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No. 445, February 1995

Updated Analysis of Longwall Pillar Stability (ALPS)
Computer Program Incorporates New
Coal Mine Roof Rating (CMRR)

Objective

To develop a comprehensive, practical method for de-
signing longwall gate entrics that considers coal pillar size,
mine roof quality, and artificial support in an easy-to-usc
compuler program.

Background

Longwall mines now account for more than 40% of
US. underground coal production, an increase from less
than 5% 20 years ago, Improved longwall performance
requires stable gate entries that provide safe access to the
Jongwall face. Several years ago, the U.S. Burcau of
Mines (USBM) developed the Analysis of Longwall Pillar
Stability (ALPS) computer program to help size chain
pillars to better protect the gate cntrics. ALPS built upon
the experience at many mines, which found that poor
tailgate conditions oflen improved significantly when pillar
sizes were increased. In much of the U.S. coal industry,
‘ALPS has become the standard technique for analyzing
longwall pillar design problems.

However, coal pillar design is not the only factor that
impacts longwall gate entry performance, For example, a
mine with strong roof rack will have better conditions (all
other things being equal) than one with a weak roof, The
entry width and the amount of artificial support can also
be important. The need was evident for a gate entry
design method that extends ALPS to account for all of
these factors.

Approach

USBM researchers conducted studies at 44 longwall
mines representing all of the major US. coalficlds

(figure 1). At cach mine, underground observations of
site geology, entry conditions, and artificial support were
recorded. Discussions with mine personnel provided
information on past experience with different gate entry
designs used at each mine. Ultimately, 69 U.S. longwall
case histories were identified, and the performance of cach
was characterized as either satisfactory, unsatisfactory, ot
borderline.

Each case was also described by several design vari-
ables. For example, the ALPS stability factor (SF) was
used to define coal pillar design. Other rating scales were
developed for primary support, secondary support, and
entry width. One of the keys to the success of the re-
search was the ability to define roof geology using the
USBM’s newly developed Coal Mine Roof Rating
(CMRR). The CMRR replaces traditional geologic de-
scriptions with a roof competence rating scale of 0 to 100.
It weighs the importance of rock strength, bedding plane
resistance, fracture spacing, and other factors in deter-
mining the overall rating. The CMRR is uniquely advan-
tageous in that it applies to all U.S. coalfields, so that
mine roofs can be compared even when their geologies are
quite different. Gathering the information needed to
compute the CMRR requires only simple field tests and
observations.

Statistical analyses of the entire longwall case history
database indicated that when the CMRR and ALPS are
combined, the performance of a longwall gate entry system
can be accurately predicted, Only three cases in the data-
base, or 6%, were misclassified or fell outside a narrow
borderline region (figure 2).

Based on these results, a simple equation was devel-
oped to guide the design of longwall pillars and gate

entries:

ALPS SFR = 1.76 ~ 0.014 CMRR,

This document was prepared by the Bureau of Mines. Neither the United States Government nor any person acting on bebalf of the United States
Government assumes any liability resulting from the use of the information contained in this document, or warrants that such use be free from

privately owned rights.



where ALPS SFp = ALPS SF suggested for design.

The analyses also confirmed that primary support and
gate entry width are esscntial elements in successful gate
entry design. Satisfactory designs in mines with weak roof
(i.c., CMRR<45) utilized entries no wider than 5.2 m (17
ft), whereas those with strong roof employed entries 5.8 to
6.1 m (19 to 20 ft) wide. ‘The relative importance of the
floor and secondary support could not be determined from
the data.

Results

The gate entry design methodology that resulted from
the study is expected to significantly aid longwall mine
planners. It is the first design methodology to focus on
the tailgate entry itself rather than only the chain pillars.
More importantly, it is based on the scientific interpreta-
tion of the ground control experience examined at more
than one-haif of all U.S. longwall mines. The method thus
makes the wealth of U.S. longwall experience available in
a practical form.

ALPS version 4.0, an update of the ALPS computer
program, has been propared to aid mine planacrs. It is
written in the C programming language and is an execut-
able file that runs directly from DOS on any PC, The
program is menu-driven and is extremely user-friendly and
accessible by those with little or no computer background.
An on-line help file is also included.

ALPS version 4.0 contains the same formulas for cal-
culating the ALPS SF as earlier versions, As before, it
may be used to analyze longwall coal pillar designs em-
ploying any number of entries or combination of pillar

Northern

LEGEND

Surtace end underatound

= cose hitories

Figure 1.—Location of geotechnical surveys of U.S. long-
wall mines conducted by USBM ground control researchers.

widths, The method does not apply to pure yield pillar
designs, however, such as those uscd on some western US.
longwalls.

A new feature of ALPS version 4.0 is the CMRR.
After selecting this menu item, users may input the
CMRR for their application, and the program will return
the suggested ALPS SF for that mine reof. Alternatively,
if the CMRR is not known, the user may browse an on-
line library of nearly 100 roof descriptions and CMRR
values, The library is sorted by State and coal seam, and
includes all major U.S. coalfields. The library can also
help users verify their own CMRR estimates against ob-
servations already made.

For More Information

Single copies of the ALPS version 4.0 computer pro-
gram may be obtained by sending a blank, formatted dis-
kette to Christopher Mark, U.S. Burcau of Mines, Pitts-
burgh Research Center, Cochrans Mill Rd., P.O. Box
18070, Pittsburgh, PA 15236-0070.

Additional information on different aspects of gate
entry design is contained in the publication "New
Technology for Longwall Ground Control. Proceedings:
U.S. Bureau of Mines Technology Transfer Seminar* com-
piled by Christopher Mark, Robert J. Tuchman, Richard
C. Repsher, and Catherine L. Simon (1994). This soft-
cover, 145-page proccedings volume may be ordered from:
GPO Superintendent of Documents, P.O. Box 371954,
Pittsburgh, PA 15250-7954. GPO stock number: 024-004-
02299-0. Price: $10.00 (international customers please add
25%). Phone: (202) 783-3238; fax: (202) 512-2250.

lower bound of borderline region

e
2 _.—— Discriminant equation
5 7 besigneaustion
3
i
8
8
2
re . .
Z
= oe
z q °
8 ° .

02 Os os o8 i} 12 14 16 18

ALPS STABILITY FACTOR
KEY

« Satisfactory cases
5 Unsatisfactory cases

Figure 2.—Design equation determined from U.S. longwall
case history database.