According to many sources such as The American Society of Civil Engineers (ASCE), expansive soils continue to damage commercial and residential structures across the United States. ASCE and the USGS have estimated that that damage in the United States includes 25% of all structures, resulting in greater financial losses than earthquakes, floods, hurricanes, and tornadoes. The losses are anticipated in that 60 percent of new homes will experience minor damage and 10 percent experience significant damage (Holtz and Hart, 1978).
In order to combat the expansive nature of the soil, some geotechnical engineers have prophesized that removing the soil, reconditioning the soil and replacing the soil can alleviate the damages.
SBSA would argue this method may have some disastrous results.
Differential movements within the zone of over-excavation are common, the lack of common material properties will allow some of the reconditioned soils to heave, and some to settle. The over excavation itself creates a bathtub condition with no drain in the bottom of the bathtub, ponding water onto the expansive soils. The depths of the active zone, the depth of wetting and the impacts of perched water are all changed as the geotechnical environment is altered. Even utility trenches can provide additional avenues of water into the clay bathtub. The hydraulic permeability of the conditioned soils, utility backfill, and the foundation backfill all play roles in the increased water into the over-excavation.
During construction, the site is fully exposed in a condition it has never before encountered, and the foundation excavations become the residing place of water. This water permeates downward to the bottom of the excavation and activates the expansive nature of the clay soils. Additionally, the water has nowhere to go. The bearing conditions of the structural fill will ultimately be compromised, and the use of spread footings would have to rely on the properties associated with saturated soils.
Consider the issues related to the creation of a new geotechnical environment when you are selecting your foundation systems, and your expansive mitigation techniques.
SBSA can aid you in the evaluation of risks associated with your decisions.
Contact Jerod Faris, PE for further information.