A hopelessly hilly or uneven garden can make setting up a garden, building a patio or completing other types of landscaping projects difficult. Fortunately, concrete retaining walls can help you create large, flat and easy-to-work-with outdoor spaces. A concrete retaining wall holds back vertical faces of earth that otherwise would slide downward and take the more natural forms of hills or slopes.
There are four main types of concrete retaining walls: gravity, semi-gravity, cantilever and counterfort. Gravity concrete retaining walls are massive, heavy structures that rely solely on their weight for holding back earth. Semi-gravity concrete retaining walls use steel tension reinforcing rods, which help minimise wall thickness. Cantilever concrete retaining walls, like semi-gravity walls, also utilise reinforcing rods. But these walls have the additional feature of a two-part base that includes an outward-reaching toe and a heel that reaches under the earth that the wall is retaining. Counterfort concrete retaining walls are the strongest. They have the same basic design as cantilever walls but also include webs of concrete, known as counterforts, which help tie the bases and main structures of counterfort walls together.
The specific height of a concrete retaining wall depends on the amount of earth you want to retain the type of wall you decide to use. A gravity retaining wall is typically sufficient and economical for heights of up to 3 m (10 feet). For a wall higher than 3 m (10 feet), you need a semi-gravity or cantilever design. A counterfort concrete retaining wall is your most economical choice for a wall higher than 7.5 m (25 feet). Before starting construction on any type of retaining wall, look over the building regulations that apply to your area. Most counties require that you have a licensed engineer design or approve the design of a concrete retaining wall that is more than 1.2 m (4 feet) high.
As a general rule, a concrete retaining wall should be at least 30 cm (12 inches) thick. However, you might need a thicker wall in many instances, such as when using a gravity retaining wall or when working with highly reactive soils. With a cantilever or counterfort retaining wall, wall thickness is typically about 10 per cent of the total wall height.
The length of a concrete retaining wall depends on the size of the area you are trying to retain. In most instances, length is approximately 50 per cent to 70 per cent of a retaining wall's height. So if your wall is 7.5 m (25 feet) high, its length should be between 3.75 and 5.25 m (12.5 and 17.5 feet).