Sand is generally a non-compactable material and tends to move around. Two inches is too thick of a layer in any case. A half inch is on the thick side - just enough to smooth the top surface ofthe crushed gravel. The reason that many do not mention sand at all is usually when they are telling you to use larger gravel without any fines in it because it will eventually slip into the spaces between the stones and cause settling.

Tbest way, in my opinion, is to use a grushed stone that has fines - there are all kinds of regional terms, but it would be a mix of 5/8" or bigger gravel with smaller gravel and the dust from the crushing all mixed in like a road base. That has strength and compacts well (with a little moisture). It does leave a coarse surface so a skim coat of sand helps the blocks keep from tipping as you set them. Because the material has fines and compacts well, the sand can not infiltrate and the wall does not settle.

Thanks for the info - in reviewing my previous research, I realize I have mentally combined techniques for wall building with those of building a paver patio (future dream). So speaking of - why do you use sand in patio construction? Wouldn't that sift down between the gravel?

Exact same purpose as laag described for the wall. For a wall, a patio, a step system etc... when one is constructing over a compacted base of 0-3/4" (synonymous with- road base, flex base, Granular A...) a layer of sand or screenings as fine as one can manage is often required to achieve the perfect level necessary to start building upon. Used sparingly, it will not affect the structural integrity of your patio base and will allow you to lay a perfectly graded patio, wall, or step system.

The sand will not sift between the gravel because the gravel you will want to use will be the same for a wall, i.e what was mentioned above by laag. It has fines in it, hence the name 0-3/4", as in 0" fines to 3/4" rock, and everything in between. These fines at the micro level are shaped like tiny triangles that bind together in a structural fashion while filling in all the voids that are found between the larger pieces of rock (or gravel). Furthermore, 0-3/4" is normally crushed lime stone. Lime is one of the key additives in concrete mix. Wet the 0-3/4" as you compact it and you will see it turn to a near solid mass.

I had the same question. It may be because when I watch videos on preparing for a retaining wall the gravel looks different than mine. I got my gravel second hand and I’m wondering if that’s why my gravel looks different? Would adding some sand to my gravel add back the smaller “fines” to the gravel?

@Shira M If you are watching Youtube videos, there are many many many videos that are just plain wrong about any subject. You do not have to have a license or professional knowledge to post a video there. Most block/paver manufacturers have their own created videos available, and websites with written how-to's that can help guide you.

The proper gravel for base is 3/4" crushed stone, and the fines are just that - tiny chipped pieces down to stone dust from the crushing. They vary in size and are needed to fill all the different sizes of voids and allow the base to compact to provide a very solid foundation. There are many similar-effect gradations of 3/4" gravel with fines that go by different regional terms that may be the only available product you can get (e.g. road base, crusher run, 3/4 minus, CA-6, etc).

You may have purchased "washed" gravel, which has the fines washed out. Sand is naturally shaped by water, so the rock granules are smoother and more rounded (not as angular as crushed rock), and adhere to a certain small range of size. So adding sand to washed gravel would not fill all the varying sizes of voids, just a specific range, which could lead to settling.

Also be aware that there is a difference between a retaining wall and a garden wall. A garden wall, like this old thread was describing as a 24" high wall, doesn't have the necessity of a full-on base and drainage detailing you need to do to hold back the earth (which a retaining wall does). You can sit a couple courses of blocks right on the soil if you want, you just may have to re-set them after they move around over the seasons.