Environmental Engineering Reference
In-Depth Information
In situ rock
refers to the rock mass that normally contains defects, such as fractures or
cavities, which separate the mass into blocks of intact rock and control the hydraulic and
mechanical properties. Classification is based on rock quality, with the mass generally
termed as competent or incompetent. Various practitioners have presented systems for
describing incompetent rock, which can contain a wide range of characteristics, but a uni-
versally accepted nomenclature has not been established.
5.1.3
Soil Groups and Classes
Geologic Bases
Geologically, soils are grouped or classified on a number of bases, as follows:
Origin: residual, colluvial, alluvial, aeolian, glacial, and sedentary
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Mode of occurrence: floodplain, estuaries, marine, moraine, etc.
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Texture: particle size and gradation
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Pedology: climate and morphology
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Engineering Bases
Classes
Soils are classified on an engineering basis by gradation, plasticity, and organic content,
and described generally as cohesionless or cohesive, granular, or nongranular.
Groups
Soils are grouped by their engineering characteristics as strong or weak, sensitive or insen-
sitive, compressible or incompressible, swelling (expansive) or nonswelling, pervious or
impervious; or grouped by physical phenomena as erodible, frost-susceptible, or
metastable (collapsible or liquefiable, with the structure becoming unstable under certain
environmental changes).
Soils are also grouped generally as gravel, sand, silt, clay, organics, and mixtures.
5.2
Rocks
5.2.1
The Three Groups
Igneous
Igneous rocks are formed by the crystallization of masses of molten rock originating from
below the Earth's surface.
Sedimentary
Sedimentary rocks are formed from sediments that have sometimes been transported and
deposited as chemical precipitates, or from the remains of plants and animals, which have
been lithified under the tremendous heat and pressure of overlying sediments or by chem-
ical reactions.
Metamorphic
Metamorphic rocks are formed from other rocks by the enormous shearing stresses of
orogenic processes that cause plastic flow, in combination with heat and water, or by the
