IV. EARTH 



1. Component Types and Class of Soils . 



Under the words "earth" or "soil," a large assortment of materials of 

 various origins is covered; for engineering purposes these are generally 

 classified as gravel, sand, silt, clay, and organic material. Most soils 

 are composed of a mixture containing two or more of these materials. 

 Different geological processes (such as alluvial, residual, glacial, or 

 loesial), and parent materials (sedimentary, igneous, and metamorphic) 

 will affect the type and nature of the soils formed. A soil can be de- 

 scribed by its grain-size classification, appearance and structure, and 

 compactness or hardness. 



There are several soil classification systems, but the most widely used 

 in engineering is the Unified Soil Classification System (USCS). The USCS 

 is presented in the ASTM Standard D2487 and MIL-STD-619A. A summary of 

 the classification system is presented in Table 2 and the general soil 

 characteristics are discussed in the following paragraphs. Table 2-3 in TM 

 S-818-1 is another useful version of USCS. A more detailed presentation of 

 the classification systems and soil properties can be found in the report 

 entitled "Geotechnical Engineering in the Coastal Zone," Callender and 

 Eckert (in preparation, 1983). 



a. Coarse-Grained Materials . Gravels and sands are known as coarse- 

 grained soils. Coarse-grained materials are such that 50 percent or more 

 of the materials by weight are retained on the No. 200 sieve. They are 

 recognized either visually and manually or by means of the following 

 parameters: 



- Effective grain size (Dig): grain size such that 10 percent by 

 weight of the materials are finer. 



D 



60 



Uniformity coefficient (C ) = ■= — 



u D 10 



2 



CD30) 



- Coefficient of curvature (C ) = Diq • ^60 



Because most soils are composed of more than one type of constituent, 

 the USCS makes the following distinctions for sands and gravels: 



- Well-graded gravel (GW) or sand (SW) : all particle sizes are 

 represented within the constituent limits, C is greater than 



4 or 6, respectively, C is between 1 and 3 and the fraction 

 smaller than the No. 200 sieve size does not exceed 5 percent. 



- Poorly graded gravel (GP) or sand (SP) : some particle sizes 

 are missing or are in excess within the constituent limits, 

 gradation requirements for (GW) or (SW) are not met, and the 

 fraction smaller than the No. 200 sieve size does not exceed 



5 percent. 



