Environmental Engineering Reference
In-Depth Information
Horizon
Description
A
Zone of leaching (eluviation)
contains the maximum amount
of organic material, or
Soil to
pedologists
(soil
profile)
O
"A" horizon of pure organic
nonmineral layers.
B
Illuviated layer in which dissolved
materials are deposited.
Contains an accumulation of
clay and sesquioxides with
small amounts of organic
matter (the duricrust zone)
Soil to
engineers
(regolith)
C
Partly weathered parent
material.
Unconsolidated
earthy
material
D
Unweathered parent soil, or
Bedrock
R
Unweathered parent rock
FIGURE 7.111
Generalized soil profile showing the morphological units and soil terminology used by pedologists and
engineers. The Modern Soil Profile includes three major zones — A, B, and C horizons.
In general, in the A horizon (zone of leaching) K, Mg, Na, and clay are removed; in the
B horizon, Al, Fe, and clay accumulate in older soils in moist climates, and Si and Ca in
arid climates.
Plant Indicators
For a given climate various plant species favor particular conditions of soil type and
ground moisture, and therefore provide useful information about geologic conditions.
Much more information on the subject should be provided in engineering publications
than is readily available as an aid to predictions.
7.8.2
Pedological Classifications
Descriptive Nomenclature (SCS, 1960)
Grain size definitions for soil components are given in
Table 5.27.
Sand, silt, and clay are
less than 2 mm in diameter; gravel, 2 mm to 3 in.; cobbles, 3 to 10 in.; and boulders, over
10 in. in diameter.
Soil texture
refers to the gradation of particles below 2 mm in diameter, and depending
upon relative percentages, soils are termed sand, sandy loam, loam, clay. etc., as given in
Figure 7.112.
Loam refers to a detrital material containing nearly equal percentages of
sand, silt, and clay.
Organic soils are described as
muck
(well-decomposed material) or
peat
(raw, undecom-
posed material).
Stoniness
describes surface conditions in terms of boulders, ranging in scale from 0 (no
stones) to 5 (land essentially paved with stones).
Rockiness
refers to the relative proportion of bedrock exposures, ranging in scale from 0
(no bedrock exposed) to 5 (land with 90% exposed rock).
Soil structure
describes the shape of individual particles as prismatic, columnar, blocky,
platy, or granular.
