A retaining wall is erected to keep back soil on a sloping area or to create a stepped effect in a garden.
It is important to remember this, the pressure created by soil and water behind the wall can determine the type and size of wall that you should construct and how long it will last.
Before deciding on the type of retaining wall to build, a number of issues need to be considered
The first is the height of the wall; most walls over 1.5 metres need extra reinforcing and an engineered design; secondly the look of the retaining wall; timber, brick or interlocking blocks.
Then the type of plants and foliage that will be located near the wall; some plants can cause the wall to lean prematurely.
Timber Retaining Walls
If you prefer the look of timber for a retaining wall it is vital that CCA treated timber is used for both softwood and hardwood to prevent rot and white ants destroying it.
The starting point for this type of retaining wall is to set timber uprights into the ground, spaced to the length of the logs or sleepers, to support the wall.
These posts should have one third the total length set vertically or on a slight backward lean into concrete.
Koppers logs or sleepers are then laid horizontally and fastened to the upright posts. Once the desired height is achieved, backfill with soil and the job is complete.
The timber can then be painted or stained, depending on the finish required.
Timber retaining walls are limited in the shape that can be created as the logs are straight. Placing the posts closer together will allow a squared curve, which is the closest you can get to a curved edge.
Always use galvanised bolts when bolting CCA treated sleepers to the posts, as zinc bolts will rust prematurely.
Brick and Block Retaining Walls
Block and brick retaining walls allow greater flexibility for design, texture and colour as the wall can be rendered, bagged and painted after it is erected or left as plain brick.
There is no chance of rot, however extra care must be taken to ensure it will not fall forward with the weight of the soil behind as well as the weight of the wall.
A retaining wall higher than one metre should have 400 mm concrete footings with reinforcing mesh to give a solid base.
When designing the wall, it is a good idea to have piers, double the thickness of the wall, spaced every 3 m along the wall.
Attach 12 mm rod to the mesh in the footings and pass it through the bricks or blocks as they are laid. This will give the wall greater strength and stability.
Once the concrete has cured for seven days, lay the bricks double width for greater strength. A 10 mm bed of mortar should surround each brick or block and always stagger the bricks as they are laid.
To allow for soil and water to pass through the wall, place vertical weep holes (a gap without mortar) in the bottom course spaced about one metre apart.
Interlocking Block Wall
This type of retaining wall is the easiest and most effective way of keeping back soil up to 1.5 metres, yet still giving the flexibility of curves, shapes, colour and longevity.
The blocks, depending on the brand, come in a range of different types. Solid blocks for the bottom row can be cut for curves.
Smaller sizes are available, which are already made for curves and hollow blocks for the rest of the wall.
A range is also available that allows you to fill with soil and plant small flowering plants in them.
These blocks are set into 50 mm thick base of medium river sand or, if the wall is over 1.5 metres high, a shallow concrete footing is preferred.
Ensure the bottom row is level and then lay the second row on top of the first, staggering the blocks so that there are no continuous vertical joints. There is a lug at the bottom rear of each block that is designed to hook over the top of the previous row of blocks.
This will give a stepped effect and increases the wall's ability to hold back soil pressure without deep footings and reinforcing.
Lay four rows, ensuring that the blocks are staggered, and then back fill with soil .Cutting the blocks can be the only difficult task, however this is made easy with precast grooves in the blocks for one half and one quarter blocks.
Place the block on a soft bed of sand and set a brick bolster into the desired groove. Then, just strike with a hammer until it breaks cleanly.
Place as many blocks as is required to achieve the desired height, backfilling every four rows, and you are ready to sit back and admire the effect.
NOTE - The most important factor to any successful retaining wall is that there is sufficient drainage behind it.
A retaining wall will act as a dam for all running water, whether it is above or underground.
This water must be channelled if you want the wall to remain in place for any length of time.
If the retaining wall is designed to keep back soil on a natural slope, a slotted drainage pipe is necessary.
(Source: Queensland Homes Building and Renovating - Vol 1, No 2 - text - Herb Kuhn)
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