TM 5-818-1/AFM 88-3. Chap. 7
IDENTIFICATION AND CLASSIFICATION OF SOIL AND ROCK
Natural soil deposits.
soils have more than 50 percent by weight retained on
a. The character of natural soil deposits is
the No. 200 sieve and are described primarily on the
basis of grain size and density. With regard to grain-size
influenced primarily by parent material and climate. The
distribution, these soils should be described as uniform,
parent material is generally rock but may include partially
or well-graded; and, if in their natural state, as loose,
indurated materials intermediate between soil and rock.
medium, or dense. The shape of the grains and the
Soils are the results of weathering, mechanical
presence of foreign materials, such as mica or organic
matter, should be noted.
material. The products of weathering may have the
b. Fine-grained soils have more than 50
same composition as the parent material, or they may be
new minerals that have resulted from the action of water,
percent by weight finer than the No.
carbon dioxide, and organic acids with minerals
Descriptions of these soils should state the color, texture,
comprising the parent material.
stratification, and odor, and whether the soils are soft,
b. The products of weathering that remain in
firm, or stiff, intact or fissured. The visual examination
place are termed residual soils. In relatively flat regions,
should be accompanied by estimated or laboratory-
determined index properties. A summary of expedient
large and deep deposits of residual soils may
tests for identifying fine-grained soils is given in table 2-2.
accumulate; however, in most cases gravity and erosion
by ice, wind, and water move these soils to form new
The important index properties are summarized in the
termed transported soils.
following paragraphs. Laboratory tests for determining
transportation, weathered material may be mixed with
index properties should be made in accordance with
others of different origin. They may be ground up or
decomposed still further and are usually sorted according
to grain size before finally being deposited. The newly
formed soil deposit may be again subject to weathering,
a. Grain-size distribution.
especially when the soil particles find themselves in a
distribution of soils is determined by means of sieves
completely different environment from that in which they
and/or a hydrometer analysis, and the results are
were formed. In humid and tropical climates, weathering
expressed in the form of a cumulative semilog plot of
may significantly affect the character of the soil to great
percentage finer versus grain diameter. Typical grain-
depths, while in temperate climates it produces a soil
size distribution curves are shown in figure 2-2. The
profile that primarily affects the character of surface soils.
knowledge of particle-size distribution is of particular
c. The character of natural soil deposits
importance when coarse-grained soils are involved.
usually is complex. A simplified classification of natural
Useful values are the effective size, which is defined as
soil deposits based on methods of deposition is given in
the grain diameter corresponding to the 10 percent finer
table 2-1, together with pertinent engineering
ordinate on the grain-size curve; the coefficient of
characteristics of each type. More complete descriptions
uniformity, which is defined as the ratio of the D60 size to
of natural soil deposits are given in geology textbooks.
the D, , size (fig 2-2); the coefficient of curvature, which
The highly generalized map in figure 2-1 shows the
is defined as the ratio of the square of the Do3 size to the
distribution of the more important natural soil deposits in
product of the D1o and D60 sizes (table 2-3); and the 15 and
the United States.
85 percent sizes, which are used in filter design.
b. Atterberg limits.
The Atterberg limits
indicate the range of water content over which a cohesive
Identification of soils.
a. It is essential to identify accurately materials
soil behaves plastically. The upper limit of this range is
known as the liquid limit (LL); the lower, as the plastic
comprising foundation strata. Soils are identified by
limit (PL). The LL is the water content at which a soil will
visual examination and by means of their index
just begin to flow when slightly jarred in a pre
properties (grain-size distribution, Atterberg limits, water
content, specific gravity, and void ratio). A description
based on visual examination should include color, odor
when present, size and shape of grains, gradation, and
density and consistency characteristics.- Coarsegrained