Retaining walls - things to think about? (Nat)

A retaining wall is a structure that holds back a mass of earth or other solid material behind it; and is designed to resist the lateral pressure (thrust) and overturning forces caused by such retained materials.  Retaining walls prevent down slope movement or erosion and provide support for vertical or near-vertical grade changes.  Retaining walls are generally made of masonry, stone, brick, concrete, steel or timber.  Railroad sleepers used to be an inexpensive retaining wall material.

Design considerations

Retaining walls need to be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift.

The most important consideration in designing and installing a retaining wall properly is the fact that the retained material is seeking to move forward and downward in a slope due to gravity.  This generates lateral earth pressure behind the wall which relates to the angle of internal friction (phi) and the cohesive strength of the retained material. 

Lateral earth pressure is typically smallest at the top of the wall and increases towards the bottom of the wall. Earth pressures will push the wall forward, or overturn it, if not properly designed.  The presence of ground water behind the wall that is not channelled away by a drainage system will also cause an additional horizontal hydrostatic pressure on the wall.

Types of retaining walls

There are several types of retaining walls including:

• Gravity
• Sheet Piling
• Cantilevered
• Anchored
• Soil nailing
• Soil strengthened, such as: Gabion meshes & Mechanical stabilization.

The Garden Wall

For a general garden retaining wall with a maximum height less than 1 metre, there are usually several simple designs and material choices available from hardware and building material suppliers.

Generally, it is good to consult with your local council for design regulations prior to building the wall.  Stone can be used for both functional and decorative walls.  Bricks are another strong material that will last a lifetime.  A timber retaining wall is also strong and cheap to build.  

For further information HIA members can contact HIA’s Building Services staff on 1300 650 620 or hia_technical@hia.com.au.   

If you would like to become a HIA member, contact 1300 650 620 or enquiry@hia.com.au

Ref No. NAT MAT 08-21

The above is intended to provide general information in summary form. The contents do not constitute specific advice and should not be relied upon as such. Formal specific advice should be sought by members with respect to particular matters before taking action.  ABN 99 004 631 752