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Underground Mine Pillars 

 
Authors: Jack Caldwell

Summary

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.

INTRODUCTION

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.

DEFINITIONS

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.

EDUCATION

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.

PUBLICATIONS

The 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.

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. Two who do are:


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.


KEYWORDS

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:


SME PAPERS

Some papers from the SME 2006 conference in St Louis:

An Evaluation of the Strength of Slender Pillars. G. Esterhuizen of NIOSH describes the performance of slender pillars in underground mine workings based on observations at metal and limestone mines, lab testing, and theoretical analyses. His conclusions are pretty obvious and well in line with what I learnt in structural engineering thirty years ago: slender pillars are more susceptible to the vagaries of nature than fat columns. His contribution is to identify the factors, explain them, and to the extent possible quantify them. I wonder if this is a case for the use of strapping and horizontal reinforcement using the HJ3 methods (see Google for more on HJ3).

Enhancement to the LaModel Stress Analysis Program. R. Hardy and K. Healsey of West Virginia University describe the computer code, LaModel, and recent enhancements. The code is a boundary-element model used to calculate stresses and displacements in thin-bedded deposits such as coal, salt, and potash mines. This quote summarizes the issues: "Many LaModel users are ultimately interested in the stability of the pillar design in their model. Prior to release 2.1, the pillar stability had to be meticulously calculated from the stresses and displacements included in the output file. With the new release a pillar safety factor calculation has been implemented and is included as one of the standard outputs of the program."

Extreme Multiple Seam Mining in the Appalachian Coalfield. C. Mark of NIOSH describes some of the over 300 case histories of coal mines that are being developed above or below old workings. He lists the factors affecting interaction of the old and the new workings and concludes "Existing multiple seam guidelines should be revised."

A Critical Analysis of Overcoring Stress Measurement from US Coal Mines. Gadde, Peng, and Rusnak describe errors induced by the proximity of the mine working on the in situ stresses estimated by overcoring methods. They show that three-dimensional stress analyses may be used to correct for the errors induced by the stress redistribution/change resulting from the proximity of the mine workings to the coring location. For those who collect acronyms, here is theirs:
Numerical In Situ Stress Correction Procedure (NISCP) -only one S.

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