A retaining wall is a structure that holds back a mass of earth or other solid material behind it.
It 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. Gabion retaining walls (pictured) are also becoming popular in Australia.
Railroad sleepers used to be an inexpensive retaining wall material.
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 down a slope due to gravity.
This generates lateral earth pressure behind the wall that 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.