Environmental Engineering Reference
In-Depth Information
5
Rock and Soil: Identification and Classification
5.1
Introduction
5.1.1
The Geologic Materials
Definitions
Precise definitions of the two general constituents, rock and soil, that are applicable to all
cases are difficult to establish because of the very significant transition zone in which rock
is changing to soil or in which a soil formation has acquired rock-like properties, or vari-
ous other conditions. In general terms, the constituents may be defined as follows.
Rock
Material of the Earth's crust, composed of one or more minerals strongly bonded together
that are so little altered by weathering that the fabric and the majority of the parent min-
erals are still present.
Soil
A naturally occurring mass of discrete particles or grains, at most lightly bonded together,
occurring as a product of rock weathering either
in situ
or transported, with or without
admixtures of organic constituents, in formations with no or only slight lithification.
Comments
The definitions given are geologic and not adequate for application to engineering prob-
lems in which the solution relates to hydraulic and mechanical properties as well as to cer-
tain other physical properties, such as hardness. For most practical engineering problems,
it is more important to describe and classify the materials in terms of their physical con-
ditions and properties than to attempt in every case to define the material as a soil or a
rock.
The most important practical distinction between soil and rock in engineering works
arises in excavations, since soil is normally much less costly to remove than rock, which
may require blasting. For pay quantities, soils are usually defined as materials that can be
removed by machine excavation, and other materials are defined as rock. This can be an
ambiguous definition since the success of machine excavation depends on the size,
strength, and condition of the equipment, as well as the effort applied by the operator.
In rock masses, the material factors of major significance in excavations, as well as in
other engineering problems, are mineral hardness, the frequency and orientations of
