Retaining walls are one of the most important structural elements in many landscape projects.
They can create level areas on sloping properties, improve access, support gardens and dramatically increase the amount of usable outdoor space.
But when retaining walls are poorly designed or constructed, problems can develop quickly — and repairing them later can be considerably more expensive than building them correctly in the first place.
After many years constructing landscapes across Sydney, we have found that retaining wall problems are rarely caused by one thing alone. Failure is usually the result of poor drainage, inadequate foundations, incorrect construction methods or a wall that was never properly designed for the load it is supporting.
Water Is Often The Biggest Problem
One of the most common causes of retaining wall failure is poor drainage.
A retaining wall isn’t simply holding back soil. After heavy rain, it may also be dealing with a significant amount of water within that soil.
If that water cannot escape, pressure builds behind the wall.
Over time, this can contribute to:
- Cracking
- Bowing or leaning
- Movement
- Soil washing through joints
- Damage to surrounding paving or landscaping
- Complete structural failure
Good retaining wall construction therefore isn’t only about what you can see.
What happens behind the wall is just as important.
Drainage Behind The Wall Matters
Depending on the wall and site conditions, appropriate drainage may include elements such as free-draining aggregate, drainage pipe, geotextile and suitable outlets.
The exact solution will depend on the project, but the principle remains the same:
Water needs somewhere to go.
Simply building a strong wall and hoping it keeps water out is not a drainage strategy.
Foundations Matter Too
A retaining wall is only as reliable as the foundation supporting it.
Poor ground preparation, unsuitable footing dimensions or construction over unstable material can all contribute to movement.
Different retaining wall systems also require different footing and foundation methods.
A small garden wall and a substantial structural retaining wall are not the same thing and shouldn’t be treated as though they are.
The Wall Needs To Suit The Site
Retaining walls can be constructed from many materials, including:
- Reinforced masonry
- Concrete sleepers
- Timber sleepers
- Natural stone
- Other engineered wall systems
There isn’t one material that is automatically best for every property.
The right system depends on factors including:
- Wall height
- Loads behind the wall
- Soil conditions
- Drainage
- Available space
- Access
- Desired appearance
- Budget
- Engineering requirements
Material should be selected as part of the overall solution rather than simply because it is the cheapest or most fashionable option.
What Is Behind The Wall?
This is an important question that homeowners don’t always consider.
A wall supporting a garden bed is very different from a wall supporting a driveway, building, swimming pool or another substantial load.
The higher the wall and the greater the loads involved, the more important proper design becomes.
Depending on the circumstances, structural engineering and approvals may also be required.
Warning Signs To Look For
Existing retaining walls can sometimes provide early signs that something is wrong.
These may include:
- Cracks becoming larger
- Walls leaning outward
- Separation between sections
- Water constantly appearing through or beneath the wall
- Soil washing away
- Paving moving nearby
- Posts or sleepers noticeably shifting
Not every crack means a wall is about to fail, but movement should not simply be ignored.
Why Cheap Retaining Walls Can Become Expensive
Retaining walls are an area where saving money during construction can become very expensive later.
If a wall fails, repairing it may involve removing:
- Gardens
- Paving
- Fencing
- Drainage
- Stairs
- Other completed landscape elements
Access may also become more difficult once the rest of the property has been landscaped.
That is why retaining walls should be treated as part of the structural foundation of a landscape rather than simply another decorative feature.
Final Thoughts
A good retaining wall should do something fairly unremarkable:
It should stay where it was built.
Achieving that requires the correct design, suitable foundations, appropriate construction methods and, importantly, effective drainage.
The finished wall may be what everyone sees, but much of the work determining its long-term performance is hidden underground and behind it.
When planning a landscape project involving retaining walls, getting those unseen details right is one of the most important investments you can make.
