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No. 443, November 1994

Design Practices for Multiple-Seam
Room-and-Pillar Mines

Objective

Provide room-and-pillar operators with practical in-
formation and guidelines concerning multiple-seam mine
design to reduce ground problems associated with the
interaction of adjacent workings.

Background

Interactions of multiple-seam operations can cause
ground problems resulting from the transfer of stress,
strata displacement, and caving due to subsidence. Such
interactions are a common occurrence, resulting in loss of
coal reserves and increased operating costs. Studies
estimate that 140 billion metric tons of coal, representing
68 percent of the minable reserves in the United States,
are subject to multiple-seam mining. In many instances,
mining sequence is based primarily on availability and
economics, with little regard for the effects mining would
have on coalbeds above and below the one being mined.
These practices could have strong implications for resource
conservation. For instance, West Virginia, Virginia, and
Kentucky have over 90 minable coalbeds, many of which
are classified as “low sulfur.” Many coal analysts speculate
that the 1992 Clean Air Act and new compliance coal
standards may shift future mining to these reserves. But
without competent design strategies, interactions between
vertically adjacent operations will increase the difficulty
and expense of mining.

Effective mine planning and design are essential for
avoiding ground problems related to multiple-seam mining.
To avoid higher mining costs, operators should focus on
adopting practices and procedures that prevent and con-
tol interactions in multiple seams. The U.S, Bureau of

Mines (USBM), in an effort to improve mine planning, is
investigating multiple-scam room-and-pillar design and
development.

Approach

Factors that influence interactions between operations
can be classified as either "geologic" or "mine design” pa-
rameters. The geologic parameters include the depth, in-
terburden thickness and physical characteristics, coalbed
thickness and physical characteristics, immediate roof-and-
floor stratigraphy, and in situ stress fields. The minc
design parameters include the seam sequence, pillar size
and strength, entry widths and roof spans, percent extrac-
tion, mining height, geometric layout of the workings, sup-
port methods, and the time delay between mining scams.
Optimization of mine design factors is the primary means
for controlling interactions between operations.

Of the design factors, three are considered primary
and have significant influence in seam interaction. These
factors are very closely related and should be weighed
equally for effective mine planning. First, the sequence or
order in which the seams will be mined will determine
the type of interaction. Second, the design of pillars and
entries will determine the magnitude of interaction. Third,
the geometric layout of the workings will determine the
location of interaction. Other parameters fixed by the ge-
ologic environment, such as depth and interburden thick-
ness, will influence interaction magnitude and location and
must also be considered in the design process.

Empirical investigations involving case study docu-
mentation and analysis have constituted most of the
USBM research and have provided important informa-
tion in the development of design procedures. However,
computer-based numerical models are gaining more

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research attention because they can provide insight into
relative stress transfer and distribution in multiple seams.
The analysis of diffcrent designs using numerical models
has considerable potential in helping operators find sotu-
tions to complex multiple-scam interactive problems. The
USBM’s MULSIM/NL model is a boundary clement mod-
el for calculating stresses and displacements in tabular
deposits, The MULSIM/NL model was used to evaluate
stress distribution and transfer for design problems that
are commonly encountered in multiple-seam room-and-
pillar layouts.

Accomplishments

Some primary findings from these investigations are
summarized as follows:

1. To ensure optimum ground conditions, coalbeds
should be mined in descending order. This extraction
order prevents coalbeds from being damaged by caving
and other strata displacements caused by subsidence.

2. Two basic approaches are available for pillar design:
yield pillars and conventional pillars. For the most part,
further study is required to assess the performance of yield
pillars under multiple-scam conditions. However, there
are several conventional pillar design approaches available

Gob-solid coal boundary

Direction of development mining

Gob-solid coal boundary

a Direction of retreat mining

to the operator that have demonstrated their success in the
field.

3. There are two basic approaches to laying out room-
and pillar panels in successive seams: superpositioned
panels or offset panels. Superpositioned panel arrange-
ments will be uscd, in most instances, for all extraction
sequences, Offset arrangements will be used to avoid
interactions when geologic conditions are not favorable,

4, High-stress zones are usually encountered in the
lower mine when mining occurs beneath an isolated
barricr pillar or a gob-solid coal boundary in the upper
mine, Stress can be reduced in the lower mine pillars by
retreat mining from the gob to the solid side of the
boundary and supporting the barrier edge with a row of
pillars.

For More Information

USBM Information Circular 9403 and Reports of
Investigation 9056, 9066, 9173, and 9176 provide more
detail on multiple-seam room-and-pillar design. For a
copy of these reports or for additional information
concerning the USBM multiple-scam research program,
contact Gregory J. Chekan, U.S. Bureau of Mines, Pitts-
burgh Research Center, P.O. Box 18070, Cochrans Mil!
Road, Pittsburgh, PA 15236; telephone (412) 892-6749

Upper mine

(abandoned)

Lower mine
(active)

Upper mine
(abandoned)

ms Lower mine
(active)

Studios show that stress in the lower mine pillars can be reduced when you are mining beneath a gob-solid coal boundary in a overlying
mine by following two design criterla: (1) Develop pillars from the solid to the gob side of the boundary, and then retreat piliara from
the gob to the solid side, and (2) support the boundary edge with a row of pillars.

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