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Retaining Walls

Retaining walls are mainly used to contain a slope in the garden or to form terraces or different levels.  But what do you need to know about retaining walls?

Designing Retaining Walls

When designing retaining walls, it is important to remember that the material behind the wall is attempting to move forward down the slope.  This is due to gravity.  Therefore, pressure from the soil is created behind the wall.  Lateral soil pressures are usually lowest at the top of the wall and highest at the bottom.  If these pressures are not properly accounted for, the wall will be pushed forward or even overturn.  Also drainage must be properly addressed as the pressure of the water will create additional stress on the retaining wall.

You have several options when it comes to materials used in the construction of retaining walls, with masonry, stone, brick (link to brick), concrete (link to concrete), vinyl, steel or timber being the most popular.  Railroad sleepers used to be popular but they have fallen out of favour due to environmental concerns and the fact that they decompose over time.

In the planning stages of the retaining wall, the designer will have to consider the structural requirements of the wall, the soil conditions on the property, what the best materials to use are, the cost effectiveness of the wall, and what impact it will have on the environment.  Of course, they will also need to design a wall that fits in with the existing house and landscape (link to landscaping and gardening).

Types of Retaining Walls

There are several types of retaining walls including:

• Gravity retaining walls – these are retaining walls that depend on their own weight as well as any soil that is resting on the concrete in resisting lateral earth forces.  They often set so they are leaning slightly backwards in order to improve stability.
  
• Semi gravity retaining walls – are a specialised form of gravity retaining walls and they include some tension reinforcing steel so that the thickness of the wall is minimised without losing strength, and requiring extensive reinforcement.
  
• Cantilever retaining walls – are made from reinforced concrete and they are made up of a thin stem and a base slab.  The base slab is also divided into two parts – a heel and a toe.  The heel is located under the backfill.
  
• Counterfort retaining walls – are similar to cantilever retaining walls but they have thin vertical concrete webs running at regular intervals along the back of the wall.  These webs are called counterforts and they are used to tie the base and the slab together.

Approval for Retaining Walls

Depending on the size of the retaining wall and what you intend to use it for, you may need to get council approval or a building permit before you can begin building it.  Any wall that is over one metre high requires that you call in the services of a structural engineer.  Retaining walls, including the footings, also need to be completely within the bounds of your property.

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