USING THE MULTIPLE-SEAM FACTOR

The MS values in figure 4 must be used in conjunction
with ALPS to estimate the development load on the lower
mine gate road pillars. Details on the ALPS method are
contained in Bureau IC 9247 (4), which includes an ap-
pendix that gives step-by-step guidelines for using ALPS
in a single-seam case. From the stresses calculated using
ALPS, the multiple-seam stresses can be determined.
ALPS is also available as user-friendly software, which can
be obtained from the Bureau upon request.? The software
is accompanied by a user’s guide that provides instructions
for using the program and interpreting the output. A step-
by-step example of how to use the output generated from
ALPS to estimate the multiple-seam stresses and stability
factors on the lower mine gate road pillars is given below.

Problem

Mine A, the upper mine, has an average overburden of
800 ft. The mine uses a three-entry gate with 70- by 80-ft
pillars and 20-ft-wide entries. The longwall panel is 700 ft
wide. The lower mine, Mine B, will also use the same size
pillars and entries, and the gate roads will be superposi-
tioned as shown in figure 2. The mining height in both
seams is 6 ft. The interburden between the two mines is
50 ft. Determine the stress on the lower mine pillars dur-
ing development and the resulting stability factors.

Solution

Step 1: Figure 5 gives an example of the output from
the ALPS program for the upper mine once the above in-
formation is entered into the "analysis mode." Figure 6
gives the output for the lower mine. The first part of the
calculations involves using the "development loading" and
isolated loading” for the upper mine. These values are
found in figure 5 under the heading "Additional Output"
and subheading "Design Loadings On Pillar System.” In
this example, the development loading for the upper mine
is 0.234E+08 lb/ft of gate entry, and the isolated load is
0.632E+08 Ib/ft of gate entry.

Step 2: The next step is to convert these linear loads to
a force per unit area, pounds per square inch, using the
following equations:

Lec
144A,

@

og=
and

oF , ©

144Ap

3A copy of the program can be obtained by sending one double-
sided, double-density diskette to the Pittsburgh Research Center, U.S.
Bureau of Mines, Cochrans Mill Road, Pittsburgh, PA 15236. Please
reference this publication.

where Oq = average stress on pillar due to devel-
opment load, psi,

0; = average stress on pillar due to isolat-
ed load, psi,

Lg = development load, 0.234E +08 lb/ft,
L, = isolated load, 0.632E+08 Ib/ft,
C = crosscut spacing, 100 ft,

and Ag = total area of pillars, 11,200 ft.

Substituting the values into the equations, the stress for
the development load and the isolated load are 1,450 psi
and 3,290 psi, respectively.

Step 3: The development stress is then subtracted from
the isolated stress to yield the isolated abutment stress,
which is 2,470 psi.

Step 4: The proper MS must be selected from the
graphs in figure 4, In this example, refer to graph 44
for a three-entry gate road. The interburden is 50 ft and
from the curve for a 70-ft-wide pillar the factor is approxi-
mately 0.72. This factor is then multiplied by the isolated
abutment stress (2,470 psi) giving 1,780 psi, which is the
amount of stress transferred to the lower mine pillars.

Step 5: Step 2 is repeated for the lower mine, using
only the development load from figure 6, which is
0.248E+-08 lb/ft of gate entry. Converting this linear load
to a force per unit area using equation 4 gives 1,540 psi.

Step 6: The values in steps 4 and 5 are added:
1,780 psi + 1,540 psi = 3,320 psi. This value is the devel-
opment stress on the lower mine pillars and accounts for
the stress transferred from the overlying gate roads.

Step 7: This step involves calculating the pillar stability
factor to determine if the pillar can adequately support the
development stress. The unit pillar strength is divided by
the development stress calculated in step 6. Several for-
mulas are available for calculating unit pillar strength, the
least conservative being the Bieniawski-Pennsylvania State
University formula (11).

o [ose +036: i} ()

where o

unit pillar strength, psi,

0, = in situ coal strength, 900 psi (7),

<z
"

least width of pillar, in,

£

height of pillar, in.


UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT
deg degree Ib/ft pound per foot
ft foot Ib/f® pound per cubic foot
ft? square foot pet percent
in inch psi pound per square inch

DISCLAIMER OF LIABILITY

The U.S. Bureau of Mines expressly declares that there are no warranties express or implied that apply to the software
described herein, By acceptance and use of said software, which is conveyed to the user without consideration by the
Bureau of Mines, the user hereof expressly waives any and all claims for damage and/or suits for or by reason of personal
injury, or property damage, including special, consequential or other similar damages arising out of or in any way
connected with the use of the software described herein.



Surface

transferred to
lower mine on
development

Development

load
FRONT VIEW

Additional
abutment load
from lower

seam minin
Upper mine a

Mined-out
longwall panel

Interburden

Lower mine

Not to scale KEY
ECB Gate road
WH Coal

Figure 7.Abutment from upper mine transferred to lower mine gate roads at Isolated load.

35 V1
g KEY
i Single-seam case PA
aw 30, ZZ) Multipie-seam case P77] a 1.3 3
b - 2
= 52 4isz
328 ~
=% Ee
an = 20 41.9 a
a
2 =
« 4725”
=>
=
: 3.8

Development Headgate Bleeder Tailgate isolated
LOADING CONDITION

Figure 8.—Average pillar stress and stabllity factors for multiple-seam and single-seam case at each
stress condition.


transfer between workings. The combination of these two
attributes was used to estimate the magnitude of stress
that the lower mine gate road pillars would experience.

ALPS analyzes five stress conditions that gate road
pillars experience as the first and second longwall panels
are mined. Stress is calculated as the total load divided by
the load-bearing area of the pillar. These stress condi-
tions are as follows: (1) the development stress, which oc-
curs when the gate road pillars are initially mined; (2) the
headgate stress, which occurs when the first panel is di-
rectly adjacent to the target pillar; (3) the bleeder stress,
which occurs when the first panel has been mined; (4) the
tailgate stress, which occurs when the second panel is adja-
cent to the target pillar; and (5) the isolated stress, which
occurs when the second panel has been mined. Conditions
2 through 5 are each separate stresses and they must be
added to the development stress to obtain the total stress
for that condition.

‘As shown in figure 3, the stress conditions that apply
for analyzing superpositioned gate roads are 1 and 5 for
the upper mine and 1 for the lower mine. ALPS was used
to calibrate the MULSIM/NL model for these conditions.
The model was calibrated assuming a 21° abutment angle
on a 700-ft-wide panel, and using a three-entry gate road

Surface

consisting of two 60- by 80-ft pillars and 20-ft-wide entries
at 750 ft of depth. At this depth, the pillars have an ALPS
stability factor of 1.0 at isolated loading. Stability factors
ranging from 1.0 to 1.3 are recommended for sizing pillars
for tailgate protection using ALPS (7, 10). Through a
systematic procedure, MULSIM/NL input was gradually
modified until the best fit was achieved for the two stress
conditions. To illustrate, MULSIM/NL predicted 91.2 pet
of the average pillar stress calculated by ALPS at develop-
ment. For the isolated case, the MULSIM/NL model fit
was 89.1 pet of the predicted ALPS average pillar stress.
An exact match to ALPS could not be achieved by the
MULSIM/NL model without making input parameters un-
realistic. Two factors were responsible for this. First, in
most boundary element models, some stress is lost along
the boundaries of the model. Second, the model places
more stress on the adjacent longwall panel than ALPS
does because of the inherent lateral stiffness of the homo-
geneous, isotropic overburden assumed in MULSIM/NL.
However, achieving an exact match was not necessary
since, in the final analysis, model output from the multiple-
seam results was normalized to these single-seam results.

The model input parameters for overburden, inter-
burden, coal, and gob that best resembled output data

FRONT VIEW

Upper mine

Development
load

Abutment
angle

\

‘Abutment
~ load

r

Overburden

Mined-out
longwall panel

|
I Interburden
|
i

Lower mine H

Not to scale

KEY

ECR Gate road
WE Coal

Figure 3.—Stress conditions for upper and lower mine when gate roads are superimposed.


INTRODUCTION

Longwall mining in the Appalachian Coal Region is
becoming more economically advantageous as experience
in its use is gained. The high productivity of longwall min-
ing illustrates its potential to be the future of underground
coal production. The high extraction efficiency and coal
recovery of longwall mining make this system very advan-
tageous compared with room-and-pillar methods in re-
ducing interactions between multiple-seam mines.

There are two fundamental planning decisions a long-
wall operator must make when mining multiple seams:
first, the order or sequence in which the seams will be
mined and, second, whether the gate road pillars will be
offset or superimposed. A descending order of mining is
preferable, because the severe ground problems associated
with subsidence interactions should be avoided in most
cases (I-4)2 The decision to offset or superimpose gate
roads when mining multiple seams has been a topic of
much concern in longwall design. Model studies have
shown that superpositioning of gate roads produces the
most adverse stress conditions on the gate road pillars
and that in most cases offsetting pillars is preferable (5).
However, there may be instances in which superposition-
ing of gate roads is required. Offset gate roads may create
high stress concentrations on the longwall face, causing
unacceptable production delays. Geologic factors, such as
faults or sandstone channels within the coalbed, may ren-
der a block of coal unminable, thereby shifting gate roads
to a superpositioned arrangement. Superpositioning may
also be required near the perimeter of a coal property
to maximize recovery. Contingency plans to superposition
gate roads may be adopted under these special circum-
stances, but pillars must be properly designed to contend
with load transfer from the overlying gate road pillars.

The major advantage of offsetting is that the gate roads
in the lower mine can be developed under the upper mine
gob, as shown in figure 1. When offsetting is practiced,
designing pillars as in a single-seam case should be suf-
ficient, because, with offsetting, load from overlying pillars
is transferred to the longwall panel. This may or may not
be a desirable situation, depending on the magnitude of,
the resultant stress concentration on the longwall face
and the ability of the longwall supports to withstand this
additional load.

When gate roads are superpositioned, as shown in fig-
ure 2, the longwall face has the advantage of operating
beneath the upper mine gob, but gate road pillars must
then be properly designed to withstand load transfer from

"Italic numbers in parentheses refer to items in the list of references
at the end of this report.

the upper mine. The purpose of this study, conducted by
the U.S. Bureau of Mines as part of its health and safety
program, was to develop a method for estimating the mag-
nitude of load transfer on the lower mine gate road pillars
when superpositioning is practiced. This method should
assist longwall operators in formulating mining plans and
sizing pillars in the lower mine.

The use of numerical methods for predicting interac-
tive problems is receiving more attention from researchers
for application as a design and planning tool. Numerical
methods, combined with case study results and theoretical
and statistical analyses, can be used to develop optimum
mining plans for different multiple-seam conditions. The
computer programs MULSIM/NL and Analysis of Long-
wall Pillar Stability (ALPS) were used in this research to
develop multiple-seam mining plans specifically for super-
imposed longwall gate roads.

MULSIM/NL is a boundary element model, developed
by the Bureau, for calculating stresses and displacements
in tabular deposits (6). The model provides the capability
to analyze many coal mining situations and to determine
the effects of the three-dimensional stress redistributions
caused by mining in cither single or multiple seams. The
program can be used to evaluate the following: safe pillar
sizes, ground control problems caused by stress concen-
trations, the effects of different geomechanical properties
and in situ stress fields, load transfer from adjacent min-
ing operations or multiple seams, entry deformations, and
energy changes resulting from pillar development and
extraction.

ALPS (7) is a design method developed by Bureau re-
searchers that can estimate the strength of longwall pillar
systems and the loads applied to them. This method is
empirically based, and it has been verified by back-analysis
of more than 100 case studies. ALPS was used to cali-
brate the MULSIM/NL model to generate baseline data
for a single-seam case. These baseline data were then
used to estimate load transfer and safe pillar designs when
two seams are longwall mined.

Safe and productive longwall mining in multiple-seam
situations requires that gate roads be properly designed to
withstand the loads applied to them. The development of
design methods based on numerical models has consider-
able potential in helping operators find solutions to com-
plex multiple-seam interactive problems. The application
of modeling methods coupled with sound engineering
judgment and experience will improve coal conservation,
resource recovery, and the health and safety of miners.


ANALYSIS AND DESIGN CONSIDERATIONS FOR SUPERIMPOSED
LONGWALL GATE ROADS

By Gregory J. Chekan! and Jeffrey M. Listak’

ABSTRACT

The U.S. Bureau of Mines is investigating longwall panel layouts to maximize coal recovery and
minimize interactive problems in multiple-seam operations. When coalbeds are longwall mined in
descending order, the transfer of stress from overlying gate roads is a major design constraint affecting
pillar stability in the lower mine. The lower mine gate road pillars must be properly designed to
withstand the additional load transfer if gate roads arc superpositioned in successive seams. The
Bureau's MULSIM/NL model, a boundary element computer program, was used to analyze load
transfer mechanics for superpositioned gate road pillars. Analysis of Longwall Pillar Stability (ALPS),
an empirically based design method for longwall gate road pillars, was used to calibrate model input
parameters. ALPS provided a basis to verify model trends and to recommend limits for safe pillar
design when superpositioning longwall gate roads.

The attributes of the MULSIM/NL model and ALPS were combined to develop a modified method
for estimating pillar stress for multiple-seam cases. The modified method uses a multiple-seam factor
(MS) to estimate the stress on superpositioned lower mine gate road pillars. Numerical analysis shows
that MS depends on the interburden thickness and pillar width.

‘Mining engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.
ig engi


SUMMARY

The purpose of this report is to provide mine opera-
tors with guidelines for determining safe pillar designs for
multiple-seam mining situations. Because of the high de-
gree of geologic and mechanical property variability expe-
rienced in coal and coal measure rocks, proof of a mathe-
matical model’s accuracy for mine design is not as easily
determined as in other engineering disciplines. However,
since very little information exists to help mine operators
design pillars for multiple-seam applications, the use of a
model makes available a tool from which the initial design
process may begin.

The mathematical model used in this report was cali-
brated by the design method ALPS, which uses empirical
data obtained from single-seam longwall mine operations.
Although the model is based on ALPS, which has been

very successful at determining gate road pillar stability
in single seams, the results obtained from the use of this
model have not been compared with actual in-mine
multiple-seam situations. Therefore, to add validity to the
model's results, an investigation will be conducted to verify
that mines with superimposed longwall gate roads follow
the trends predicted by the model. If discrepancies are
evident, it may be necessary to modify the model’s input
parameters to refine the results. This does not mean that
the MS’s generated by the model cannot be used without
confidence. It does mean, however, that one should be
cognizant of the fact that this model, as well as all mathe-
matical models, should be viewed as a tool to solve prob-
lems, Therefore, these guidelines should not be used with-
out site-specific experience and sound mining judgment.

REFERENCES

1, Webster, S., C. Haycocks, and M. Karmis. Subsidence Interaction
Effects in Multi-Seam Mining. Paper in Stability in Underground
Mining, ed. by A. B. Szwilski and C. O. Brawner (Proc. 2d Int. Conf. on
Stab.

Underground Min., Univ. KY, Lexington, KY, Aug. 6-8, 1984).
. Eng. AIME, 1984, pp. 589-604,

2. Heiug, S. M., and S. S. Peng. Design Guidelines for Multiple-
Seam Mining, Part 1. Coal Min., v. 24, No. 9, 1987, pp. 42-46.

3. Chekan, G. J.,R. J. Matetic, and J. A. Galek. Strata Interactions
in Multiple-Seam Mining—Two Case Studies in Pennsylvania. BuMines
RI 9056, 1986, 17 pp.

4. Chekan, G. J., R. J. Matetic, and D. L. Dwyer. Effects of Aban-
doned Multiple-Seam Workings on a Longwall in Virginia. BuMines
RI 9247, 1989, 15 pp.

5. Su, H,, S. S. Peng, and S. M. Hsiung. Optimum Mining Plans for
‘Multiple-Seam Mining. Dep. Min. Eng, WV Univ., Morgantown, WV,
Final Rep., Sept. 1986, 130 pp.

6. Zipf, R.K. MULSIM/NL Theoretical and Programmer's Manual.
BuMines IC, in press.

7. Mark, C. Pillar Design Methods for Longwall Mining. BuMines
IC 9247, 1990, 53 pp.

8. Merritt, P. C. As Time Changes, So Do Longwalls. Coal Min.,
v. 96, No, 2, 1991, pp. 40-49.

9. Haycocks, C., M. Karmis, E. Barko, J. Carman, B. Ehgartner,
S. Hudock, and S. Webster. Ground Control Mechanisms in Multi-
Seam Mining (grants G1105050 and G1115511, VA Polytech. Inst. and
State Univ.), BuMines OFR 7-84, 1983, 328 pp.

10, Mark, C. Practical Aspects of Longwall Pillar Design. Paper in
Proceedings, Tenth Conference on Ground Control in Mining. WV
Univ., Morgantown, WV, 1991, pp. 1-13.

11. Bieniawski, Z. T. Improved Design of Room-and-Pillar Coal
Mining (Dep. Energy grant DE-FG01-78ET-11428, PA State Univ).
DOE/ET/11428T1, 1982, 165 pp.


ENGINEERING DESIGN AND GEOLOGIC INPUT PARAMETERS

In the Eastern Coal Region, geologic conditions are
best described as anisotropic. Depth, interburden thick-
ness and other physical characteristics, and coalbed thick-
ness vary widely among the different geologic regions.
Mines in some extreme geologic environments, such as
those associated with bumps, require specialized design
techniques to contend with seam interaction. In this study,
engineering and geologic parameters more typical of the
Eastern Coal Region were selected for analysis. The en-
gineering and geologic parameters designated for this
study are given below with basis for their selection.

LONGWALL PANEL AND PILLAR DIMENSIONS

‘The longwall panel and pillar dimensions used in this
study were chosen to generally represent current condi-
tions in the field. The 1991 longwall census from Coal
Magazine (8) was used as a basis for selection. The crit-
ical dimension for the longwall panel, as used in the mod-
el, is the panel width. There were 96 operating longwall
faces in 1990, with 14 pct in the 500- to 599-ft range,
31 pet in the 600- to 699-ft range, 35 pct in the 700- to
799-ft range, 14 pet in the 800- to 899-ft range, and 6 pet
in the 900- to 1,000-ft range. The average panel width was
approximately 707 ft. Based on these figures, a panel
width of 700 ft was selected for analysis in the model.

‘The census (8) also shows that 40 pet of the operators
use a three-entry gate road, 34 pct use a four-entry gate
road, 3 pet use other configurations, and the configura-
tions used by the remaining 23 pet are unknown. Based
on these figures and to narrow the extent of the investiga-
tion, both three- and four-entry gate roads with equal-sized
pillars were selected for study. To cover a representative
sample of pillar sizes in the field, pillar widths ranging
from 50 to 100 ft were evaluated using the model.

GEOLOGIC INPUT PARAMETERS
The geologic parameters include depth, interburden

characteristics, coalbed characteristics, in situ stresses, and
geologic discontinuities. Of these, depth and interburden

characteristics, which include thickness, strength, and elas-
tic modulus, are the critical factors influencing stress
transfer. Statistical analysis of case study information and
photoelastic model results show that interactive probability
is most sensitive to the relationship between depth and
interburden characteristics (9). ‘The next most important
parameter is coalbed characteristics, which include thick-
ness, coal strength, and dip, because these factors influ-
ence the loading behavior of the pillars. In situ stresses,
such as a high horizontal stress, may affect overall stress
magnitude, but the transfer of stress is more directly re-
lated to the normal cover load, which is a function of
depth. Geologic discontinuities, such as clay veins, are
more likely to cause localized instability in the workings
than to affect the overall transfer of stress.

For this study, the MULSIM/NL model has two short-
comings. First, geologic discontinuities cannot be re-
presented in the model. Second, individual strata that
characterize the overburden and interburden cannot be
represented, so a generic modulus is chosen that depicts
the overall lithology. Assuming that the overburden and
interburden are one homogencous, isotopic material
makes the strata reactions stiffer than is actually the case.
Therefore, the elastic modulus of the material is lowered
to more closely approximate a stratified rock mass. Phys-
ical properties of strata in the MULSIM/NL model are
represented by the elastic modulus and Poisson’s ratio.
Elastic modulus values, in pounds per square inch, for the
overburden and interburden, coal, and gob are given be-
low. A Poisson’s ratio of 0.25 was assumed for all cases.

Overburden and interburden .... + 800,000
Gob . eee eee eee eeee 1,500

The coalbeds were assumed to be 6 ft thick. This figure
was chosen because it represents an average operating
height from a recent survey of longwall productivity (8).
The coalbeds were also assumed to be flat-lying deposits
with no dip. The assumed density of the overburden was
162.5 Ib/ft?.

CALIBRATING THE MODEL TO ALPS

ALPS (7) is a design method that estimates the strength
of longwall gate road pillars and the stress they experience
during development and mining of the longwall panels.
ALPS was used to calibrate the MULSIM/NL model, be-
cause ALPS is empirically based on field measurements
conducted at 16 longwall panels, and has been verified
by back-analysis of over 100 case histories. The basic

approach was to use the best attributes of both ALPS and
the MULSIM/NL model to determine the magnitude of
stress transfer between gate road pillars. ALPS’ strength
lies in its ability to estimate different abutment stresses
and pillar stability factors at each stress condition. The
MULSIM/NL model's strength is its ability to develop
relative trends and to calculate the magnitude of stress


BUREAU OF MINES
INFORMATION CIRCULAR/1992

Analysis and Design Considerations
for Superimposed Longwall Gate
Roads

By Gregory J. Chekan and Jeffrey M. Listak

UNITED STATES DEPARTMENT OF THE INTERIOR



Table 1.—Multiple-seam factor (MS) for pillars at varying depths with Interburden of 50 ft

Pillar size and Upper mine Lower mine
MS (/o,) Depth, ft ciw PSi__ay PS yy PSI__Depth, ft oy Psi oqy PSI o% PSI
60- by 80+ pillar
O71. 250 1,075 428 647 300 970814456
070.. 350 1,520 599 921 400 1,330 «685645
450 1,966 771 1,195 500 1,689 «= «857-832
069 ......... 550 2,410 942 1,468 600 2,046 1,028 1,018
650 2,850 1,114 1,736 700 2,404 1,200 1,204
750 3,204 «= 1,285 + 2,009 800 2,763 1,371 1,392
60- by 120+ pilla
O71 250 1,057, 407 650 300 945 488 457
350 1,483 569 914 400 1,209 «651648
450 1,919 732 1,187 500 1,653 «813.840
550 2,351 895 1,456 600 2,004 976 1,028
650 2,785 1,058 1,727 700 2,356 1,139 1,217
750 3,214 1,221 1,993 800 2,708 1,302 1,404
650 2,176 999 1,177 700 1,969 1,075, 894
750 2,515 1,151 1,364 800 2,262 1,229 1,033
850 (2,851 1,306 1,545, 900 2,554 1,382 1,172
950 3,190 1,460 1,730 1,000 2,847 1,536 1,311
1050 3,526 1,613 1,913 1,100 3,138 1,600 1,448
1,150 3,865 1,766 «2,099 1,200 3,432 1,843—(1,589
1,250 4,202 «1,920 2,282 «= 1,800 3,723 1,996 1,727
1,350, 4,541 2,074 (2,467, 1,400 4,017 2,150, 1,867

Table 2.Multiple-seam factor (MS) for pillars at varying depths with interburden of 25 ft

Pillar size and Upper mine Lower mine
MS @/o,) Depth, ft oi PSI cay PS! Gq PSI Depth, ft oy psi om PSI oy psi
60- by 80-ft pillar:
0.87. . . 275 1,184 471 713 300 1,135 514 621
0.86. 375 1,626 642 984 400 1,539 «685845
475 2,064 814 1,250 500 1,940 «857 ‘1,083
575 2,506 985 1,521 600 2,344 1,028 1,316
675 2,946 «1,156 1,790 700 2,748 1,200 1,546
75 3,386 = 1,327 2,059 800 3,150 1,371 1,779
67522531087 1,216 700 2,161 1,075 1,086
775 2,587 1,192 1,395, 800 2,475 1,229 1,246
875 2,925 1,345 1,580 900 2,792 1,382 1,410
975 3,259 ° 1,498 = 1,761 1,000 3,106 1,536 1,570,
1,075 3,597 1,651,946 «1,100 3,424 «1,690 1,734
1,175 3,932 1,805 2,127 1,200 3,738 1,843 1,895
1,275 4,270,958 «2,312 1,300 4,055 1,996 2,059
1,375 4,604 2,112 2,492 1,400 4,371 2,150 2,210
The above analysis showed that the two most important _a depth of 1,100 ft. Therefore, for the first model run the

parameters influencing MS, in order of sensitivity, are
interburden thickness and pillar width. Model runs were
then performed for interburdens ranging from 25 to 300 ft
in 25-ft increments and for pillar widths ranging from
50 to 100 ft in 10-ft increments. The depth to the lower
seam was kept constant, because table 1 shows that depth
does not influence MS. The depth selected for a partic-
ular pillar size was based on the stability factor of 1.0 for
tailgate loading using ALPS. For instance, for a pillar
width of 80 ft, an 80- by 80-ft pillar was modeled. This
pillar reaches a stability factor of 1.0 at tailgate loading at

lower mine depth was kept constant at 1,100 ft and the
depth to the upper mine was 1,075 ft, giving an inter-
burden of 25 ft. For the second model run the lower mine
was again kept constant at 1,100 ft, and the depth to the
upper mine was changed to 1,050 ft, giving an interburden
of 50 ft. This procedure was continued until an interbur-
den of 300 ft was reached. At 300 ft of interburden, stress
dissipation was such that MS became negligible—less than
0.15 for all pillar widths. Table 3 shows how MS was
generated for a 60- by 80-ft pillar at each interburden
using equations 1, 2, and 3,


from ALPS are given in the previous section on geologic
input parameters, All model input parameters were as-
signed linear elastic property codes to best simulate the
behavior of the ALPS method. ALPS failure criterion as-
sumes that the pillars do not yield, but instead support the

weight of the different stress conditions until these stresses
exceed the strength of the pillar. The stability of the pil-
lar, which is represented in ALPS as a stability factor,
becomes critical as the factor approaches 1.0 to 1.3.

ANALYSIS OF SUPERPOSITIONED GATE ROADS

The analysis of superimposed gate roads assumes that
mining is completed in the upper seam with gob on both
sides of the gate roads and that the pillars in both seams
are of equal size. As shown in figure 3, the stress on the
upper mine pillars is equal to the development stress at
that depth plus the stress generated by the two side
abutment loads. The stress on the lower seam pillars is
equal to the development stress at that depth plus a cer-
tain percentage of the stress that is transferred from the
upper mine pillars. To determine the percentage of stress
that is transferred from the upper to the lower mine the
following equations are used.

For the upper mine:

Sau = iv ~ Four @
where qy = abutment stress for upper mine
pillars, psi,

Giy = isolated stress for upper mine pillars,
psi,
and au = development stress for upper mine
pillars, psi.
For the lower mine:
= oy - Cap 2
where , = amount of stress transferred to lower
mine pillars, psi,

oy = total stress on lower mine pillars, psi,

and @q = development stress for lower mine
pillars, psi.
Also,
%
MS =—!, Q)
o,

where MS = multiple-scam factor, which is the percentage
of abutment stress transferred from the upper to the lower
mine pillars,

The MULSIM/NL model generates stress values for
Ow Fay Oy and oy. Values for o,, and a, are calculated
from equations 1 and 2, and MS, for a given interburden,
is calculated from equation 3.

These three equations were used to analyze the effects
of varying depths, interburden thicknesses, and pillar sizes
on MS. All analyses assumed a three-entry gate road with
20-ft-wide entries. The analysis was first conducted for a
60- by 80-ft pillar at an interburden of 50 ft for depths
ranging from 300 to 800 ft for the lower mine. A depth of
800 ft represents the design limitation for a 60- by 80-ft
pillar using ALPS, because the stability factor at this depth
is approximately 1.0 at tailgate loading.

The results of this analysis are given in table 1. These
data show that for a 60- by 80-ft pillar and at 50 ft of in-
terburden between the mines, MS has little variation, rang-
ing between 0.69 and 0.71. This lack of variation indicates
that when interburden is kept constant MS is independent
of depth.

To determine if pillar length and percent extraction
affect this factor, the pillar width was kept constant at
60 ft and length was increased to 120 ft. The extraction is
40 pet for a 60- by 80-ét pillar and 36 pet for a 60- by
120-ft pillar. The results for a 60- by 120-ft pillar are giv-
enin table 1, As shown, MS had little variation, indicating
that, within the range of variables tested, MS is independ-
ent of the pillar length and percent extraction.

To determine if pillar width affects MS, the width was
increased to 100 ft. The same analysis was performed on
a 100- by 100-ft pillar keeping the interburden at 50 ft.
The depths ranged from 700 to 1,400 ft for the lower
mine, because the stability factor at the greater depth is
approximately 1.0 at tailgate loading. Table 1 shows the
results of this analysis. MS in this case was higher (0.76),
indicating that MS varies slightly depending on the width
of the pillar.

To determine how interburden thickness affects MS, the
same analysis was performed on a 60- by 80-ft and a 100-
by 100-ft pillar, but changing the interburden to 25 ft. The
results for each pillar are given in table 2. MS increased
to 0.87 for the 60- by 80-ft pillar and to 0.89 for the 100-
by 100-ft pillar, indicating that MS is very sensitive to the
interburden thickness.


Surface

Upper
mine
FRONT VIEW
Lower
mine
Not to scale
KEY
ECB Gate road
~— Stress contour
Figure 1.—Longwalll panel and gate roads offset.
Surface
= BESS S2 SS SU MSs: (Per
mine
| | | | FRONT VIEW
es ee ee eee
mine
Not to scale
KEY
ELE Gate road
Coal
Gob

~<— Stress contour

Figure 2.—Longwall panel and gate roads superimposed.


CONTENTS

Page
Abstract . . 1
Introduction . 2
Engineering design and geologic input parameters 4
Longwall panel and pillar dimensions . . 4
Geologic input parameters . . 4
Calibrating the model to ALPS . + 4
Analysis of superpositioned gate roads . 6
Using the multiple-seam factor . . 9
Sensitivity analysis . n
Summary ... 14
References .. 14
1. Longwall panel and gate roads offset 3
2. Longwall panel and gate roads superimposed . 3
3. Stress conditions for upper and lower mine when gate roads are superimposed . s
4. Multiple-seam factor for three- “entry and fourentry gate roads . 8
5. Output from ALPS program for upper mine . 10
6. Output from ALPS program for lower mine . . . u
7, Abutment from upper mine transferred to lower mine gate roads 1 at isolated load . . 13
8. Average pillar stress and stability factors for multiple-seam and single-seam case at each stress condition. 13
TABLES
1, Multiple-seam factor (MS) for pillars at varying depths with interburden of 50 ft . 7
2, Multiple-seam factor (MS) for pillars at varying depths with interburden of 25 ft . 7
3. Multiple-seam factor (MS) for 60- by 80-ft pillar at varying interburdens 8



weenie ANALYSIS OF LONGWALL PILLAR STABILITY (ALPS) — *#%%#iicex

NAME OF MINE A Mine
MINING HEIGHT (ft) 6.0
MINING DEPTH (ft) 800
IN SITU COAL STRENGTH (psi) 900
ABUTMENT ANGLE (deg) 21.0
PANEL WIDTH (ft) 700
ENTRY WIDTH (ft) 20.0
CROSSCUT CENTER/CENTER (ft) 100.0
ENTRY CENTER / CENTER (ft) - PILLAR 1 90.0
ENTRY CENTER / CENTER (ft) - PILLAR 2 90.0

*eeeRE ALPS STABILITY FACTORS *******

DEVELOPMENT LOADING
HEADGATE LOADING
BLEEDER LOADING
*** TAILGATE LOADING
ISOLATED LOADING

HeenRw
HNAHO
HOW

FOR PILLAR WIDTH (ft) = 70.0
AND PILLAR LENGTH (ft) = 80.0
WIDTH/HEIGHT RATIO = 11.67
UNIT PILLAR STRENGTH = 4356

PILLAR LOAD BEARING CAPACITY (lbs per ft of gate entry)= 3.51E+07

TOTAL PILLAR SYSTEM LOAD BEARING CAPACITY
(Ibs per ft of gate entry) = 7.03E+07

DESIGN LOADINGS ON PILLAR SYSTEM (lbs per ft of gate entry)

FOR DEVELOPMENT LOADING = 0.234E+08
FOR HEADGATE LOADING = 0.330E+08
FOR BLEEDER LOADING = 0.426E+08
FOR TAILGATE LOADING = 0.572E+08
FOR ISOLATED LOADING = 0.632E+08

INDIVIDUAL PILLAR LOADING (psi)
LOADING PILLAR PILLAR PILLAR PILLAR
CONDITION 1 2 3 4
DEVELOPMENT 1448 1448 0 0
HEADGATE 2304 1751 0 0
BLEEDER 3160 2054 0 0
UNIT PILLAR
STRENGTH 4356 4356 0 0

NOTE: INDIVIDUAL PILLAR LOADINGS DO NOT CONSIDER LOAD
TRANSFER DUE TO PILLAR YIELDING!

Figure 5.—Output from ALPS program for upper mine.


In this example, the least width of the pillar is 840 in,
and the height of the pillar is 72 in. Substituting these val-
ues into equation 6 gives a unit pillar strength of 4,360 psi.

Step 8: To calculate the stability factor, the unit pillar
strength value in step 7 is divided by the development
stress in step 6: 4,360/3,320 = 13. The recommended sta-
bility factor for pillars using this formula ranges from 1.5
to 2.0 (17). By this analysis the 70- by 80-ft pillar in the
lower mine is slightly underdesigned to accept the devel-
opment stress as well as the additional stress that is
applied as the longwall panels are mined.

There are two options to consider under these condi-
tions. First, a larger pillar can be used in the lower mine.
If the center entries are superimposed, this would cause
the lower mine gate roads to be slightly offset in rela-
tion to the upper mine, thereby shortening the panel
width. In this case, a pillar at least 80 ft wide would be
required in the lower mine. Steps 1 through 4 for the up-
per mine would remain the same. Step 5 would require
running ALPS for an 80- by 80-ft pillar and a 680-ft panel.
This gives a development stress of 1,495 psi. Adding
1,780 psi from step 4, which is the amount of transferred
stress, gives a development stress on the lower mine pillars
of 3,275 psi. From step 7 the unit pillar strength is cal-
culated to be 4,896 psi. From step 8 this gives a stability
factor of 1.5.

Second, pillars can be designed in both mines simul-
taneously so similar sizes are achieved in both operations.
This requires planning before mining begins and a con-
scious effort to longwall mine both seams knowing that
pillars in the upper mine may be slightly overdesigned

to achieve properly designed pillars in the lower mine,
Working steps 1 through 8 for 80- by 80-ft pillars in both
mines gives a stability factor of 1.6 for the lower mine pil-
lars on development. Depending on local roof and floor
conditions, this stability factor can be adjusted accordingly
within the specified range of 1.5 to 2.0.

ALPS stability factors for development loading in a
single-seam case are usually over 2.5. For instance, the
stability factor for the 70- by 80-ft pillar at 850 ft of depth
in the preceding example is 2.83 (fig. 6). In a single-seam
case, this high stability factor is necessary to maintain a
factor between 1.0 and 1.3 after both longwall panels are
mined. In a multiple-seam case, the stability factors for
development loading do not need to be as high. As shown
in figure 7, part of the abutment load on the lower mine
pillars after both panels are extracted (isolated load) is
that transferred on development from the upper mine.

To further illustrate, MULSIM/NL was used to analyze
the development and abutment stresses for both a single-
and a multiple-seam case. The results are shown in fig-
ure 8. A three-entry gate road with 60- by 80-ft pillars,
20-ft-wide entries, and a 700-ft-wide panel was used in the
analysis. The depth was 750 ft. The first model run was
for a single-seam case to attain stress values for the five
different stress conditions. The second model run was for
a multiple-seam case. The mine depth remained at 750 ft
with a similar set of gate roads 50 ft above. As shown, the
development stress in the multiple-seam case is more than
twice that of the single-seam case; the stability factor is
2.9 for the single-seam case and approximately 1.5 for the
multiple-seam case. However, at isolated loading the sta-
bility factors are nearly equal at 1.1.

SENSITIVITY ANALYSIS

A sensitivity analysis was conducted to determine if
changing the model input parameters would affect MS.
Elastic modulus values for the rock mass, coal, and gob
were increased and decreased by 25 pct from the values
given in the "Geologic Input Parameters" section. For in-

stance, the rock mass elastic modulus in the original model’

‘was 800,000 psi. Model runs were made with the modulus
at 1 million psi and then at 600,000 psi. The seam height
was also increased and decreased from 6 ft with model
runs made at 4 ft and 8 ft.

The analysis was performed on a three-entry gate road
using 60- by 80-ft pillars, 20-ft-wide entries, and a 700-ft
panel. This analysis showed that the stress values for

pillars would change, but this had little effect on MS when
the data were normalized. Changes in the rock mass and
coal modulus values followed similar trends. Increasing
the modulus increased the multiple-seam pillar stresses,
and decreases caused decreasing stresses, Changes in the
gob modulus followed a reverse trend. Changes in seam
height followed a similar trend as that of the rock mass
and coal. The 8-ft seam increased pillar stress, and the
4-ft seam decreased pillar stress, Changing modulus and
seam height values in one seam and not the other also had
little effect on MS. For all model runs, the change in MS
for all interburdens, moduli, and seam heights was less
than 2 pet.


Library of Congress Cataloging in Publication Data:

Chekan, G. J. (Gregory J.)
Analysis and design considerations for superimposed longwall gate roads / by
Gregory J. Chekan and Jeffrey M. Listak.

p. cm. — (Information circular; 9305)

Includes bibliographical references (p. 14).

1. Ground control (Mining)—Computer simulation. 2. Longwall mining—
Computer simulation. 3. MULSIM/NL. 4. ALPS (Computer file). I. Listak,

Jeffrey M. Il. Title. III. Series: Information circular (United States. Bureau of
Mines); 9305.

TN295.U4 [TN288] 622 s—dce20 —[622’.28]_ 91-42048 CIP



Information Circular 9305

Analysis and Design Considerations
for Superimposed Longwall Gate
Roads

By Gregory J. Chekan and Jeffrey M. Listak

UNITED STATES DEPARTMENT OF THE INTERIOR
Manuel Lujan, Jr., Secretary

BUREAU OF MINES
T S Ary, Director


Table 3.—Multiple-seam factor (MS) for 60- by 80-ft pillar at varying Interburdens

(MS o4/ou,) Upper mine Lower mine

Depth, ft iy PSI_ ay PSi_ gy PSI_Interburden, ft_ Depth, ft om Psi oq, Psi psi
0.86 . 778 3,386 1,327 2,059 25 800 «3,150 1,371 1,779
0.69 . 750 3,294 1,285 2,009 50 800 2,763 1,371 1,302
0.48 . 725 3,231 1,242 1,989 75 800 2,325 1,371 954
0.39 . 700 3,138 1,200 1,938 100 800 2,117 1,371 746
0.31. 675 3,046 1,156 1,890 125 800 1,947 1,371 876
0.24. 650 2,948 1,114 1,834 150 800 1,807 1,371 436
018. 625 2,844 14,071 1,773 175 800 1,693 1,371 «322
0.14. 600 2,735 1,028 1,707 200 800 1,603 1,371 «232
0.10. 575 2,627 985 (1,642 225 800 1,528 1,371 87
0.06 . 550 2519 942 1,577 250 800 1,469 1,371 98
0.03 . 525 2,408 = $001,508 275 800 4421 1,371 50
0.01. 500 2,292 857 1,435 300 800 1,384 1,371 13

Figure 4 shows the data generated from these analyses. _ estimate the development stress on the lower mine pillars
These graphs are used in conjunction with ALPS to and resulting stability factors.

8 iN KEY KEY
~\ Pillar width, ft N Pillar width, ft

KEY
Pillar width, ft

KEY
Pillar width, ft

MULTIPLE SEAM FACTOR
°

4
2
° 60 120 180 240 300 0 60 120 180 240 300

INTERBURDEN, ft

Figure 4.—Multiple-seam factor for thr

ntry (AB) and four-entry (C-D) gate roads.


weKKKKEKEK ANALYSIS OF LONGWALL PILLAR STABILITY (ALPS) = ***#####%%

NAME OF MINE B Mine
MINING HEIGHT (ft) 6.0
MINING DEPTH (ft) 850
IN SITU COAL STRENGTH (psi) 900
ABUTMENT ANGLE (deg) 21.0
PANEL WIDTH (ft) 700
ENTRY WIDTH (ft) 20.0
CROSSCUT CENTER/CENTER (ft) 100.0
ENTRY CENTER / CENTER (ft) - PILLAR 1 90.0
ENTRY CENTER / CENTER (ft) - PILLAR 2 90.0
wee ALPS STABILITY FACTORS **##***
DEVELOPMENT LOADING 2.83
HEADGATE LOADING 1.97
BLEEDER LOADING 1.51
*** TAILGATE LOADING 1.11
ISOLATED LOADING 1.01
bec b eee e eee eeenneeeeeees ADDITIONAL OUTPUT.........---e02 ee eee
FOR PILLAR WIDTH (ft) = 70.0
AND PILLAR LENGTH (ft) = 80.0
WIDTH/HEIGHT RATIO = 11.67

UNIT PILLAR STRENGTH = 4356
PILLAR LOAD BEARING CAPACITY (lbs per ft of gate entry)= 3.51E+07

TOTAL PILLAR SYSTEM LOAD BEARING CAPACITY
(lbs per ft of gate entry) = 7.03E+07

DESIGN LOADINGS ON PILLAR SYSTEM (lbs per ft of gate entry)

FOR DEVELOPMENT LOADING = 0.248E+08
FOR HEADGATE LOADING = 0.356E+08
FOR BLEEDER LOADING = 0.465E+08
FOR TAILGATE LOADING = 0.630E+08
FOR ISOLATED LOADING = 0.698E+08

INDIVIDUAL PILLAR LOADING (psi)
LOADING PILLAR PILLAR PILLAR PILLAR
CONDITION 1 2 3 4
DEVELOPMENT 1539 1539 0 0
HEADGATE 2480 1888 0 0
BLEEDER 3422 2237 0 0
UNIT PILLAR
STRENGTH 4356 4356 0 0

NOTE: INDIVIDUAL PILLAR LOADINGS DO NOT CONSIDER LOAD
TRANSFER DUE TO PILLAR YIELDING!

Figure 6.—Output from ALPS program for lower mine.