What characteristic do expansive clays exhibit?

Expansive clays, also known as shrink-swell clays, are characterized by their high plasticity index. This means that they have a significant ability to change shape when water is added or removed. These clays have a high proportion of clay minerals like montmorillonite, which can absorb water and swell considerably when wet. Conversely, when they dry out, they shrink and can form cracks. This property of high plasticity index is crucial in geotechnical engineering and soil management because it influences the behavior of these soils under varying moisture conditions, making them suitable for unique engineering considerations, such as foundation design and land use.

The other characteristics mentioned do not accurately reflect the properties of expansive clays. For instance, low plasticity index would imply less variability in shape with moisture change, which is not the case. Increased density typically refers to soils under compaction rather than the natural state of expansive clays. High water retention, while somewhat related, does not capture the characteristic expansion and contraction associated with the high plasticity index. Thus, recognizing that expansive clays have a high plasticity index is fundamental to understanding their behavior in various engineering and environmental contexts.