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

Clays are plastic fines. They may have high strength when dry but very low strength when wet. They are difficult to compact when wet and are hard to drain, a property of clay called plasticity. One measure of the plasticity of a clay is the liquid limit. In general terms, the higher the liquid limit, the more compressible a clay will be and the more likely it is to heave when its water content increases.

To estimate the sand or clay nature of a soil in the field, some simple tests can be used. If you rub the moist soil between your thumb and forefinger, sand will feel gritty, while silt or clay will feel "greasy" or slick. The Earth Manual, a U.S. Bureau of Reclamation publication, describes the following procedure (the toughness test) for estimating the plasticity of clay: collect a 1/2-cubic-inch sample of soil that is finer than a No. 40 sieve (fine sand) and add water until it has the consistency of putty. Roll the soil into threads 1/8 inch in diameter; then fold the thread again and re-roll repeatedly until the thread stiffens and crumbles--this is when it has passed the plastic limit. The tougher the thread is as it nears the plastic limit, the more likely it is that the clay in the soil will complicate the construction. Highly organic clays have a very weak, spongy feel at the plastic limit.

Swell/consolidation tests are used to measure the volume-change behavior of soils when they become wet after application of a load. In the soils report, clay soils are characterized as having low, moderate, high, or very high swell potential. In Colorado, Table 2 is frequently used as a guideline for the amount of volume change that poses a significant risk of swelling enough to damage a foundation or slab on grade. Slab-on-grade heave of between 1 and 3 inches is considered normal for areas with swell potential in the low to moderate range. Heave of slabs of 5 inches or more is possible for soils with high or very high swell potential. The geotechnical engineer can estimate heave on a site-specific basis using the results of swell, suction, and other tests.

Foundation recommendations

Geotechnical reports usually include a short description of the foundation alternatives that have been considered or recommended, followed by detailed design criteria for the recommended foundation system or systems. Some geotechnical consultants will recommend a single foundation system. Others prefer to present a menu of options, noting the advantages and disadvantages of each, so that the owner or structural engineer can choose a preferred alternative.

For lightly loaded structures, such as residential houses, the common foundation types include spread footings, reinforced slabs on grade, and drilled piers. For a housing development it is not uncommon for several foundation types to be recommended within the same development. The geotechnical engineer makes a judgment and recommends the foundation type based on subsurface conditions, local availability, contractor preference, and the engineer's experience. Given all these variables, the very same single-story building with a slab-on-grade floor that is designed with a post-tensioned slab foundation in Texas might be designed as a drilled pier foundation with a floating slab on grade in Colorado.