Environmental Engineering Reference
In-Depth Information
in feldspars and other impurities to form a clayey sandy overburden, generally not more
than a few meters thick.
Shales: Freshwater
Freshwater shales are composed of clay minerals and silt grains and decomposition is gen-
erally limited to impurities. Weathering is primarily mechanical, especially in temperate
or cooler zones. The characteristic weathering product is small shale fragments in a clay
matrix, usually only a few meters in thickness at the most.
Triassic shales, considered to be freshwater deposits in shallow inland seas, predomi-
nantly contain inactive clays, and normally develop a thin reddish clayey overburden.
Limestones and Other Carbonates
Composed chiefly of calcite, the pure limestones are readily soluble and do not decompose
to soil. It is the impurities that decompose, but normally there is no transition zone
between the soil and the rock surface (see
Figure 6.22)
,
as is normal for other rock types,
and the rock surface can be very irregular as shown in
Figure 6.85
and
Figure 6.86.
The
residual soils in warm, wet climates are typically clayey and colored red (“terra rossa”) to
yellow to reddish brown; and in cooler, less moist climates, grayish brown.
Marine Shales
Significance
Marine shales, particularly of the Tertiary, Cretaceous, and Permian periods, normally
contain montmorillonite. They are the most troublesome shales from an engineering view-
point because of their tendency to form unstable slopes and to heave in excavations (see
Sections 9.2.6 and 10.6.3). The montmorillonite commonly was deposited during periods
of volcanic activity
(Figure 6.90a).
Well-known troublesome formations include the
Cucaracha shales (Tertiary) encountered during the construction of the Panama Canal, the
Cretaceous shales covering large areas of the northwestern United States and adjoining
FIGURE 6.86
Decomposition profile in dipping carbonate rocks. Decay and solution proceed along the joints. The impure
strata from soil deposits which may fill a cavity when collapse occurs. (From Deere, D. U. and Patton, F. D.,
Proceedings of the 4th Panamerican Conference on Soil Mechanics and Foundation Engineering
, San Juan, Vol. 1, 1971,
pp. 87-100. With permission.)
