What are retaining walls?
Retaining walls are structures that hold back, or retain, the earth behind them. They are very common in properties that are on sloping sites and are generally constructed either:
• As part of the building structure to enable floors to be built at one level – quite often requiring the ground to either be excavated or for soil to be imported to increase the height of the ground;
• As part of general landscaping or driveway construction – in order to create level areas on a sloping block.
This type of construction is often known as “cut and fill” as shown below.
What are Retaining Walls made of?
Retaining walls can be constructed of any type of material. Common materials used are masonry (blocks or bricks), concrete and timber. Sometimes large rocks are also used simply as a “mass” or “gravity” wall. Regardless of the material, it is advisable and often necessary in accordance with Council regulations, to have the retaining wall designed and certified by a structural engineer, especially if it is over 1.0 m in height.
Three Important Things to Look For
If a retaining wall is constructed of a rigid material such as concrete or masonry the design and construction need to be in accordance with Australian Standards in order to minimise any cracking. While some movement can be expected which may lead to minor cracking, anything over 2 to 3 mmm wide is a sign of possible structural damage.
Generally vertical construction joints are incorporated every 6 to 10 m along the wall in order to manage shrinkage and/or expansion of the building material. For internal walls
these joints should be sealed with a flexible waterproof jointing compound to stop moisture seeping through.
- Tilting from vertical alignment
Walls are usually built vertically and so therefore if a wall is tilting or leaning over there is a potential structural issue. Walls start to lean over when they can no longer hold back the pressure of the soil behind. If a wall has started to lean over then it is most likely a sign that the wall’s integrity is compromised. Sometimes it can take years for a wall to display such signs.
When walls are built outside exposed to the weather it is not necessary to waterproof the rear surface. Quite often there are seep holes incorporated into the wall that allow water to pass through the wall. This alleviates the build-up of water pressure behind the wall. The only negative side to this type of construction is that often the seeping water leaves stains on the wall which can be unsightly. To avoid this the rear surface of walls are often sealed against moisture preventing water from seeping through.
It is advisable that all retaining walls have a free drainage layer immediately behind them to assist with minimizing build-up of water pressure.
Walls constructed as part of a building that form an interior surface require special attention. The main concern with these type walls is the seepage of moisture through them which can result in unsightly staining of internal wall finishes (plaster or paintwork) or build up mildew and mold. The rear of all internal walls should generally be lined with a waterproof membrane and have free draining material immediately behind it. Usually internal walls will not have seep holes installed but rather look to stop any water from getting through. This will then prevent any moisture from seeping through and possibly affecting the finished surface inside the building.
In addition to the membrane, free draining material is placed against the rear surface. This minimizes the potential build-up of water pressure. This drainage material needs to be sloped and directed to a point where the water can escape, either through open drains or plumbed-in drainage into the stormwater. Often internal walls end up below natural surface level (such as in basements) which then requires installation of a collection point or sump, which will then need a pump to be installed to get rid of the water. Without any of the above is it likely that moisture problems will exist.