Some movement in a retaining wall is normal, especially in the first year or two after construction. However, certain signs indicate something far more serious — and in New Zealand's hilly, wet terrain, a failing retaining wall can pose a real risk to life and property. Here's how to read the signs.
What Is Normal Settlement?
Settlement is the gradual, minor movement that occurs as the soil behind and beneath a wall compacts over time. You might notice:
- Very slight leaning (a millimetre or two) that hasn't changed in years
- Minor surface cracking in mortar joints that remains stable
- Small gaps at the ends of the wall where it meets a fence or structure, present since construction
If these signs appeared shortly after the wall was built and haven't changed noticeably since, they're likely cosmetic. Keep an eye on them, but they may not require urgent action.
Warning Signs of Structural Failure
The following signs suggest your wall may be structurally failing and need professional assessment as soon as possible:
1. Visible Leaning or Tilting
A wall that is visibly leaning away from the retained soil — especially if the lean is getting worse — is a serious red flag. Use a spirit level or plumb bob to check. Any lean greater than about 25 mm over the height of the wall warrants urgent attention.
2. Horizontal or Stair-Step Cracking
Horizontal cracks in a concrete block or masonry wall are particularly alarming. They often indicate the wall is bending under lateral soil pressure — a sign the wall may be close to collapse. Stair-step cracking along mortar joints in brick or block walls can also signal movement beyond normal settlement.
3. Bulging or Bowing
A wall that bulges outward in the middle, rather than remaining flat, is under excessive pressure. This can happen when drainage behind the wall has failed, causing water to build up and dramatically increase the load on the wall.
4. Water Seeping Through or Pooling at the Base
Retaining walls need adequate drainage to function safely. If water is weeping through the wall face, pooling at the base, or the soil behind the wall is saturated, the hydrostatic pressure may be overwhelming the wall's design capacity. This is a common cause of wall failure in New Zealand's wetter regions.
5. Soil Movement or Slumping Behind the Wall
If the ground behind the wall is cracking, dropping, or slumping, the wall may no longer be doing its job. Look for cracks in nearby paths, driveways, or garden beds running parallel to the wall.
6. Exposed or Damaged Footings
If the base of the wall is becoming exposed through erosion, or if you can see cracking or crumbling at the footing level, the structural foundation of the wall is compromised.
A Quick Comparison
| Sign | Likely Settlement | Possible Failure |
|---|---|---|
| Minor mortar cracking, stable | ✅ | |
| Horizontal cracking in blocks | ⚠️ Urgent | |
| Slight lean, unchanged for years | ✅ | |
| Lean that is visibly worsening | ⚠️ Urgent | |
| Water seeping through wall face | ⚠️ Investigate | |
| Soil slumping behind wall | ⚠️ Urgent | |
| Small end gaps, present since build | ✅ | |
| Bulging or bowing wall face | ⚠️ Urgent |
NZ Building Code and Consent Considerations
In New Zealand, retaining walls over 1.5 metres in height (or lower walls with surcharge loads such as driveways or buildings nearby) generally require a building consent. Under the NZ Building Code, retaining walls must meet structural performance requirements — meaning they must safely retain soil under expected loads, including saturated conditions.
If your wall was built without consent when one was required, or if it was built to a poor standard, it may not have the capacity to handle the loads placed on it. Always check with your local council if you're unsure about consent history.
BRANZ provides technical guidance on retaining wall construction and performance that can help you understand what good practice looks like.
What to Do If You Suspect Failure
- Don't ignore it. A failing retaining wall can collapse suddenly, especially after heavy rain.
- Keep people away from the area below the wall until it has been assessed.
- Document the damage with photos, noting the date. This helps a professional track progression and may be useful for insurance purposes.
- Contact your council if the wall is on or near a boundary, or if it supports a public area.
- Get a professional assessment promptly.
When to Call a Professional
If you spot any of the urgent warning signs above — worsening lean, horizontal cracking, bulging, soil slumping, or water ingress — call a qualified retaining wall builder or structural engineer immediately. This is not a situation to monitor and wait on.
Even if you're unsure, a professional assessment is always worthwhile. A qualified tradesperson can determine whether the wall needs repair, reinforcement, or full replacement. They can also advise on improving drainage, which is one of the most common causes of retaining wall failure in New Zealand.
You can find experienced, local retaining wall builders on tradedirectory.co.nz to get a professional opinion quickly.
Licensed Building Practitioner (LBP)
