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Foundations Explained
Let's start with soil types found in Houston.
- Expansive Clay Soils:
Expansive clays will swell/ heave when wet and contract/consolidate when dry. If the foundation system is in an active area (a shallow foundation), the foundation will move as moisture conditions fluctuate in the active area.
- Select Fill/ Loam:
Select fill is normally defined as a sandy loam that shows little change with moisture variations. A building pad properly built with select fill/loam will support the foundation. Problems could occur if erosion develops this might cause changes in the bearing capacity of the soil.
- Sand:
Sand will not change as moisture conditions change. However, sand will erode if drainage around the lot allows water to work its way under the foundation. Sand can will also fall in cracks created by drying soil conditions and cause the foundation to move horizontally.
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Rock:
Rock can erode and expand slightly if it is made up of a low density shale. In some slope conditions, fractures or faults in the rock can allow sliding and failure will occur if not properly pinned with tie back anchors. Foundation settlement and movement can be caused by building on expansive clay, compressible or improperly compacted fill soils and/or improper maintenance around foundations cause foundations to settle move and often fail.
Whatever the cause, settlement can destroy the value of structures and may even render them unsafe if the problem is not resolved. In any case, water is most often the cause in the vast majority of expansive soil problems. Specific constituents of certain soils tend to swell or shrink with variations in moisture. The extent of this movement varies from soil to soil.
Those soils highest in clay content are generally more susceptible while those lowest in clay content are the least affected. In some areas the movement in insignificant; in others, it is quite pronounced. When unstable soils are used as a base for a foundation, the tendency for movement is transmitted to the foundation. Since soil movement is rarely uniform, the foundation is subjected to a differential or upheaval. The problem shows up in both slab and pier and beam type foundations.