HOW TO CONSTRUCT A BETTER GARDEN RETAINING WALL
Retaining walls can form complete physical and visual barriers for a long period of time. They are more common than one would think in urban design used as an alternative to hedges and to define garden beds, while in rural areas and country houses they can be used to raise and level an otherwise sloping area, providing flat land around houses, or in an idyllic country setting to make a ha-ha to provide for house protection while preserving open views to the surrounding landscape.
There are several uses for retaining walls:
- As a garden design feature, to define garden beds, stairs and multi leveled spaces
- As a physical barrier when required as a alternative to hedges
- To retain soil and make a surface level
- To maintain privacy from lower leveled observing points in relation to the garden, as from streets or public spaces below hill houses.
When planning a wall, always think about the following:
Where the Wall is Positioned or Located
The wall must follow the shape of the embankment it is retaining, as close as possible, to avoid carting large quantities of soil either in or out of the work site.
What the Wall is Constructed from
This is often determined by what materials are available. In some locations rock is pleantiful and inexpensive; but in other places wood, brick or concrete may be easier to obtain.
The cost can vary with different types of construction. So can the visual appearance. Some types of construction will last for decades or centuries with little deterioration, but others deteriorate within years.
Dry walls (without cementing) need a solid base/foundation. A mortared wall needs a concrete strip foundation and weep holes for drainage.
Position the wall to Optimise Soil Stability
All walls should slope back into the embankment (i.e. this slope is called "the batter"). A minimum batter should be approx. 1 cm for every 6 cm in height. Ideally, the ground at both the top and bottom of a wall should be fairly flat, to minimise erosion. The batter is essential for wall stability in all retaining walls above 1 m height.
Ensure there is Good Drainage both Above and Below any Wall.
This factor is obviously more critical in clay soils. A spoon drain may be built at both the top and bottom of the wall. Sub surface drains might also be used in these positions. If surface drainage is allowed to run over the top of the wall, it can cause bad erosion behind and at the base of the wall, very quickly.
Tips for Brick or Rock Wall Building
- Avoid horizontal or vertical lines between the stones.
- Mix the colours of the stones.
- Larger stones should tend to be placed towards the bottom of the wall.
- Roughly 1 ton of rubble stone is needed.
- About 1 ton of veneer stone will cover 55 sq. ft.
- If mortar is being used for veneering, use a mix of 4 parts sand to 1 part cement.
For a structurally strong brick wall it should be two bricks thick. If only one brick thick, regular buttressing is needed or the wall can be staggered with panels stepped forward and back, or built in a serpentine shape (although both these styles may be too decorative for some tastes).
A wall two bricks thick should have a concrete foundation. This involves digging a trench, the depth depending on the height of the wall and the type of subsoil. On average it should be approximately 235 cm deep and twice as wide as the wall. Upon this the bricks are built up in a method called `bonding'. Bonding involves laying the bricks both lengthways and crossways this binds the wall and gives it strength. Provided the wall does not exceed 6 ft it should not need buttressing.
If built in a damp position, the wall will last longer if a damp proof course is built in. It should be built in the base of the wall, about 150 mm above ground level. Suitable materials are hard brick, asphalt and bituminous sheet.
Innovative products for retaining walls have appeared recently in the market that can offer economic solutions when large retaining walls are needed, and they are offered in a range of appearances, from soil like looks to veneer type facing. One of the options is with material made of recycled car tyres. This option is good for large areas as well as for smaller ones, although they need to be installed by professional companies and materials are not available for the home owner or landscaping companies, yet. Walls can be built with many shapes and wall heights, allowing for design flexibility. If you are also keen to favour environmental solutions, this can be one of them.
Building Timber Walls
Wood walls are a popular and cheaper alternative to hard walls. They may not last so long but can be built in a range of attractive styles. Different timber is available in different areas although all timber will need to be treated with a wood preservative to prevent the wood rotting. Remember though that plants do not like fresh timber preservative and will die if they come into contact with it. It sometimes takes up to a year before climbers can grow on treated wood. Do not use timber with bark on it, for as it decays it provides a trap for moisture and insects.
Wooden retaining walls should never be more than 1.2 m tall. Wood should be either a type which resists decay (such as red gum or yellow box), or else treated with a chemical to prevent decay or damage by insects, fungus etc.
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