How to Safely Excavate Around a Concrete Retaining Wall Footing

Excavating around a concrete retaining wall footing is one of the riskier DIY tasks a homeowner can attempt — and in many cases, it's work best left to a licensed professional. The footing is the structural base that keeps the wall upright and the soil behind it stable, so disturbing the ground around it without proper planning can cause the wall to shift, crack, or collapse entirely.

Understand What You're Dealing With First

Before you pick up a spade, you need to understand the wall's construction. A concrete retaining wall footing typically extends below the finished ground level — often 300–600 mm or more depending on the wall height and soil conditions. The footing spreads the load and resists overturning forces from the retained soil.

Key questions to answer before you start:

• How tall is the retaining wall? In New Zealand, retaining walls over 1.5 m high (or over 1 m if they support a surcharge such as a driveway or building) generally require a building consent. Check with your local council — don't assume.
• Do you have existing plans? If the wall was consented, your council may hold plans showing footing depth and dimensions.
• What services are buried nearby? Water, stormwater, gas, and electrical lines are commonly found near retaining walls. Always call Dial Before You Dig (0800 248 344) or visit beforeudig.co.nz before any excavation.
• What is the soil type? Clay, sandy, or loose fill soils behave very differently and affect how quickly an excavation can become unstable.

Building Consent and Legal Requirements

Under the NZ Building Code and the Building Act 2004, excavation work that affects the stability of a neighbouring property, a building, or a retaining structure may require consent. MBIE Building Performance provides guidance on Schedule 1 exemptions, but these have specific limits — always confirm with your local council before starting. Getting this wrong can result in costly remediation and legal liability.

Plan for Ground Stability and Shoring

This is the most critical safety step. When you excavate beside or below a footing, you remove the lateral support the surrounding soil provides. The wall and the retained soil above can become unstable very quickly.

Key principles:

• Never undercut the footing. Excavating below the base of the footing removes its bearing capacity and can cause sudden collapse.
• Work in small stages. Excavate in short sections (no more than 1–2 m at a time) rather than opening up a long trench all at once.
• Shore or batter the excavation walls. For any excavation deeper than 1.5 m, WorkSafe NZ requires that the excavation be either battered back to a safe angle or properly shored with timber, steel trench boxes, or hydraulic shoring. Even shallower excavations can collapse in poor soils.
• Keep spoil (excavated soil) well away from the edge — at least 1 m back — to avoid surcharging the trench walls.
• Never work alone in or near a deep excavation.

Step-by-Step Safe Approach for Shallow Work

If you're doing minor work — for example, exposing the top of a footing to inspect it or install drainage — and the excavation is shallow (under 600 mm), here is a general approach:

1. Mark out services using Dial Before You Dig information and hand-dig cautiously near any marked lines.
2. Start away from the wall and work inward carefully, using a spade rather than a mattock or mechanical equipment close to the footing.
3. Expose the footing face gradually, checking for cracks or movement in the wall as you go. Stop immediately if you see any cracking or hear any movement.
4. Maintain a clear escape route from the excavation at all times.
5. Inspect the footing condition — look for cracking, spalling concrete, exposed or corroded reinforcing steel, or signs of previous movement.
6. Backfill promptly once your work is complete, compacting in layers to restore support.

Drainage Considerations

Retaining walls often fail due to water pressure building up behind them. If you're excavating to install or repair drainage (such as a ag-pipe or no-fines concrete drainage layer), this is worthwhile maintenance — but the excavation must be done carefully. The NZ Building Code clause E1 covers surface water management, and proper drainage behind retaining walls is essential to long-term performance. BRANZ has useful guidance on retaining wall drainage best practice.

Using Mechanical Excavation Equipment

Using a mini-excavator or digger near a retaining wall footing significantly increases the risk of damage. The vibration alone can disturb the footing and the retained soil. If mechanical excavation is necessary:

• Keep the machine well back from the wall and hand-dig the final 500 mm closest to the footing.
• Use an experienced operator who understands the risks.
• Never use a machine to lever or push against the wall or footing.

When to Call a Professional

You should engage a licensed excavation contractor or geotechnical engineer if:

• The retaining wall is over 1 m tall or retains a driveway, building, or slope.
• The excavation will be deeper than 1.5 m.
• The soil is loose, sandy, or waterlogged.
• You can see existing cracks or movement in the wall.
• A building consent is required for the work.
• You are unsure about any aspect of the job.

A structural or geotechnical engineer can assess the wall's condition and specify safe excavation methods. For retaining walls that require consent, work must be carried out or supervised by a Licensed Building Practitioner (LBP). You can find qualified local excavation contractors and LBPs on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – Retaining Walls and Consent
• WorkSafe NZ – Excavation Safety
• BRANZ – Building Research
• Licensed Building Practitioners