What Are The Methods Of Classifying Soils In Foundation Design?

It is necessary for the foundation engineer to classify the site soils for use as a foundation for several reasons:

1. To be able to use the database of others in predicting foundation performance.
2. To build one's own local database of successes (or any failures).
3. To maintain a permanent record that can be understood by others should problems later develop and outside parties be required to investigate the original design.
4. To be able to contribute to the general body of knowledge in common terminology via journal papers or conference presentations. After all, if one is to partake in the contributions of others, one should be making contributions to the general knowledge base and not be just a "taker."

The Unified Soil Classification System (USCS) of Table 2-1 is much used in foundation work. A version of this system has been standardized by ASTM as D 2487 (in Volume 04.08: Soil and Rock; Dimension Stone; Geosynthetics). The standardized version is similar to the original USCS as given by Casagrande (1948) but with specified percentages of sand or gravel passing specific sieves being used to give the "visual description" of the soil.

The original Casagrande USCS only classified the soil using the symbols shown in Table 2-1 (GP, GW, SM, SP, CL, CH, etc.), based on the indicated percentages passing the No. 4 and No. 200 sieves and the plasticity data. The author has always suggested a visual description supplement such as the following:

It is evident in this table that terms "trace" and "with" are somewhat subjective. The soil color, such as "blue clay," "gray clay," etc., is particularly useful in soil classification.

In many areas the color—particularly of cohesive soils—is an indication of the presence of the same soil stratum as found elsewhere. For example the "soft blue clay" on the soil profile of Fig. 2-4 for Chicago has about the same properties at any site in the Chicago area.

In foundation work the terms loose, medium, and dense, , and consistency descriptions such as soft, stiff, very stiff, etc., are also commonly used in foundation soil classification. Clearly, all of these descriptive terms are of great use to the local geotechnical engineer but are somewhat subjective.

That is, there could easily be some debate over what is a "medium" versus a "dense" sand, for example. The D 2487 standard removed some of the subjectiveness of the classification and requires the following terminology:

< 15% is sand or gravel use name (organic clay, silt, etc.)
15% < x < 30% is sand or gravel describe as clay or silt with sand, or clay or silt with gravel
> 30% is sand or gravel describe as sandy clay, silty clay, or gravelly clay, gravelly silt

The gravel or sand classification is based on the percentage retained on the No. 4 (gravel) sieve or passing the No. 4 and retained on the No. 200 (sand) sieves. This explanation is only partial, as the new standard is too lengthy to be presented in detail.

Although not stated in D 2487, the standard is devised for using a computer program3 to classify the soil. Further, not all geotechnical engineers directly use the ASTM standard, particularly if their practice has a history of success using the original USC system.

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