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....
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About Drainage & Base
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. Everything in this group is about getting water out and getting the base right.
The four parts, and what each one does
| Product | Job | Sizing rule |
|---|---|---|
| Drainage gravel | Forms the free draining zone behind the sleepers | 300mm wide zone, full height, full length |
| Agi pipe | Collects the water and carries it to a discharge point | 65mm to 1m high, 100mm at 1m and above |
| Geo fabric | Stops fines migrating into the gravel and clogging it | Full wall length plus overlap on joins |
| Cement and concrete bags | Sets the posts into the pier holes | About 16 bags per post per metre of pier depth |
A worked example
A 10 metre wall, 1 metre high, in 2380mm sleepers needs 6 posts. That comes out at roughly 3m3 of drainage gravel, 10 metres of agi pipe, 10 metres of geo fabric plus overlap, and about 96 bags of concrete for six 450mm pier holes 1 metre deep. Double the wall height and the gravel volume doubles with it.
The wall calculator works all of that out from a length and a height, and the drainage guide sets out quantities per lineal metre if you want to check it by hand.
Build order
- Line the excavation behind the wall with geo fabric in a U shape.
- Lay the agi pipe at the base, graded so water runs to an outlet that actually goes somewhere.
- Backfill with drainage gravel, never soil or road base.
- Fold the fabric over the top of the aggregate and cover the last 200mm with soil.
The drainage management guide covers the sequence and the cross section diagram shows how it all sits together.
Two things before you order
Bulk 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. Check Delivery and Installation before you order.