Environmental Engineering Reference
In-Depth Information
Materials
Cohesive
Clays and silty clays.
(Usually found in lake
beds, marine terraces,
and clay shale areas.)
Cross sections
Profiles
Land surface
Gully profile
Low, uniform gradient
Weathered soil
profile
Moderately cohesive.
Weakly cemented sand-
clays.
Coastal plains and
many bedrock areas.
Loose soil
Compound gradient
Moderately cohesive
silt. (Primarily loess
and alluvial silt
deposits. Also fine
volcanic ash falls.)
Fins and pinnacles
near head
Very steep,
coarse
material,
few fines
Noncohesive granular
materials. (Often found
in terraces and
outwash plains.)
Well-graded
mixtures
FIGURE 7.2
Gully characteristics: cross sections, profiles, and associated soils. (From Way, D., Terrain Analysis , 2nd ed.,
Dowden, Hutchinson & Ross Publ., Stroudsburg, PA, 1978. With Permission.)
7.2.2
Igneous and Metamorphic Rocks
General
Products of Decomposition
Igneous and metamorphic rocks are composed of silicates and oxides as summarized in
Tables 6.13 and 6.14. Silicates decompose to yield primarily clay soils. Feldspars produce
kaolin clays generally of light colors ranging from white to cream. Ferromagnesians (see
Table 5.2) produce clays and iron oxides, which in humid climates give residual soils a
strong red color, or when concentrations are weaker, yellow or brown colors. Black, highly
expansive clays occur under certain conditions. Muscovite (white mica) is relatively stable
and is often found unaltered, since it is one of the last minerals to decompose (biotite,
black mica, decomposes relatively rapidly).
Quartz, the predominant oxide, essentially undergoes no change, except for some slight
solution, and produces silt and sand particles.
Effects of Climate (see also Section 6.7.2)
Temperate and semitropical climates tend to produce predominantly kaolinite clays, and
occasionally halloysite and hydrated and anhydrous oxides of iron and aluminum. The
end products usually are sand-silt-clay mixtures with mica, with low activity.
 
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