What type of soil is most susceptible to shrink-swell behavior?

Clay soil is most susceptible to shrink-swell behavior due to its unique properties. When clay is wet, it absorbs water and expands significantly, leading to swelling. This process occurs because clay particles are flat and plate-like, which allows them to hold water in between their layers. When the moisture content decreases, the clay loses water and contracts, resulting in shrinkage. This cyclic behavior can cause serious issues for structures and foundations built on or in clay-rich soils, as it can lead to ground movement and shifting.

Other types of soil such as sand, silt, and gravel typically do not exhibit the same degree of shrink-swell behavior. Sand, for example, has larger particle sizes and a coarse texture, which allows it to drain quickly and reduces the water retention that contributes to expansion and contraction. Silt can retain some moisture but does not swell as much as clay. Gravel has a very coarse structure with large particles that also do not retain water effectively, further minimizing any potential for shrink-swell action. Thus, clay soil is the most affected by this phenomenon, making it the correct answer.