Environmental Engineering Reference
In-Depth Information
In tropical climates the decomposition is much more intense and rapid, in some loca-
tions reaching depths of 100 ft or more in foliated rocks. The predominant end products
are silty clay mixtures with minor amounts of sand. The common red tropical clays con-
tain hydrated oxides of iron and aluminum in the clay fraction. The type of clay is a func-
tion of the parent rock minerals and time and can vary from inactive to active as given in
Figure 6.70
and
Table 5.30.
In hot climates with alternating wet and dry seasons, as decomposition advances, the
clay minerals are essentially destroyed and the silica leached out. The material remaining
consists of aluminum hydroxide (bauxite, in pure form), or hydrous iron oxide, such as
limonite. This is the process of laterization (see
Section 7.7.2)
.
In hot climates with low to moderate rainfall, mafic rocks, in an environment of poor
drainage, produce black, highly expansive clays (the “black cotton soils” of Africa).
Prediction of Soil Type
Known factors required for the prediction of the soil type are the basic rock type (from a
geologic map), climate, and topography. The procedure is as follows: Determine the min-
eral composition from
Table 5.7
for igneous rocks or
Table 5.13
for metamorphic rocks.
Determine the possible physical extent of the formation on the basis of typical forms vs.
the rock type from
Table 6.4,
and evaluate the influence of topography from terrain analy-
sis. Evaluate the relative stability of the minerals in terms of the end product from
Table
6.13.
Estimate the soil composition and activity from Figure 6.70, and consider the climate
factor as described above and given in
Figure 6.69
in terms of the topographic expression
as it relates to formation drainage (see
Section 6.7.2).
Typical Residual Soil Profiles and Characteristics
General
Soil profile and engineering characteristics at a given site can be extremely variable, even for
a given rock type, because of variations in mineral content, rock structure, and topography.
Sialic Rocks
Granite, syenite, etc., decompose to mixtures of quartz sands and kaolin clays with mus-
covite mica fragments. Biotite mica decomposes to produce iron oxides that impart a red-
dish or brownish color. Clays are of low activity. Boulders form in massive to blocky
formations as described in
Section 6.7.3.
Mafic Rocks
Gabbro, basalt, periodotite, etc., decompose primarily to plastic red clays in tropical cli-
mates, and black expansive clays in hot climates with low to moderate rainfall and poor
drainage. The black clays are often associated with basalt flows.
A typical decomposition profile of basalt in south central Brazil is given in
Figure 6.81.
Some typical engineering properties including index and strength values are given in
Table 7.5.
“Porous clays,” a type of collapsible soil, are common in the residual soils of
basalt in Brazil (as well as sandstones and tertiary clays, see
Section 10.5.2).
Foliated Rocks
Decomposition in gneiss varies with the original rock (prior to metamorphism,
Table 5.12),
and can range from red micaceous sandy clay (low activity) to micaceous sandy
silt depending upon the amount of feldspar and biotite in the original rock. Foliations
