Geology Reference
In-Depth Information
joints should not be used interchangeably in empirical equations as this
could lead to an overestimation of
field scale strength by perhaps
10 degrees in many cases.
There is some evidence that frictional resistance for rock joints is
dependent on loading rate, and this may be signi
cant for aseismic
design and for understanding response to blasting. For a block of rock
sitting on an inclined plane, given a value for static friction, one can
calculate the horizontal acceleration necessary to initiate movement
and when the block should stop, given a particular acceleration time
history, as illustrated in
Figure 5.27.
This type of calculation is the
basis of the Newmark (1965) method of dynamic slope stability ana-
lysis, which is used to calculate the distance travelled, as discussed in
more detail in
Chapter 6.
Hencher (1977) carried out a series of
experiments and found that initiation of movement was generally
later than anticipated (or did not occur), implying greater peak fric-
tional resistance than predicted from static tests. The effective friction
for initiation increased with the rate of loading (
Figure 5.28).
The
implication is that if the loading is very rapid and reversed quickly
(as in blast vibrations), shear displacement might not occur, despite the
supposed critical acceleration being exceeded. However, once move-
ment was initiated, Hencher found that the distance travelled was
higher than anticipated from static strength measurements and inter-
preted this as re
ecting rolling friction and the inability of strong
frictional contacts to form during rapid sliding. Hencher (1981a)
suggested that for Newmark-type analysis, residual strength should
Critical
acceleration
..
k
..
High
..
..
k
“
φ
b
”
Low
t
1
Time
t
0
t
0
-
Predicted initiation of sliding
t
1
-
Block comes to halt
