c. Silt. Silt is a relatively fine-grained material that lacks plasticity and is composed of individual particles
ranging from 0.004 millimeter to 0.07 millimeter in diameter.
d. Clay. Clay is a cohesive, highly plastic material made up of microscopic particles less than 0.004
millimeter in diameter.
e. Organic Mater. Organic matter consists of relatively fine-grained, partially decomposed animal or
In order to more clearly define the specific parameters for each of these individual soil types, it is necessary to
separately discuss the classification of soils based on the grain size as well as the consistency of the component
2. Classification of Soils Based on Grain Size. Soils may be classified as gravels, sands, silts, or clays based
solely on the size of the individual component grains. As previously mentioned, gravels range in size from 4.7
millimeters to 76 millimeters in diameter, sands range from 0.07 millimeter to 4.7 millimeters in diameter, silts
range from 0.004 millimeter to 0.07 millimeter in diameter, and clays are comprised of particles with diameters
less than 0.004 millimeter.
In reality, soils are seldom made up of a single component; that is, pure gravel, sand, silt, or clay is rarely
encountered. More commonly, soils consist of particles of various sizes mixed in varying proportions. The
component grain sizes and their respective proportions may be determined by Materials Quality Specialists
(Military Occupational Specialty (MOS) 51G) through the use of sieve analyses and/or settling tube analyses.
a. Sieve Analysis. A sieve analysis begins by recording the weight of an oven-dried soil sample on DD
Form 1206 (see figure 3-1). Next, an attempt is made to pass the soil through a series of standard-sized sieves
containing progressively smaller openings (see figure 3-2, page 3-6). Particles with diameters larger than the
openings of a particular sieve are not allowed to pass through the sieve and are said to be "retained" at that point.
The weight of material retained on each of the sieves is recorded on DD Form 1206 (see figure 3-1). The weight
of the material passing each of the sieves may then be determined by subtracting the weight of material that has
been retained on each of the preceding sieves from the weight of the original sample. These weights are normally
converted to percentages by dividing them by the weight of the original sample and multiplying the quotient by
Common sieves used by military engineers in a sieve analysis include 2-inch, 1 1/2-inch, 1-inch, and 3/4-inch
sieves that have 2-inch, 1 1/2-inch, 1-inch, and 3/4-inch openings respectively, as well as US Standard Number 4,
10, 20, 40, 100, and 200 sieves, which contain 4, 10, 20, 40, 100, and 200 openings per linear inch respectively.
The US Standard Number 4 sieve serves to separate gravel-sized particles from those particles that are smaller
than gravel, whereas the Number 200 sieve is used to separate coarse materials (gravel- and sand-sized particles)
from fine materials (silt- and clay-sized particles). In some instances, fine materials (silts and clays) adhere to the
coarse materials (gravels and sands) to the extent that they cannot be separated by dry sieving. In such cases, it is
necessary to soak the sample in water and attempt to wash it through the Number 200 sieve. Both the material
retained on the sieve and the material that has passed through the sieve must then be oven-dried and the sieve