Environmental Engineering Reference
In-Depth Information
Depth
in feet
below
surface
Description of shale
Index
properties
(a)
Shale,
highly weathered (a)
20
Wn = 34
LL = 83
PL = 40
25
Zone of complete
disintegration
30
Shale,
highly fractured
(b)
35
Wn = 31 - 51
LL = 77 - 116
PL = 34 - 43
Zone of advanced
disintegration
40
Shale,
moderately
fractured (b)
45
Zone of medium
disintegration
(c)
50
Shale, intact (c)
Wn = 22 - 32
LL = 61 - 92
PL = 31 - 40
55
Joints, few with
large extent
60
65
70
75
Unweathered
FIGURE 6.90
Typical profile in Pierre Shale, near Forest City, South Dakota. Note bentonite seam in upper photo.
activity is 2.8, and the content of particles smaller than 2
µ
m is 50%. In any formation,
however, large variations in LL and PI can be found.
Strength:
Disintegration occurs rapidly when fresh rock is exposed to moisture in the air
during excavation and is accompanied by a rapid reduction in strength. Typical properties
of some marine and clay shales are given in
Table 3.38.
Residual shear strengths
ϕ
r
, as
determined by direct shear tests, vs. the liquid limit for various marine shales from the
northwestern United States are given in
Figure 6.91.
Interbedded Shales
Marine Shales
Sandstone interbeds retard deep weathering that proceeds along exposed shale surfaces.
In the northwestern United States, Banks (1972) observed that slopes capped with or
underlain by resistant sandstone strata had inclinations ranging from 20 to 45
o
whereas
slopes containing only marine shales ranged from 8 to 15
o
.
Dipping interbeds of sandstone and shale have caused severe problems in a development
in Menlo Park, California (Meehan et al., 1975). The shales have decomposed to a black,
expansive clay. The differential movement of foundations and pavements founded over alter-
