Which Way Is It Moving?
Guidelines for Diagnosing Heave, Subsidence and Settlement
Ron Kelm, P.E. | Nicole Wylie, P.E. | Forensic Engineers Inc. | Houston TX | www.forensicengineersinc.com
- The three common foundation movement types requiring foundation repair in the Houston area are heave (up), subsidence (down) and settlement (down).
- All three movement types in the Houston area are commonly misdiagnosed. Misdiagnosis is minimized when a forensic engineer performs a Level C investigation and when foundation monitoring is implemented.
- Heave is the most difficult movement type to mitigate while settlement is the simplest movement to diagnose and mitigate.
- At least a dozen symptoms are common for each type of foundation movement to help the forensic engineer identify the movement type. Regardless, when heave is present there will be an available moisture source nearby; when subsidence is present there will be mature trees or other large vegetation nearby; and when settlement is present the ultimate shear strength of the clay supporting a slab-ongrade at the grade beam bearing level will be below 0.5 TSF.
- Remedial foundation work varies depending on the type of foundation movement present: Unless monitoring shows the movement has stopped, heave repairs often do not work and are usually not warranted by repair contractors unless the entire foundation can be lifted above the soil’s potential upward movement elevation; subsidence repairs work for a period but are often difficult to maintain because offending trees are not removed or heavily pruned on a regular basis; and settlement repairs have the best chance for no call-backs from a warranty standpoint. However, settlement occurs rarely in the Houston area as compared to heave and subsidence.
- Forensic engineers need to be aware there may be other types of foundation movement that do not fit the symptoms of the three common movement types described in this paper. Two examples given in this paper are active fault slippage and root heave.