Expansive clays (e.g. montmorillonite/smectite) cause significant engineering problems due to their extreme shrink–swell potential — they expand dramatically when wetted (absorbing water between clay platelets) and contract when dried. This volume change damages foundations, roads, and buried structures. The activity index (plasticity index / % clay fraction) quantifies expansion potential. Black cotton soil (vertisol) in Pakistan is a classic expansive clay causing structural damage.