A common question for a retaining wall: what is the best backfill type for my retaining wall? You will find a plethora of answers online. All of them are well intentioned for their particular case. Let’s discuss the different backfill zones and what backfill type works best for each.
Definitions
Base Material
The base material is the foundation of the retaining wall. It also provides a level surface for placing your blocks. For more information on foundations, check out this post.
Wall Rock
Wall Rock is only applicable to block walls. This refers to the material used to fill hollow blocks. This adds weight to the retaining wall while keeping the blocks themselves relatively light. It also aids in friction resistance between the blocks. This is an essential component to hollow blocks. Not all blocks are hollow.
Drainage Stone
The drainage stone will be installed at the back of the retaining wall and will extend 12-in behind the wall. Drainage stone allows for water to freely flow through to the drain pipe and out the face of the wall. For more on how drainage stone fits into a drainage system, check out this post.
Retained Soil
If your wall is on a fill site, you will need additional backfill behind the drainage stone. Ideally, you will want to use what you have on your property, but this is not always possible.
Acceptable Backfill Types
Base Material
Rock screenings, 3/4″ minus, crush and run (or ABC stone), paver base, 3/4″ crushed rock, #57 or #78 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. Rock screenings or minus material make leveling the initial retaining wall blocks easier, but clean crushed rock will perform just as well if not better over time.
Wall Rock
Wall rock must be angular. This eliminates pea gravels and river rock. I recommend a well graded compactable aggregate that is 0.25″ – 1.25″ ideally. This includes 3/4″ crushed rock, #57 or #78 stone, clean crushed limestone, or Class I or II backfill.
Drainage Stone
Drainage stone must allow water to drain through. The amount of fines (material passing through a No. 200 sieve) should be less than 10%. You may be tempted to use pea gravel or river rock as drainage stone. However, the smoothness of this material will make it difficult to retain at the end of the wall or if you ever need to remove a block. In addition, smooth material is difficult to compact. I recommend a well graded compactable aggregate that is 0.25″ – 1.25″ ideally. This includes 3/4″ crushed rock, #57 or #78 stone, clean crushed limestone, or Class I or II backfill.
Retained Soil
Ideally, you want to use what you have on your property. Do not use clay, organic material such as decomposing mulch, garden soil, top soil, or any other soft soil. If you need to purchase backfill, look for processed fill, gravel, sand, or screenings.
The base material, wall rock, and drainage stone all require a quarried backfill. Although you could use different materials for each, I recommend sticking with just one material for all three zones. This should be a clean crushed rock, like a #57 or #78 stone, 3/4″ crushed rock, or clean crushed limestone.
Where to Buy
For a smaller project, crushed rock can be bought from a typical hardware store. At a Lowes or Home Depot, crushed rock is purchased by the 0.5 cu. ft. bag. Look into their delivery options if you do not have a large vehicle.
If the quantity of crushed rock exceeds 1/4 cu. yd. (6.75 cu. ft) and you have access to a truck, research landscape supply companies such as this one in Raleigh. A 1/4 cu. yd. is typically the minimum order amount at a landscape supply company. You will pay a fraction of the price compared to a typical home improvement store. Landscape supply companies will also deliver for larger quantities, typically over 5 cu. yds.
