[freebeard]

Dirt is a mix of clay, silt and sand with organic material. The proportions probably matter.

https://en.wikipedia.org/wiki/Expansive_clay

Originally Posted by DDG

Expansive Soil Problems and Solutions
http://www.foundation-repair-guide.c...sive-soil.html
Getting Control of Expansive Soil. Expansive soil, also called shrink-swell soil, is a very common cause of foundation problems. Depending upon the supply of moisture in the ground, shrink-swell soils will experience changes in volume of up to thirty percent or more.