If a soil sample passes the 3-inch sieve but doesnot pass the No. 4 sieve, the larger particle size is lessthan 3 inches and the smallest size is larger than 1/16inch. This soil is classified as gravel.Soils that pass the No. 4 sieve but are retained onthe No. 200 sieve are classified as sands. Sands arefurther broken down as coarse sand or fine sands.Coarse sand passes the No. 4 sieve and is retained onthe No. 40 sieve. Fine sand passes the No. 40 sieve andis retained on the No. 200 sieve.Any soil, passing the No. 200 sieve, is classifiedas fine-grained.Soil GradationGradation describes the distribution of differentsize groups within a soil sample. A well-graded soil(fig. 15-60) is a soil sample that has all sizes ofmaterial present from the No. 4 sieve to the No. 200sieve.Poorly graded soil may be uniformed-graded(fig. 15-61) or gap-graded (fig. 15-62). If a soil isuniformed-graded, most of its particles are about thesame size. An example of this is a sieve analysis inwhich sand size No. 20 is the only size present.If a soil is gap-graded, at least one particle size ismissing. An example of gap-graded soil is one inwhich a sieve analysis reveals that sand sizes No. 10and No. 40 are missing. All other sizes are present.Soil CompactionCompaction is pressing together soil particles toform a consolidated mass with increased stability.Figure 15-60.—Well-graded soil.Figure 15-61.—Uniform-graded soil.Figure 15-62.—Gap-graded soil.15-36
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