PILLAR EXTRACTION USING CONTINUOUS MINERS

Overview

Now that longwall equipment has been developed to a stage where it is able to cope with most conditions satisfactorily, longwall mining is the most widely used method of secondary extraction. However, there are still mines and occasions where longwall mining is not suitable e.g.

• Size of deposit or lease, or scale of market does not warrant high cost, high capacity longwall mining.
  
• Mine operators are unable to raise the high capital cost required to purchase a longwall.
  
• Geological structures (faults, etc) limit block sizes to below economic limits for longwall mining
  
• Mine lease has areas remaining around longwall blocks too small for longwall extraction but amenable to "remnant mining" with continuous miners.
  
• Partial extraction only is permitted because of the potential effect of full extraction on surface features.
  
• Small areas of extraction may be required for specific purposes eg stress relief to protect adjacent areas of particular importance.

In such cases, pillars may be extracted by methods using continuous miners.
There are several methods of secondary extraction employing continuous miners, all of which involve splitting the pillars or blocks formed by first workings using minimal (but adequate) short term supports and/or mining a series of short, unsupported "lifts" off long "splits" that are driven with the continuous miner. The length of the lifts are controlled so that the roof remains intact while the continuous miner is extracting the lift and no personnel are required to work under the unsupported roof.
Pillar extraction with continuous miners poses a risk of loss of roof control in the immediate working area or extracted area ("goaf") resulting in the continuous miners and even the operators being buried. The development of mobile roof supports ("breaker line supports" or "BLS's"), has largely ameliorated this risk, but the safe application of such pillar extraction methods is still very heavily dependent on the judgment and experience of individuals employed.
Some methods of pillar extraction using continuous miners are:

• Pillar splitting where roadways are driven through the pillars formed on first workings; the remnant pillars left would not normally be stable over extended areas. In some cases "pockets" are driven into the pillars with no supports rather than drive roads fully through.
  

  Some possible methiods of partial extraction

• Pillar stripping where slices are taken off the edge(s) of pillars, effectively creating extra wide roadways which also would not normally be stable over extended areas.
  

  Pillar stripping, another possible method of partial extraction

• "Split and fendering" methods where roadways ("splits") are driven, leaving a 6-10m thick strip of coal (a "fender") between that roadway and the goaf. When the split reaches the limit of the block, the miner pulls back then cuts into the fender removing most of its width (referred to as a "lift"); a small section of coal is frequently left at the end of the lift, known as a "stook". This process is repeated so the miner retreats back to the original panel roadways removing most of the coal from the fender. The process usually includes removing the first working pillars in the same way. The whole procedure is then repeated with further splits and fenders formed parallel to the previous until the whole block has been removed. The width of the block (length of the split) is typically governed by the wheeling distance of the shuttle car from the conveyor boot (shuttle car cable lengths) and ventilation practices adopted when driving the split. There are several variations to this basic system including the Wongawilli system and the Old Ben system, etc.
  
  

  A Method of "full" extraction using continuous miners

Note that although these systems appear simple a considerable degree of experience is required to operate them safely and efficiently; when it is required to use them intermittently, finding suitably experienced personnel may be problematic.