Retaining Wall Inspections
In the excitement of inspecting your potential dream home it’s easy to focus on the house itself. You might have cast a cursory glance over the garden and outdoor area. You might even have admired a nicely landscaped garden around a rock retaining wall. But unfortunately these aesthetic touches sometimes conceal a major money pit or potentially deadly hazard. A teetering, unstable retaining wall or a hastily and poorly-repaired retaining wall may be lurking under a foliage disguise. Retaining wall inspections identify any faults in retaining walls that could lead to harm.
Note that Action Property Inspections is a specialist service for pre-purchase building inspections in Brisbane. We do not offer retaining wall inspections as a stand-alone service.
Retaining walls aren’t there for their good looks. They perform a number of critical functions. For that reason, if a retaining wall is unstable, the consequences can be disastrous, even deadly. When you gleefully accept the keys to your new property you’re taking on the responsibility of ensuring that your retaining wall is stable and safe. Getting a retaining wall inspection can set your mind at ease and assist you to avoid a financial nightmare.
Role of a retaining wall
Whilst retaining walls can have the effect of terracing a piece of land so as to make it more visually appealing and easier to maintain, they do a lot more than just that. In short, a retaining wall is built to hold back earth or soil and in some cases to redirect the water draining down a sloped surface. In doing so, they can assist in preventing landslips or landslides. Retaining walls are often used to carve out a flat area of land from a slope so that the land is made suitable for buildings, roads, driveways and paths. Retaining walls are often needed where:
- There’s been some change made to a slope whereby its natural integrity has been lost, or
- The slope itself is inherently unstable; for example, it might be made up of loose gravel or rocks.
Different types of retaining walls
Most retaining walls fall into one of three main categories. Sometimes features from more than one category are present in a retaining wall.
Weight or gravity retaining walls
Like the name suggests, a weight or gravity retaining wall uses gravity to fight gravity. It relies on its sheer weight to firmly anchor it in place and stop the soil slipping. For that reason, weight or gravity retaining walls can be incredibly thick and heavy and often made of materials such as stone, concrete or brick. Gabions (wire baskets of stones or rocks) are an example of a gravity or weight retaining wall. Because of the weight needed to make a gravity retaining wall effective, they can be quite costly and necessitate a lot of material and space.
Pile retaining walls
Piling or pile retaining walls rely on piles (poles or planks) or sheeting driven into the ground. Effectively the weight of the slope below the pole, plank or sheeting is used to counteract the weight of the slope above, much like a lever. In a way, these retaining walls are a bit like icebergs with up to 2/3 of the pile doing the hard work of holding back the soil below the surface of the ground.
Cantilevered retaining walls
Cantilevered retaining walls often rely on steel–reinforced concrete components shaped like upside-down T’s wedged or cut into the slope. The weight of the slope above pressing down on the horizontal part (the ‘footing’) anchors the wall into place and counteracts the natural flow of soil and water downhill.
Gravity, pile and cantilevered retaining walls can also be further strengthened by being anchored into place with cables that have been bored into the slope and secured with mechanical fasteners (like barbs on a fish hook) or through the injection of pressurised concrete into the anchor point so that the cable cannot pull free.
These are just some of the types of retaining wall you might come across. There are many others.
Materials used to build retaining walls
Retaining walls can be built from a wide array of materials. Examples of different types of retaining walls include:
- Timber or sleeper retaining walls
- Brick retaining walls
- Concrete or concrete sleeper retaining walls, stone-clad concrete or brick retaining walls, crib retaining walls
- Stone or rock retaining walls, some mortared, some unmortared dry stone retaining walls, stacked stone retaining walls
- Block retaining walls, including besser block retaining walls and interlocking block retaining walls, segmental block retaining walls.
Each has its own advantages and disadvantages and some are more suitable for a particular site than others. If the materials used to construct the retaining wall are unsuitable for the soil, the site, the height of the wall or the load that it will bear, your retaining wall could be a disaster just waiting to happen.
Other things that can go wrong with retaining walls
There are many things that can go wrong with a retaining wall. Unfortunately, the problems can be very difficult for the untrained eye to spot. Often, the structural integrity of a retaining wall can be completely compromised before the problem becomes obvious, leaving you with a massive financial headache and a dangerous situation. Sadly, some defects in retaining walls only become apparent when they fail completely and someone is injured or property damaged. You might be insured but your policy may be voided in such a scenario. You could even be sued. Retaining wall inspections should be conducted by suitably qualified building inspectors as soon as any changes to your wall become apparent.
Here are just a few of the issues you should be aware of in relation to retaining walls:
Retaining walls and the laws of physics
For a retaining wall to be safe and do its job properly, it must have been built in a manner that takes into account the pressures and forces that will be at play, as well as the angle and composition of the slope. Gravity is the primary force at play in relation to a retaining wall.
The composition of the slope (ie. what kind of material makes up the slope) is also very important in determining the safety of a retaining wall. This is because different materials have different angles of repose. Essentially, if you took a jar of sand and poured it onto a table, you’d notice that there’s a point at which you can’t make the pile any higher because the sand keeps sliding down the slope. It has reached its critical angle of repose. The angle of repose of a particular material also depends on how dense the material is and how much friction it generates.
A slippery substance such as very fine sand or silt will have less friction and slide much more easily and will therefore have a much lower angle of repose than other soils or materials. These are all factors that need to have been considered in the design and construction of a retaining wall. If the angle of repose is exceeded in the design and construction of your retaining wall, it’s inherently unstable.
Pests and retaining walls
If your retaining wall is constructed from wood or sleepers then it may be susceptible to attack from pests such as termites. This is a problem that is common with wood that is in close proximity to damp earth. A retaining wall serves to hold back and redirect water as well as soil in a slope and, for that reason, termite attacks can undermine the strength and stability of the wall.
Timber is also particularly susceptible to rot and attack from fungi, and may actually start to decompose under the right conditions. It is crucial that you investigate whether the timber used in a retaining wall is treated or untreated and whether it has started to rot.
Trees and retaining walls
Trees and their root systems can compromise a retaining wall in several ways. The weight of a tree located higher up a slope from a retaining wall increases as the tree grows. That tree may not have even been planted at the time that the retaining wall was built so its position and weight may not have been factored into the original design of the retaining wall. Also, a tree’s root systems can be very invasive and play havoc with the structure of the retaining wall, causing cracks, lifting of the wall and, potentially, toppling. Not only that, but tree roots can also penetrate the drainage system of your retaining wall, rendering it useless.
Inadequate or no drainage
A retaining wall should always incorporate an appropriate system of drainage to minimise the risk of water damage. The drainage system must be sufficient to cope with the water that naturally drains down the slope as its previous path will have been disturbed and cut off by the positioning of the wall. A build-up of water behind a retaining wall causes hydrostatic pressure. That is, water will find the shortest route downhill and can cause erosion.
A build-up of water can also lead to problems with rising damp and mould if it is in close proximity to an exterior house wall. In addition, damp conditions can create an ideal conduit for termites in a timber retaining wall. Certain soil types do not cope well with excess moisture, such as reactive clay, which has a tendency to expand when saturated and contract again when dry. This can contribute to movement in the wall and lead to bowing and eventually, collapse.
All these risks can either be avoided or minimised by the addition of an appropriate system of drainage at the time of design and construction of the retaining wall. In certain circumstances, additional measures such as waterproofing should also be incorporated into a retaining wall. If your conditions are particularly damp, a timber retaining wall may not be as good an option as concrete or a wall built from stone or other materials. Of greatest concern is the rise in DIY retaining walls where the inclusion of any system of drainage has been completely overlooked by the weekend builder.
Cracking and subsidence
Once a retaining wall has been constructed, allowances should be made for a degree of subsidence or settling of the soil adjacent to the retaining wall. This means that the soil that was disturbed in the building of the wall becomes compacted. Some subsidence is normal. However, there is a point at which subsidence can undermine the structural integrity of your retaining wall, particularly if it makes the ground so uneven that water pools on the surface of the ground above the wall.
Activity further up or further down the slope
Even a well-constructed retaining wall can become unstable if further excavation work up or down the slope affects the weight of the soil that the wall is holding back or otherwise destabilises the site. This often occurs in housing estates where new properties are being built adjacent to existing homes and their older retaining walls.
The danger signs to look out for
A qualified building inspector will be on the look out for the obvious and not so obvious signs of a failing retaining wall. Bowing or cracking retaining walls are symptomatic of a serious problem. Similarly, if the height of a retaining wall has become uneven, that may also indicate that the retaining wall has potentially become unstable or is failing. The pooling of water in places along the ground at the top of the wall might also be evidence of something seriously amiss with your retaining wall.
Not all damage is immediately obvious, especially to the untrained eye. Sometimes special equipment such as moisture detection equipment will be needed to really assess whether or not the structural integrity of a retaining wall has been compromised. While retaining wall inspections can be conducted by a building inspector, when there is so much at stake, there are instances where an experienced engineer should be called upon to assess the state of your retaining wall and suggest possible solutions.
Those solutions may include repairing the retaining wall, reducing its height so as to reduce the load it is being subjected to or completely rebuilding the retaining wall. An expert can also advise you as to the likely costs of remedying the situation. Those costs can then be factored into your offer and can be taken into account in your decision whether to purchase the property. It’s the only sure-fire way to ensure that your dream property doesn’t conceal your worst nightmare.