low bearing strengths and little value as construction material. Although landform distribution maps are generally
very good sources of information, they are normally produced for a limited function and area; therefore, they may
be difficult to locate, obtain, and reproduce.
c. Geologic Maps. In the absence of more reliable data, geologic maps can be used to indicate possible
overlying soil types and their geographic distributions. This is done by examining the geologic map to determine
the rock types present and subsequently estimating the most likely weathering products derived from those rock
types (see Lesson 2).
d. Topographic Maps. Topographic maps may supplement soil maps, landform distribution maps, or
geologic maps by providing indirect data concerning the soils of an area. This indirect data may be obtained by
analyzing the landforms and/or drainage patterns present on the map to determine the soil types most likely to be
present. The relationship between drainage pattern and geologic materials is described in Lesson 2.B.1.a.(1)(b),
3. Aerial Photography. Analysis of aerial photographs is another good way to obtain soil information. For best
results, the photographs should be of high quality and reasonably large scale (1:20,000 or larger). Interpretation is
carried out by studying soil patterns resulting from the nature of the parent rock, the mode of deposition of the
sediments, and the physical, climatic, and biological environments. One limitation of aerial photography is that,
although soils are three-dimensional, the aerial photograph reveals only the two-dimensional soil surface;
therefore, interpretation is somewhat restricted.
4. Soil Reports. Various types of soil reports may also be available. For example, the United States Soil
Conservation Service has published reports concerning the soils of numerous counties in the United States. These
reports are primarily tailored for agricultural users; however, the more recently published ones also contain
information concerning the engineering properties of soils. Many foreign countries have also published soil
reports that may be useful in analyzing a particular area of interest.
PART B - CLASSIFICATION OF SOILS
Although numerous soil classification schemes exist, engineers commonly use only those systems that base
classification on the grain size and consistency of the component materials. In this way, several general soil
categories may be identified.
1. General Soil Categories. Soils may be considered to belong to one of five broad categories based on the
grain size, consistency, and chemical characteristics of the component particles.
a. Gravel. Gravels are coarse materials ranging from 4.7 millimeters to 76 millimeters in diameter.
b. Sand. Sand grains are also coarse, ranging in diameter from 0.07 millimeter to 4.7 millimeters.