A contractor's guide to geotechnical engineering: knowing how to decipher a soils report and make sure that it reflects field conditions will save everyone time and money

Concrete Construction, May, 2002 by Thomas A. Chapel

Basement floor systems and slabs on grade

If basements or slab-on-grade floors are planned on swelling soils, the preparation of the subgrade soils is very important. Since slabs on grade are generally very lightly loaded, comparatively low swell pressures can result in damaging heave. Slab-on-grade basement floors are usually recommended in areas where swell potential is characterized as low or moderate. Structurally supported floors are more commonly recommended in basement areas where swell potential is high or very high, or for walkout and garden-level basements on lots underlain by soils with moderate, high, or very high swell potential.

In areas where soils are unsuitable bearing materials, one alternative to deep foundations and structurally supported floors is to treat the subgrade soils. A practice that is becoming more widespread is to subexcavate the expansive soil, moisture-treat the soil, and then re-compact it. This practice will reduce the swell potential of the soil and permit construction of shallow foundations and slab-on-grade floors. The geotechnical engineers will specify that the soil be subexcavated to a depth below the zone of probable wetting or to a change in lithology where swell potential is not considered a problem. Field and laboratory testing is used to calculate the expected heave based on assumptions of wetting depth.

Mitigation of soft or otherwise unstable soils below slabs on grade has generally been by removal and replacement of the soils, by replacement using geotextiles or geogrids, or by crowding coarse, crushed aggregate into the subgrade. Another alternative, that has seen some success, is to inject water into the soils on a grid pattern at depths of up to 16 feet to reduce the swell potential of the soils in place.

Other slabs on grade

Garage slabs, driveways, and sidewalks are normally constructed as slabs on grade. These slabs should be constructed to reduce the likelihood that ground heave will lift the slabs or that excessive settlement will occur. Various properties of the soils and environmental conditions will influence the amount of movement and other performance characteristics of slabs supported by expansive soils. Increases in the moisture content in expansive soils will cause heaving and may cause cracking which could result in the need for the slab to be repaired, maintained, or replaced. One approach is to place loose backfill under structurally supported slabs. This fill is more likely to settle than to swell and can thus allow some 'heave of the underlying soils. Another approach is to cast the slab over void-forming materials.

Slabs on grade should be isolated from the foundation or be designed so that slab movement does not affect the foundation. A short pier or footing bottomed out at least 3 feet below grade can be used if movement is acceptable, while 8- to 10-foot piers can reduce the potential movement. Patios can also be built with a thickened edge, although frost heave may affect these slabs. If there is a basement adjacent to a porch or deck, the footings or short piers should not be founded ill the backfill adjacent to the basement walls because the backfill is normally not densely compacted, and there is a risk of settlement.