Final answer:
Clay-rich soils are characterized by their shrink-swell behavior, not by constancy in volume, absence of cracks, or absence of engineering challenges. They pose significant engineering challenges and have the capacity to undergo volume changes as they expand when wet and shrink when dry.
Step-by-step explanation:
Clay-rich soils are known to shrink and swell with changes in moisture content. None of the provided statements accurately describe these types of soils; rather, it's important to understand that such soils change in volume due to moisture levels, often exhibiting cracks during dry periods and posing engineering challenges due to their inability to maintain consistent conditions. These soils will exhibit shrink-swell behavior, which means they expand when wet and shrink when they dry out.
As the soil dries, it loses water and thus volume, leading to the formation of cracks. This process can create substantial engineering challenges because structures built on clay-rich soils can become damaged as the ground moves. Additionally, the soil may also present well-developed horizons, which are layers that form as a result of various natural soil processes. Despite this characteristic, it is the shrink-swell capacity and the engineering issues related to it that are particularly significant in the context of this discussion.