A FACT: 100% of longwall roof falls are caused by people.
A probability: 90% of roof falls are avoidable.
A hypothesis: the powered roof support system on a longwall is probably the most physically abused, grossly neglected and totally misunderstood integration of leading edge technology that exists today, and you can’t buy one for less than $US10 million.
Another fact: longwall roof falls cost the industry millions of dollars in lost revenue and contracts every year.
An amazing fact: an 800-tonne longwall roof support can support the weight of two fully loaded Boeing 747400 Jumbo Jets. Typically we use 140 supports on a longwall — which represents an awful lot of 747s!
Do I have your attention?
If so, you probably fit into one of the following job categories: longwall crews, longwall supervisors, longwall managers, longwall engineers, longwall owners, longwall manufacturers, geologists. This article has really been written for you.
How much do you love your longwall?
How about taking five minutes of your valuable time to answer the following questionnaire. Answer truthfully, add up your score, and then read about what it means.
The questions:
1.Does your longwall suffer broken and friable roof conditions?
* a) Never.
* b) Occasionally.
* c) Frequently.
* d) Only on the other shift.
2. Has your longwall suffered a major roof fall (ie, interruption to production) in the past six months?
* a) No.
* b) Yes.
* c) Yes, but it was geological.
* d) Yes, but it was caused by the other shift.
3. Does your longwall have “spalling coal” with large lumps falling onto the AFC?
* a) No.
* b) Yes, occasionally.
* c) Yes, frequently.
* d) Only when the supports are left back over the weekend.
4. Have your longwall operators and staff been trained on longwall strata control?
* a) Yes, training is continuous.
* b) Yes, but only the original crews when the longwall was first installed.
* c) Not specific training.
* d) Little or no training.
5. Does your longwall have hydraulic leaks and bypasses on support cylinders, valves, hoses, etc?
* a) None at all.
* b) Yes, but they are reported and fixed within days.
* c) Yes, a constant problem. Some are left for longwall moves.
* d) Yes, but there is a good supply of emulsion to make up.
6. On the support leg cylinders, are all the minor stages extended BEFORE the major stages?
* a) Yes, on all legs.
* b) Yes, on at least 50% of legs.
* c) They all vary, don’t know.
* d) Why? Does it matter?
7. What is the typical system pressure from the hydraulic pumps to the longwall?
* a) Over 320 bar.
* b) Over 300 bar.
* c) Less than 300 bar.
* d) Don’t know.
8. Is guaranteed set or positive set installed on your longwall? Is it turned on?
* a) Yes and always on.
* b) Yes, some are on, others are off.
* c) Yes but it’s turned off.
* d) No, no system.
9. What is the distance from the canopy tip of the supports to the coal face, before the shearer has cut?
* a) 300-500mm.
* b) 500-1000mm.
* c) 1000-1500mm.
* d) More than 2000mm.
10. Are support pressures monitored on the supports?
* a) Yes, underground and on the surface with alarms.
* b) Yes, underground and surface but no alarms.
* c) Yes, underground only.
* d) No.
How did I do?
For every answer marked (a), you score five points; (b) three points; (c) two points; and (d) nil.
If you ended up with a score of 48 or more, you either have a brand new longwall or you haven’t been underground for a while.
If you finished with a score of 38-47, your roof support system is operating reasonably efficiently, at about 75% of its capacity. There is still room for significant improvement through fine tuning and education of personnel.
If you had a score of 30-37, it is very likely that your longwall performance can be significantly improved with limited capital expenditure, and with a training program. Your longwall system is operating at less than 70% of its capacity to hold up the roof.
A score of 20-29 signifies that you are operating the support system at less than 50% of its capacity. In other words, your longwall support system is grossly inadequate. Moderate capital expenditure and training of personnel required. The situation is probably recoverable.
If you could manage only below 20, then you have a serious problem with your relationship (with the longwall!).
The modern longwall roof support is at the forefront of hydraulic, structural and electronic technology. There is no other industry that subjects equipment to the massive forces that are exerted on a longwall.
Roof problems continue to be a problem, however, and in most cases they can be avoided. At the start of this article I said that 100% of roof falls are man made and there is always a reason for a roof fall. Roof falls are not “acts of God” and they are certainly not caused by geology. Geology is just one of the factors to be taken into account when we plan to work a particular seam.
Put very simply, “bad roof” exists because we fail to adequately support the overlying strata. This can be for many reasons, ranging from low system pressure to poor operating techniques to bypassing leg circuits. But, the strata was not bad before mining commenced. Bad roof is generally mining induced.
Spalling coal occurs because the roof converges. The roof converges because the support allows it to do so .The best support we have on the coal face is the coal itself, (it’s been there a long time), and if it spalls away, the powered roof support has more to hold up, resulting in more convergence, and so on.
As with all complex systems, it is often a combination of smaller problems that cumulatively cause a much bigger problem.
* This article first appeared in the September 2000 edition of Australia's Longwalls.
