Highwall Mining Backfill

This report studies the potential of using backfill to increase the coal recovery. It is developed from the premis that placed fill will provide pillars with additional confirming pressure thereby increasing both their load leaving capacity and stability.

Backfilling CHM entries using colliery rejects is demonstrated in this study as being both technically and economically viable. However the target benefit of backfilling, defined as allowing 70% plan area recovery of 4m to 8m thick coal seams cannot be met for 4m single pass CHM mining. This is due to adverse mechanisms of slender pillar deformation being determined for pillars with a slenderness ratio (w/h) of less than 0.50. This constraint would limit recoveries (plan area) to 64%. In many seams where the seam strength (UCS basis) is less than 7MPa to 8MPa, this level of recovery exceeds the 52% to 58% recovery that might be achieved without backfill.

For two-pass highwall mining of seams greater than 4m in thickness, it was found that by using a 3% cement stabilised paste backfilled that cures to 1MPa crushing strength, significant plan area recoveries of up to 64% are possible by staged backfilling of the lower pass. The backfilled stabilised lower pass pillars isolate support requirements to the upper pass pillars, hence 8m thick seams can be mined with 2m thick pillars.

Without backfill, a 8m thick seam could only be highwall mined for a 30% plan area recovery.

The findings from this research study are considered to demonstrate, with some certainty, that field trials to confirm backfilling specification and pillar support performance is warranted. Given the ubiquitous properties of washery rejects, most sites would qualify for field trials. A major opportunity to realise the benefits of backfilling is suggested for Wambo Colliery, where high slimes contents tailings have proved problematical and costly to dispose. Such materials could form high density fluidised slurries or high solids content pastes for use as backfill material, capable of generating good pillar confinement.

Sufficient "seed" information on reject preparation and slurry transport and discharge into CHM entries is presented to work up a more detailed proposal for field trials.