Q. We are a small residential contractor in southern Arkansas, building slab-on-grade monolithic foundations reinforced with steel and welded wire fabric. During the past couple of years we started having trouble with slab settlement and cracks in the concrete, so we started pouring wider and deeper footings, putting almost one-third more concrete in the foundation than before. However, the weight of the concrete is only adding to our problems. We had geotechnical tests done, and we now know the problem is caused by the type of soil we are building on. But some of the methods we have been advised to use to correct the problem are cost prohibitive. Is there any way to build a foundation on poor soil without digging a footing to China to prevent this settling and cracking?
A. With greater demand for new homes, increasing numbers of them are being built on poor-quality soils. It's important to determine whether your problem is caused by expansive soils or by compressible soils, and that requires a qualified geotechnical engineer.
Expansive soils are clays that usually contain montmorillonite or bentonite minerals. These minerals cause excessive clay swelling when wetted, or shrinkage when dried. When wetted, the soils heave, or move upward, exerting pressure on basement floors and footings that can cause structural damage. Swelling soils can also exert enough lateral pressure on basement or retaining walls to damage them. When the soils dry and shrink, footings settle, but the interior of the structure doesn't move because it takes a long time for soil in that area to dry. This differential settlement also causes structural distress (Ref. 1).
Compressible soils may contain organic material, soft clay or other components that cause the soil to settle when loaded. Geotechnical engineers will be particularly suspicious of any dark-colored soils because the color, and perhaps an organic odor, indicate possible problems with excessive settlement.
If you build slab-on-grade houses on poor-quality soils, you have several design options. These include heavily reinforced slabs on grade, structural slabs supported by piles or piers, and post-tensioned slabs (Ref. 2). A structural engineer can help you choose the most economical option for avoiding distress caused by the poor soils. There are code requirements for such slabs, which usually must be designed by a registered professional engineer.
Another possibility is to excavate the poor soil, fill the footing trench (which may be several feet deep) with flowable fill, then place footings on this fill. This technique is currently being used in the Northwest (Ref. 3). Your concrete supplier should be familiar with flowable fill, which is also called controlled-density fill, controlled-low-strength material (CLSM), and lots of other terms. Check with local building-code officials to see if this technique is allowed in your area.
1. Warren K. Wray, Editor, So Your Home Is Built on Expansive Soils, American Society of Civil Engineers, Reston, Va., 1995.
2. ACI 332R-84, "Guide to Residential Cast-in-Place Concrete Construction," American Concrete Institute, Farmington Hills, Mich., 1984. 3. Morris Huffman, "Hitting Paydirt When There's Poor Dirt," The Concrete Producer, November 1998, pp. 767-769.