The short answer
Most residential caliche pads in the Valley are built 8–12 inches of compacted caliche over a prepped subgrade, placed in lifts and compacted as you go. Shops, metal buildings and anything heavy — or anything on bad expansive clay — should be built to an engineer's or geotech's spec, which can call for more depth, select fill, or moisture conditioning.
What determines the thickness
- What's going on it — a house slab, a metal shop, and a barndominium with heavy point loads don't ask the same thing of the pad.
- The soil underneath — the Valley's expansive clay swells and shrinks with moisture. On bad clay, a thicker or engineered pad (sometimes with select fill or a moisture barrier) does more than piling on caliche.
- Drainage and elevation — part of a pad's job is to lift the slab above the surrounding grade so water runs away from it. That target elevation often sets the depth as much as structure does.
- The engineer's spec — if there's a slab design, it wins. Build to the number on the plan, not a rule of thumb.
Compaction matters as much as depth
A thick pad that wasn't compacted right is worse than a thinner one that was. Caliche has to go down in lifts — usually 6 to 8 inches at a time — with each lift compacted before the next goes on. Dump two feet of caliche and roll the top and the bottom stays loose, which is exactly where settlement cracks come from.
Moisture is the other half. Caliche compacts best near its optimum moisture content — too dry and it won't bind, too wet and it pumps. On engineered pads, a testing lab shoots density (typically 95% standard Proctor) on each lift so there's proof it's right, not just a promise.
Elevation and drainage
In the flat Valley, water doesn't leave on its own. A pad built dead level with the yard will sit in standing water after every big rain. We build pads up above the surrounding grade and slope the ground away on all sides so the slab stays dry. Getting this right at pad stage is far cheaper than fixing drainage after the house is up.
Building soon?
If you have an engineer's slab design, send it and we'll build to spec and arrange density testing. No plan yet? We'll help you get a pad sized right for what you're putting on it. See our building pads & caliche page.