Footing depth is one of the most critical — and most frequently underestimated — aspects of retaining wall construction. A wall with inadequate footings will lean, crack, or fail, often within a few years of installation.

This guide covers the minimum footing depths required for retaining walls in Victoria, how soil type affects your requirements, and what you need to know before you start digging.

Why Footing Depth Matters

A retaining wall footing does two things:

1. Anchors the post or wall base against the lateral (sideways) pressure of the retained soil
2. Distributes the load of the wall and soil into stable ground below

The deeper the footing, the more resistance it has to being pushed over. This is why taller walls require deeper footings — the soil pressure increases with wall height, and the footing needs to counteract it.

Minimum Footing Depths by Wall Height

These are practical guidelines for concrete sleeper retaining walls with steel H posts in typical Melbourne soil conditions:

Wall Height	Minimum Post Footing Depth	Total Post Length
Up to 600mm	600mm	1.2m minimum
600mm–1.0m	800mm	1.8m minimum
1.0m–1.2m	1.0m	2.1m–2.4m
1.2m–1.5m	1.2m	2.7m–3.0m
Over 1.5m	Engineer to specify	Engineer to specify

Important: These are minimum guidelines. In poor soil conditions, on slopes, or where water pressure is a factor, deeper footings are always better.

How Soil Type Affects Footing Requirements

Not all soils are equal. Melbourne's geology varies significantly across the metropolitan area, and your soil type has a direct impact on footing requirements.

Clay Soils (Common in Melbourne's Outer Suburbs)

Clay is the most challenging soil type for retaining walls because:

• It expands when wet and contracts when dry, creating movement
• It retains water, increasing hydrostatic pressure behind the wall
• It has lower bearing capacity than rock or gravel

In clay soils, increase footing depth by at least 200mm beyond the minimums above, and ensure your drainage system is excellent.

Sandy or Gravelly Soils

Sandy soils drain well and exert less pressure on retaining walls. Minimum footing depths are generally sufficient, but check that the sand is stable and not prone to erosion.

Rock

If you hit rock at shallow depth, you may not be able to achieve the minimum footing depth. In this case, consult an engineer — there are alternative footing methods for rocky ground.

Fill Soils

If your site has been filled (common on new estates and subdivided blocks), never rely on fill for footing support. Dig through the fill to undisturbed natural soil below.

The One-Third Rule

A widely used rule of thumb in the trade is:

Footing depth = at least one-third of total post length

So if you need 1.5m of post above ground, you want at least 750mm in the ground — meaning a minimum 2.25m post. Most builders round up to the next standard length.

This rule works well for typical conditions. For walls over 1.2m, or in challenging soil conditions, always get engineering advice.

Footing Width and Concrete

The footing hole diameter should be at least 3 times the post width. For a 100mm UC post, that means a minimum 300mm diameter hole.

Fill the hole with concrete — rapid-set concrete is convenient for post footings and reaches working strength within a few hours. Key points:

• Mix concrete to a stiff consistency — it should hold its shape, not flow
• Ensure the post is plumb before the concrete sets
• Brace the post while the concrete cures (at least 24 hours before loading)
• The top of the concrete footing should be slightly above ground level and sloped away from the post to shed water

Frost and Seasonal Movement

Melbourne doesn't experience significant ground frost, so frost heave is not a major concern. However, the clay soils common in Melbourne's outer suburbs do move seasonally — expanding in winter and contracting in summer. This is another reason to go deeper than the minimum in clay soils.

When Do You Need a Building Permit in Victoria?

Under the Victorian Building Regulations, a retaining wall generally requires a building permit if:

• The wall is over 1 metre high (measured from the bottom of the footing to the top of the wall)
• The wall is within 1.5 metres of a building, boundary, or easement
• The wall is on a slope or in a situation where failure could affect neighbouring properties

If your wall requires a permit, you'll also need engineering drawings and a certificate of compliance. FPM Building Supplies has engineering certificates available for our concrete sleeper and H post systems, which can simplify the permit process.

Practical Tips for Getting Footings Right

• Use an auger or post hole digger for clean, straight holes — hand digging rarely produces a hole straight enough for a plumb post
• Check for underground services before you dig — call Dial Before You Dig (1100) to locate any pipes or cables
• Don't rush the concrete cure — wait at least 24–48 hours before backfilling or loading the posts
• Keep records — photograph your footings before backfilling; this documentation is useful if you ever need to prove compliance

Materials for Retaining Wall Footings

FPM Building Supplies stocks everything you need for retaining wall footings:

• Rapid-set concrete — sets in 15–30 minutes, ideal for post footings
• General purpose concrete mix — for larger poured footings
• Steel H posts — hot-dip galvanised, multiple sizes
• Post caps — to protect the top of each post

Visit us at 32 Fullarton Dr, Epping VIC 3076, or call +61 431 235 919 to discuss your project requirements.