If you've noticed hairline cracks running along your drywall, a door that suddenly won't latch, or a driveway slab that seems to be tilting, expansive soil could be the culprit. Across the Bay Area and Sacramento Valley, clay-rich soils shrink and swell with the seasons — and that movement is one of the most common, and most misunderstood, threats to a home's foundation. What Are Expansive Soils? Expansive soils contain clay minerals — often montmorillonite or similar smectit