The foundation of a retaining wall is a key component in the wall’s longevity. A good foundation ensures the wall does not sag or settle. It will ensure the finished wall is level and straight. A good foundation prevents frost heave in colder climates. Here are the essential steps for building a strong foundation.
Before you Dig
An electrical wire, gas pipeline, communications line, or other utility service could be just a few inches below the ground surface. Before you begin any excavating, you must contact Call811. You can either visit call811.com or call 811. When you contact Call811, they will direct you to your local center. They will then contact all the utility companies that have lines on your property. The utility companies will send out a representative to put up flags or spray paint along the utility lines. Typically the utilities have 2-3 days to mark their facilities. There are usually 7-8 utility providers that are contacted. Confirm all utilities respond to the request prior to beginning your project.
It is your job to ensure no excavating occurs within the 18-24″ tolerance zone around the markings. If this is not possible, by law you cannot use mechanical equipment within the tolerance zone until the facility is completely exposed.
Mark the Face of Wall
Mark the front face of your retaining wall. You can use garden stakes, string, rope, a hose, or spray paint. For walls in a straight line, a string* tied between two stakes works best. Use a line level* to level the string so you can use it to measure the depth for your trench.
How Deep do I Dig?
With the wall layout marked, begin digging a trench. The depth of the trench includes the base material and embedment depth.
Base Material
The base material is the foundation of the retaining wall. It also provides a level surface for placing your blocks. The quantity of base material depends on the wall height (including embedment). Note, for block walls, the width (W) needs to be at least twice the width of a block, even if that is greater than the values below.
| Wall Height (H) | Base Material Width (W) | Base Depth (d) |
| 0-2 ft | 12-in | 4-in |
| 2-4 ft | 18-in | 4-in |
| 4-6 ft* | 24-in | 6-in |
*Walls greater than 4-ft require engineering.
Rock screenings, paver base, ¾” rock, #57 stone, and Class I, II, or III backfill are all acceptable forms of base material. Do not use smooth river rock, pea gravel, or uniform sand for the base material. Round material like this does not hold its shape, is difficult to compact, and will make it difficult to level the blocks.
Embedment
The amount of buried wall at the front face is called embedment. The front of the wall is partially buried to provide additional stability and to minimize the potential of undermining the wall from erosion. The table below assumed a flat grade in front of the wall.
| Tall Wall Height | Minimum Embedment |
| 0-2 ft | 4-in |
| 2-4 ft | 6-in |
| 5 ft* | 7-in |
| 6 ft* | 8-in |
*Walls greater than 4-ft require engineering.
A toe slope is sloping ground at the front of the wall. The general recommendation for wall embedment with a toe slope is to bury enough blocks so that from the base of the wall a 5-ft horizontal bench to daylight is created. A bench to daylight means you should be able to draw at least a 5-ft horizontal line from the bottom of the wall to the ground surface. To determine the embedment depth, take 5-ft divided by the slope change per one foot in elevation change. So if there is a 3-ft horizontal to 1-ft vertical (3:1) slope in front of the wall, take 5-ft divided by 3-ft for a minimum of 1.66-ft of embedment that is required.
The minimum depth of the trench is the depth of base material plus the embedment depth. If there is any soft soil, organic material (mulch, roots, or dark soil), or slick clays, remove this material. This soft material will either settle over time causing a dip in your wall, or cause your wall to slide out during a rain event. Remove and replace with additional base material.
Compact the Foundation
Use a 4-ft level to ensure the trench is level. If the ground is dry, lightly water it before compacting. Water helps the soil lock in place during compaction. Just don’t use too much or you will oversaturate your foundation soil. Ensure there is no standing mud or standing water on the foundation soil before compacting. If there is mud, remove it and replace with base material. Then, compact the foundation with two passes of the plate compactor. Note, use ear protection and a dust mask anytime you are using the plate compactor.
Place & Compact the Base Material
Place the base material in the trench, spreading it with a rake. Bring the base material close to the bottom of wall elevation. Add some water to moisten and do not oversaturate. A few seconds on a shower setting with a garden hose along every couple feet of base material will do. After the base material is moist, compact it with two passes of the plate compactor.
If your wall is less than 2-ft in height and you opt not to use a plate compactor, it is recommended that you compact the base material in 2” lifts (or layers of backfill). All walls greater than 2-ft in height need a foundation compacted by a plate compactor.
Leveling the Foundation
Leveling the base material is a critical step. Lay a PVC pipe across the width of your trench. This is called a screed pipe. The screed pipe should be roughly as wide as your trench. Use the torpedo level to ensure the screed pipe is level. Set a second screed pipe approximately 9-ft from the first and level it.
Now, place a 10-ft 2×4 spanning across the two screed pipes. Use a 4-ft level on top of the 2×4 to ensure both screed pipes are set to the same elevation. Then add or remove base material as needed to the top of your screed pipes. If more than 1-in of loose base material is added, recompact with the plate compactor.
Use the 2×4 to scrape across the top of the two screed pipes to ensure the entire base is level. Avoid putting too much pressure on the screed pipes or they may be knocked out of level. Repeat this process along the entire length of the trench or until you reach a step location. Remove the screed pipes prior to starting the wall.
With a properly prepared foundation, the rest of the retaining wall can be built.
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