Civil Engineering Reference
In-Depth Information
f
(a)
1.0
90
0.9
b
c
89
0.8
0.7
z
c
85
0.6
H
0.5
80
0.4
f
70
p
0.3
60
50
40
0.2
30
20
10
0.1
0
10
20
30
40
50
60
70
80
90
p
(b)
Ratio (
b
c
/
H
)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0
10
p
10
20
30
40
50
60
70
20
30
40
50
60
70
80
Figure 6.6
Critical tension
crack locations for a dry slope:
(a) critical tension crack depth
relative to crest of cut;
(b) critical tension crack
location behind crest of cut.
80
90
found that the tension crack resulted from small
shear movements within the rock mass. Although
these individual movements were very small, their
cumulative effect was that there was a significant
displacement of the slope surfaces—sufficient to
cause separation of vertical joints behind the slope
crest and to form “tension” cracks. The fact that
the tension crack is caused by shear movements
in the slope is important because it suggests that,
when a tension crack becomes visible in the sur-
face of a slope, it must be assumed that shear
failure has initiated within the rock mass.
It is impossible to quantify the significance of
tension cracks since their formation is only the
start of a complex progressive failure process
within the rock mass, about which little is known.
It is quite probable that, in some cases, the
improved drainage resulting from dilation of the
rock structure, combined with the interlocking of
individual blocks within the rock mass, could give
rise to an increase in stability. However, where
the failure surface comprises a single discontinu-
ity surface such as a bedding plane daylighting
in the slope face, initial movement could be fol-
lowed by a very rapid decrease in stability because
a small amount of movement could result in a
reduction in the shear strength from the peak to
the residual value.
