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

The largest single source of cost overruns and construction-related claims by contractors historically has been unanticipated subsurface conditions encountered during construction. As new development spreads farther from established building areas and as rebuilding on previously developed sites increases, soil-related construction problems can be expected to increase.

Even on routine construction projects, contractors often must solve earth-related construction problems quickly and without the benefit of additional engineering investigation. This article describes selected parts of a geotechnical report and provides some practical ways to evaluate geotechnical conditions. It focuses on three key parts of an investigation: subsurface conditions, foundation recommendations, and slab-on-grade recommendations.

Geotechnical investigations and reports

Thirty minutes spent reviewing the geotechnical report for a project before beginning work can save hours later and reduce the risk of problems during construction. Not all geotechnical reports are created equal, however. Geotechnicai engineers generally explore a site in several increasingly more detailed investigations, and it's important to recognize which type of report you've got.

A preliminary report characterizes the subsurface of a site in a broad sense using widely spaced borings. From it you will find the depth to water and to bedrock, the prevalent soil types and their engineering properties, excavation and slope criteria, and potential geotechnical problems. It may identify bearing capacity ranges and preliminary pavement thicknesses, but detailed design criteria for specific structures or pavements are not generally included.

A design-level geotechnical investigation is more extensive and includes borings or sample locations at key points within the building envelopes.

Subsurface conditions

This section of a geotechnical report provides detailed information about the subsoils at the site. The soil profile is described as a series of soil or bedrock layers, and the report describes the physical and engineering properties of those soils. The soils are described in "big picture" terms that focus on the predominant soil or on the soil that controls the design. For example, a 10-foot-thick sedimentary layer might be described as sand with clay lenses if sand is the predominant soil. This judgment is based on the relationship between samples from different locations vertically and horizontally, on the overall geologic environment, and on the impact the soils are expected to have on construction.

As a contractor, you should compare the appearance of the site with the descriptions in the geotechnical report. In some cases, for example if the site has been re-graded since the geotechnical investigation, groundwater conditions may have changed, site-grading fill may have more or less swell potential than the original soil, or other changes may have occurred that would impact foundations. Pointing out such inconsistencies earlier rather than later will save everyone time and money.

The soil descriptions in the report will include a summary of the information needed for design and construction: the predominant soil type (such as sand or clay), a secondary descriptor of the soil (for example, silty), a description of the consistency of the soil (very soft to hard), the relative moisture (dry, slightly moist, moist, very moist, or wet), the colors, and some secondary characteristics (such as "with gravel lenses").

The soil consistency (clay soils) or relative density (sand soils) is determined from a standard penetration-resistance test performed during drilling. This test is performed by driving a standard soil-sampling device into the soil using a 140-pound hammer falling 30 inches. The number of blows required to drive the sampler 12 inches (the standard penetration number) is recorded as N. So you can see that the description used for soil consistency is not just a qualitative description but represents the actual strength (see Table 1, below, and "Checking Soil Consistency," p. 36).

Soil descriptions provide important information about why certain design recommendations are made later in the report. For example, if a soil is described as "soft" or "fill," it may explain why the engineer recommends a footing foundation be placed at a lower elevation or why a foundation or slab on grade should be placed on an engineered fill. Fill is a special case in that it can be onsite material or imported and may or may not be acceptable for bearing the building's weight, depending on its history. An "engineered fill" is one that has been tested to document its suitability as a bearing stratum. If a fill has an unknown history or has been placed without testing, it is usually unsuitable as a bearing stratum because its properties are unknown and may be variable.

Since soils are generally a mixture of gravel, sand, silt, and clay, they are classified according to which size fraction they have in greatest abundance. Soil particles larger than about 1/4 inch (retained on the No. 4 sieve) are classified as gravel, those between the No. 200 sieve and the No. 4 sieve are classified as sand, and those finer than the No. 200 sieve are either silt or clay, depending largely on their chemical composition.