Most retaining wall failures aren't caused by bad materials or poor construction — they're caused by inadequate site preparation. Skipping steps at the beginning leads to expensive problems later: walls that lean, crack, bow or collapse entirely.

This guide covers everything you need to do before a single post goes in the ground.

Step 1: Check If You Need a Permit

In Victoria, retaining walls over 1 metre in height generally require a building permit. Some councils have lower thresholds, and walls near boundaries, structures or drainage easements may require permits regardless of height.

Before you start:

• Contact your local council to confirm permit requirements
• Check if the wall is within a drainage or sewerage easement (contact Melbourne Water or your local water authority)
• Identify any planning overlays that may apply (heritage, vegetation, flood)

Building without a required permit can result in a demolition order — the cost of doing it right upfront is always less than the cost of doing it twice.

Step 2: Call Before You Dig

This is not optional. Underground services — gas, water, electricity, telecommunications and sewer — run through residential properties in locations that aren't always obvious.

Call Dial Before You Dig on 1100 (or visit www.1100.com.au) to get a free plan of underground services in your area. Allow 2–3 business days for the response.

Hitting a gas or electrical service is dangerous and expensive. Hitting a sewer or water main can result in significant repair costs and liability.

Step 3: Assess Your Soil

Soil type has a major impact on retaining wall design and drainage requirements.

Sandy or loamy soil: Free-draining, lower lateral pressure on the wall. Standard post depths and drainage are usually sufficient.

Clay soil: Poor drainage, high lateral pressure when wet, can expand and contract significantly. Clay soils require deeper posts, more robust drainage and often engineering assessment for walls over 600mm.

Fill soil: Unpredictable — can contain voids, organic material or debris. Always investigate fill soil carefully before building on it.

If you're unsure about your soil type, dig a test hole 600mm deep and observe the soil profile. If you find heavy clay or suspect fill, consider getting a geotechnical assessment before proceeding.

Step 4: Plan Your Drainage

Drainage is the single most important factor in retaining wall longevity. Hydrostatic pressure from water trapped behind a wall is the number one cause of wall failure.

Before excavating, plan:

• Where will water behind the wall drain to?
• Is there a natural low point or existing drainage outlet?
• Will you use ag pipe, weep holes or both?
• Is there a risk of water flowing from neighbouring properties?

A good drainage plan includes:

• Ag pipe (slotted, sock-wrapped) at the base of the wall, sloping to an outlet
• 150–200mm of drainage aggregate (20mm blue metal) behind the wall
• Geotextile fabric to prevent soil migration into the drainage layer
• Weep holes through the wall face at regular intervals (one per 1.8m minimum)

Step 5: Mark Out the Wall Line

Use string lines and stakes to mark the exact position of the wall. Check:

• The wall is parallel to any boundaries or structures it needs to align with
• Post positions are marked at the correct spacing (typically 1.8m centres for 2.4m sleepers)
• The wall line allows for adequate drainage space behind it (minimum 300mm between the wall and the cut face)
• There is sufficient clearance from any structures, trees or underground services

Step 6: Organise Equipment and Access

Retaining wall construction requires excavation — either by hand or machine. For walls over 10 lineal metres or in difficult soil, hiring a mini excavator is usually worthwhile.

Equipment checklist:

• Post-hole digger or mini excavator
• Concrete mixer or access to pre-mixed concrete
• Level and string line
• Compactor (for backfill)
• Safety equipment (hard hat, steel-capped boots, gloves, eye protection)

Access: Consider how materials will be delivered and moved to the work area. Concrete sleepers are heavy (a standard 2.4m sleeper weighs 80–120kg depending on thickness) and require mechanical assistance or multiple people to handle safely.

Step 7: Order Materials in Advance

Don't start excavating until your materials are on site or confirmed for delivery. Leaving an open excavation for days while waiting for materials is a safety hazard and can cause soil collapse.

Materials to order:

• Concrete sleepers (calculate using our online estimator)
• Galvanised steel posts (H or C posts, sized for your wall height)
• Drainage aggregate (20mm blue metal)
• Geotextile fabric
• Ag pipe (slotted, sock-wrapped, 100mm diameter)
• Concrete for post footings (N20 minimum)
• Backfill material (if required)

Step 8: Safety Planning

Retaining wall construction involves heavy materials, deep excavations and power tools. Before you start:

• Ensure all workers are aware of underground service locations
• Brace excavation walls if they are deeper than 1.5m or in unstable soil
• Never work alone in a deep excavation
• Keep children and pets away from the work area
• Have a first aid kit on site

Ready to Start?

Once your permits are in order, services are located, drainage is planned and materials are on site, you're ready to start building. For a step-by-step construction guide, see our article on How to Build a Concrete Sleeper Retaining Wall.

FPM Building Supplies stocks everything you need for your retaining wall project from our Epping warehouse. Contact our team for a materials quote and project advice.