Expansive soils (shrink swell clays)

Where do they occur?

Expansive soils can occur in almost any location in the Mountains, Piedmont, and Coastal Plain of North Carolina.  The greatest potential, however, is in the Carolina terrane and the Triassic basins (see map below).  The rocks of the Triassic basins are sedimentary rocks that were deposited about 220 million years ago when the supercontinent Pangea began to rift, or break, apart.  As the basins formed, rivers deposited sediment that would eventually fill the basin and become sedimentary rock.  Most of these sedimentary rocks are reddish-brown sandstone, siltstone, and mudstone.  The soil that forms from these rocks as they weather are acidic, friable (crumble easily), have poor drainage capabilities, and have a potential to shrink and swell.

Why do they occur?

Shrink-swell of clay soil occurs because of moisture content.  A rainy season can cause the clay soil to retain moisture and cause it to swell.  A dry season or drought can cause the soil to shrink, or decrease in volume.

Why is this important to you?

Soils that shrink and swell can cause property and infrastructure damage.  Swelling, or soil expansion, can cause the ground to heave causing damage to buildings, roads, and other structures.  Soil shrinkage can cause structure settlement or subsidence.  If you think your property is underlain by clay soils that are capable of shrinking and expanding, contact your insurance company for more information.

Map showing approximate extent of Triassic basins and Carolina terrane in North Carolina.