Getting the footings right is one of the most critical steps in building a retaining wall. A post that isn’t embedded deep enough — or isn’t set in enough concrete — will rotate under soil pressure, causing the wall to lean, bow, or fail. This guide covers footing depth, concrete volume, and the key variables that affect both.

For a full build walkthrough, see our guide on how to build a concrete sleeper retaining wall.

The Rule of Thumb: One-Third in the Ground

The standard rule of thumb for retaining wall post embedment is that the in-ground portion should be at least one-third of the total post length. This means:

• 1.5m post — minimum 500mm in the ground (1.0m above ground)
• 1.8m post — minimum 600mm in the ground (1.2m above ground)
• 2.1m post — minimum 700mm in the ground (1.4m above ground)
• 2.4m post — minimum 800mm in the ground (1.6m above ground)

This is a minimum guide for standard residential conditions. In heavy clay soils, near boundaries, with surcharge loads, or for walls over 1.0m, greater embedment depth may be required. Always confirm with a structural engineer for walls over 1.0m. See our full guide on how deep retaining wall posts should be.

Factors That Increase Required Embedment Depth

• Heavy clay soils — clay exerts significantly more lateral pressure than sandy soils. Increase embedment depth in clay.
• Surcharge loads — driveways, vehicle parking, or structures above the wall increase the forces on the posts. Engineering sign-off required.
• Taller walls — the taller the wall, the greater the overturning moment on each post. Embedment depth must increase accordingly.
• Wider post spacing — wider spacing increases the load on each post, which may require deeper embedment.
• Proximity to boundaries or structures — walls near boundaries or structures typically require engineering certification, which will specify embedment depth.

Footing Hole Dimensions

The footing hole should be:

• Diameter: approximately 300mm for standard C-channel posts; 350–400mm for H-beam posts. The hole needs to be large enough to allow concrete to flow around the post and achieve adequate cover on all sides.
• Depth: embedment depth plus 100–150mm below the post base. The concrete below the post base provides a bearing pad that resists the post being pushed down under vertical load.

Use a post hole digger, hand auger, or excavator to dig the holes. In rocky ground or hard clay, a hydraulic auger attachment on a mini-excavator is the most practical option.

Check for underground services before digging. In Australia, call Dial Before You Dig (1100) to get a service location report for your site before any excavation.

How Much Concrete Per Post Footing?

The volume of concrete required per footing depends on the hole diameter and depth. As a practical guide for standard residential retaining wall posts:

• 300mm diameter hole, 600mm deep: approximately 0.042m³ — roughly 2 bags of 20kg rapid-set concrete
• 300mm diameter hole, 700mm deep: approximately 0.049m³ — roughly 2–3 bags
• 300mm diameter hole, 800mm deep: approximately 0.057m³ — roughly 3 bags

These figures are for the gross hole volume. The post itself displaces some of this volume, so actual concrete consumption will be slightly less. As a practical rule, allow 2–4 bags of 20kg rapid-set concrete per post for standard residential walls, and order 10–15% extra to account for variations in hole size and soil conditions.

See our concrete and cement range.

Concrete Mix and Curing

For post footings, use either:

• Rapid-set concrete — sets in 15–30 minutes, allowing you to continue work the same day. Ideal for most residential retaining wall post footings. Available in 20kg bags.
• Standard concrete mix — slower setting (24–48 hours to initial set, 7 days to full strength). More economical for large volumes but requires longer wait before loading the posts.

Do not load the posts with sleepers until the concrete has achieved adequate strength — 24–48 hours for rapid-set, 7 days for standard mix. Loading posts before the concrete has cured can cause them to rotate out of plumb.

Setting Posts Plumb

Before pouring concrete, check each post for plumb (vertical) in both directions using a spirit level. Brace the post temporarily while the concrete sets. Check alignment along the wall line before the concrete sets — correcting alignment after the concrete has cured is difficult and time-consuming.

Pour the concrete around the post, filling to slightly above ground level. Slope the top of the concrete away from the post to shed water and reduce moisture ingress at the post base.

When to Get Engineering Sign-Off on Footings

The footing depths in this guide are general guidance for standard residential conditions. Get engineering sign-off on footing design when:

• Wall height exceeds 1.0m
• Surcharge loads are present above the wall
• Soil is heavy clay, reactive, waterlogged, or unstable
• Wall is near a boundary or structure
• Council approval requires engineering certification

A structural engineer will specify the required footing depth, diameter, and concrete strength for your specific site conditions. See our guide on retaining wall height limits and council approval to understand when engineering is required.

Frequently Asked Questions

How deep should retaining wall post footings be?

As a general rule, the in-ground embedment should be at least one-third of the total post length. For a 1.0m wall, this means a minimum of 500mm in the ground (1.5m post). In heavy clay soils or for taller walls, greater embedment is required — confirm with a structural engineer.

How much concrete do I need for retaining wall post footings?

Allow 2–4 bags of 20kg rapid-set concrete per post for standard residential walls. Order 10–15% extra to account for variations in hole size. For large projects, calculate the gross hole volume (pi × radius² × depth) and convert to bags.

Can I use rapid-set concrete for retaining wall post footings?

Yes. Rapid-set concrete is the most practical choice for residential retaining wall post footings. It sets in 15–30 minutes, allowing you to continue work the same day. Do not load the posts with sleepers until the concrete has achieved adequate strength — at least 24 hours for rapid-set.

Do I need to put gravel in the bottom of the footing hole?

In well-drained soils, concrete directly on the base of the hole is standard practice. In waterlogged or poorly drained soils, a 50–100mm layer of compacted gravel at the base of the hole can improve drainage around the post base and reduce moisture-related corrosion risk.

Ready to Order?

Browse our concrete and cement range, or see our full steel post range including C-channel posts and H-beam posts. Contact our team if you need help confirming the right post and footing specification for your project.