A retaining wall can be useful for both practical and aesthetic purposes; either to terrace a sloping site, to create usable space, and simply to create profile and interest in an otherwise flat and featureless garden. But whatever the reason for its creation, a retaining wall needs to be well made to ensure it remains intact, safe, and trouble-free.
Good design really begins below the ground and behind the wall, so if you’re wanting a trouble free wall that will look good for years to come then consider the critical elements outlined in this article.
To that end, keep reading for some practical tips on designing, constructing, and waterproofing a retaining wall, along with some of the problems that can occur if it isn’t done right.
What is a retaining wall?
As its name suggests, a retaining wall retains earth laterally when there is a drastic change in elevation, preventing it from spilling onto the lower level. Retaining walls can range from simple raised garden beds to large walls several metres in height, built to control erosion, redirect water, and prevent landslips.
A retaining wall is normally needed when a slope has lost its natural integrity or is composed of inherently unstable materials such as gravel or sand. Most retaining walls fall into one of three categories:
- Gravity retaining walls – these rely on their own sheer weight to anchor them in place and prevent the earth from slipping. They are normally thick and heavy and made from materials such as stone, concrete and brick, and they are often tilted back slightly towards the soil they are supporting to compensate for the pressure.
- Pile retaining walls – these rely on piles or sheeting driven into the ground which counteract the weight of the slope above from below the surface of the ground, much in the way a lever works.Taller walls of this type often need to be supported with a tie-back cable planted in the soil behind the wall to help keep it vertical.
- Cantilevered retaining walls – these rely on steel–reinforced concrete structures cut into the slope. The weight of the slope presses down on their footings to counteract the downhill flow of earth.This extra support at their base means these walls can be made much thinner than gravity walls.
Types of retaining walls
Just as there are different categories of retaining walls, the materials they are made from also differ widely. Various types of retaining walls include:
- Rock bank – a large bank of rocks and boulders which uses the natural slope already in place, but at a less acute angle. It can be sewn with natural vegetation for added strength and erosion resistance.
- Sleeper wall – one of the most popular walls due to its cost benefits and easy installation. Timber does eventually rot however, so concrete sleepers can be a more durable option.
- Masonry wall – a crushed rock or concrete footing upon which layers of rendered block work or brick are stacked on top of each other and capped with a finished capping paver. These can be built higher than other retaining walls, but require serious footings and drainage.
- Gabion wall – reinforced steel cages are filled with carefully selected rubble, stone or pebbles (much like a drystone wall in a cage). These walls are very permeable, making them ideal for sites with drainage or soil issues.
- Dry stone wall – a mortar-free retaining wall built from carefully chosen rocks and stones. It requires a very broad base for strength and stability (at least one third of the height of the wall).
How to Waterproof a Retaining Wall
There are a number of important steps to follow when building a retaining wall. It is a major piece of construction and if not designed and built correctly, it can become an eyesore at best or in the worst case scenario, a dangerous hazard.
1. Check regulations
Depending on where you live in Australia and the height of your planned retaining wall, you may require council permission to build it. In some areas, any wall more than 600mm high or costing more than $5,000 can require a council permit and if it’s more than $16,000, you may need builders warranty insurance as well, so check with your local authorities before commencing any work.
As you will be digging down for the foundations of your wall, you will also need to check your plans against available plumbing and electrical diagrams to make sure there are no pipes or cables running under the area (severing public utility cables is not only dangerous, but can attract heavy fines for repairs).
If your wall is being built close to your property boundaries, you will also need to consult with your neighbours to ensure they are okay with your plans and that your wall will not be a threat to their property from erosion or water runoff. And if your wall will be supporting large trees or load-bearing structures, you may be wise to engage an engineer in its construction.
2. Check soil type
When designing your retaining wall, it is important to look at the composition of the slope it will be holding back. This is because different materials such as clay and sand have a different ‘angle of repose’ (the steepest angle of descent to which they can be piled without slumping).
The angle of repose depends on how dense the material is and how much friction it generates (i.e. a slippery substance like sand or silt will have less friction and slide more easily than other materials). If the angle of repose is exceeded, your wall will be inherently unstable, so the type of material being retained will need to be a consideration in the design and construction of your wall.
3. Create a solid base
One of the most important fundamentals in good wall construction is creating a solid base. The base of a retaining wall should be set below ground level and made from compacted soil and a layer of at least 150mm of compacted sand and gravel.
This will ensure that the wall remains flat, meaning more contact between the materials used in its construction, meaning more friction and ultimately more strength.
And the higher the wall, the further below ground level it should be set. As a general rule, one tenth of the wall’s height should be underground (i.e. if the wall is 500mm high, 50mm should be underground).
4. Ensure good drainage
Effective drainage is vital for a retaining wall, otherwise water pressure known as hydrostatic pressure will build up behind the wall and lead to bulging or cracking.
Ways to achieve good drainage include using at least 300mm of a granular material such as gravel in the backfill (the material directly behind the wall). Compacting the backfill as you go will also help to direct pressure downward, rather than against the wall.
Other ways to create good drainage include installing a perforated pipe along the inside bottom of the wall that feeds into a storm water drain and creating small weep holes in the wall which will allow water to drain out through them.
5. Consider function and height
Retaining walls hold off soil when there is a drastic change in elevation. They’re usually necessary on steep sites to create safe, usable space for gardens, buildings, and driveways. Many homeowners also choose to build low retaining walls to section off different areas of the garden for functional and aesthetic reasons. Retaining walls under a metre high are well within the scope of your average DIY-er. For anything above this height, or to serve as a foundation for buildings and driveways, you should seek professional advice before you start building.
Generally, the higher the wall the deeper below ground the wall begins. A good rule of thumb is to have one tenth of the height of the wall below ground level. A 900 mm high wall should have a base course at least 90 mm below ground level. Posts for cantilevered walls should be 100 mm below ground level for every 100 mm of the wall height. Also remember to dig back another 300 mm behind the wall for gravel backfill.
6. Get consent from council
Poorly built retaining walls can bulge, crack, or lean and become an eyesore. In more serious cases they can also topple, which is why councils like to see your plans before you begin any major work. Always check with your local council to see if you need consent for building a retaining wall. If building close to a boundary line you may also have to consult with the neighbours.
7. Check plumbing and cables
Check your plans against plumbing and electrical diagrams to ensure that you don’t get any nasty surprises when you start digging. It is also important to choose the right type of digging equipment. If the project is large in scale then there are a lots of different excavators that need to be reviewed although if it is a small residential wall then you could just choose from a range of mini diggers that may suit the project.
A well designed retaining wall will not tip over and supports the earth behind it and any loads that may be applied, such as that from cars and buildings. It will also prevent water buildup behind the wall, which increases the lateral load on the wall. Gravity walls, usually made of stone or concrete, rely on their mass to withstand the lateral pressure of the soil behind them. Cantilever walls are stabilised by their footings or vertical poles in the ground, and rely on the strength of their construction materials.
9. Use the force
Use gravity to your advantage by stepping the wall back towards the soil. On this point, consider your backfill. Retaining walls look as if they have to hold back great masses of earth, but they only have to hold back a small wedge. Rather than filling that wedge with moisture-loving soil that may threaten your wall, fill it with sandy, gravelly materials that compact easily and allow water to drain down and away from your wall. Compacting the backfill as you go directs pressure downward rather than against the wall. For proper drainage you need at least 300 mm of gravel or similar material directly behind the wall. If you’re planning on landscaping behind the wall, allow for at least 150 mm of topsoil above the gravel.
10. Waterproof the retaining wall
As well as good drainage, a retaining wall also needs to be waterproofed to help reduce the buildup of hydrostatic pressure. Waterproofing also protects the back of the wall from moisture, which can remain behind after the runoff is gone and slowly seep through the wall, eventually discolouring the face & possibly affecting the integrity of the wall.
So it’s essential to use a waterproofing membrane on the back of your retaining wall, just as you would on a basement wall where the same condition occurs. There are various types of waterproofing membranes available on the market including Cosmofin and Wolfin from Projex Group, both of which are suitable for below & above ground applications.
If you have designed your retaining wall properly, it should become a permanent feature, but if there is an issue with the materials or construction, the following problems may be encountered:
- Pests – if your retaining wall is made using timber sleepers, they can be susceptible to pests such as termiteswhich can undermine the strength and stability of the wall. Rotting and fungal attack are also common problems when wood is placed in close proximity to damp earth. To guard against these things, you should ensure that the sleepers you use in your wall are made from treated rather than untreated timber.
- Trees – these can compromise your retaining wall in several ways. If they are on the slope above the wall, their weight will increase as they grow, putting added pressure on the wall. Their root systems can also be highly invasive, cracking and lifting the wall and, potentially toppling it over. Tree roots can also penetrate the drainage system in search of water and cause it to become blocked and useless. It is therefore important to factor any nearby trees into your wall design, including their future growth potential.
- Inadequate drainage – failure to include adequate drainage in the design of your wall such as gravel backfill and drainage pipes can lead to a build up in hydrostatic pressure. Signs to look out for include rising damp and mould and pooling of water at the top of the wall. One way to help alleviate this problem is to drill some weep holes in your wall to allow some of this internal moisture to escape. But ideally, you should design your wall drainage based on the amount of runoff the slope experiences, observed over a period of time.
- Poor soil conditions – certain soil types do not cope well with excessive moisture. Reactive clay for example tends to expand when saturated and contract when dry, which would create bowing and movement in the wall and could eventually undermine its structural integrity. The materials in the slope should be analysed prior to construction and factors included in the design which would compensate for any soil problems (i.e. removing the clay and replacing it with a more stable material).
- Destabilisation – you can expect a certain amount of subsidence (sinking or settling) once your retaining wall is built while the disturbed earth on the slope slowly compacts over time. But if you undertake further heavy landscaping work above your new retaining wall, it is possible to destabilise the area even further and cause a problem with your wall (as is often seen in housing estates where new homes are built next to established ones). Therefore, it is wise to make your retaining wall the last part of your landscaping project so that the weight it supports remains consistent.
Australian residential property includes a high proportion of sloping sites and these can create challenges when trying to design gardens with usable spaces. So knowing how to construct a strong, effective retaining wall that will stand the test of time can be a valuable asset for every homeowner and budding landscaper to have.
This guide has hopefully given you a basic insight into the steps involved in designing and building your own retaining wall. You can find out more about the materials you’ll need by visiting your local landscaping supplier and for more information on waterproofing, contact Projex on 02 8336 1666 during business hours, or use our online contact form today.