Newstan Colliery, NSW, Australia

A roof fall occurred on tailgate 22 of the Newstan Colliery in New South Wales. The fall occurred adjacent to downthrown fault zone trending diagonally across the longwall block. The zone had been identified from Awaba Colliery bord and pillar workings in the Great Northern seam. The immediate roof of the seam included the Nobby’s Tuff, which is prone to water softening and is overlain by several meters of weak, interbedded coal seams. Water was visible entering the roadway from the strata in the cavity above the seam.

The fall material consisted mainly of fine broken coal and had completely blocked the 3.4m high by 5.2m wide heading.

The mine decided to recover the roadway by consolidating the fallen material and establishing a beam in the roof cavity. Having successfully completed many similar applications in the past, Minova Australia was contacted to provide a solution that would allow the roadway to be recovered and the fall to be successfully negotiated. The mine estimated the volume required for filling by considering the estimated volume of the material mucked out and visual estimates of the dimensions of the cavity and roadway.

To recover the roadway, it was decided to build a partition in the roadway using an ash block stopping located about 2m back from the fall lip and fill the roadway to the roof line using Durafoam, a lightweight, high yield foaming grout. This was done to enable the establishment of a 2.5m thick, high strength beam at roof level using Minova’s CMT grout selected for its high volume yield and rapid strength gain.

Durafoam is a fast setting grout and can be placed at a rate of up to 24 cubic meters per hour using a proprietary placing pump. The grout can achieve a compressive strength of 0.25 megapascals after 24 hours and 0.4MPa in seven days.

The CMT grout can be tailored by Minova Australia to achieve a range of strengths. In this case the mine requested a target strength of 5MPa at seven days. CMT grout also gels in a relatively short time of between nine and 11 minutes depending on ambient temperatures. A previous application at Newstan Colliery demonstrated that CMT grout could be used to establish a suitable medium for anchoring resin bolts.

Once the block seal had been completed, the Durafoam was pumped into the void from the inbye end of the fall via four 40mm diameter polylines. Two of the lines were placed on each rib at a height of approximately 2.5m above the roof line and two were placed on each rib at the roadway roof line. The fast-gelling nature of the Durafoam ensured that there was no noticeable hydrostatic load placed on the stopping.

Once the Durafoam had been placed, six breather holes, spread evenly across the heading, were drilled approximately 300mm from roof level at an angle of 60 degrees from horizontal until the cavity was intercepted. CMT grout was then pumped into the cavity via a previously positioned polyline until grout bled through one of the drill holes. These drill holes were then left in place to provide longer term drainage of water from the cavity. The complete job required 18 metric tonnes of Durafoam and 21 tonnes of CMT grout.

Initially it was thought that a cutter head attachment on an Eimco load-haul-dump would be required to cut through the Durafoam but it was found that most of the excavation could be done with the LHD bucket.

Following a risk assessment, the mine developed a support regime that included the installation at maximum 1m intervals of parallel flanged channels bolted to the roof with 6m long flexibolts angled over the solid pillar to act as a sling for the grout beam. This was a necessary precaution as the condition and stability of the strata in contact with the grout was unknown. The maximum cut out distance was 1.5m.

The ABM25 bolter/miner was relocated in the tailgate roadway and commenced excavating the face. The fall recovery was completed safely and successfully with no problems experienced with roadway support or stability of the CMT grout beam in the fall area.

Click here to read Part 1 on Minova’s work at the Crown 3 mine in Illinois.

Published in the December 2006 American Longwall Magazine