What is the primary characteristic of expanding clay?

The primary characteristic of expanding clay is its ability to swell when it absorbs water, which is attributed to its specific mineral composition. This type of clay typically contains minerals such as montmorillonite and bentonite, which have a layered structure that allows them to absorb water between their layers. When these minerals come into contact with moisture, they expand significantly, leading to a notable increase in volume. This property is critical for understanding how expanding clay interacts with construction foundations, retaining walls, and septic systems, as it can cause heaving and shifting if not properly accounted for.

In contrast, a clay soil that contracts when dry points to a more general characteristic of clays but does not specifically address the swelling property that defines expanding clay. Sandy soil retaining water is not characteristic of clay at all; rather, sandy soils typically drain quickly and do not exhibit swelling. Finally, a soil with no clay particles does not align with the topic, as expanding clay is, by definition, a type of clay soil. Thus, the correct response highlights the essential swelling characteristic that makes expanding clay particularly noteworthy in soil engineering and construction contexts.