Types of Expansive Soils

Moderate Expansion Type Soil

• Moderate expansion properties occurred in well-drained soils containing Mafic and Felsic rocks, according to Thomas, Baker and Zelzany. Davidson and Cecil soil types showed moderate levels of soil swelling and contraction when water was added or removed from the soil. These types of soils pose low threat to building structures.

Moderately HIgh Expansion Type Soil

• Clayey, shale and sandstone---containing Triassic rocks and Coastal Plains sediments, respectively---ranked moderately high expansions, reports Thomas, Baker and Zelzany. Mayodan, Creedmoor, Craven and Peawick soils tested as moderately high expansion soils. Some threat is posed to building structure and design may need reinforcement plans for approval when building with moderately high expansion soils.

High Expansion Type Soil

• High expansion was recorded in deep-to-rock Frederick soils containing Carbonate rocks, states Thomas, Baker and Zelzany. Deep-to-rock soil structure requires digging deep before bedrock is struck. Building foundations will likely need plans for reinforcement when constructed in high expansion soils.

Very High Expansion Type Soil

• Thermally altered shales, shallow-to-rock soils and poorly to moderately drained soils, containing Triassic rocks, Carbonate rocks and Mafic rocks, respectively, exhibited very high expansion properties. Shallow-to-rock soils require only a small amount of digging before bedrock is struck. Kelly, Carbo, Iredell, Waxpool and Jackland soils tested as very high expansion soils. Buildings constructed in these soils will almost certainly need reinforcement planning for building site approval.