Gravity block retaining wall foundation question

[LucyBIT]

Hi all--we are replacing a crappy retaining wall along one side of our backyard. We had a guy with a bobcat out yesterday to remove the old wall, plus several tree stumps/roots, also excavated a few feet into the yard to clear the ground for the new wall. We're using these with adhesive: http://www.homedepot.com/p/Pavestone-RockWall-11-5-in-x-7-in-x-4-in-Pecan-Concrete-Retaining-Wall-Block-87524/204643740

The exposed ground where the wall will be built is clay, and immediately adjacent is our neighbor's poured concrete driveway. The wall is approx. 72 feet  long and the height of the wall will be 18-24" above ground/driveway surface (not including any material underground). Following best practices for drainage, ex. gravel behind the wall, drain pipe (tile?) at the base behind the wall, fabric between gravel and dirt, etc.

Question: For the foundation, under the first course, do we need to lay down base and/or gravel, or is just sand sufficient? We haven't been able to find a straight answer--most articles say for a higher wall than we're building, sand is not sufficient, but they always get fuzzy at our actual height.

We haven't dug out a trench yet so we're not super committed to any foundational course of action, ha. The old wall had a few semi-buried ties, so part of the length is already about 4-5" below the driveway surface, but we'll need at least 4" of that for the sunken blocks so we can dig out more to make room for base if needed.

Thanks in advance and let me know if more info is needed!

[ncornilsen]

You will want to put down 3-4" of crushed rock (3/4minus) that you tamp/compact afterward. Otherwise, you will probably get settling and distortions in your wall over time.

[soccerluvof4]

^ This. I would put down 4" of TB and wet it then compact it. Then next day do it again till solid 3-4" compacted. Basically it will be like building the wall on a concrete slab.