Back to Basics – Retaining Walls

     Often times when I am driving through a neighborhood, I will see aspects of someone’s landscaping that I admire. Other times I see things that aren’t quite right. I have been tempted on more than one occasion, to snap a picture of a retaining wall that has fallen into disrepair and is moving outward or downward. I think to myself that I could use these pictures to demonstrate what happens when a retaining wall is installed incorrectly. After thinking about it for a minute or two, the sound of the horn of the car behind me forces me to move on and get to where I am going. So, I’ve decided that rather than point out things that were done wrong, I will explain how they should be done. This post, the second in a series will be an informative explanation on proper techniques for installing a retaining wall. Complex hardscapes installations such as retaining walls require a fair bit of experience and knowledge and there are several “tricks of the trade” that come into play during construction, but my hope is that this information be useful when tackling a project like this on your own, or will at least give you the information you need to make an educated decision when choosing a landscape contractor for your next retaining wall project.

     Before I begin to outline best practices, it is important to note that soil types, wall material, and other factors will affect how a wall should be constructed and when building a retaining wall that is higher than 4’ high or will be retaining something like a driveway, you should consider hiring an engineer to specify the method of construction.  With that being said, here are a few basic steps to follow for a successful retaining wall installation.

1)      Base preparation. The most important part of the construction process is the base. Typically you should have a 12” thick by 12” wide compacted gravel pad under any retaining wall. The pad should not be set on topsoil, but on firm sub-grade material. In some cases the base of the wall will need to step upwards. In these cases, begin the wall at the lowest part of the slope. Make sure that the base is as level as it can be since this is what the wall material will be sitting on.

2)      Installing the wall material. The first course of wall material will need to be set below grade. Typically you should bury at least 1-2 courses of stone or block to keep the base of the wall from moving forward. As you lay each piece of the first course, make sure it is level both side to side and front to back. As you install each subsequent course, backfill behind the wall as you go. Never install a retaining wall freestanding and backfill later. The backfill you use is also very important. Typically, you will want to install a 12” wide layer of drainage gravel directly behind the wall. Never install soil directly behind a wall. Soil becomes saturated with water and will cause the wall to fail during the freeze thaw cycle. It is also a good idea to install a perforated PVC draintile behind the wall at the base and allow the end to “daylight” at the end of the low point in the wall. Between the drainage gravel and the rest of the backfill soil, weed barrier should be installed to keep the gravel free of soil which will clog it up. If your wall is larger than 4’ high or is retaining a large amount of soil, Geo-Grid may need to be incorporated. This woven fiber material is used between the layers of the wall and run back into the slope to add strength to the wall system. Each site and scenario is different and if you are not familiar with how a Geo-Grid system works, you should consult a professional.

3)      Capping it. Once you have the wall constructed and properly backfilled, it’s time to install the finishing touch. Every manufactured wall system has a cap that is used on top of the walls. Typically this cap is glued down onto the top course of block. For stone walls, mortaring the last cap course in a good way to keep the top from moving should someone stand on it.

     Building a retaining wall can be both challenging and exhausting. Having one installed by a professional can be expensive, but having to install it twice will be twice as expensive. If you are considering install a retaining wall on your own, consult with a local wall material distributor, or check out some online resources such as those at www.unilock.com. Do your homework before you start ordering material or digging. If you are considering hiring someone to install one for you, make sure you ask lots of questions on how the wall will be built. Find out if an engineer should be utilized to make sure the wall is constructed soundly. A retaining wall is an investment in your home and like any investment it pays to do it right.