Conditions Contributing to Foundation Movements
A number of conditions may cause distress to foundations constructed on expansive clay soils in this area of Texas. Several major factors influencing soil movements are:
Ø Pre-Construction Conditions. Large trees, thickets, bushes, or other vegetation requiring large amounts of moisture from the soil that were removed to build the structure tend to make these soils drier than the adjacent areas. These desiccated pockets have a much higher potential for swelling than do the adjacent, less desiccated areas.
Ø Fence Lines, Trails, and Tracks. These surface features typically have the vegetation worn away, leaving only bare or thinly covered strips, which are much drier than the soil on either side. Like the desiccated areas caused by pre-construction vegetation, these areas will often swell more than other areas.
Ø Slopes. Slopes comprised of active expansive soils sometimes have a tendency to migrate downhill as the soils experience shrink-swell cycles.
Ø Cut and Fill Sections. These sections will often experience differential soil movements because of variations in compacted densities, soil interfaces, and unequal moisture migration.
Ø Drainage. If rainfall runoff is allowed to pond or collect adjacent to a structure built on expansive soil, the structure may be subjected to distress caused by the soil beneath the structure swelling as a direct result of increased soil moisture content. Lot surfaces must be graded to provide drainage away from the building. Gutter discharge must often be collected and piped away from the structure. Collection drains are sometimes required on hillside or terraced lots. Care should always be taken with sewage and water utility lines to ensure that leaks do not develop beneath the slab. A sub slab release of water can be very damaging to a structure.
Ø Time of construction. If the slab is cast at the end of a lengthy dry period, it may experience greater uplift around the edges when the soil becomes wetter at the conclusion of the dry period. Similarly, a slab cast during a wet period may experience greater edge drop during a subsequent period of dryness.
Ø Post-Construction. A number of post construction practices can occur to cause distress to structures founded on expansive clay soils. Planting flowerbeds or shrubs next to the foundation can sometimes cause a net increase in the soil moisture content in this area, resulting in soil expansion near the perimeter of the house in the vicinity of the beds. Planting shade trees closer to the structure than a distance equal to the mature height of the tree will often allow the tree roots to grow beneath the foundation and withdraw moisture from the soil; the results will be soil shrinkage in the region of the roots. Redirecting surface runoff channels or swales, landscape plantings, and new fencing can also result in improper drainage as detailed above.
Ø To minimize movements in the soil due to post-construction and climate related factors, the following preventative maintenance procedures are recommended:
o When building a house, have an expert carefully review the soils report for your lot and plan accordingly. The foundation design and construction should reflect the specific site properties.
o Initial landscaping should be done on all sides of the house. Positive drainage away from the foundation must be provided and maintained. No standing or ponding water should be allowed near the house.
o Watering near the structure should be done in a uniform, systematic manner, providing moisture on all sides of the foundation to constantly keep the soils at a reasonably equal moisture content. Areas of the soil that do not have ground cover may require more moisture, as they are most susceptible to evaporation. Ponding or trapping of water in localized areas adjacent to the foundation can cause differential moisture levels in subsurface soils and should be avoided. Splashblocks or gutter extensions should be used at all gutter downspouts, or the water collected and piped away from the building.
o Studies have shown that trees within twenty feet of foundations have caused differential movements in foundations. Trees will require more water in periods of extreme drought and provisions for an adequate supply of water must be made.
o During extreme hot and dry periods typical of Texas summers, adequate water must be provided to the soils and vegetation near the structure. Close observations should be made around foundations to ensure that ample water is being provided to keep the soil from separating or pulling back from the foundation. The goal should be to maintain a relatively constant soil moisture content near your house at all times.
The homeowner should carefully consider these factors and evaluate any other indications of adverse conditions affecting the structure. If necessary, a structural evaluation of your home and yard may help avoid damage to your most valuable investment.
Call our office today to schedule your structural evaluation. (817) 447-8450