Building retaining walls

Marc Worner deals with the possibility of a slippery slope.

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A retaining wall is a structure that holds or retains earth behind it.

It's rare to find even ground on a landscape site. Retaining walls effectively level even the hardest of inclines or declines, allowing the transformation of an empty slope into an enclosed area for outdoor leisure, a decorative garden or playground.

The most important consideration in proper design and installation of retaining walls is to counteract the tendency of the retained material to move downslope due to gravity. If not properly addressed, earth pressures will push the wall forward or overturn it. Also, any groundwater behind the wall not dissipated by a drainage system will cause enormous hydrostatic pressure on the vertical surface.

First up

There are many materials that can be used to create retaining walls, like reinforced concrete blocks, poured concrete, treated timbers, rock, boulders, railway sleepers, and bricks.

Some are easy to use, others have a shorter lifespan, but all retain earth.

We need to know what type of soil we're retaining before we know what type and height of retaining wall we can build, and there are several questions to be answered before a project begins.

For example:

• Is the soil sand, clay, or something in between?

• What's behind the wall - a slope, a driveway, a garden or something much heavier like a pool?

• Will the ground above or below the wall be level or sloped?

• What will be the height of the wall?

• All these factors will determine whether a gravity retaining wall or a reinforced retaining wall is needed.

Gravity or reinforced?

Gravity retaining walls depend on their own weight and setback/batter (the distance a course will be offset back into the slope from the course beneath it) and are typically shorter in height.