Last week, the blog discussed an example of poor engineering judgment. Another example of bad engineering advise was at a home built on the side of a steep hill which ran down to a creek. It was estimated that the builder brought in over 15 foot of fill material to level the lot prior to installing the foundation. As is usually the case, the fill material was not properly compacted so, over time, the fill material consolidated causing the slab foundation to settle several inches. (Even if the dirt the builder imported – the fill material – had been properly compacted, the foundation would have still settled because gravity, over time, would have pushed the dirt downhill. Only a solid retaining wall would correct this problem.)
Within a few years of moving into his brand new home, the homeowner started noticing indications of severe foundation settling. This resulted in numerous slab cracks, out of level floors, sheetrock cracks, brick mortar cracks and out of level doors.
He retained an engineer to design a repair for the foundation. The engineer specified the installation of about 50 drilled concrete piers that only went to the 12 foot depth. In other words, the piers on half of the home were installed completely within the fill material. Needless to say, 1/2 of the foundation continued to be unstable for several years, requiring continual “adjustment” of the piers. The only remedy for this foundation is to replace the existing piers with piers that are founded in a stable bearing stratum. However, because of the expense, the homeowner has not been able to do so.
It should be said that a post construction pier will typically only prevent vertical movement (downward or settlement movement) and not horizontal movement. Some extreme topographies require a stable retaining wall to stop horizontal movement of the soil.
So, a foundation situated on the side of an extreme slope needs special attention from an experienced engineer who understands the forces of gravity.
Both of these homeowners had retained structural engineers but unfortunately, the engineers did not understand the basics of soil behavior.
Jim McNeme, P.E.
For more information on foundation movement, go to www.GeoDFW.com