There is a neighborhood in North Dallas that is well known for its highly expansive clay soils and its poor quality foundations. Most of the foundations of the homes in this neighborhood have had foundation repairs. Several years ago, a homeowner asked me to investigate his continuing foundation movement. Several years prior to calling me, he had the foundation repaired by a well known foundation repair company in Dallas. That company recommended the installation of 40 or so drilled concrete piers. The piers installed were double shaft concrete piers (8” diameter) that were to extend “to a depth of 9 feet or to rock whichever came first”. Well, rock in that neighborhood was over 40 feet down, i.e., the piers did not extend to a stable stratum and the piers were so unstable so the home was continuing to experience significant differential foundation movement to the point that doors were jammed shut, sheetrock was always cracking, etc. The severity of the problem varied with the seasons (wet springs and dry summers).
In North Texas, the soils tend to dry out every summer. It is generally accepted that they dry to depths of 12 to 15 feet. They may not get bone dry, but they get drier than during the rainy seasons. As expansive clays dry out, they shrink and anything within that zone of moisture change will move. This includes piers. Every summer, some of the piers would settle an inch or so but every winter/spring, they would rebound back upward. He said the foundation used to recover better but lately, the settlement is getting worse and staying bad. The only remedy was to install new piers of a known quality and disconnect the old piers from the foundation. This cost him about $35k. The new piers I specified were steel pipe piers that went to depths of about 40 feet. The foundation has been performing properly since.
Jim McNeme, P.E.
For more information on foundation movement, go to www.GeoDFW.com