Civil Engineering Reference
In-Depth Information
in the overturned siltstone beds at the crest of the
pit. When mining on the 1840 m bench, a series of
cracks formed on the 1860 m bench and a move-
ment monitoring program was set up in February
1975 using a combination of wire extensometers
(Figure 13.3(b)) and surveying of prisms. This
monitoring system was used to control the min-
ing operation with the objective of mining back
to the final wall so that the coal could eventually
be mined to the bottom of the pit. Figure 13.6(b)
shows the sensitivity of the slope movement to
mining at the toe of the toppling slope, and typ-
ical regressive behavior as soon as the shovel was
pulled back. This experience was used to establish
the criterion, based on hourly movement read-
ings, that mining would be halted as soon as
the rate of movement reached 25 mm per hour.
When this rate reduced to 15 mm per day over a
period of about 10 days, mining recommenced.
Using this control procedure, mining contin-
ued towards the final depth of about 1700 m
with the slope moving at an average rate of
6 mm per day.
In April 1976, the slope started to accelerate,
and over the next two months total movement
of about 30 m occurred on the hillside above
the pit and the maximum velocity reached nearly
a meter per day (Figures 13.6(c) and (d)). The
acceleration on the slope movement plots gave
an adequate warning of deterioration stability
conditions and mining was abandoned. The area
with the greatest movement was on the toppling
beds along the crest of the pit, and in early June
1976 two separate slope failures occurred with
a total volume of 570,000 m
3
. After the failures,
the monitoring system was re-established which
showed that the rate of movement was gradu-
ally decreasing, and after a month it was decided
to restart mining at the bottom of the pit. This
decision was also based on borehole probe meas-
urements, which showed that the circular slide
surfaces, associated with the toppling at the pit
crest, daylighted in the upper part of the pit slope
(Figure 13.6(a)). Therefore, mining at the base of
the pit would have little effect on stability.
The
analyzed to help predict the time of failure
once the progressive stage of slope movement
has developed (Zavodni and Broadbent, 1980).
Figure 13.7 shows the semi-log time-velocity plot
in feet per day preceding the slope failure at the
Liberty Pit. On this plot it is possible to identify
the velocities at the start
V
0
and the mid-point
V
mp
of progressive stage of movement. A constant
K
is defined as
V
mp
V
0
K
=
(13.2)
Study of six carefully documented slope fail-
ures shows that the average value of
K
is
7.21,
with a standard deviation of 2.11. For example,
Figure 13.6(d) shows a
K
value of about
−
−
7
(
0.07
/
0.01).
The general equation for a semi-log straight line
graph has the form
−
C
e
St
V
=
(13.3)
where
V
is the velocity,
C
is the intercept of the
line on the time axis,
e
is the base of the nat-
ural logarithm,
S
is the slope of the line and
t
is
the time. Therefore, the velocity at any time is
given by
V
0
e
St
V
=
(13.4)
Combining equations (13.2) and (13.3) gives
the following relationship for the velocity at
collapse
V
col
:
K
2
V
0
V
col
=
(13.5)
The use of equation (13.4) in conjunction with
a time-velocity plot allows an estimation to be
made of the time of collapse.
For example,
from Figure 13.6(d) where
K
7 and
V
0
=
0.01 m/day, the value of
V
col
is 0.49 m/day. Extra-
polation of the velocity-time line shows that this
rate will occur at about 61 days, which is very
close to the actual day of collapse.
=−
type
of
movement
monitoring
data
shown
in
Figures
13.6(c)
and
(d)
has
been
