Lesson 2/Learning Event 3
Learning Event 3
USE VISUAL EXAMINATION PROCEDURES TO CLASSIFY SOILS
It is not uncommon for the military engineer to find himself/herself in the field without laboratory soil
testing facilities. Even where laboratory tests are to follow, field identification tests must be made
during the soil exploration to distinguish between the different soil types encountered. In this way
duplication of samples for laboratory testing will be held to a minimum. Several simple tests that you
may use in field identification are described in this lesson. Each test requires a minimum of time and
equipment, although you will seldom need all of them to identify any given sample. Using these tests,
you can estimate the soil properties and classify the materials. You should recognize such classification
as an approximation, since even experienced personnel have difficulty estimating detailed soil properties
with a high degree of accuracy. The material which follows is intended as an aid in the identification
and classification of soils according to the unique Soil Classification System.
The experienced engineer can get a lot of information from a visual examination. It is possible to
determine the color, grain size, and grain shape of the coarse-grained portion of a soil, and estimate the
grain size distribution. To observe these properties, a sample of the material is first dried and then
spread on a flat surface.
In field soil surveys, color is often helpful in distinguishing between various soil strata. Color is also
useful for identifying soil types. Since the color of a soil often varies with its moisture content, you
must always record the condition of the soil when you determine color. There is generally more contrast
in these colors when the soil is in a moist condition, with all the colors becoming lighter as the moisture
contents are reduced. In fine-grained soils, certain dark or drab shades of gray or brown, including most
black colors, are indicative of organic colloidal matter (OL, OH). In contrast, clean and bright looking
colors, including medium and light gray, olive green, brown, yellow, and white generally are associated
with inorganic soils. Soil color also may indicate the presence of certain chemicals. Red, yellow, and
yellowish brown soil colors may be a result of the presence of iron oxides. White to pinkish colors may
indicate presence of considerable silica, calcium carbonate, or aluminum compounds in some cases.
Grayish blue, and gray and yellow mottled colors frequently indicate poor drainage.
The maximum particle size should always be estimated for each sample considered, thereby establishing
the upper limit of the grain size distribution curve for that sample. To aid in determining something
about the lower limit of the grain size distribution, it is useful to know that the naked eye can normally
distinguish the individual grains of soil down to about 0.07 millimeters.