Digging near an existing retaining wall is one of the riskier DIY excavation tasks a homeowner can attempt. Done carelessly, it can undermine the wall's foundations, cause sudden collapse, and result in serious injury or significant property damage. With the right approach, careful planning, and a clear understanding of your limits, small-scale hand-digging can be done safely — but there are situations where you must stop and call a qualified professional.
Understand What You're Dealing With First
Before you pick up a spade, take time to understand the wall itself. Retaining walls hold back a mass of soil (called the retained fill), and that soil exerts constant lateral pressure on the wall. The wall's stability depends on:
- Its foundation depth — how far below ground the base sits
- Drainage behind the wall — poor drainage dramatically increases pressure
- The type of construction — timber, concrete block, poured concrete, or stone
- Any tiebacks, deadmen, or geogrid reinforcement buried behind or beneath it
Older walls may have no engineering documentation at all. If you don't know how the wall was built, treat it as potentially fragile.
Check for Building Consent Requirements
In New Zealand, retaining walls over 1.5 metres in height (or lower walls with surcharge loads such as driveways or buildings above) generally require a building consent. Any excavation that could affect a consented retaining wall — or that requires a new or modified wall — may also need consent. Schedule 1 of the Building Act lists some exemptions for low retaining walls, but the rules are specific and vary by situation. Always confirm with your local council before starting work that could affect an existing wall or require a new one. You can find guidance at MBIE Building Performance.
Locate Underground Services First
Before any digging, you must identify buried services — water pipes, stormwater, sewer lines, gas, and electrical cables. Contact Dial Before You Dig (0800 248 344) and check with your local council for service plans. Striking a gas line or electrical cable can be fatal. This step is non-negotiable.
Key Safety Rules for Digging Near a Retaining Wall
Keep Your Distance from the Foundation
As a general rule, never excavate closer to the wall than the height of the wall without professional advice. For example, if the wall is 1.2 m tall, stay at least 1.2 m away from its base. This is a conservative rule of thumb — actual safe distances depend on soil type, wall construction, and surcharge loads. When in doubt, stay further back.
Work in Small Sections
- Dig in short sections (no more than 1–1.5 m at a time) rather than opening up a long trench all at once
- Backfill and compact each section before moving to the next where possible
- Never leave an open excavation adjacent to a retaining wall overnight without proper shoring
Watch for Warning Signs
Stop immediately and evacuate the area if you notice:
- Cracking appearing or widening in the wall
- Leaning or bulging of the wall face
- Soil slumping or moving behind or beneath the wall
- Water seeping from unexpected locations
These are signs the wall may be close to failure. Do not re-enter the area until a structural engineer or experienced retaining wall contractor has assessed it.
Manage Soil and Spoil Carefully
Don't pile excavated soil (spoil) close to the top of the retaining wall. This adds surcharge load and increases the pressure the wall must resist. Truck spoil away or store it well clear of the wall's influence zone.
Hand Dig Near the Wall Face
If you must work close to the wall, use hand tools — a spade, mattock, or hand trowel — rather than mechanical equipment. Vibration from machinery can loosen mortar, shift blocks, or disturb the retained fill.
Soil Types Matter
Different soils behave very differently when excavated:
| Soil Type | Risk Level Near Retaining Wall | Notes |
|---|---|---|
| Firm clay | Moderate | Can stand temporarily but softens when wet |
| Sandy or gravelly soil | High | Loses support quickly, prone to collapse |
| Fill (imported or mixed) | High | Unpredictable, may contain voids |
| Weathered rock | Lower | Generally more stable but check for fractures |
New Zealand's BRANZ publishes useful guidance on soil behaviour and construction near slopes.
Personal Protective Equipment
Always wear:
- Steel-capped boots
- Hard hat when working near any wall or overhead hazard
- Hi-vis vest if machinery is operating nearby
- Gloves and safety glasses
Refer to WorkSafe NZ for excavation safety requirements — trenches deeper than 1.5 m have specific legal requirements around shoring and safe access.
When to Call a Professional
You should always engage a qualified excavator or structural engineer if:
- The retaining wall is over 1 metre tall
- You need to dig within the wall's influence zone (generally within a distance equal to the wall height)
- The wall shows any existing cracking, leaning, or drainage problems
- You're using mechanical excavation equipment anywhere near the wall
- The soil is loose, sandy, or unknown fill
- There is a building, driveway, or heavy load above the wall
- You're unsure about the wall's construction or foundation depth
A licensed excavator has the experience to assess risk on-site and the equipment to work safely. You can find qualified local excavation contractors on tradedirectory.co.nz.
For walls that are part of a consented structure, you may also need a Licensed Building Practitioner (LBP) to oversee or carry out restricted building work. Check the LBP register to find a practitioner in your area.
Licensed Building Practitioner (LBP)
