The foundation repair contractor’s perspective
The foundation repair business grew out of the house leveling business. To this day you can still see company trucks that say something like XYZ Foundation Repair & House Leveling.
The houses being leveled were built in before World War II. They were crawl space houses. The foundations came in many variations but virtually none were slab foundations. Slab foundations were not a viable alternative until a new transportation technology made its debut soon after WWII: ReadyMix concrete trucks.
These foundations, called block and pad or pier and beam, consisted of a concrete footing resting in a shallow hole, or a concrete pad usually a few inches thick and maybe 14 inches square. Regardless, it was an easily accessed shallow foundation.
Over time, the piers or the pads would settle. The problem was that they did not all settle at the same rate so that eventually the house frame would become distorted creating door issues and drywall cracks.
Eventually, a house leveling company would be hired to relevel the house. The beams that had originally rested on the concrete footings would be lifted and then shimmed into position. As the house got older, the beams would typically sag between the concrete supports. This would be compensated for by adding additional supports.
Regardless of the details of the original construction, the goal of the house leveler was to find low areas and then lift them to make the floor more level.
Today, foundation repair contractors view their job in a very similar vein. Just as a carpenter drives nails, a foundation repair contractor sees his job as finding low areas, typically around the perimeter, and lifting those low areas.
Let me repeat for emphasis: foundation repair contractors fine low areas and mechanically lift them.
The engineering perspective
The engineering perspective, as it is embedded in the slab foundation design methodology, views the slab as a beam on a mound of expansive soil. As the clay mound absorbs moisture, it pushes upward against the concrete beam while the house pushes downward on the swelling soil mound. Typically, the perimeter of a slab is lower in elevation than the middle area. This is especially true after the slab is around seven years old.
The engineer thinks of a slab foundation as a kind of shock absorber that results in mitigating damage to the house that would otherwise develop. The term mitigate does not mean eliminate. Expansive soils are so powerful that to expect that no distress will develop over the life of the house is unrealistic.
An experienced structural engineer