Upheaval – revisited again
I recently inspected a slab foundation of a 10 year old house in an unincorporated area in North Texas (no local building codes). The residence had been experiencing foundation movement to the point that the front door was jammed shut, numerous large sheetrock cracks existed throughout the house and much of the foundation was obviously out of level. The homeowner said that sometimes the front door would work properly (during the dry summer months) but other times, it could not be used (typically during the winter/spring or after a few days of rain).
The homeowner also said that when the home was being built, the builder installed numerous concrete piers around the outer perimeter of the foundation and that they were 6 to 8 feet deep, however, no piers were installed under the middle of the slab. I asked the owner if the builder had obtained a site-specific soil report for the foundation design engineer to use. He said no, that the builder had over 20 years experience in construction and said he did not need an engineer or soil report.
I determined that the piers installed by the builder were so shallow that the expansion of the clay soil (during the wet weather) caused the clay to grab the sides of the piers and push the piers up. During the dry weather, the soils would shrink and allow some subsidence of the piers. Unfortunately, the upheaval each year was much more severe than the settlement so that, each year, the outer edge of the foundation moved a little higher than the previous year. When I inspected the foundation the outer perimeter of the slab was over 4” higher than the middle.
There were two possible remedies to this, neither of them delightful. One remedy would be to install piers both around the outer perimeter of the foundation AND throughout the interior. The interior piers would then be used to raise up the interior to match (as close as possible) the slab elevations around the outer perimeter of the house. Then the unstable builder installed piers would be cut loose from the foundation and a void would be left under the slab to allow some movement of the soils. This technique, though used somewhat commonly (usually when the builder is threatened with a lawsuit) has some limitations to its success and requires the homeowner to move out of the home for a few weeks and it also devastates the below slab plumbing (and other things).
The other possible remedy may have been less invasive and less costly but it involved a two step phased approach to the repair. IF a void remained under the outer portions of the slab (because the piers pushed the foundation up), it may have been possible to expose all the piers and cut them free of the foundation and attempt to lower the slab back down to its original position. This is a rather risky technique because sometimes the soil under the slab expands up and leaves no void space between the slab and the dirt, so the slab could not be lowered. So before this technique is attempted, further investigation would be required. Another potential problem with this approach was that when the soil moved again, so would the foundation.
In the end, the homeowner decided that he was ok with the foundation as it was and was not interested in spending any further money on it.
It would not have taken much very much additional money for the builder to install the piers properly (to a stable bearing stratum) but apparently the builder did not understand all that was involved in building a slab-on-pier foundation in expansive clay soils (even though he claimed that he had been building homes for over 20 years and never had a problem with one of his foundations).
The primary lesson to be learned here is when constructing a foundation on problem soils, the piers must be designed by an experienced structural engineer and installed by a knowledgeable builder. Also, piers must be installed into a stable bearing stratum (among other things). This requires conducting soil borings, testing and an analysis by a geotechnical engineer.
For more information on foundation movement, go to http://www.GeoDFW.com
Jim McNeme, P.E.