However, the specific mineral content of the soil on your lot--there are at least a half dozen types of clays, for instance--and other factors that will affect the moisture balance must be taken into account in designing and pouring your foundation. Not to mention, the type of foundation you're considering--slab, piers, piles, shallow.

You should have a soils engineer's report before undertaking any work.

Good luck on your project!

--Pier and grade beam;
--Mat slab

Your soils engineer will also have recommendations about any pre-wetting or drying prior to construction. Just hire an experienced professional and let him/her worry about the rest. You will be at rest a lot longer this way!

My basement is 12ft deep/tall in very expansive clay soil (the worst). The 1ft thick poured concrete walls and footings were designed by a structural engineer, who used quite a bit of rebar. The rebar design consists of two parallel layers (6 inch separation) of a grid made out of #4 spaced 8 inches apart vertically and 12 inches apart horizontally. The footings are up to 6 foot wide where the concrete basement ceiling is open (staircase), making this part of the basement wall a cantilevered retaining wall.

It's backfilled with a 3ft wide section of free draining crushed concrete all the way up to grade, all around the basement.

The basement was built during the Texas drought in the summer of 2010 and has since experienced very wet conditions and right now another dry spell. No cracks whatsoever.