually wins.

We stock both: socked agi pipe and unsocked agi pipe, in the drainage collection.

Pipe diameter by wall height

We carry 60 mm, 65 mm and 100 mm coils in both socked and unsocked, in 20 m and 50 m lengths depending on the diameter.

Two honest qualifications. First, both of the published specifications quoted above nominate 100 mm for the walls they cover, so 100 mm is never the wrong answer and is the safer choice on a long run, a wet site or anywhere the catchment behind the wall is large. Second, where an engineer has produced a design for your wall, that design governs, and above the approval threshold in your area there should be one. See our Queensland council approval guide.

Grade and outlet

The pipe has to fall. Published minimum grades differ by source, so here they are side by side:

• ABCB Housing Provisions: uniform fall of not less than 1:300, discharging into an external silt pit or sump
• Concrete Masonry Association of Australia: minimum uniform grade of 1 in 100, connected to the stormwater system
• NATSPEC based council specification: even line and uniform grade of 2 per cent minimum fall towards the outlet

These are minimums from different documents covering different wall types. More fall is easier to achieve at set out than it is to retrofit, so if the site allows it, take the steeper number.

The outlet has to be lawful. Connecting a subsoil drain into a council stormwater system, or discharging it across a boundary, may need council consent or a plumbing approval. The Queensland Development Code NMP 1.7 references AS/NZS 3500.3.2 for stormwater drainage in the context of retaining walls. Work the outlet out before you dig, not after the wall is up.

Gravel: 10 mm or 20 mm, and how much

Our drainage gravel comes in 10 mm and 20 mm, in volumes from 3 m3 to 10 m3.

Both sit inside the range the published guidance nominates. The Concrete Masonry Association specifies nominal 10 mm to 20 mm poorly graded gravel placed to a minimum thickness of 300 mm adjacent to the wall stem. The NATSPEC based specification is broader, calling for broken stone or river gravel of clean, hard, durable particles graded from 10 mm to 50 mm, with no more than 5 per cent passing the 9.5 mm sieve by mass.

Practically:

• 20 mm has larger voids, so it drains faster and is harder to choke. It is the default for taller walls and wet sites.
• 10 mm is easier to place into a narrow zone and to bed a pipe into without leaving voids under it. It suits lower walls and tight working room.

What matters more than the choice between the two is that whatever you use is clean and free of fines. Note the 5 per cent limit on material passing the 9.5 mm sieve in that specification. A dirty aggregate defeats the drainage zone from day one, which is the same failure the geotextile exists to prevent.

Working out the volume

Use a 300 mm wide drainage zone behind the wall, which matches the minimum thickness in the published guidance. Then:

Gravel volume in cubic metres = wall length in metres x 0.3 x wall height in metres.

That is 0.3 cubic metres per lineal metre for every metre of wall height. Some worked examples:

Order the next volume up rather than down. Running out of gravel halfway along a trench means an unwrapped, part filled drainage zone sitting open until the next delivery.

Gravel is delivery only. We supply and deliver it and there is no pickup option. Tell us the volume and the delivery suburb and our team will confirm the freight cost with the rest of your order. See delivery and installation.

Geo fabric: which face, and how much

The geotextile does one job: separation and filtration. It sits between the retained soil and the drainage aggregate so that fines do not migrate into the aggregate and clog it. The NATSPEC based specification requires a layer of geotextile between the wall units and the granular drainage layer.

Two things follow from that.

• The fabric goes on the soil side. Its purpose is to stop soil entering the aggregate, so it has to separate the two. Fabric laid against the back of the sleepers instead, with soil in direct contact with the aggregate, does not do the job.
• The joins matter. A gap or an unlapped join is a direct path for fines into the drainage zone. Lap the joins generously and follow the design or the fabric manufacturer's instruction for the overlap, rather than a number off a web page.

One caution from the standards side: not all geotextiles are interchangeable, and using the wrong fabric can allow fines through and defeat the whole system. If your wall has an engineered design, use the fabric it specifies.

Our geo fabric rolls come as 1 m x 50 m and 2 m x 50 m, in the geo fabric collection. Choose the roll width that covers the height of the excavated face plus enough to wrap the top of the aggregate zone, so a 2 m roll for taller walls and a 1 m roll for low garden walls. On length, allow the run of the wall plus the laps.

Quantities at a glance

The materials calculator runs all of this alongside the sleeper and post take off, so you get one list. It is an estimate rather than a quote.

The five ways drainage gets built wrong

• No outlet. The most common and the most expensive. A drainage zone with nowhere to discharge is a sump behind your wall.
• Fabric on the wrong face, or omitted entirely, so the soil and the aggregate are in direct contact and the zone silts up over a few wet seasons.
• Dirty aggregate. Gravel with fines in it clogs itself. This is what the 5 per cent passing 9.5 mm limit in the specification is guarding against.
• Backfilling with site clay. Excavated clay put back against the wall holds water rather than shedding it. The aggregate zone exists so that does not happen.
• Pipe laid flat or with a back fall. Water needs a continuous fall to the outlet. A dip in the middle of a run holds water and silt.

Fire ants and bulk quarry material

If you are buying gravel anywhere in South East Queensland, fire ant biosecurity applies to you as the receiver as well as to us as the supplier. Movement controls are law under the Biosecurity Act 2014, and fire ants are a category 1 restricted pest.

Regulated fire ant carriers include soil, mulch, manure, hay and mining and quarry materials, and processed materials expressly include gravel, fines, sand and crusher dust. That is most of what goes in behind a retaining wall.

• Queensland has two fire ant biosecurity zones, and they are reviewed and updated monthly. What applies to a load depends on where the material came from, where it is going, and the zones on that day. That is why we do not publish a zone map. Check the live map and the movement advice tool at fireants.org.au on the day you order.
• The 24 hour rule. Within a zone you may move regulated material if you move it within 24 hours of it being delivered, or you follow the required management steps, or it goes directly to an approved waste facility in the permitted zone.
• If you stockpile it longer than 24 hours, the obligation is yours. The storage options are off the ground and covered, such as a tarped trailer or a shed, with no chemical treatment required; or on the ground and covered on a fire ant resistant surface of concrete, bitumen or 200 micron plastic sheeting with a 30 cm treated perimeter; or on compacted treated ground with the full surface treated. The only permitted insecticide for these treatments is a bifenthrin product applied in accordance with APVMA permit PER14317 and the product label.
• The disturbance alternative is to vigorously disturb the whole stockpile by turning, crushing, washing or screening it at least every 21 days and within 24 hours before moving it.
• Records. Written records of treatment, chemical application and disturbance must be kept for a minimum of 2 years.
• Leaving the zones needs a permit. For soil and for mining and quarry materials, moving material out of the fire ant biosecurity zones, or from Zone 2 into Zone 1, is prohibited unless a biosecurity instrument permit is granted.
• Crossing into New South Wales is a separate regime again. See our Northern New South Wales page for what applies to gravel and rocks under 20 mm heading south.

The eradication program's own advice to buyers is to ask the seller whether the material has been produced and stored in accordance with the regulation. Ask us. We would rather you asked than assumed. Fire ants and non compliance can be reported on 13 22 68.

What to order

• Drainage: socked and unsocked agi pipe and drainage gravel
• Geo fabric: 1 m and 2 m rolls
• Drainage and base: the full category including bagged cement for setting posts
• Concrete sleepers and steel posts for the wall itself, with the post selection guide to size them

Send us the wall and we will do the whole take off including drainage. Request a quote with the wall length, the height, your suburb and a couple of site photos, or ring the yard.

28 Shettleston Street, Rocklea QLD 4106. Monday to Friday, 6am to 4:30pm. Phone (07) 3716 0191 or email enquiry@hybuilt.com.au.