The mine first trialled a predriven backfilled roadway with longwall 407, after using a cement fly ash mixture of 3.0-3.5 megapascal strength as backfill while extracting a dyke in the geologically tough Area 4.
The standing support in the recovery roadway consisted of the same fly ash cement mixture with some tin cans set prior to the backfill to guarantee contact with the roof, along with other support such as 8m, 63-tonne string megabolts and 10m multi-stream fibreglass dowels rated at 44t.
But difficulties with mining out the tin cans, plus the costs associated with this setup led to some more experimentation for the next longwall panel.
On a 50m section of Longwall 301 Appin trialled round concrete cylinders, using off-the-shelf concrete rated at 30mPa.
This support comprised of one block on top of another with timber in between the two blocks and on the floor and roof.
The timber was to act as a spring or buffer because concrete is very brittle.
With help from the University of Wollongong, the mine figured out what type of timber could best handle compression, and the trial worked with the shearer cutting out the concrete pillars to safely lead into the predriven recovery roadway.
While aiming to repeat exactly the same approach with Longwall 302, the concrete wasn’t quite the same and set at a higher strength.
Appin technical services manager Brad Elvy said they got a “summer mix” instead of a “winter mix”.
The mine worked with the concrete providers to
develop the best concrete mix for the job. The result was a mix for the next predriven recovery roadway Longwall 702 ranging from 29-32mPa.
Given that the predriven recovery roadway was only 5.2m wide for this panel, the mine placed two rows of pillars at 2m intervals and also opted to use mega bolts, the dowels on one side, and to remesh the final side with 2.4m roof bolts.
Before Appin started using predriven recovery roadways, Elvy said the bolt-up period from the time the mesh was installed to breaking up the AFC was never less than 24 days and was often up in the high 20s and even up to 30 days.
Elvy said if the predriven recovery roadway was designed correctly the longwall should be able to link up with the roadway without putting a bolt in the roof.
This would not only save time but improve safety by removing workers from the danger of having to support the face.
Elvy added the support standard in a recovery roadway was also much greater when done in a controlled environment.
“You find the support you are getting is done to a much better standard and you can tailor it to the conditions of your seam – so once you have driven the roadway and you can map the roadway, you can put all the support in you need to make sure you are going to get that longwall safely home.”
He said the conventional approach could also be a struggle, especially under conditions where there might be faults or dykes on the face.
“Obviously if you are managing your strata better your chock recovery is better, so then you start to find not only the benefit of reduced bolt-up timeframes but the benefit of quicker chock recovery and [it is also] far safer and more productive.”
Longwalls 407, 301 and 302 had about 500m depth of cover while Longwall 702 was at 520m.