A retaining wall does one job: it holds back soil that would otherwise move. Most of the time it does that quietly for decades. But when a wall starts to struggle, it usually tells you. The trick is knowing what to look for, and knowing when a patch-up is no longer going to cut it.
Below we cover the most common retaining wall failure signs, what they usually mean, and when it’s time to get an engineer or an experienced retaining wall contractor to take a proper look.
Why retaining walls fail
Almost every retaining wall problem comes back to pressure. Soil pushes against the back of the wall, and that pressure increases dramatically when the ground behind it gets wet. Water is heavy, and saturated soil pushes far harder than dry soil.
The usual culprits are poor or blocked drainage, a wall that was undersized for the load it’s holding, ageing timber or deteriorating materials, extra weight added above the wall (a driveway, a shed, a new slab), and ground movement that the original design never allowed for.
The warning signs to look for
Leaning or tilting
A wall that’s no longer standing straight is one of the clearest signs of retaining wall failure. Some walls are built with a slight backward lean on purpose, so the concern is a wall leaning forward, towards whatever is below it. If the lean is getting worse over time, the wall is losing its fight with the soil behind it.
Bulging
Bulging usually shows up in the middle of a wall, where it bows outward while the ends stay put. It’s common in timber pole walls and block walls where pressure has built up behind a section. A bulge means the wall is deforming under load, and that rarely reverses on its own.
Cracking
Hairline cracks in concrete or blockwork aren’t always a crisis, but cracks that are widening, running diagonally, or stepping through the mortar joints are worth taking seriously. In timber walls, look for split sleepers, rot at ground level and posts that have started to snap or shear.
Blocked or missing drainage
A healthy retaining wall lets water out. If you never see water coming from the weep holes or drainage outlets after heavy rain, or if water is pooling behind or at the base of the wall, the drainage may be blocked or may never have been installed properly. Poor drainage is behind a huge share of the failures we see.
Soil movement above or below the wall
Don’t just look at the wall itself. Check the ground around it. Cracks in the lawn or paving above the wall, a gap opening up between the wall and the soil, fences or paths that have dropped or shifted, and soil spilling out at the base are all signs the ground is moving.
Erosion and exposed footings
If soil is washing out from under or around the base of the wall, the wall is losing its support. Exposed footings or undermined posts are a serious sign, especially on sloping sites or where stormwater runs along the base.
Repair or replace: when repair is no longer enough
Some retaining wall problems can be repaired. Clearing blocked drainage, replacing a few rotten sleepers or fixing a localised crack can add years to an otherwise sound wall.
But repair has its limits. As a general rule, replacement becomes the more realistic option when:
- the wall is leaning or bulging across a large section, not just in one spot
- the ground behind it is actively moving or has already slipped
- the wall was never designed for the load it’s now carrying
- the structural parts (posts, footings, reinforcing) are compromised, not just the facing
- there’s a house, driveway, road or footpath that would be at risk if the wall let go
At that point, patching the wall is often just buying time, and sometimes not much of it. A properly engineered replacement deals with the actual cause, whether that’s drainage, load or ground conditions.
You can see how we’ve approached this on other sites in our Matau Road retaining wall and Opunake Beach retaining wall projects.
When to get an engineer or contractor involved
You don’t need to call someone every time you spot a small crack. But it’s worth getting a professional opinion if:
- the wall is noticeably leaning, bulging or has moved since you last looked
- you’ve seen new cracks in the ground above the wall
- the wall is holding up something important, like your house, driveway or a neighbour’s boundary
- there’s a public footpath, road or neighbouring property below it
- the problems appeared or got worse after heavy rain
- you’re not sure whether what you’re seeing is normal
An experienced contractor can tell you whether a wall is repairable. Where the wall is under serious load, supporting a building, or the ground has moved, an engineer will usually need to assess it and design the fix. At Excavat we work alongside engineers on these jobs regularly, so the design and the build line up from the start.
Case study: replacing a failing two-tier wall on Gaine Street, New Plymouth
We’re currently onsite at a residential property on Gaine Street in New Plymouth, and it’s a textbook example of a wall that had gone well past the point of repair.
The problem
The property had a two-tier retaining wall above the street. The ground behind it had started slipping, and the wall was leaning out towards the footpath. That created two risks at once: a danger to pedestrians walking below, and a threat to the house itself, which sits close to the top of the wall.
The process
The homeowner did everything right. She brought in an engineer to assess the wall and design a replacement, then worked through the situation with the local council, the Natural Hazards Commission (formerly EQC) and her insurer to sort out liability and cover.
It was accepted that the land had slipped, which fell under the Natural Hazards Commission’s land cover. The wall itself was covered by her insurance policy, which included specific retaining wall cover. Every policy is different, so it’s always worth checking what yours includes before you need it.
The solution
The Excavat team is building a new single-rise retaining wall, engineered specifically for the ground conditions on this site. Replacing two tiers with one engineered wall gives a stronger, more reliable result than trying to prop up the old structure.
The tricky part has been sequencing. We’ve had to build the new wall while keeping the existing one in place until it was no longer needed. Removing the old wall too early could have triggered further slipping and put the house at risk, so the work has been carefully staged to keep the ground supported at every step.
Noticed a problem with your retaining wall?
If your wall is leaning, bulging, cracking or the ground around it has started to move, don’t wait for it to get worse. The earlier a problem is looked at, the more options you usually have.
The Excavat team builds and replaces retaining walls across Taranaki, and we’re happy to come out, take a look and give you a straight answer on whether it needs repair or replacement. Get in touch about your retaining wall today.