Expansive clay swells when it gets wet and shrinks when it dries, and that movement is a common reason slabs crack here.
What the County Says
The County of San Diego says Expansive clay soaks up water and swells, then shrinks as it dries out., also called shrink and swell soils, hold clay that can take in a lot of water. They change volume a great deal as they get wetter or drier. The County says highly expansive soils occur mostly on the coastal plains, and also in valleys and on foothill slopes. The source is the County geology and soils report.
Why Water Is the Trigger
San Diego is dry most of the year. The National Weather Service puts the downtown average at 10.77 inches of rain a year, on about 43 days. So the ground spends long stretches dry and then gets soaked by a storm or a leak. That swing is what makes clay swell and shrink. Source is the NWS climate memo.
Sprinklers, downspouts and slow leaks add water in one spot. That can lift one side of a slab and not the other.
What Helps a Slab
- A well built base under the slab
- The right slab thickness for the load
- Control joints at a spacing that suits the slab
- Steel where the plan calls for it
- Water sent away from the slab, not under it
Ask the contractor what they plan for each one. A good answer is specific to your lot.
Get Your Soil Checked
For a retaining wall or a foundation, get a soils report before the design is set. For a quick look at your area, the NRCS Web Soil Survey maps soils by parcel. It is free to use.