Environmental Engineering Reference
In-Depth Information
7
Soil Formations: Geologic Classes and Characteristics
7.1
Introduction
7.1.1
Geologic Classification of Soil Formations
General
Soils are classified geologically by their origin as residual, colluvial, alluvial, eolian, gla-
cial, or secondary soils. They are subclassified on the basis of their mode of occurrence,
which refers to the landform or surface expression of a deposit, or its location relative to
the regional physiography. The various geologic classes exhibit characteristic modes of
occurrence.
Classifying soils by geologic origin, describing the formation in terms of their mode of
occurrence, and considering both as related to climate, provide information on the char-
acteristics of gradation, structure, and stress history for a given deposit. Knowledge of
these characteristics provides the basis for formulating preliminary judgments on the
engineering properties of permeability, strength, and deformability; for intelligent plan-
ning of exploration programs, especially in locations where the investigator has little or no
prior experience; and for extending the data obtained during exploration from a relatively
few points over the entire study area.
Classes and Mode of Occurrence
Residual Soils
Developed
in situ
from the decomposition of rock as discussed in
Section 6.7,
residual soils
have geomorphic characteristics closely related to the parent rock.
Colluvial Soils
Colluvium refers to soils transported by gravitational forces. Their modes of occurrence
relate to forms of landsliding and other slope movements such as falls, avalanches, and
flows.
Alluvial Soils
Alluvium is transported by water. The mode of occurrence can take many forms generally
divided into four groups and further subdivided. In this chapter the term is used broadly
to include marine deposits.
Fluvial or river deposits include stream bed, alluvial fan, and floodplain deposits (point
bar, clay plugs, natural levees, back swamp), deposits laid down under rejuvenated stream
