Drainage

Water is the single biggest reason retaining walls fail. Soil that cannot drain builds hydrostatic pressure behind the wall, and that pressure sits on top of the earth pressure the wall was designed for. Drainage is a structural matter, not a finishing detail, and it is the part of a materials list people trim first.

Drainage gravel

Free draining aggregate in 10mm and 20mm, sold in bulk from 3m3 up to 10m3. The larger the load the cheaper the rate per cubic metre, so it pays to work out the whole wall before you order rather than topping up later.

How much you need. Our calculator assumes a 300mm wide drainage zone running the full height and full length of the wall. On a 10 metre wall 1 metre high that is 3m3, which happens to be the smallest load we deliver. Double the height and you double the volume.

Ten millimetre gravel packs tighter and weighs about 1.68 tonnes per cubic metre. Twenty millimetre is more open and lighter at about 1.55 tonnes. Either works behind a wall. The 20mm drains a little faster, the 10mm is easier to shovel and rake into a narrow zone.

Agi pipe

The pipe collects the water the gravel lets through and carries it to a legal discharge point. Available socked and unsocked, in 60mm, 65mm and 100mm, in 20m and 50m coils.

• Size by wall height. 65mm suits walls up to 1m. Use 100mm at 1m and above, or on any wall where a lot of water drains toward it from uphill.
• Socked or unsocked. The sock is a filter sleeve that keeps fines out of the pipe. We recommend socked for most walls. Unsocked is fine where the pipe sits inside a geo fabric wrap at the base of the aggregate, because the fabric is already doing the filtering.
• One run, full length. Lay it at the base of the drainage zone behind the bottom sleeper, and make sure it actually goes somewhere. A pipe that ends in the middle of the yard has just moved the problem.

Pair the gravel and pipe with geo fabric so fines cannot migrate in and clog the zone. A blocked drain is the same as no drain.

Two things to know before you order

Drainage gravel is delivery only. We supply and deliver it to your site and do not offer pickup for it.

Fire ants. Bulk gravel, sand and soil are regulated material under the South East Queensland fire ant movement controls, and those obligations apply to you as the receiver as well as to us. Read Delivery and Installation before you order.

Our drainage management guide covers the build sequence and the drainage guide sets out quantities per lineal metre.