SLOPE STABILITY MONITORING
Punch longwall layout showing highwall and low wall, with gateroad access at base of highwall
the open cut highwall. The operation wanted to
understand the mechanism causing the
movement and potentially be able to predict
instability.
Mining of longwalls under or adjacent to large
voids such as stream valleys, escarpments or
cliffs is commonly associated with heavily
vegetated or steep areas, where survey access is
limited and interpretation of any surface
movement data is complicated.
The punch longwall layout provides access to
deploy radar and laser scanning technology. An
unvegetated, evenly excavated open cut highwall
creates a perfect configuration to study the effect
of longwall subsidence on a steeply dipping
surface.
Inclinometer monitoring adjacent to the
Broadmeadow Mine LW11 panel during the start
of the block confirmed that the direction of shear
movement is toward the centre of the void
created by the longwall panel.
Survey peg data indicated typical horizontal
movement, with points drawn toward the goaf
(collapsed rock above the void) as the longwall
approaches and then moving in the opposite
direction as the longwall passes underneath and
the surface settles. This traditional
understanding led barrier pillar designers to
believe that longwall subsidence would pull the
highwall toward the goaf and into an even more
stable position.
However, at Broadmeadow the observed
highwall movement and associated ground
deformation did not conform to either typical
longwall subsidence profiles or highwall
movement. Values far exceeded stability limits
used in adjacent open cut mines, indicating the
onset of failure. This outward movement, while
not affecting the global stability of the highwall,
destabilised local areas around existing defects
and geological structures.
The Maptek Sentry system was trialled
alongside a radar system on LW10 and used
64 International Mining | AUGUST 2019
exclusively for LW11. Both systems enabled near
continuous, real-time, sub-millimetre monitoring
of a full 100 m high x 500 m wide highwall. The
Maptek laser system can scan the entire wall in
six minutes, depending on the block size to be
monitored.
An advantage of laser technology is that it is
spatially referenced, allowing itinerant
monitoring. The scanner can be shifted to a new
location and maintain a correlation in the data
before and after moving.
Both laser and radar techniques enabled real-
time graphical display of total displacement and
rate of wall movement. The supporting software
provided real-time triggers or warnings of
increasing rate of movement, and videos of
displacement over time could be created. High
quality, accurate information was generated, and
large-scale highwall displacement of up to 1,000
mm was measured from 300 m.
At Broadmeadow, an unprecedented
magnitude and rate of movement was caused by
the longwall effect (1,000 mm total, with a rate
up to 1.5 mm/hr over +6 weeks), and the rate
was controlled by longwall retreat rather than
ground failure. Interpretation of the data requires
the judgement of a geotechnical engineer to
balance increased rate of movement with
increased rate of longwall retreat. The ability to
monitor and colour contour the entire highwall
allowed identification of anomalous localised
areas of movement to trigger additional
protection measures.
Punch longwall mining increases the risk of
mine inundation due to the low elevation and
wide catchment area. Large levees are
constructed to protect the open pit from flooding
by adjacent rivers. Sumps constructed against
the highwall between the headgate and tailgate
portals control rainfall and rock hazards in the pit
catchment. The sumps protect the working area
from rockfall hazards between headgate and
tailgate, but the portal areas remain exposed.
Access of personnel and materials through the
portal entries requires more stabilisation on this
part of the wall. The highwall is rehabilitated
with rockbolts and draped with rockfall mesh.
Substantial reinforced concrete portals are
installed out to 15 m from the highwall to allow
covered access. A 10 m exclusion zone is
enforced adjacent to the portals, and a 2 m high
rock bund creates a catch drain for local
rockfalls. This is standard for all punch longwall
portal accesses and is independent of the study
results.
Forward movement of the highwall is theorised
to be primarily caused by the subsiding ground;
as the strata lies down behind the longwall, a
massive forward push occurs. This is normally
confined by hundreds of metres of solid ground,
but when adjacent to an open cut void or stream
valley, the longwall shoves the bedded ground
forward like a stacked deck of cards. The
maximum movement is near the surface and
decreases downward due to leverage and
frictional resistance from the weight of
overburden and proximity to the subsidence
trough.
Underground planning engineers seek to
optimise barrier pillar widths, but traditional
design methods may not account for highwall
stability. While the global highwall stability was
maintained at Broadmeadow, localised failures
remobilising along joints or faults can be
triggered around pre-existing geological
structures, cling-ons or blast cracking. Various
controls are suggested if mining is within 300 m
of an open pit:
n Scan and map highwalls prior to planning
portal locations and longwall stop positions to
identify all potential structures that could be
affected and put specific controls in place.
n Allow space on catch benches and portal pads
for adequate bunding against the slope toes
to manage pit slope failures.
n Ensure that infrastructure placement on
highwall benches and pads allows for
potential ground movement, with concrete
portal entries set further off the highwall.
n Cater for access and restricted access to catch
benches.
n Catch drains should be accessible and
regularly cleared to maintain capacity
“The punch longwall layout and the open cut
slope stability monitoring technology provide a
near-perfect scenario for monitoring the effect of
highwall movement due to an approaching
longwall, where subsidence will push ground
forward when adjacent to a void. Barrier pillar
sizes in punch longwalls can be minimised with
an understanding of the mechanism, appropriate
design and deliberate controls. Additional
controls can be very effective for working in close
proximity to a highwall or void.” IM