Retaining walls are great for many purposes like preventing erosion and walling off an area where you can better use your space. However, they are only as good as their construction. A retaining wall that doesn’t have good drainage won’t add much value or use to your home.
How do you stop dirt from seeping through a retaining wall? You have to add better drainage. Specifically, you need to add a French drain. That includes a perforated pipe angled downward to prevent dirt clogs in it.
Read further about how drainage can resolve a host of problems with retaining walls.
Problem Retaining Walls
Drainage is a common problem with retaining walls. Engineers say you know a wall has improper drainage behind the wall when it looks like it’s being pushed by dirt from over the top or if it’s leaning forward.
Retaining walls are designed to hold up something. It may be dirt in landscaping or it may be used as an external basement wall. There is pressure against it constantly along with bad drainage makes it shift, sink, and weaken the wall.
Eventually, the wall will be so damaged that it will be expensive to fix or replace.
Drainage is important to eliminate soil and water seeping through retaining walls as well as protect the wall from damage.
One key concern when you think about drainage is the condition of the soil. You must accommodate for that. You don’t need as much drainage in sandy or well-draining soils. However, areas that stay wet or have a lot of clay will need more drainage.
Those with serious drainage issues may need to call an engineer or drainage company to develop a remedy for the problem. A contractor may suggest things but it takes someone with drainage knowledge to find a permanent solution.
Many home experts advocate for a French drain system along with a special waterproof coating that protects the side of the wall against dirt. Some believe those two things will eliminate all water and soil seeping through the retaining wall.
What Is a French Drain?
A French drain is a perforated pipe laying on gravel in a covered trench. It helps water flow out of the area and into a pipe that goes to a storm drain or some other appropriate place.
The pipe is at the base of the wall on the dirt side. The pipe slopes down and typically will connect to a solid pipe for water to flow to a drain, dry creek bed, or some other place.
What Is a Membrane?
There are liquid waterproofing membranes that coat the dirt side of the retaining wall before it’s backfilled. It’s easy to apply because you put it on the wall just like pain. You pour it into a paint tray and use a paint roller to apply it. You have to let it cure but the cure time is set by the manufacturer.
Tips for Fixing Drainage
Check the Footing
Every retaining wall needs a footing drain. This is the drain at the foundation and where most say the French drain should go. It includes a pipe leading out from the wall so water flows away from the wall. It’s a small investment that prevents big problems.
You will have to do some digging to see if there is a footer. You can also check to see if there is proper gravel too. It’s okay to do some minor excavating as long as you don’t hurt the structure of the wall. Try to keep your digging to one or two spots.
Get the Correct Pipe
Even though most love the French drain, you have two types of pipes involved in any retaining wall drainage. There is the perforated pipe and the solid pipe. The perforated works well behind or under the retaining wall while the solid pipe works best once you get into the yard to carry water away.
Get Enough Gravel
Good drainage needs crushed stone or gravel that is between 1/2 to 3/4 inch in size. This is the base and the backfill of the wall. The standard is to add a minimum of 12 inches of gravel for backfill but some claim adding 24 inches is even better for optimal draining and to maintain structural integrity.
Gravel also needs to be clean gravel to be used for backfill. Dirty gravel prevents water flow. While the process is the same, backfilling a wooden retaining wall has some additional steps so be aware and ask experts about wooden walls.
Say Yes to Compacted Soil
Most of the time, you don’t want compacted soil in your yard because it won’t absorb water. However, you want compacted soil behind your retaining wall. Soil that soaks up water causes a lot of pressure on retaining walls and making them less permeable prevents water from damaging your wall.
Check your soil and add clay and compact it if necessary.
Use Landscape Fabric
Landscape or filter fabric should be used with retaining walls. It’s a permeable cloth used between soil and gravel. It keeps weeds from growing as well as dirt from preventing the gravel from draining well. This fabric helps your retaining wall maintain its structural integrity.
Landscape fabric is cheap so make sure you have plenty of it if you are fixing your wall or rebuilding it.
Create Weep Holes
Weep holes are tiny holes spaced evenly along the bottom of the retaining wall. It allows water to seep there underground and that prevents pressure from building.
You can add them to an existing retaining wall. You can use a core drill with a drill bit and drill holes into existing materials like stone, concrete, or brick.
The trick is where to place them. That depends on the size of your wall. Walls six feet or taller need 3 to 4-inch in diameter weep holes spaced out every three to four feet. Smaller walls need holes that are 1 1/2 inches in diameter and spaced every eight feet.
They are drilled near the base of the wall. The one problem you may encounter is a retaining wall backfilled with soil instead of gravel. Soil doesn’t allow enough drainage through the weep holes.
How do you fix erosion under a retaining wall?
Erosion happens when the water has nowhere to go. You may need to rework your wall to pack sand, gravel, and filter fabric behind the stones to all water a way to escape.
How do you waterproof a timber retaining wall?
Use Liquid Rubber. Apply it directly to the backside of the wall. It will make a seamless waterproof membrane and will also bridge cracks that form over time.
What’s the most important thing about drainage behind a retaining wall?
Stone or gravel should fill the first 12 inches of space behind a retaining wall. This is to allow for drainage and help water flow to the drains and weep holes.
How deep should a French drain be behind a retaining wall?
A French drain should be between eight inches and two feet deep.
What can happen if I don’t fix my retaining wall that leaks?
Soil and water leakage is the first sign you have a drainage problem. Failing to fix the problem will cause the wall to eventually lean and shift. It will eventually crack and even start to crumble.