What is the main disadvantage of using clay as a foundation material?

The main disadvantage of using clay as a foundation material is its unpredictable behavior. Clay soils can exhibit significant changes in characteristics due to moisture content variations. They tend to expand when wet and shrink when dry, which can lead to differential settlement and instability in structures built on them. This behavior can create challenges in ensuring the long-term durability and integrity of a foundation.

In contrast, low cost is often an advantage rather than a disadvantage, as clay can be readily available and economical. High drainage capability does not typically apply to clay, as it tends to retain water rather than allowing it to drain efficiently. Additionally, while clay can bear loads to some extent, its load-bearing capacity is often less reliable due to the potential for movement and settling. Thus, the unpredictability in behavior stands out as the primary concern for using clay in foundation applications.