Retaining walls are very common, from a basic rock or timber retaining wall, to more elaborate concrete ones. They are usually built to hold back soil and retain its pressure, or act as support for a slope by terracing the slope. Retaining walls can also look attractive and enhance any garden, but if you plan on building one, thorough planning is required.
The higher the retaining wall the more stress it incurs, from the soil and water behind it. Therefore, many local building authorities have strict regulations regarding the maximum allowed height of retaining walls, and you may need to check with them first. As a rule of thumb, 1 m height is about the maximum for most retaining walls. Smaller walls are certainly easier to build. Common materials used for retaining walls are timber, rocks/stones, bricks, and concrete.
There are several typical models or types of retaining walls, such as the gravity wall, the piling wall, the anchored wall, and the cantilever wall. Here, we will just concern ourselves with the basics of building retaining walls.
The first stage is of course to draw a detailed plan of the proposed site. Decide if the wall is to be straight or curved, and angled or vertical. Do not build retaining walls on filled areas; only on the cut-out portion of a terrace.
Before you terrace a slope, you need to calculate the amount of slope. Use a level string line with the help of a spirit level (to make sure the string is level) from the slope top to the slope bottom. At the slope bottom, measure the vertical height of the string line to the ground. This is the “fall.” Also, measure the distance from both points of the string line (slope top to slope bottom). If the distance is 5 m and the fall is 1 m, this means the land drops 1 m for every 5 m of slope. The “fall” height determines the height of the retaining wall, while the horizontal distance determines the size of the terrace.
Excavating and cutting the terrace is best done by hiring a backhoe and driver. Mark the areas where the digging is to proceed, and take note of any underground pipes or other installations.
Proper water drainage is essential in order to prevent water buildup in the soil behind the terrace, and there are several ways to accomplish this. The best way is through using agricultural pipe which is placed behind the retaining wall, and channels water runoff to escape holes at some points in the wall. Purchase socked pipes that can filter out the soil from clogging it.
Rough sand or gravel should be placed behind the retaining wall and as the buffer material between the agricultural pipes. Also, if you are building concrete retaining walls, weep holes can be placed at intervals in the brickwork which help drain off even more water. Simple retaining walls using timber or rocks do not need weep holes, as there are already many gaps within them. As an added measure for more elaborate retaining walls, you can construct open drains using PVC or kiln pipes which will help to drain away more water.
To fill up the area behind the retaining wall, you can use sandy or light soil. Try to avoid clayey soils, because they have a tendency to absorb water and then swell, putting stress on the wall.
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