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Improving Cavity Prediction on
Longwall Faces
ACARP C21013 Extension
Aims of Study
Develop reliable convergence and leg
pressure TARPs from shield monitoring
data that can give a pre-warning of
impending roof control problems
Determine the accuracy of the TARPs
An estimate of the duration of the pre-
warning
An indication of operational controls
Mines Where Analysis of Convergence, Tilt and
Pressure Monitoring has been carried out to date
Austar - CSIRO Tilt Sensors
Newlands - CSIRO Tilt Sensors
Narrabri – Caterpillar Tilt Sensors
Broadmeadow – Caterpillar Tilt
Sensors
Dendrobium – Joy Tilt Sensors
West Wallsend – Leg pressure only
Leg Pressure TARPs
A manual set pressure of ≤ 40 tonnes/m2
(about 180 Bar)
set pressures less than this are not
adequate to eliminate tension in the
subsiding roof plate
when are inadequate manual set
pressures a result of cavities and when
do they cause cavities? – it is critical to
answer this question
Leg Pressure TARPs
A Pre- yield initial (within the first 5
minutes) loading rate of ≥ 15
bar/minute
this can be a very fragile signal,
particularly when high set pumps
are enabled
it infers rather than measures post-
yield convergence
Convergence TARPs
A cumulative convergence over 3
consecutive load cycles of ≥ 200 mm
Convergence in a single load cycle of
≥ 80 mm
An average convergence rate over the
entire cycle of 50 mm/hour
Convergence TARPs
The pre-yield loading rate TARP
would indicate a pre-yield
convergence rate trigger of about 1.5
mm/min
not determinable with current OEM
sensors and difficult to determine
with CSIRO sensors without filtering
Significant Convergence Event
A post-yield convergence rate of 5 mm/minute has
been shown to be indicative of the onset of a major
convergence event
outside of the resolution of current OEM
sensors
not a TARP because occurs too late in the build
up to the event
measurements could nevertheless significantly
improve our understanding of the interaction
between the shield and the strata
October 25, 2016 9
How Much Notice of an Event?
October 25, 2016 10
How Much Notice of an Event?
Accuracy of the Trigger Levels
A detailed analysis indicated that the trigger levels
were successfully initiated before cavities occurred
about 80% of the time in the majority of the back-
analyses carried out.
The majority of the 20% where cavity detection in
advance was not successful were false positives
At Broadmeadow mine for LW9 the accuracy of the
trigger levels was about 80% for the first third of
the longwall and then apparently dropped to about
40% (this did not occur at any of the other mines)
TWAP for LW 9 Broadmeadow
October 25, 2016 12
Pre-Yield Loading Rate for LW 9
October 25, 2016 13
Convergence for LW 9
October 25, 2016 14
Average Closure Rate for LW 9
October 25, 2016 15
Cumulative Convergence for LW 9
October 25, 2016 16
Broadmeadow LW9
An analysis indicated that in the majority of
the cases where trigger levels were initiated
the tip height increased
It was hypothesised that cavities did occur
in the majority of the occasions where the
trigger levels were initiated but that
adequate set pressures were achieved by
the operators
Comparing with LVA TARPSBroadmeadow LW9 August 2014
October 25, 2016 19
Cumulative Success in Detecting Cavities for
LW 9
TWAP and Initial Loading Rate Maps for West
Wallsend Mine
October 25, 2016 20
West Wallsend Pre-yield Loading
Rate
Further Work
Confirm cavity detection using tip height is
valid
Improve cycle delineation within LVA
including sub shear TARP monitoring
Clearer delineation of the geological causes
of high level periodic weighting
Controls other than set pressure