Why Foundations Fail:
It is not unusual to see a foundation in the DFW area move in response to moisture variations that normally attend seasonal changes. Heaving of a foundation that is induced by seasonal considerations usually occurs during the cool wet winter months. Heaving can lift a foundation off the piers that are supporting the foundation. Settlement of a foundation that is induced by seasonal considerations usually occurs during the hot dry summer months when the clay soils dry out to a depth of 10 to 15 feet. The drying (desiccation) occurs because of both surface evaporation and transpiration (water being removed from the soil by trees and shrubs).
Settlement can also be caused by a loss of soil strength. Soil strength decreases as the water content of a soil increases. A general rule of soil strength is that wet soils are weak and dry soils are strong. It is possible that portions of a slab foundation that have previously not moved can experience settlement due to desiccation of the soil. If a portion of the foundation has been previously underpinned, those portions should not settle.
As soils dry they shrink and consolidate. A foundation usually settles, during the dry summer months, especially along the outer perimeter. Usually the winter months are wet so the expansive clay soils tend to swell and raise the foundation. If the soils get too wet, soil strength will become too low to support the load of the house and the soils will deform and the house will settle.
Soil upheaval many times is caused by water being allowed to pond near a foundation. Excessively wet soils will lose their strength and cause a house to settle. This is why it is essential that the homeowner ensure that the ground surface around the entire foundation be sloped to allow water to flow rapidly away from the foundation. It is recommended that the ground surface be sloped at a rate of one inch per linear foot. The ground surface near the foundation should be sloped to drain away from the house at this rate for a distance of at least 6 to 8 feet. There should be no place around the perimeter of the house where water is allowed to pond near the foundation.