09/11/2017
After the Flood - Your Foundation©
By: Fred Willcox
www.willcoxinspections.com
I have been asked a number of questions by people who are concerned about the effects of the flooding on their foundation. Let me give you some ideas of what you may expect. Soils are too diverse for anyone to know exactly what will happen to your foundation after the heavy rains and flooding. What is explained below are simply general principals. Be aware that almost all soils are a mixture of soil types and that while a surface soil may be classified as a “clay”, all clays do not have the same characteristics, strengths or shrink/swell potentials.
If your foundation is supported primarily on an expansive clay, you may experience an “edge lift” or “cupping” effect. This is due to the swelling of the clay soils around the perimeter of the foundation and that fact that the clays here have a low permeability and percolation rate. As the water slowly makes its way into the clay around the edges of the foundation, the clay expands and pushes the perimeter of the foundation up. If the water were able to travel quickly through the clay the entire foundation might be lifted.
The best response to this foundation movement is patience. Given time, when normal weather patterns resume, the clay will dry and shrink back down. Most of the time the perimeter of the foundation will move with the soil returning your foundation to a more “level” condition. Be aware that this may take several years to occur.
If you were to raise the center of the foundation to match the perimeter by any of the several leveling methods on the market, when the soil dries around the perimeter of the foundation the center of your foundation will be higher in elevation than the perimeter. Your foundation leveling contractor will then tell you that whatever they did is working perfectly and that you just need to pay them to do more work.
If, before the flood, your foundation had settled around part or all of the perimeter of the foundation and the foundation has returned to a more “level” condition after the rain, if probably means that your foundation is supported on an expansive clay. If so, your foundation is an excellent candidate for a foundation irrigation system. Be aware that watering the soil around your foundation requires a great deal of knowledge and constant monitoring of the performance of the foundation. Water causes expansive clays to swell. The clays do not stop swelling just because your foundation has reached the point where it is “level”. Clays stop swelling when they reach their maximum swell potential. The amount of water needed to maintain the foundation it its “level” condition changes with the weather. While it takes a great deal of education and effort to use a foundation irrigation system, it can be done. It is also preferable to use foundation irrigation over mechanical leveling procedures. Foundations are not actually designed to be “leveled” by the use of piers, pilings or hydraulic pressure. Mechanical “leveling” should always be a last resort not a first impulse reaction to differential foundation movements.
If your foundation is supported on cohesionless soils, gravel, sand or silt, you may experience the opposite. Gravel, sand and silt are surface soils in Harris County and throughout southeast Texas, despite rumors that all of soil of Harris County consists of expansive clay. A general map of the surface soils in Harris County that I drew is shown below.
Sands and silts exist in the southern parts of Harris County and there are pockets of clay in the northern part of Harris County. The drawing simply gives you an idea of the soils distribution and classifications.
Cohesionless soils, when subjected to flooding, may consolidate which can cause voids to form under the foundation. The foundation can “settle” into the voids caused by consolidation of the soil mass. Consolidation will normally occur quickly, a matter of a few days, in silt and more slowly, possibly in a matter of a few weeks, in sands and gravel. It all depends on how well consolidated the mass is and how well contained the mass is.
If your foundation settles into these voids, depending on the level of the deformation, you may need to “level” the foundation. Leveling of cohesionless materials, particularly after a flooding event, is often best performed by the use of urethane foam. The foam fills the voids, helps to consolidate the soils, can stop additional consolidation or erosion and lifts the foundation to a more “level’ condition.
