Environmental Engineering Reference
In-Depth Information
24
°
0.44
Legend
0.40
Normal stress
(tons/ft
2
)
30
8
6 and 10
Formation or group
Colorado
Claggett
Bearpaw
Fort Union
Pierre
Dawson (Chatfield Dam
near Denver Colorado)
20
°
0.36
0.32
(1)
0.28
NOTE : (1) Tests performed for design of
chatfield dam(USAE, Omaha District, 1968)
15
°
0.24
0.20
10
°
9
0.16
8
7
°
6
0.12
5
°
4
°
3
°
0.08
0.04
10
30
50
70
90
110
Liquid limit
130
150
170
190
210
FIGURE 6.91
Summary of residual direct shear strength vs. liquid limit for various marine shales of the northwestern United
States. (From Banks, D.C., Proceedings ASCE, 13th Symposium on Rock Mechanics, University of Illinois, 1971,
pp. 303-328, 1972.)
Sandstones Containing Thin Shale Beds
Relatively steep overall stable slopes are characteristic of sandstones interbedded with
thin shale layers, but such slopes are subject to falling blocks of sandstone when the shales
contain expansive materials. Thin seams of montmorillonite in the shale expand, fractur-
ing and wedging blocks of sandstone loose. Differential weathering of the shale causes it
to recede beneath the sandstone as shown in
Figure 6.48,
resulting in a loss of support and
blocks falling to the roadway.
References
Badgley, P. C.,
Structural Methods for the Exploration Geologist
, Harper & Bros., New York, 1959.
Banks, D. C., Study of clay shale slopes,
Stability of Rock Slopes
,
Proceedings of ASCE, 13th Symposium
on Rock Mechanics
, University of Illinois, 1971, pp. 303-328, 1972.
