Environmental Engineering Reference
In-Depth Information
Canada
(Figure 6.87),
the Permian shales of central and western Brazil, and the Cretaceous
shales of Bahia, Brazil.
Weathering Processes
Marine shale formations have characteristically been prestressed by high overburden
pressures, often as great as 100 tsf or more. When uplifted and subjected to erosion, stress
release and the resulting strains cause intense fracturing in the mass. Water enters the frac-
tures and the montmorillonite clay minerals expand to break the mass into numerous
small fragments as shown in
Figure 6.88,
thus making it susceptible to further weathering
and reduction to a soil. Weathering, however, is primarily mechanical, although some
chemical decomposition occurs.
Landforms
Landforms are characteristically gently rolling topography with shallow slopes, often of
the order of 8 to 15
o
, with the shallow-depth materials subjected to sloughing and sliding
movements as shown in
Figure 6.89.
Characteristics
Profiles:
Colors are predominantly gray to black. A typical profile from a core boring in the
Pierre shale (Upper Cretaceous) shows a weathering profile consisting of four distinct
zones
(Figure 6.90)
.
It is the amount of montmorillonite in a given zone that has the great-
est effect on the intensity of disintegration because of nonuniform swelling. The Pierre
shale is found in North and South Dakota (see
Figure 6.87)
.
Basic and Index Properties:
Some natural water contents from the Pierre shale are given in
Figure 6.90;
in the unweathered zones it is about equal to the plastic limit. In
the medium hard, fractured, weathered zone, typically LL
77 to 116%, PI
34 to 74%,
FIGURE 6.89
Typical gently rolling landform in the Pierre Shales near Forest City, South Dakota. (Note the local natural
slope failures resulting from gully erosion.)
