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2003 by CRC Press LLC 15 Soil Relationships and Classification 15.1 Soil Classification Grain-Size Characteristics of Soils Atterberg Limits and Plasticity The Unified Soil Classification System (USCS) The AASHTO Classification System 15.2 Weight, Mass, and Volume Relationships The Phase Diagram Volume Relationships Weight and Mass Relationships Unit Weight Density Specific Gravity Conversion of Unit Weight and Density Weight-Volume Problems Involving Defined Quantities Weight-Volume Problems Involving Only Relationships Equations among Relationships 15.1 Soil Classification There are two soil classification systems in common use for engineering purposes. The Unified Soil Classification System [ASTM D 2487-93] is used for virtually all geotechnical engineering work except highway and road construction, where the AASHTO classification system [AASHTO M 145-87] is used. Both systems use the results of grain-size analysis and determinations of Atterberg limits to determine a soils classification. Soil components may be described as gravel, sand, silt, or clay. A soil comprising one or more of these components is given a descriptive name and a designation consisting of letters or letters and numbers which depend on the relative proportions of the components and the plasticity characteristics of the soil. Grain-Size Characteristics of Soils Large-grained materials such as cobbles and boulders are sometimes considered to be soil. The differ- entiation of cobbles and boulders depends somewhat on local practice, but boulders are generally taken to be particles larger than 200 to 300 mm or 9 to 12 in. The Unified Soil Classification System suggests that boulders be defined as particles that will not pass a 12-in. (300 mm) opening. Cobbles are smaller than boulders and range down to particles that are retained on a 3-inch (75 mm) sieve. Gravels and sands are classified as coarse-grained soils; silts and clays are fine-grained soils. For engineering purposes, gravel is defined as soil that passes a 3-inch (75 mm) sieve and is retained by a No. 4 sieve (4.75 mm or 0.187 in.) or No. 10 sieve (2.00 mm or 0.078 in.), depending on the classification system. Sand is defined as soil particles smaller than gravel but retained on a No. 200 sieve (0.075 mm or about 0.003 in.). Soils passing the No. 200 sieve may be silt or clay. Although grain-size criteria were used in some older classification systems to differentiate silt from clay, the two systems described herein make this differen- tiation based on plasticity rather than grain size. Thomas F. Wolff Michigan State University The grain-size characteristics of soils that are predominantly coarse grained are evaluated by a sieve analysis. A nest of sieves is prepared by stacking sieves one above the other with the largest opening at the top followed by sieves of successively smaller openings and a catch pan at the bottom. Opening sizes of commonly used sieves are shown in ... View Full Document

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