So you’ve decided to build a retaining wall. Great!
Retaining walls come in all shapes and sizes. They can be straight, they can be curved. They can be tall and they can be short. Some of them can be as tall as a house while some barely come out of the ground. It all depends on their intended use.
When building any structure, setting a proper foundation is very important. It sets the stage for everything that comes after it. An improper foundation will mean problems for everything built on top of it.
What are some important keys of a proper foundation?
When digging a trench for your wall, the trench should be sufficiently deep and wide. We recommend at least 18 inches deep and wide enough for the width of the block that you will be using plus 2-4 inches in front and 12 inches behind. At this point, you will have dirt as a surface. You will want to compact the dirt with hand tamper or a plate compactor.
Next line the bottom with permeable landscape fabric. Afterwards, you will want to fill the trench with aggregate, aka crushed rock. We recommend 3/4″ crushed concrete. 3/4″ is the approximate size of the individual rocks. Crushed concrete is the material that it’s made from. The reason we want crushed concrete is due to its geometric characteristics and properties, namely that the the sides are angular and ‘jagged’ and they lock together easily. Afterall, you don’t want your material to shift and move once they are set. We will want to apply 6″ of this aggregate but only 2″ at a time. Every 2″, you compact what’s there before adding another 2″.
How do you know you’ve put in 2″ of material vs 1.5″ or 3″? You can use a variety of methods. Some of the easiest and simplest involve a string, wooden stakes, and a string level. You want the string level and above the trench and then use a tape measure or ruler to determine vertical distances.
To do this one only needs the use of a dead blow hammer (not too heavy), a small level (aka torpedo level), and lots of patience. The blocks should be perfectly level to the ground. They should also be perfectly level with the blocks next to them. Their front and back faces should line up perfectly straight. The tops of the blocks should be perfectly level with each other as well.
To accomplish this requires the adding or removal of small amounts of aggregate beneath the blocks. It also requires the gentle use of the deadblow hammer. If you follow all these steps, you should now have a solid foundation to build a sturdy wall that is straight and true.
Forming the Foundation
Ok so you have your first course set. You may have noticed a big space behind your wall. This is for drainage.
First we’ll want the soil compacted, just like the base. Then we will set down permeable landscaping fabric so that it lines the back face of this trench. This fabric will prevent dirt and debris from entering the trench. Make sure to leave enough to cover on top as well if you plan putting vegetation on top. Remember the point is to keep the dirt and other debris away from the aggregate. If you need more than a single piece of fabric, make sure to overlap the edges by at least 6-12″.
Now we are going to fill this area with 3/4″ aggregate, the same stuff we used for the base of the wall. Just as before, you will want to do this 2″ at a time so you can compact it. Any more and you risk the compactor not being able to reach far enough. At this point, many contractors will also put in a drain tile system. Unless you are using mortar to seal all the cracks, any water will naturally percolate through the aggregate and then through all the spaces between the blocks. The entire system acts as a giant drain and prevents water and more importantly, mud from building up behind the wall. This buildup of what’s known as hydrostatic pressure is what causes many walls to fail.
At this point you will add layer upon layer building to the desired height. The wall should lean slightly back when viewed sideways.
Finishing Your Retaining Wall
Once you’ve reached your desired height, you will want to use cap stones for your last layer/course. The cap stones are glued on with adhesives. This hides the irregular features of the insides of your blocks from view as well as add a nice finishing touch to the overall look. The adhesives helps prevent these stones from falling off.
The top of the fill behind the wall, whether you used aggregate all the way or decided to put some soil and then vegetation, need to be level with the top of the capstone. Otherwise you will have erosion and gradual weakening of the wall.
Any space left in your trench in front of the wall can be filled with dirt and then vegetation for a nice clean look. And there you have it. A job like this can take hundreds of hours easily for just a few hundred square feet. However, a retaining wall is one of the most beautiful and useful structures you can add to your property and is well worth the investment.