This review gives an introduction to the topic of underground mine pillars. It includes information available on the web such as underground mining courses, publications related to pillars and underground mining, consultants specializing in pillar design and construction, and discusses ways to improve productivity in underground coal mines.
Mine pillars are so much a part of an underground mine that most mines would not exist without them. There is an enormous quantity of e-information on mine pillars. So this is but a brief introduction to the topic, sources of information, and people who can help you. If I leave out your company or omit your expertise, please let me know
The InfoMine Dictionary
has twenty-four definitions related to mine pillars. These are my favourites:
- Pillar: A column of coal or ore left to support the overlying strata or hanging wall in a mine, generally resulting in a "room and pillar" array. Pillars are normally left permanently to support the surface or to keep old workings water tight. Coal pillars, such as those in pillar-and-stall mining, are extracted at a later period.
- Pillar boss: In bituminous coal mining, a person who supervises the work of robbers in removing pillars of coal that were left to support the roof of working places during mining operations.
- Pillar robber: (1) In anthracite and bituminous coal mining, one who breaks down and rips out with a pick, pillars of coal left to support the roof in rooms when the usual mining was being done.
- Pillar man: A person who builds stone packs in mine workings.
MINE PILLAR TYPES AND DESIGN
Mine Pillars can be classified under two categories:
1. Room & Pillar Mining Method:
The size of the mine pillars (width to height ratio, W:H) serves an important criteria behind the success of a room and pillar mining method. There is always a challenge to select the optimum size as smaller W:H may cause pillars to collapse while a very large W:H will leave behind significant quantities of profitable mineral. This ratio is usually lower for hard rock metal mines and higher for coal and potash mines. Because these pillars are left in the mine, the usual recovery is 70-75% in hard rock mines, 60% in coal mines and 50% in potash mines. These recovery rates can be improved if the pillars are extracted also in the retreat mining method. Here are some publications that can widen your knowledge on this subject:
- Stability Analyses of Underground Room and Pillar Stopes by Means of Convergence
- Monitoring and analysis of room-and-pillar mining with continuous miner in Estonian oil shale mines
- Methods of mining, University of Ketucky
- Design methods to control violent pillar failures in room and pillar mines, NIOSH, Karl & Mark
Google Images shows many images of room and pillar mining methods.
2. Other Metal Mining Method:
Three types of mine pillars are known to exist here - Crown Pillar, Rib Pillar and Sill Pillar. Here are their definitions I found in the InfoMine Dictionary of Mining, Mineral and Related Terms
- Crown pillar: An ore pillar at the top of an open stope left for wall support and protection from wall sloughing above.
- Rib Pillar: A pillar whose length is large compared with its width.
You can also define them in this way: Crown pillars are pillars left to support the upper haulage level; Sill pillars are pillars left to support the lower haulage level; Rib pillars are vertical pillars left between stopes at the same level. Although these pillars are very common in any mine stoping method, I could not find much descriptive information in my limited search on the Internet. Below are a few publications and links which may help you in understanding these concepts:
EduMine deals with pillars in underground mines in these courses: (1) Design for Underground Metal Mines 2 - Design Guidelines
; (2) Practical Rock Engineering 1 - Introduction
; (3) Underground Mining Methods and Equipment
; and (4) Underground Mine Backfill 1 - Introduction
Here is a summary of pillars from the course on Practical Rock Engineering.
- Typical Problems: Progressive spalling and slabbing of the rock mass leading to eventual pillar collapse or rockbursting.
- Critical Parameters: (a) Strength of the rock mass forming the pillars (b) Presence of unfavorably oriented structural features. (c) Pillar geometry, particularly width to height ratio. (d) Overall mine geometry including extraction ratio
- Analysis Methods: For horizontally bedded deposits, pillar strength from empirical relationships based upon width to height ratios and average pillar stress based on tributary area calculations are compared to give a factor of safety. For more complex mining geometry, numerical analyses including progressive pillar failure may be required.
- Acceptability Criteria: Factor of safety for simple pillar layouts in horizontally bedded deposits should exceed 1.6 for "permanent" pillars. In cases where progressive failure of complex pillar layouts is modeled, individual pillar failures can be tolerated provided that they do not initiate "domino" failure of adjacent pillars.
InfoMine Library has over 60 technical publications related to mine pillars.
Amazon.com list over 300 publications for sale under the keywords "mine pillar." The vagaries of search engines include this book in the list: The church, the pillar and ground of the truth from 1838 for $18. The same author published A Presbyterian clergyman looking for the church in 1849. Now there's history.
www.books.google.com lists 1,361 books that, in part, deal with mine pillars. Only the first 335 could be easily accessed, and they all seem relevant.
Some publications are also mentioned in Mine Pillar Types and Design section above.
CONSULTANTS & SPECIALISTS
Most specialists in rock mechanics will be able to design a pillar systems for your underground mine. Most do not list this skill as one of their premier services. These are the some of the consulting firms which have mentioned room and pillar mining as one of their areas of expertise:
Itasca International - Their consulting services include Room and Pillar mining.
Snowden Group - They claim to have expertise in stope and pillar mining system and in coal mines.
Mining One Pty Ltd. - An Australian mining firm with specialization in Underground Mine Planning & Design, Underground Pillar Design & Roof Support.
Rokdok is another firm providing consulting services in pillar mining methods. They offer a range of services to the coal and soft rock industries, and their experience includes design of pillars and pillar extraction sequencing.
EFFECTS ON PRODUCTIVITY
Here are ways to improve productivity in underground coal mines. The suggestions are well documented in a seminal paper Roof bolting and mining: are your cycles in tune?
by E.B. Kroeger and M. McGolden published in the January 2007 issue of Mining Engineering
. One needs to be a member to access the paper, but you can access for nothing this paper by the same authors on pretty much the same topic: Increasing underground coal mine productivity through a training program
Here is their conclusion:
There are many variables that have an affect on the productivity from an underground room-and-pillar coal mine. One of the goals of this paper was to illustrate the need for mines to model their production systems, including their roof-bolting cycles, an determine if further improvements can be made. Another goal was to help mining operations quantify the benefits of specific changes so they can make more informed decisions and focus their attention on the most critical issues. These critical issues can then be incorporated into a miner-productivity-training program to increase the productivity from the mine.
The work by the authors, their colleagues, and the related sponsor goes some way to achieving the goal of safer, more cost-effective mining.
The paper reports on work funded by the Illinois Clean Coal Institute. Here is another related paper from this site that is well worth reading: Development and Demonstration of alternative room-and-pillar mining geometry in Illinois by Chugh et al. Another related paper is Best practices and bolting machine innovations for roof screening by Robertson et al.
For those of us who don't have easy access to a library, the following keywords are usually successful in pulling up a multitude of results of one kind or another: room and pillar, roof support, rock bolting
On the topic of roof bolts in coal mines, some good resources are:
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