House foundations or other parts of a home that are in contact with soil sometimes settle. This occurs when the soil that is loadbearing has inadequate density or when the soil shrinks because the moisture content decreases. House foundations or other parts of a home in contact with soil can also be heaved. This takes place as part of a series of events: A soil dries during drought and consequently shrinks and cracks. If drainage is poor these cracks allow deep penetration by water to partially saturated soil lower down. The partially saturated soil expands when the moisture content increases and heaves shallow foundations and floor slabs.
If the frame of a house does not begin to distort until after three or more years of satisfactory performance, it is doubtful that the distortion is caused by full-depth foundation settlement, which is always evidenced by matching cracks. Cracks occur at each side of a portion of the foundation wall that is undergoing downward movement caused by soil bearing failure. Settlement cracks are nearly always vertical, and they should not be confused with cracks that occur when a wall is subjected to lateral movement from soil pressure. Lateral movement always produces diagonal cracks at each end of a foundation and vertical cracks at the center area of the wall. This is common when a basement wall is backfilled full depth before the top of the wall is fastened to the floor joists.
Heaving soil beneath a house creates some signs that are similar to those exhibited by a foundation that is settling. However, there are unmistakable differences between these two types of foundation movement. A single crack always indicates upward movement of a foundation. In a house that is undergoing distortion from heaving soil, the basement floor slab will always exhibit a convex appearance across its length and breadth and it will have very noticeable cracks along, and approximately 3 feet out from, the face of the foundation.