Expansive Clay Soil and Concrete Cracking in San Diego

What the County Says

The County of San Diego soils report says expansive soils, also called shrink swell soils, hold clay minerals that soak up water. They change volume a lot as the water content changes.

It says highly expansive soils in the county occur mostly in the coastal plains. They are also found in valleys and on foothill slopes near Ramona, Escondido, Rainbow and northeast of Vista.

Why Weather Is Not the Usual Cause

In cold places, water freezes inside concrete and breaks it apart. The National Weather Service says downtown San Diego averages 64.4 degrees a year. All measurable precipitation at the official station has fallen as rain.

So when a San Diego slab cracks, look at the ground under it first. Wet winters and dry summers make clay swell and shrink. Tree roots and a thin base add to it.

How Contractors Build for Clay

A thicker, well packed base gives the slab a steadier bed. Steel in the slab holds cracks tight. Control jointsA control joint is a cut or groove that gives a slab a planned place to crack, so the crack runs straight and out of sight. give the slab planned places to crack. Good drainage keeps water from soaking the clay next to the slab.

Ask the contractor to explain each of these for your job, in writing.

Check Your Own Lot

Soil changes block by block. The NRCS Web Soil Survey lets you look up the mapped soil for a parcel. For walls and foundations, ask for a soils report from a soils engineer.

More Guides

• Permit Guide
• Approach Permits
• General Contractors
• Check a Contractor