robert trueman - golder associates

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