The dyke, varying in thickness from 1-3m and measuring up to 300mpa in places, had been encountered in the previous block but because of the mine layout, there was no option except to go round it. The current plan is to remove the dyke with explosives and then backfill the void prior to longwall mining. Backfilling is necessary because of faulting associated with the dyke.

As the longwall approaches failure of the excavated roadway is a concern because of the broken nature of the roof and the risk of the fault plane not allowing load transfer across the opening. Because of this risk it was decided to totally backfill the opening and monitor roof behavior as the longwall mines through it and then use that data to determine the level of support required for subsequent excavations.

"With the earlier block we couldn't take the outbye side of the block because that would have impacted on the stability of the adjacent return roadways," said mine manager Bob Gordon.

In current longwall block 404, the longwall is currently 1.0km from the dyke which is being removed ahead of the longwall with P1 explosives. An Eimco is then cleaning up the opening which measures 3.5x3m. The dyke removal is progressing at an average of 4m per day, 2 shifts per day. It is being extracted from the tailgate side and is currently 75m across the block and the plan is to extract 250m of the dyke, which is the width of the longwall. Extraction is scheduled to be completed by the end of May. Two months have then been allocated for the backfilling process with the longwall scheduled to begin passing through the area in early September.

Gordon said the best material to fill the void was still being assessed and would probably consist of a combination of flyash or coal waste with cementatious mix. Roughly 2,500-3000 cubic meters of fill will be pumped into the void from a surface borehole. Depth of cover is 500m.

Because of Appin's gas content and the unavailability of a P5 explosive, a risk assessment was carried out prior to the plan being implemented and a training package was developed with input from Orica. The plan involved the use of P1 explosive to excavate the dyke only. The in-seam drainage holes in the dyke zone, now two years old, are filled with water within 20m of the shotfiring to make sure there is no methane present.

If the method proves successful it will be used in the next four blocks where the dyke is present. As mining moves through the filled zone it will be monitored with pressure cells to ascertain the load transfer. Once this is known a more informed decision will be made about how much support to introduce into the void left by the removed dyke in future blocks. Gordon said it may be possible to use only cuttable cans and bolts for support in the next dyke void zones.

A modified Jumbo attached to an Eimco is being used to drill the holes for shotfiring. An outcome from the risk assessment was that P1 was not to be used in coal at Appin. To enable efficient shotfiring to be carried out the contractors, Allied, developed a trencher which also attaches to an Eimco so that a free face in the coal is created in the face next to the dyke. The dyke is then drilled and fired.