Why do retaining walls need drainage?
For a retaining wall to properly do its job of retaining soil, it needs some type of drainage. Water is heavy, and over time soil that is wet and heavy will push against your retaining wall. Adding drainage reduces the amount of that heavy soil, and will help make the retaining wall last longer.
If this Water is not drained away, it will create problems during freezing temperatures. As the water freezes it expands, and will cause the retaining wall to move or shift. While this may not structurally affect the wall, it won’t look straight anymore, reducing the aesthetic look of your retaining wall.
Types of drainage for retaining walls
A proper drainage system consists of one 4” pipe behind the base of the wall called the Toe Drain. If you have a small wall, and Geogrid is NOT required than you really only need 1 pipe
If you have a larger wall and Geogrid is required than you will need two drains, the Toe drain, and the Heel drain.
On the far end of the wall there should be a clean out cap for future maintenance, and the low end of the pipe should be tied into an appropriate drainage system, such as city storm drain, in ground concrete sump, perimeter drain system, or ditch.
The best method is to connect directly to your existing drainage system or storm connection.
Redirect the water away from your wall, and get it off your property or at least to an acceptable designed area.
Gravel is essential for the retaining wall base to provide a solid level foundation to build upon.
Gravel drains water away to eliminate soil erosion, decomposition of the wall material, and to remove the standing water below the wall system to prevent frost heave in the winter time.
When retaining walls are built, the area is excavated and gravel is brought in to replace all of the soil that was excavated.
Some contractors do not put enough gravel behind their walls due to the cost of having to excavate a lot of soil, and replacing that soil with gravel.
The problem with this is that if you cheat the excavation area and do not add enough gravel, your retaining wall will fail.
Gravel provides a solid level of foundation to build the wall on, and will allow water to drain through the gravel, providing proper drainage.
When there is no standing water, there will be no pressure put against the wall, and no frost heave in the winter time.
Wood retaining walls and Concrete retaining walls both need an adequate gravel base and a proper drainage area behind the wall.
What happens if I dont add drainage?
The Majority Of Retaining Walls Fail Because Of A Lack Of A Proper Drainage System Behind The Wall.
Common problems with retaining wall drainage are that there either no drains at all, pipes are not protected from migration of fine materials (think clay soil plugging up the pipe), or the drains don’t drain to a good location.
A proper drainage system is composed of a minimum of 16” of clear draining gravel directly behind the wall, and a drainpipe at the bottom of the wall moving the water away.
The pacific northwest has a lot of rain, and if a retaining wall is not built correctly with proper drainage, will fail.
A good contractor will ensure that there is sufficient drainage behind your wall so that it lasts decades or more, and you can rest easy knowing that your wall is safe and secure.