How to Build Retaining Walls | Complete Guide to Constructing Retaining Walls Using Wall Block
There are a lot of steps to the process of building a retaining wall that will last and most failures that you see would have skipped one or more of these steps involved in its construction. This can lead to serious consequences as walls can hold up important live loads like people or cars moving on the surface it is retaining or dead loads like a building.
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When in doubt, refer to the manufacturer's engineered drawings and specifications for the wall construction. In our area and as a rule of thumb, walls that are to be built taller than approximately three feet require engineered stamped drawings and permits before building. Make sure you do your research before you begin.
We start with the excavation, the width is a minimum of 6” before the wall block, plus the depth of the wall block, plus a minimum of 12” past the wall block. You will also notice that beyond the 12” mark the trench tapers up further past this mark. This will vary from wall to wall depending on the height, so it is best to refer to the engineered drawings for your wall. Most manufacturers have this available for each of their wall blocks.
After our excavation, our subsoil gets compacted and amended if needed. This is followed by being lined with a non-woven geotextile fabric for separation and filtration. The fabric wraps around the entire trench and on top of the eventual backfill.
The base material is then installed in lifts based on how many inches your compactor can compact at a time. We use a Granular “A” or ¾” angular crushed stone down to fines for our retaining wall bases, but you can opt for a ¾” angular crushed clean stone or ASTM #57.
The base course of the wall is then installed ensuring each piece is level and in line with one another. A string line can help guide this process. We use a rubber mallet and a torpedo level for this step and take our time to get this base course perfect. Failure to do so will become increasingly evident the more the wall is built up.
You will also notice that we are stepping this wall up from time to time. This is because without doing so while following the slope of our final grade would cause the depth of our base to fall out of our specified 6 to 8 inches. By stepping up our excavated area, we are then able to step up our stones while keeping a 6 to 8 inch base below each stone that is laid. The number of step ups need to be planned ahead of time during the excavation phase by measuring the slope of the area.
The wall can begin to be built up once that first course is completed, checking to make sure the wall is being built level. Wall blocks must be staggered, so that the lines from row-to-row do not line up with one another. There should be an offset of at least one third of a block. This adds to the strength and stability of the retaining wall.
A perforated drainage pipe is installed in behind the wall with approximately 4” of separation from the compacted dense base material. This allows for the drainage of any water that enters into the system. The drain also needs to be daylighted or exited out through the face of the wall at a maximum of every 50 feet. If you are using a dense graded base and filling in behind the wall with a clean stone, you will want to separate these two layers with a non-woven geotextile.
The backfilling of the wall is completed in stages as the wall is built. We backfill using a ¾” angular crushed clear stone that has no fines in it and backfill to the level below the row of wall we have just completed and compact it in lifts depending on how much our compactor is rated for.
If it is required in the engineered drawings, a uniaxial geogrid is installed. It helps to stabilize the aggregate behind the wall, preventing movement and pressure added to the wall. This is rolled out perpendicular to the wall and cut, not rolled out parallel to the wall. This is because a uniaxial geogrid has strength only in one direction and that direction needs to be extending into the wall and not along the wall. The geogrid never overlaps on a retaining wall installation. It is sandwiched in between the wall units and extends into the backfill to the length that is specified in the engineered drawings. At the top, the non-woven geotextile returns to the wall and topsoil is added with a swale to control the flow of water away from the wall. A cap is then added and adhered to the wall to finish it.
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