Published in the June 2011 Australian Longwall Magazine
Keeping a productive longwall in good, safe working order depends on many things:
- Good standards
- Good well trained operators
- Good well trained management
- Good effective maintenance
- Good discipline
- Good effective communications
- Safe working procedures
- High availability coal clearance system
- Fitness for purpose of the equipment for the mining conditions
Get all these things right and you will have a good safe productive longwall. Drop one off, and you will probably get away with it. Drop two or three and you will probably experience regular problems. Drop four or five off and I guarantee you will go off the rails before very long, and keep going off the rails.
You will have noticed in my previous articles that I
am passionate about “prevention is better than cure” and that most longwall falls are man made.
It is a fact of life however, that (to quote a famous politician) s**t happens, and when it does it happens BIG time at huge cost to the bottom line and even worse, roof falls are very hazardous. Make no mistake, even a small piece of mudstone in free fall is potentially lethal and you may not even hear it coming.
Some of you reading this article may recall that way back in the late 80s and early 90s, I was occasionally regarded as an “expert” in fixing up big holes on longwall faces. This was before the advent of modern cavity filling products and super glue. In fact, roof cavities were an every day occurrence back in the land of the “Pomms” where I learnt my trade. In reality, I was never an “expert” but I had seen an awful lot of roof falls and I had some very good mentors at “fixing” them up in my early days. It could also be argued (justifiably so) that we “Pomms” were good at causing the roof falls too!
In the 1970s, back in “Pommieland”, longwall cavities were filled with wooden chock timber, split bars and even straw bails. The brave coal miners had to stand on top of the canopies in the middle of huge cavities and build chock timber to the roof!
Fortunately times have changed and such methods are no longer allowed or required.
So, what happens when s**t happens? Did you know that in the majority of cases the following apply?
1. Roof falls can get bigger through lack of human intervention
2. Roof falls can get bigger through (some) human intervention
3. You can reduce the risk of the roof fall getting bigger following the “Golden Rules”
4. You can get through many “broken roof areas” without cavity fillers and super glue, by following the Golden Rules
5. Recovery from longwall roof falls is not rocket science. It is common sense. There is no magic formula, but the “Golden Rules” provide essential guidelines.
Remember, it is not the cavity itself that stops the longwall. It is all the rock that falls down to form the cavity that is the problem!
The Golden Rules
1. Keep the face straight: even with a cavity on the face, keep it straight. Inducing bends will affect the break line of the goaf and can result in extending the cavity. Bends are very difficult to manage, so just keep it straight!
2. Do NOT chase the roof: keep the support canopies at the height where the normal roof horizon should be.
3. Maintain all canopies in the correct attitude: maintain a skin to skin contact to prevent ingress of material from above and to prevent the canopies “interlocking”
4. “Double chock” the supports: wherever possible in a cavity, double chock and jam the canopy tips into the coalface (at the correct horizon). This blocks off the cavity and prevents more material falling onto the AFC. It also increases support density when the roof is being caught, or where heavy face spall has occurred.
5. Turn OFF guaranteed set systems in the cavity area: this is to prevent the supports from jack-knifing and losing tip contact.
6. Turn ON guaranteed set system: as soon as possible, once the roof is caught, have guaranteed set or positive set restored.
7. Use base lift: invariably in cavities there is a lot of debris in front of the pontoons. This can build up and restrict the support advance creating bends on the face.
8. Establish a lip: keep cutting at the correct horizon to establish a “lip”. Use the canopy tip up feature to get under the lip and provide maximum tip set load even when the legs cannot be pressurised. The tip up feature is a function of the large stabilising cylinder (or compensating ram) between the goaf shield and the canopy.
9. BI-DI Cutting: Whenever possible in large cavities, BI-DI. This allows you to DOUBLE CHOCK very quickly and close off the cavity.
10. Keep the belts running: Do not stop the conveyor system. The longwall should be given top priority. DO NOT STOP THE BELTS.
11. Use TARPS: there must be triggers for various actions in a cavity or bad roof situation. Make sure EVERYBODY knows them.
12. Glue the coal seam: The coal seam is the best roof support you have. Glue the coal seam (and the roof) BEFORE you get a cavity using the latest resin injection technology.
13. Establish a regime of glue, cut “x” shears, glue again until the roof is re-established.
14. Horizon Control: Pay special attention to roof and floor horizon. Where supports are double chocked, the lead drum must be dropped below the canopies.
15. Ground Control Specialists: There are a small number of companies in Australia that can provide you with the products and the people to glue the strata and fill the void. They are VERY good at what they do. However, reaction time is critical. If you think you might need them, you probably will, so get them on standby.
16. Don’t stop: once you start cutting, keep going. Do not stop for routine maintenance, crib, shift changeovers or public holidays.
17. Do stop: if you run out of the “glued zone” and the roof is still falling in.
In summary: Prevention is always better than cure, but when it does hit the fan, follow the simple golden rules to minimise your risk. This is a far more economical option than waiting for “it” to happen.
