Drainage - If rainfall run off is allowed to pond or collect next to a structure built in expansive soil, the structure may be subjected to distress. This is caused by swelling of soils beneath the structure to increased soil moisture content. Lot surfaces have been graded by the subdivision developer to drain away from the structure. Excess run off should not be collected and disposed of by carry a discharge pipe beneath the structure. Care should also be taken with sewage and water utility lines to ensure that leaks do not develop beneath the slab. Time of Construction - if the slab is placed at the end of a lengthy dry period, it may experience greater uplift around the edges when the soil becomes wetter or the conclusion of the dry period. Similarly, a slab cast at the end of a wet period may experience greater drying around the edges during the subsequent period of dryness. Post-construction - A number of post-construction practices beyond the control of the design engineer and the inspector can occur to cause distress to the structures and foundation, when built on expansive clay or other soils. Planting flower beds or shrubs next to the foundation and keeping these areas flooded will generally cause a net increase in soil moisture content and result in soil expansion around the foundation perimeter in that vicinity. Planting shade trees closer to the structure than a distance equal to half the mature height of the tree will allow the tree roots to penetrate under the foundation and withdraw moisture from the soil; the result will be soil shrinkage in the region of the roots. Redirecting surface runoff channels or swales by the owner can result in improper drainage as detailed above.

To minimize movements in soils due to post-construction factors that are not climate required, the following homeowner maintenance procedures are recommended:

1. Initial landscaping should be done on all sides adjacent to the foundation and drainage away from the foundation should be provided and maintained.

2. Watering should be done in a uniform, systematic manner as equally as possible on all sides of the foundation to keep the soil moist. Areas of soil that do not have ground cover may require more moisture as they are more susceptible evaporation. Ponding or trapping of water in localized areas adjacent to the foundations can cause differential moisture levels in subsurface soils.

3. Studies have shown that trees within 20 feet of foundation have caused differential movements in foundations. These will require more water in extreme periods of drought and, in some cases, a root injection system may be required to maintain moisture equilibrium. A large tree can remove as much as three hundred (300) gallons of water from the soil in one (1) day.

4. During extreme hot and cold periods, close observations should be made around foundations to insure that adequate watering is being provided to keep soil from separating or pulling back from the foundation.

 

Most new homes in Texas are constructed using a concrete slab-on-grade foundation. This is the most commonly used type of foundation system in Texas. This type of foundation can be reinforced with conventional mild steel reinforcing, post-tension tendons or a combination of the two. Most builders in the Houston area use post-tension along with some reinforcing in the foundations they build. Most homes typically have this type of foundation system. All slab-on-grade foundations are designed to sit on top of the ground and float or flex with movement in the bearing soils. The foundations are built with certain amount of rigidity; however, they are allowed by normal design parameters to deflect and bend a certain amount. Typically, all foundation movement is caused by some changes in the bearing soils beneath and directly surrounding the home. The most common and important changes in the soils is the amount of moisture content they contain. The reason that moisture content is so important is that most of the soils in the Houston area contain substantial amounts of clay, and clays have the capacity to expand with changes in their moisture content. This is why stabilization of the bearing soils moisture is important, the more stable the soils' moisture content, the less movement caused by shrinkage and swelling of the clays in the soil. This, in turn means that less ground movement places less stress on your foundation system. When a home is constructed, the moisture content beneath the foundation is fairly uniform and evenly distributed. If the moisture content remains constant or if it changes in a uniform manner, then any foundation movement induced should be reasonably consistent and even, and, therefore should not damage the home.

The two most common causes of uneven moisture distribution are incorrect perimeter and yard drainage and inconsistent watering, which can dry out the soils or over-saturate them. Typically, the center area of your foundation system will maintain fairly constant moisture since it is not subjected to the climatic changes surrounding the house.

(An occasional plumbing leak has been found to induce foundation movement. However, in our experience, this rarely occurs.) Moisture content at the home's perimeter can change if not properly controlled. Many homeowners are unaware of the fact that the way they water and landscape their yards directly affects foundation performance. Homeowners have a responsibility to properly maintain their home's foundation, lawn, trees and shrubs. To help the homeowner do this, I have listed several procedures and recommendations below which, if followed, will enhance the stability of the foundation system (but is not a guarantee).

1. Be certain that the yard around the home slopes away from the foundation, standing or ponding water next to the foundation can cause undue and unnecessary movement.

2. Even and consistent watering could be performed regularly and increased during dry or "drought periods". Watering should be done around all sides of the home. If a sprinkler system is installed, it should water the entire perimeter; however, zoning the system is recommended where over saturation can occur around various portions of the home. During dry periods if watering just the foundation is of concern, a soaker-hose laid approximately eighteen (18) to twenty-four (24) inches away from the foundation can be allowed to drip moisture slowly into the soils several hours a week. This procedure has been used successfully in the past.

3. Trees and shrubs can absorb large quantities of water and their root systems can undermine your foundation if not watered regularly. Homes that have existing trees may require additional measures. It is typically recommended that new trees be planted one-half the canopy width of the mature tree away from the foundation.