Retaining walls are useful and attractive features in many yards, but they are prone to failure if proper drainage is not considered. Some professional installers find that nearly 80 percent of the retaining walls they see fail at some point due to improper drainage. Whether you are planning to install your own retaining wall or have someone else do it, understanding how drainage should work will ensure the best possible end result.
Why Proper Drainage is Important for Retaining Wall Performance
A retaining wall is built to hold back a certain amount of soil. The design of a properly-built retaining wall resists the force of the soil behind it and prevents caving, buckling, or leaning. However, retaining walls are built with a certain capacity in mind. When water gets behind the wall, it puts additional pressure on the structure, which can result in the overall force exceeding the wall’s ability to resist.
This is what good retaining wall drainage looks like:
When the wall is no longer able to resist the force behind it, the weakest part will give. Sometimes, this means that the wall will develop a bulge at this weak spot. In other cases, water pressure behind the wall may cause it to lean outward. If the pressure is consistent and forceful enough across the wall, it may even buckle.
Conversely, this is what bad retaining wall drainage looks like:
Okay, let’s dive into all the details of this diagram and how to make your retaining wall drain successfully.
How to Set Up Your Retaining Wall to Drain Properly
The process of making sure that a retaining wall drains properly is relatively simple, but it is not very flexible. Although there are exceptions, most retaining walls require gravel backfill, soil compaction, pipe or toe drains, and weep holes. Together, these four features will provide adequate drainage for most designs. Only a few types of walls will not require all of them. We recommend getting a professional to help you install, but if you want to DIY, these steps below and other tips in our in-depth DIY guide can help you along the way.
Backfill simply refers to the soil directly behind the wall. For proper drainage, the first 12 inches of space behind a retaining wall should be filled with crushed stone or gravel. This is so that when water gets into the space, it does not become bogged down in soil but instead can flow down the wall to the drains or weep holes. Many designs allow for the top six inches of space behind a wall to be filled with native soil, so that the area can be used for planting.
Soil compaction is most important for creating strength, but it also makes the dirt less permeable and reduces the ability of water to seep into the space behind the wall. The soil behind the layer of gravel backfill is the target for this step. Proper compaction involves the use of a plate compactor or similar tool, not just stomping on the soil with your boots.
Pipe drains, also known as toe drains, are perforated pipes that collect water along the length of the wall and drain it to the outside. Some of these drains are vented through the front of the retaining wall, while others may run the length of the wall and drain out to the sides. Pipe drains are usually installed at the base of the wall, but taller walls and those with additional drainage needs may require multiple pipe drains at varying heights. Pipe drains must be vented at least every 30 to 50 feet, and the use of rodent mesh is frequently necessary to prevent debris from getting into the pipes and clogging them.
Weep holes are openings in the front of the wall that allow moisture to escape. They may be positioned at the base of the wall for structures with minimal need for drainage, or in a grid pattern across the entire wall. Water flowing from weep holes can create unsightly stains if the retaining wall material is not properly protected, so the use of waterproof overlays, varnishes, and other moisture-protection substances is necessary.
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Avoiding excess moisture during the construction process is also important. In some cases, it will be necessary to divert drainage along the slope above the wall while construction is under way. Ideally, retaining walls should not be installed when it is likely to rain. If this cannot be avoided, the use of tarps will help to keep the water out of the area when construction is not actually on-going.