Common Operational Problems

Gate Roads Misaligned

If the main and tailgates are driven off line far enough it is possible the longwall block is too wide or the space between the gate road pillar ribs is too narrow for the face equipment to fit. The only solution if this occurs is to strip coal off the rib(s) in question to allow the wall to fit and install additional support to cater for the extra width. It is best for such alignment problems to be identified early, preferably while the development machinery is still at that location so it can be used to correct them. On occasions a relatively small misalignment (which the face equipment should be able to handle) becomes a problem if the face creep is not controlled when the face reaches that point. Road widening can then only be carried out by jack-picking, shotfiring or removal of maingate equipment to allow machine access.

Face Misaligned

This can be a case of the face not being straight or of too great a difference between main gate and tail gate positions (face not at right angles to the gate roads). In extreme cases this can result in the same effect as misalignment of gate roads as described in the cutting process section and the face equipment will be too short to reach both edges of the block. Too great a curve along the face can result in there being insufficient flexibility between the pans and they lock together when being pushed over. Straightening curves on the face will mean some sections of the face do not advance as rapidly as others and it is possible for roof control to be problematic at such places.
In all cases it is best to identify the misalignment before serious problems are caused and carry out straightening cuts or fly cuts as appropriate. If the stage of the face being too short has been reached the ribs will have to be stripped at one or both ends of the face to allow correction cuts to be made. If pans are locked, it is usually possible to free them with some "jiggling" of pans and chocks. Loss of roof control will have to be rectified by means described below.

Loss of Control of Roof Along Face

This can result from:

• Face misalignment
  
• Inadequate support capacity
  
• Inadequate hydraulic pressure
  
• Failure to properly set supports (i.e. low initial setting pressures)
  
• Excessive delays in face advance
  
• Localised area of weak roof
  

The loss of roof control can vary from broken roof over the supports, through minor loss of roof material ahead of the supports, up to major cavities ahead of the supports. More often than not, this is a progressive deterioration and the sooner action is taken to correct the issue, the better. Measures that can be taken or issues to be faced include:

• Correction of face misalignment which has been dealt with above
  
• Inadequate support capacity - an issue which cannot be corrected and a new set of supports is called for. A decision would have to be taken to persevere with bad roof conditions for the rest of the block or to stop and attempt to recover the equipment. Inadequate set pressures can lead to roof failures which replicate those due to inadequate support capacity, hence routine monitoring of set pressures is required to prevent such problems and/or eliminate this cause.
  
• Inadequate hydraulic pressure may result from a problem with pumps, but is more often a result of poor support maintenance and an accumulation of leaks (internal and external) preventing correct operation. The ability of chocks to operate up to the design yield load should be checked regularly.
  
• Whilst excessive delays may have already occurred for a variety of factors, it is essential that they be evaluated to identify root causes and develop counter-measures to eliminate potential re-occurrences and/or plans for more effective introduction of control measures in the event of a re-occurrence.
  
• Localised areas of weak roof cannot be avoided, but some roof reinforcement may be called for (local bolting, grout injection, etc).
  

Once a degree of control is lost, the steps in recovery are usually:

• Avoid all unnecessary delays and try and maintain continuous production (24 hour, 7 day operations if necessary) to keep the face advancing as steadily as possible.
  
• When moving supports try and ensure they are advanced while still in contact with the roof (pressure relieved but not lowered) to retain broken roof material in place as much as possible
  
• Advance supports as close as possible behind the cutting machine and keep the unsupported roof area to a minimum
  
• Place timber in cavities above the supports if possible to maintain as uniform a roof loading as possible
  
• If it is considered the area of broken/fallen roof is unlikely to be caught, then stop the face and carry our secondary support. This again may vary from localized bolting to grouting, including strata injection ahead of the face.
  
• If a major cavity has formed it may be necessary to stop for a longer period and fill the cavity with some type of grout or light-weight cement to prevent further loss of strata and to give an artificial roof for supports to act against.
  
• In extreme cases where supports have become ironbound (with or without a cavity ahead of them), it will be necessary to jack-pick or shotfire the roof above to release them.

Loss of Roof Control in the Gate Roads

This would normally be a result of inadequate roadway support design, a localized area of weak roof or high stress or excessive delays in face advance. Again the result can vary from nuisance broken roof up to major falls or the roof lowering onto the longwall maingate equipment. In the tailgate it is frequently possible to continue operating the face even past a fall as only the tailgate drive extends into the gate road (possibly supports as well if the face has crept towards the tailgate). A shearer can clean-up in front of this drive to allow it to move forward, provided the fall has stabilized.
The maingate however is full of equipment and is a major work area for personnel, so any roof deterioration has to be made safe and cleaned-up in order to continue operations.
If roof problems become apparent in the tailgate it is often quite easy to install additional support to prevent a major failure; often passive support such as timber chocks or cans (possibly cuttable cans which have a resin or fibre-glass tube instead of steel) are quickest and easiest. If the roof closes to the extent that chocks and/or the tailgate drive cannot lower enough to pass through it may be possible to cut the roof with the shearer to create the height. If the roof has failed totally and a fall has occurred, it may be possible to continue without securing the fall, as noted above.
Whatever options are considered for the tailgate, the points to be borne in mind are:

• It must be possible to continue operating without exposing personnel to unsafe roof; is it possible to continue without access to the area in question?
  
• Is ventilation still adequate or is the tailgate airway too restricted?
  
• Are there alternative egress considerations which require the tailgate to be maintained for safe travel?
  
• Is there roof in place for the longwall supports to bear against?
  

For maingate roof problems, the considerations are much the same, but in this case:

• Personnel access will always be required so the roof must be made safe and there are always chocks and equipment in the roadway; by-passing it is not an option
  
• Any fall would have been on the conveyor or maingate equipment, so would have to be recovered. Any cavity will need to have some arrangement made to provide a "roof" for the longwall supports to bear against, which can be done by filling with lightweight concrete, erecting a steel frame, placing timber over the chocks when they reach the cavity, etc
  
• Lowering roof usually affects equipment ahead of the face (in particular the crusher) so using the shearer to gain height is not an option; the roof height will have to be re-established by either brushing, jack picking or shotfiring.
  

If roof stability problems occur in roadways adjacent to main or tailgates, those roadways are likely to be required in the future. Recovery of poor roof areas or falls will most likely be necessary, but may not be for the current longwall. In such cases recovery work may be scheduled later providing there is no further deterioration.

AFC tipped forward

It has been noted that the shearer attitude depends on the AFC. If the AFC is tipped towards the face the shearer drum will tend to cut deeper into the floor and this will cause the AFC to tip even more when it is pushed forward. If this is allowed to continue it can reach a point where it becomes very difficult to recover.
This situation can result from:

• Poor control of the floor horizon by operators e.g.cutting too deep
  
• Soft floor, allowing the toe of the pans to dig in
  
• Floor heave under the rear of the pans
  

To correct the problem, the attitude of the AFC has to be corrected, usually by lifting the pans and packing underneath with some material (generally timber) and/or by jack-picking and removing floor material from under the pans. The support canopies can be used as lifting devices to lift pans but care must be exercised to ensure the correct attachments and appropriate slings or chains are used to do this safely. It can be a long, slow process.
In some cases, there are two attachment points for the push cylinders at different levels on the pans which can assist in directing the push up or down to some extent, as required.
It is equally possible, though less common, that the AFC will tilt up at the face and the longwall will tend to climb into the roof. Provided this is not a result of poor or faulty design, this is usually quite easy to correct by cutting below the pan level.