Austin Flooding-Structural Foundation Damage?

With the recent record rainfall in the Austin metropolitan area, I have been getting a lot of questions from homeowners that seem to have a common theme:  Can my slab-on-grade foundation survive the recent deluge of rainfall and flooding?  Well, the answer is not particularly simple.

You see, shallow bearing structural foundation systems have one single purpose:  to safely separate habitable areas from the exterior environment and limit damage or distress to brittle interior and exterior building materials.   By design, these slab-on-grade foundation systems are stiff enough to buffer potentially damaging soil and subgrade material movement, yet flexible enough to protect the supported structural elements from unsafe or excessive planar tilting. The structural strength of a slab-on-grade foundation system is determined by its ability to bear allowable design loads, while foundation performance measures how closely a foundation achieves its intended purpose.

Now, the native bearing soils in the local Austin area can be expansive.   On drying, clay soils shrink. On wetting, clay soils expand.  And, it is true that slab-on-grade foundation flexure and beam rotation would be reasonably expected over the life of a typical Austin area home.     At any given time, many homes, and perhaps most Austin homes built after 1960,  likely have interior/exterior distress caused by climatic moisture driven slab-on-grade vertical displacement (a.k.a. “settlement”).  Settlement, as the term is used here, can often be traces to very small changed in humidity or bearing soil moisture content.  This type of damage is merely cosmetic and is demonstrative of the serviceability of the foundation.

Bit what about large amounts of rainfall in a short period of time.  Is my foundation at risk of being damaged or even failing?  Simply put, the answer is yes.  If your foundation system bears on clay, large amounts of water deposited around the  slab can cause the bearing soils to swell causing its edges to lift and crack-and in the worst case, foundation failure.   So how can you help to prevent your foundation from potentially failing under heavy rainfall conditions?  Here are a few tips:

  1. Check your foundation drainage. Assure that the surface water drains away from the foundation along its perimeter, and no low areas allow water to pond for longer than a day or so after a heavy rain.
  2. Understand how your foundation system works. The basic purpose of your house foundation system is to safely separate habitable areas from the exterior environment and limit damage or distress to interior/exterior brittle building materials. One of the crucial underlying design principles of shallow bearing slab on grade foundation systems is its “single unit� behavior. To do their job, slab-on-grade foundation systems act as single structural elements with uniform stiffness. These foundation systems are designed to respond uniformly to resist upward or downward movement caused by soil pressure from below, and heavy structural loads from above. In doing so, these slab-on-grade foundation systems are stiff enough to buffer potentially damaging soil and subgrade material movement, yet flexible enough to protect the supported structural elements from unsafe or excessive planar tilting.
  3. Know the limitations of your foundation system. The weight of your house walls, roof, appliances, furniture, or perhaps fireplace stone or brick are compressive loads that act downward on top of your foundation floor. These compressive loads average perhaps 300-500 pounds per square foot. Conversely, soil pressure acts upward from beneath your foundation system. Soil pressures average perhaps 3000-5000 pounds per square foot and pose a greater damage threat to your foundation system than do compressive loads.
  4. Understand your local soil conditions. Sand is non- plastic and quite stable. Clay is plastic and subject to volumetric changes with the addition or removal of moisture. When water is added to clay soil it expands, and when water is removed from the same soil, it shrinks. As previously mentioned, expanding clay can produce tons per square foot of pressure on the underside of the foundation system, while the weight of a typical house produces less than 500 pounds per square foot average along its perimeter. With that said, excessive water input to clay soil poses the greatest threat to lightly loaded, shallow clay bearing foundations systems.
  5. Get an independent engineer to evaluate your foundation. A competent structural engineer can provide an unbiased opinion and technical guidance based upon what the structural status of your foundation system. If your foundation system is structurally sound, then it is likely that structural stabilization using underpinning systems will not be needed. If the engineer finds that your foundation is not structurally sound, then the proper repair will likely require stabilization such as underpinning.
  6. Install roof gutters to direct water away from the foundation perimeter to prevent deep saturation of the foundation system bearing soils;
  7. Assure that any source of water input above or below the foundation bearing surface drains away from the foundation perimeter to prevent water ponding or bearing soil over-saturation.