Environmental Engineering Reference
In-Depth Information
silts. Below the A-line and less than a 50 LL are inorganic silts and very fine sands with
low plasticity. In the same region but above the A-line are inorganic clays of low to
medium plasticity. (See Figure 2.6.)
When developing a sampling plan the soils in the area may be described by either soil
science or engineering terminology. It is important to be able to understand both types of
descriptions and use these to determine how sampling should be done. In most cases both
descriptions will be valuable. In situations in which gravel is prevalent the engineering
FIGURE 2.6
The A-line used by engineers to describe fine soil fractions. Data
taken from Ref. 14.
descriptions will be very useful in deciding on the type of sampler to be used [18].
2.4.2.
Structure in Soil
Figure 2.4 shows the components in an idealized, well-developed soil profile. All of the
loose material—including water-saturated layers—above rock is called the regolith. The
material above the saturated zone is called the vadose zone. The material in which active
soil development is taking place is called the solum. The characteristics of these areas is
determined by both texture and structure.
The texture of soil is extremely important because it controls important characteristics
related to the movement of water and the sorption and retention of contaminants. Texture
is not, however, the only factor in movement in the soil. Soil particles, sand, silt, clay,
and organic matter do not act independently of each other, but are cemented together to
form secondary particles called peds, which are the soil structure. The cementing agents
are organic matter, clay, microbial gums, and various cations.
Different sizes and shapes of peds are found in different soil horizons. The different
kinds of peds and their typical location in a soil profile are given in Figure 2.4. Note that
platy structure or peds can be found in any horizon, but are commonly found between the
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