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In the Magazine

Retaining Walls

In the Garden > Gardening Tips, Books, Techniques and Tools

Don looked at some of the materials commonly used to build retaining walls. Treated pine log or sleeper walls are popular, cheap and easy to erect, and they are used for both small and quite substantial walls. However, they don't last well. Examples shown in our segment had rotted or collapsed, indicating that timber is not a good material for a retaining wall. A far better choice for any type of retaining wall is masonry such as brick, stone or concrete.

Cut and fill

Many problems are seen with retaining walls on cut and fill sites. The cut and fill method of construction is commonly used to build concrete slab houses on sloping land. Builders cut into a hillside or slope, then use the material removed from the slope to form part of a level area (see diagram). This material is held in place by a retaining wall. When there is inadequate drainage these sites flood during heavy rain. Cut and fill and poor retaining wall construction can be a recipe for disaster.

Timber retaining walls

Don does not recommend either railway sleepers or treated pine for building retaining walls, particularly where reactive clay soils are present (these are clays that expand when wet and contract when dry). Railway sleepers are susceptible to termite and fungi attack, and will eventually rot. If you plan to use treated pine in the ground, it is important to select treated pine rated for in-soil use. There are different ratings of treated pine, ranging from Hazard Level Code 1 (H1) which indicates the lowest level of treatment, to 6 (H6) which identifies the highest level of treatment. Ratings below 4 (H4) are not suitable for use in contact with the ground. It is important that cut ends are also treated with a timber preservative to protect them from rot or insect attack.

Masonry is best

Bricks, prefabricated concrete or stone retaining walls with weep holes and built on a reinforced concrete footing are the best choice for retaining wall construction. Ideally, all retaining walls over 1m high should be battered back into the ground. That is, they should slope backwards around 10 to 15 to prevent collapse (see diagram). All retaining walls over 1m tall should be constructed by professionals.

When it comes to retaining walls there is no room for compromise. Retaining walls that have fallen or are falling down are very difficult to repair, and can pose a danger to you, your property and passers by.
For more information

Contact the building information service in your area or consult your local council.

Copyright CTC Productions 2001


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