Wet and dry cycles
Clay can swell as it absorbs water and shrink as it dries. Uneven moisture changes can create changing support conditions.
Why consistency matters
Localized wetting from drainage or leaks and localized drying from vegetation or weather can create uneven conditions.
Do not diagnose from soil alone
Knowing that clay is present does not prescribe a repair. Foundation design, drainage, plumbing, landscaping, and the actual movement pattern all matter.
Related reading
See foundation drainage, tree roots and foundations, and settlement.