Slope Performance
Monitoring
In addition to monitoring by visual inspections, comprehensive displacement monitoring has
been implemented using survey prisms and inclinometers. Currently, during extraction of the
buttresses and the mining of the lower benches, ground movement radar (Slope Stability Radar -
SSR) has also been employed.
Survey Prism Monitoring
The current PB8S prism monitoring network consists of prisms read automatically
approximately every six hours. Data are fed back to the geotechnical engineering and pit control
offices by telemetry. Slope displacement monitoring shows movement over much of PB8S and
significant effort has been made to determine the mechanisms causing this movement and the
consequent risks.
Displacement records for these prisms are presented in Figure 3.4 and show the following:
Long term movement rates of 0.02 mm/day on the upper slope (prisms 111022 and
114013).
Long term movement rates of 0.03 mm/day in the centre of the slope through a keel
structure (prism 117023).
Short term movement rates of between approximately 0.1 mm/day and 0.2 mm/day on
the lower benches of the slope in closer proximity to the EFF (prisms 122019 and
124005).
All movement rates have slowed as the rate of mining in the lower benches has slowed.
Some correlation between rainfall events and periods of increased movement,
particularly where relatively shallow, smaller scale, instability mechanisms develop
lower in the slope in proximity to the EFF.
Displacement rates of 0.2 mm/day were recorded over the five months prior to the adjacent
instability which was triggered by 82 mm of rain fall. Displacement rates of 2.2 mm/day were
recorded at the time of collapse. Regarding mechanisms causing movement, the measured
ground displacements and the time and rate of movement over the majority of the slope are
compatible with the initial response of a rock mass to excavation (that is elastic rebound,
relaxation and dilation of the rock mass) for large open pits (Zavodni, 2000). However,
monitoring results prior to the Feb 13 slip indicate that, where multi-batter scale instability is
controlled by weak planar structures, displacement rates similar to those expected as an initial
response to mining may result in slope collapse. Total displacements of only 40 mm were
recorded prior to the Feb 13 slip. instability likely to cause movement in that particular sector of
the slope. Daily, weekly and quarterly monitoring reviews are performed, based on an
assessment of movement of prisms within these groups
Inclinometers
A total of seven inclinometers have been installed to various depths on PB8S, and are principally
designed to monitor displacement on the 320 Faults and the major keel structures as well as the
EFF. Inclinometers are manually read once every week or two, depending on previous
movements and location on the slope. Movement has been recorded on all but one of the
instruments, with the greatest movement being recorded in holes which intersect the EFF in the
central area of the PB8S. These movements show a strong correlation with large rainfall events
and mining rates.
Slope Stability Radar (SSR)
During mining of the lower benches of PB8S, Slope Stability Radar (SSR) has been employed to
provide additional monitoring of the safety hazard associated with possible bench to multi bench
instability triggered by mining or rainfall. The principal advantages of the radar over prism
monitoring are its coverage of the complete area of slope being monitored and shorter intervals
between scans (15 minutes versus six hours). To date, the radar monitoring results have
identified ongoing movement only in the area of the Feb. 13 slip. The SSR is operated in a safety
critical mode. That is, critical displacement levels are set, at which movement alarms will be
triggered in the pit control and geotechnical offices. In practice, alarm trigger levels have been
set after consideration of the type and size of failure expected and the need to prevent numerous
false alarms due to atmospheric interference.
