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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.
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.
Segmental Retaining Walls
Retaining walls are required when there is a grade change in a space that needs to be retained or in order to provide a flattened usable space. Segmental retaining walls or SRWs provide a versatile solution to numerous applications in retaining soil whether it is a garden wall or a larger space. Segmental retaining wall blocks are concrete modular blocks that interlock to one another. Segmental block retaining walls are versatile in both their construction and aesthetics. Hardscape manufacturers continuously produce new sizes, shapes, and color blends to provide numerous possibilities for a custom built retaining wall.
Typically retaining walls built higher than approximately 3 feet above grade require a permit and stamped engineered drawings, but this will vary depending on where you live. There are numerous variables to consider when constructing a retaining wall, including:
: The load that the retaining wall will be put under will depend on what is within its proximity. This can include both live and dead loads. A live load is any force that is changing like people walking in the proximity of the wall, cars, or even hydrostatic pressure caused by water. Dead loads are forces that do not change like a structure built within the proximity of the wall. The more force being placed on the retaining wall, the more complicated the build and engineering of the wall will need to be in order to withstand the pressure over time. This force is referred to as the surcharge on the retaining wall.
: This needs to be considered for any retaining wall, as any water that enters the system in behind the wall needs to be drained outside of the system. Water is a significant force that will add pressure to the wall from behind it and will push it forward, especially during freeze-thaw cycles. That is why a clear stone with no fines is used behind retaining walls to prevent the water from being retained. The clear stone allows the water to flow immediately through it where it would be picked up by a perforated drainage pipe and exited through the face of the wall.
: The lean of a retaining wall towards the area it is retaining can add to the strength and stability of the retaining wall by setting back the retaining wall blocks at a specified angle built into the retaining wall blocks.
Segmental Retaining Wall Construction
This is a cross section of this particular retaining wall build that we are constructing in this video. There is a lot to consider when looking at a drawing of a retaining wall whether provided by the manufacturer or an outside engineered drawing.
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. The wall block will be buried or embedded a minimum of 6″ below the final grade. This will vary from wall to wall depending on the height and slope that the wall is being built on, 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. There is an argument that with the right subsoil, the fabric is not necessary along the vertical portion of the trench as the fabric may become clogged with the fines, preventing proper filtration, and adding to the surcharge that is being applied to the wall. For this example, we are installing it as shown with the final piece being installed on top of the backfill material.
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. In order to speed up this process of levelling, the base can be screeded using a ¼” angular crushed chip or HPB.
How to Build a Retaining Wall on a Slope
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. If possible, it is easier to work from the bottom of an area and step up rather than stepping down.
The wall can begin to be built up once that first course is completed, checking to make sure the wall is being built level. Not all wall blocks are manufactured equal depending on the tolerances of manufacturers. 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.
Some wall units have cores in which you fill in with ¾” angular crushed clean stone as you build the wall up. Some wall units have their own locking system built into the unit in the form of a rib, but others you will be using a plastic clip or rod of some sort to form that lock from one row to the next.
Corners of your wall should be planned for by alternating the piece that will be met, stitching the two walls together and providing strength to the corner. Rock-faced walls may require you to chisel corner units to create the rock face. This is done by using a chisel and hammer and progressively increasing the strength to which you hammer the chisel into the stone while moving your way all the way around the stone until it breaks off. There are also guillotines that can help you with this. Most walls have specified corner units to use. And if a wall is not a rock-faced wall, then you need to plan for the corners appropriately to ensure they can be stitched together at that corner.
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. This may extend into the structural subsoil beyond the clear stone. To do this, the non woven geotextile will wrap towards the retaining wall where the uniaxial geogrid will begin, and then again continue once the geogrid has been installed. This allows the geogrid to be installed past the clear stone area and into the structural subsoil.
Some walls may include a setback which, in combination with geogrid, allows for walls to be built taller to help withstand the force of the surcharge beyond the wall. This angle that the wall is setback at is referred to as the batter.
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. We glue our caps to walls using a polyurethane adhesive that expands into the pores of the material creating a firm yet flexible seal. We use strips of glue that are perpendicular to the wall face, not parallel. This allows any water that makes its way under the cap to escape.
When built correctly, walls are designed to create a functional space out of a sloping yard allowing homeowners to make the most of the space that they have. They can create features and raised patio for spaces and provide a lot of value to the property. However, any properly built retaining wall comes at a cost.
There is no specified amount to which a retaining wall will cost. Every market is going to be different, the product that is installed will vary, the job sites and access will never be the same from one project to another. If you are a homeowner, you should know that anywhere you read an average amount that a retaining wall will cost is unrealistic because the person writing it is not in your market. Additionally, if you decide to proceed with the lowest bidder you should ensure they are building it to the manufacturer’s specifications. Ask them for the drawing that they are building it from and ensure that they follow that. The best way would be to proceed with a legitimate contractor that you can trust and comes recommended so that you do not have to put up with these headaches.
For contractors, any project you complete for the first time will come with a learning curve. Start small with a small and simple garden retaining wall and take copious notes during the construction process including your production rates. How long did each step in the process of the construction take you? Write notes on each of these steps, what you used, and any other important information. You can then use that information to quote future retaining walls. Continue with this note taking on future projects to get to an average that you can become increasingly accurate with your quotes.
Before long, you will make a name for yourself in your market as a segmental retaining wall contractor.