There was a time in the late 70s and early 80s that most Austinites thought of a cracked slab as a catastrophic situation. Just the term “cracked” implied something that could not be repaired. Fortunately, the public now better understands that slabs are not fine china and the effects of settling can usually be reversed. That is the good news. The bad news is that our recent cycle of extreme drought to extreme flooding can be quite problematic for foundations, especially in areas with expansive soil.
When fluctuating moisture content in soil causes the ground to shrink or expand, that expansion is defined by what we refer to as the plasticity index (PI). Bedrock would have a PI of close to zero. Extremely expansive clay might have a rating closer to 30. In theory we would hope that all foundations are engineered according to the PI of the specific site, but realistically that has not always happened.
Even when slabs have been well-designed, the addition of extreme moisture can create settling problems. Here are just a few of the moisture problems I see regularly:
- Runoff – rainfall does not quickly drain away from the slab. Water is allowed to effectively penetrate under the slab.
- Broken water or sewer pipes in or under the foundation.
- AC condensation lines creating pools of water next to the slab.
I compare the problem of too much moisture to mud pies. Add enough water and dirt becomes a liquid. The same thing can happen to the soil supporting a foundation . . . especially in the case of a pressurized water leak. In the case of a major water leak, the slab may heave as the soil swells . . . or it may do exactly the opposite, and sink as the soil turns to a liquid state and can no longer provide support. Luckily, most moisture related problems are curable, with perhaps the exception of underground springs or floodwater.
Once the water problems are corrected, most slabs can be repaired with engineered piers. Foundation companies now utilize a number of different methods to install piers for slab stabilization:
- Drilled piers – Pier holes are actually drilled and piers are created by installing rebar and pouring concrete. If no bedrock is found, then the piers can be made with bell-bottoms.
- Concrete pressed piles – Concrete cylinders are stacked and pressed into the soil until they reach the point of refusal.
- Steel pressed piers – Short pieces of 2.5″-3″ steel pipe are fitted together and are pressed into the ground until they hit a point of refusal. Steel piers can be pressed deeper into the soil than the concrete pilings.
Concrete pilings are the most common in the Austin market, but there are shortcomings. First, I have seen the 6″x12″ concrete cylinders shatter during installation . . . it sounds like a small grenade. Second, due to the larger diameter and increased friction, the concrete piers may not penetrate as deep as other types of piers.
Recently I listed a home that had extensive interior settling . . . despite the earlier installation of thirty-six piers. The perimeter of the house had no cracks, but the interior of the home was very broken up. The homeowner informed me that the foundation repair company did not install the interior piers as the engineer required. Another foundation company was hired to complete the work per the engineered plan and the successful result was remarkable. I mention this because occasionally homeowners choose to only do part of the work recommended by the structural engineers. If they are occupying the home, they may not want the chaos caused by the installation of interior piers. That usually results in more problems, not less. It pays to have the engineer closely monitor the work while it is in progress.
Jeff Stewart, CCIM, SRES
Stanberry Commercial, REALTORS