Fundamental Soil Mechanics - Mathematics and Statistics for the Environment - page 309

12.4 SOIL CHARACTERISTICS
When we refer to the characteristics of soils, we are referring to the mechanical characteristics,
physical factors important to environmental engineers. Environmental engineers focus on the char-
acteristics of the soil related to its suitability as a construction material and its ability to be exca-
vated. Simply put, the environmental engineer must understand the response of a particular volume
of soil to internal and external mechanical forces. Obviously, it is important to be able to determine
the soil's ability to withstand the load applied by structures of various types and its ability to remain
stable when excavated. From a purely engineering point of view, soil is any surficial (near the sur-
face) material that is unconsolidated enough to be excavated with tools (from bulldozers to shovels).
The engineer takes into consideration both the advantages and disadvantages of using soil for engi-
neering purposes. The obvious key advantage of using soil for engineering is that there is (in many
places) no shortage of it—it may already be on the construction site, thus avoiding the expense of
hauling it from afar. Another advantage of using soil for construction is its ease of manipulation; it
may be easily shaped into almost any desired form. Soil also allows for the passage of moisture, or,
as needed, it can be made impermeable.
The environmental engineer looks at both the advantages and disadvantages of using soil for
construction projects. The most obvious disadvantage of using soil is its variability from place to
place and from time to time. Soil is not a uniform material for which reliable data related to strength
can be compiled or computed. Cycles of wetting and drying and freezing and thawing affect the
engineering properties of soil. A particular soil may be suitable for one purpose but not for another.
Stamford clay in Texas, for example, is rated as “very good” for sealing of farm ponds but “very
poor” for use as base for roads and buildings (Buol et al., 1980).
To determine whether a particular soil is suitable for use as a base for roads or buildings, the
environmental engineer studies soil survey maps and reports. The environmental engineer also
checks with soil scientists and other engineers familiar with the region and the soil types of that
region. Any good engineer will also want to conduct field sampling to ensure that the soil product
he or she will be working with possesses the soil characteristics required for its intended purpose.
Important characteristics of soils for environmental engineering purposes include the following:
• Soil texture
• Kinds of clay present
• Depth to bedrock
• Soil density
• Erodibility
• Corrosivity
• Surface geology
• Plasticity
• Content of organic matter
• Salinity
• Depth to seasonal water table
The environmental engineer will also want to know the soil's density, space-volume and weight-
volume relationships, stress and strain, slope stability, and compaction. Because these concepts are
of paramount importance to the engineer, these concepts are discussed in the following sections.
12.4.1 s oil W eight -v olume and s paCe -v olume r elationships
All natural soil consists of at least three primary components or phases: solid particles (miner-
als), water, and air (void spaces between the solid particles). The physical relationships (for soils in
particular) between these phases must be examined. The volume of the soil mass is the sum of the
volumes of three components, or