Environmental Engineering Reference
In-Depth Information
mass, resulting in folding and fracturing, which create systems of faults, joints, and other
discontinuities. Differential weathering and erosion attack the mass, resulting in terrain
features indicative of various conditions, and provide the basis for their prediction.
Faults are closely associated with shallow-focus earthquakes. Their recognition and pos-
itive identification, as well as the determination of their activity, are critical elements in
earthquake engineering studies. Faults also create a major weakness zone in rock masses.
Joints, the most common form of discontinuity, control rock-mass behavior in most sit-
uations. A description of their spatial orientations and characteristics is necessary for the
solution of rock-mechanics problems, particularly in closed and open excavations, and in
the construction of concrete dam foundations and abutments.
Residual stresses result in strains and rock bursts in excavations and are an important
element of rock-mass conditions. Unless specifically sought by means of
in situ
instru-
mentation, their prediction is difficult, although their incidence is often associated with a
particular set of conditions.
Weathering causes rock masses to undergo disintegration and decomposition, com-
pletely changing their characteristics and producing new materials ranging from altered
minerals along joints, to soft and decomposed rock, and finally to residual soil. Various
rock types develop characteristic profiles under given climatic conditions, thus providing
the basis for the prediction of soil types and their general properties when climate and
rock type are known.
Soil Formations: Geologic Classes and Characteristics
(Chapter 7
)
Soils are classified by origin such as residual, colluvial, and alluvial; subclasses are based
on their mode of occurrence or deposition such as fluvial and lacustrine, and subdivided
as stream channel deposits, floodplains, and tidal marshes. Soils of various origins and
modes of occurrence or deposition have characteristic terrain features of landform,
drainage patterns, and vegetation, which are used as indicators to provide the basis for
their identification, the anticipation of structural features such as stratification, and the
estimation of their characteristic engineering properties. The origin and mode of occur-
rence of the soils most significantly affect their gradation characteristics as well as their
stress history.
Residual soil characteristics reflect parent rock type. For example, relatively thin
deposits of inactive clays are normal in most sedimentary and sialic igneous rocks; thick
deposits of inactive clays are normal in foliated rocks in moist climates; active clays
develop from mafic rocks and marine shales in moist climates. The landform reflects the
parent rock characteristics.
Colluvium frequently originates from residual soils, but can originate from any soil for-
mation on slopes. Colluvium develops characteristic landforms and leaves scars of
denuded vegetation and fresh surface materials after major slope failures. Colluvial soil
slopes can be expected to be unstable, and the formation may be found overlying weak
alluvia.
Alluvium refers to channel deposits that relate to stream shape and gradient, which
influence the carrying capacity of the stream. Only boulders and cobbles remain in the
steep gradients of young streams; gravels and sands in the moderate gradients of mature
streams; and sands, silts, and clays in the small to nil gradients of the pastoral zone (except
during flood stages). The fluvial environment creates floodplains, terraces, and other such
terrain features as back swamp deposits and oxbow lakes, all with their characteristic soil
types. Rivers terminate in lakes and oceans to deposit deltas or to have their sediments
carried by currents for deposition along shorelines such as beaches and spits, or offshore
in deep waters where accumulations may reach great thicknesses. The types of materials
