One of the most frequent questions I get from my consumer clients is:  Do I need to get my house slab repaired, and if so, what is the proper repair method?

As you can imagine, there are a myriad of variables that need to be considered before foundation repair is warranted.  The fact is, knowing how slab-on-grade foundation systems work will help optimize the process of determining whether or not your foundation needs to be repaired.  In any case, if you think you may need your foundation repaired, you will be in a better position to assist the structural engineer in determining the health of your foundation, and which repair method is best for you.  So, here are some items worth investigating before taking the significant economic step of getting your foundation repaired:

Get your house plumbing system statically tested.  Assure your drain system (and domestic water supply, irrigation system, etc.) is leak free. Should plumbing leaks be present, get a flow test done to determine the leak discharge water volume.

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.

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.

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 the winner is…?  You get the idea.

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.

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 underpinning will not be needed. If the engineer finds that your foundation is not structurally sound, then the proper repair will likely require underpinning.