When building a retaining wall, the layout, materials, elevation, and method of construction are all important. None of these are as important as preparing the base for the wall correctly. Most retaining walls fail due to two issues: poor base preparation and poor drainage.
In this post we will discuss how TriGem Homes and Gardens prepares the base for a precast retaining block wall under 4 ft. in height. I will start off by saying that you should always consult your local building department, as the building code for retaining walls differ across cities and counties in Georgia. Some will require a building permit and after a certain wall height some will even require engineering and supporting documentation. You should also be aware of where the measurements for the wall height start. Many counties across Georgia start the measurement of the height of the wall from the bottom of the footer or base material.
Once we have sold a retaining wall and the layout as well as the materials have been selected, it is time to start the excavation. Keep in mind that all precast retaining wall manufacturers will require at least one block buried below finish grade. This is known as the base block, and it is important because it will help stabilize the wall. Think of this as the anchor that resists the pressures of the world behind it! No pressure!
Tip: Make sure you call 811 to locate your utilities prior to excavation.
Below the base block is your compactable base material, when planning your excavation, you should start with a trench 6 inch deeper than the height of your block and at least 12 inches wider than the overall width of your block. This dimension will allow for a 6-inch-deep layer of base material and a 12-inch layer of drainage gravel behind the block. As I mentioned earlier, poor drainage is the second reason why retaining walls fail. The 12-inch layer of drainage gravel will allow for the water to flow easier.
After we have determined the beginning and finish elevation of the wall, we excavate a trench using the guidelines above. We use crush-n-run as our base (also called class 5 in other parts of the country). Crush-n-run is basically a mix of angular stone with fines that when compacted creates a hard and stable base. We start with a minimum 6-inch layer that we compact and level front to back as well as left to right. It is critical that the base is leveled perfectly as any imperfections will be amplified in the base block and all the rows above.
Once you have your base leveled, it is time to add your base block. Depending on the type of block you are using, some are smooth on the bottom, but others have a shoulder along the back edge. This shoulder hangs down, so that as you stack the block, it fits over the block below it locking the two rows together. This cantilevers the block to the rear and creates what is called the batter or percent of angle that the wall leans into the retained material. This shoulder, however, will need to be chipped off for the base block row.
Once again, I cannot stress how important it is to get the base material and base block as leveled as possible. Depending on the height of the retaining wall, you can spend as much time leveling the base and the base block as it takes to build the wall itself.
Once the row of base block is completed, the next step is to fill in behind the wall with wall rock. We always use #57 granite stone that has a size between ½ and ¾ of an inch. There should be an area at least 12 inches wide behind the base block and every row of blocks afterwards. Now that the base block has been filled in behind with gravel, it is time to toe it in. Any space between the front face of the block and the native soil needs to be filled in and compacted. The compacted soil in the front as well as the gravel in the back will stabilize the base block as you add additional rows above.
For length and the sanity of the post reader, this post is condensed to only the base prep. There is so much more information that you need to consider when building a retaining wall. As I mentioned in the beginning, height is one consideration, but method of construction also depends on the reinforcements needed, the soil type, and the surcharge of the load being retained. The surcharge can be a structure, parking area, or even the slope above the retaining wall.
Building a retaining wall is much like running a marathon, you pace yourself and concentrate on your technique so that you can have a strong finish in the end.