How to plan your DIY retaining wall project
A DIY retaining wall project could add an astounding element to your landscaping that increases functionality, improves aesthetics and ultimately boosts the value of your property.
Tiered gardens, terraces, built-in seating, raised patios and pergolas, water features and even ponds are all possible with a sturdy, reliable retaining wall. Here are some factors to consider when deciding on the right retaining solution for you.
Do I need Council approval?
Different zones have different requirements. It’s best to check with your local council prior to construction to ascertain if approval or building permits are required. Find your local government in these handy directories:
Do I have a Cut Site or a Fill Site?
A cut site is when you are cutting into the natural slope of the land to create a stepped site with your wall. A fill site is when the wall is installed at the lowest part of a gradual slope and then filled to level it with the highest part. Different sites will require slightly different procedures and equipment.
What kind of soil am I working with?
To test the soil at the site, take a handful from at least 300mm below the surface and squeeze.
- Organic soils will stick together but won’t hold – these should only be used as top soil and not to build the wall itself.
- Clay soils will form a ball – they retain moisture and add pressure behind walls, so they may require additional reinforcement, but can still be used in your project.
- Sandy soils won’t stick at all – they are granular and allow for good drainage, making them ideal for retaining walls.
Do I need to compact my soil?
Soil should only be compacted away from the zone of influence and should not be compacted against the retaining wall itself, or in any way that could affect the back fill or drainage capabilities. Fill sites would generally require some compacting to stabilise the fill, but cut sites generally will not, as you are excavating through already stable soil.
Do I need any specific equipment/machinery?
Equipment/machinery needed will depend on the type and purpose of your DIY retaining wall project and which kind of site you have.
For instance, a fill site will probably need compacting to stabilise the soil, so a plate compactor/vibrating plate may be needed, whereas a cut site may require an excavator if it is of a significant size. If the wall is high and uses heavy materials, consider plant hire to lift and stack the wall materials to avoid injury or unsafe labour practices.
How do I measure my site?
A detailed site plan, drawn to scale, will be useful when estimating and calculating the materials you’ll need. Measure length and height of the proposed wall, and how much weight it will need to retain to help determine the most appropriate retaining solution, and the measurements of materials needed.
Once you’re ready to install your retaining wall, clear the area and prepare your site. Use stakes to and spirit-levelled string lines as a guide that matches your site plan measurements.
Why is waterproofing important?
Timber will be more susceptible to water damage than cement. However, whichever material you choose should still be waterproofed before backfilling, to increase material longevity. Use a commercial waterproofing or bituminous product such as SikaTite BE, to ‘paint’ the underground portion of the material. Then, use a commercial grade polyurethane sealant, such as Sikaflex Pro, to seal any gaps or crevices.
How do I correctly backfill my wall?
Drainage is highly important, as trapped moisture can lead to irreversible damage and a premature weakening of the wall.
At the base of your wall, before backfilling, lay an agricultural line or pipe to allow release of underground seepage. Ensure you also lined it with an external filter sock to prevent silt from entering and clogging it. This pipe should exit at the sides where it can run off; avoid cutting into your wall to fit the pipe through, as it can undermine the strength of the wall.
You should also use backfill that’s granulated to allow for adequate drainage; denser soils that hold moisture will add dangerous pressure to the wall.
Is angling or stepping back my wall important?
Whether you install your wall straight or at a slight angle/step depends on how high the wall is. Gravity works against a wall that is exactly perpendicular to ground level, and this pressure is mostly directed at the lower part of the wall.
To get gravity working with you, ensure your materials are at a very slight angle; basically, the bottom of the wall should be slightly more outward than the top. However, if the wall is only a very small height – for a low garden bed, say – it isn’t an issue you really need to worry about.
Written by Evelyn Kandris
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