2. Assess the relationship between time-dependent be-
havior of rib and the time factors involved with the in-
stallation of support.

3. Apply current support technology and develop new
rib support technology using innovative systems, materials,
and procedures.

4. Develop more user-friendly computer software to
evaluate rib support design using empirical and analytical
data.

REFERENCES

1. US. Mine Safety and Health Administration (Dep. Labor).
Accident Reports. Mining Information Systems Division, Information
Data Base, covers 1983-89.

2. Farmer, I. A. Induced Cleavage During Longwall Mining—
Genesis, Effect, and Control. Paper in Proceedings of the 9th Plenary
Scientific Session of the International Bureau of Strata Mechanics
(Varna, Sweden, June 18-21, 1985). Balkema, 1986, pp. 151-156.

3, Wu, M. M,, G. A. Robbins, R.A. Winsche, and F. P. Burke. Low-
‘Temperature Coal Weathering: Its Chemical Nature and Effects on
Coal Properties. Energy and Fuels, v. 2, No. 2, Mar-Apr. 1988,
pp. 150-157.

4. Woodruff, S. D. Theory and Application of Rock Mechanics
to Roof Control and Support Problems. Ch. in Methods of Working
Coal and Metal Mines. Pergamon, v. 1, 1964, pp. 368-379.

5. Hoek, E., and J. W. Bray. Basic Mechanics of Slope Failure. Ch.
in Rock Slope Engineering. Gresham Press, Surrey, England, 1974,
pp. 28-33.

6. Stacey, T. R. A Simple Extension Strain Criterion for Fracture
of Brittle Rock. Int. J. Rock Mech. in Sci. and Geomech. Abst., v. 18,
1981, pp. 469-474,

7. Brace, W. F., and J. D. Byerlee. Recent Experimental Studies
of Brittle Fracture of Rocks. Paper in Proceedings of 8th Symposium
on Rock Mechanics, Failure, and Breakage of Rock. Univ. MN, 1968,
pp. 58-81.

8. Brown, E. T. Brittle Fracture of Rock at Low Confining
Pressures. Paper in Proceedings of the 1st Australia-New Zealand
Conference on Geomechanics (Melbourne, Australia, Aug. 9-13, 1971).
Inst. Eng., Aust. N.Z. Inst. Eng,, 1971, v. 1, pp. 31-36.

9. Jeremic, M. L. Influence of Shear Deformation Structures in
Coal on Selecting Methods of Mining. Rock Mech., v. 13, No. 1, 1980,
pp. 23-38.

10. Kripakov, N. P., L. A. Beckett, and D. A. Donato. Loading on
Underground Mine Structures Influenced by Multiple Seam Interaction.
Paper in Proceedings of International Symposium on Application of
Rock Characterization Techniques (New Orleans, LA, Mar. 1986).
AIME, 1986, pp. 225-236. .

11. Wilson, A. H. Stress and Stability in Coal Ribsides and Pillars.
Paper in Proceedings of Ist Annual Conference on Ground Control in
Mining (Morgantown, WV, July 1981). WV Univ., 1981, pp. 1-12.

12, Barron, K. An Air Injection Technique for Investigating the
Integrity of Pillars and Ribs in Coal Mines. Int. J. Rock Mech. in Sci.
and Geomech. Abst., v. 15, No. 2, 1978, pp. 69-76.

13. Hucka, V. J., and S. P. Singh. Geological Aspects of Stability in
Underground Coal Mines. Paper in Proceedings of st AIME-SME In-
ternational Conference on Stability in Underground Mining (Vancouver,
British Columbia, Canada, Aug. 16-18, 1982). AIME, 1983, pp. 165-180.

14, Babcock, C. O. Constraint Is the Prime Variable in Pillar
Strength. Paper in Proceedings of 4th Conference on Ground Control
in Mining (Morgantown, WV, July 22-24, 1985). WV Univ., 1985,
pp. 105-116.

15. Wang, F. D., W. A. Skelly, and J. Wolgamott. In-Situ Coal Pillar
Strength Study (contract H0242022, CO Sch. Mines). BuMines OFR
107-79, 1976, pp. 19-26.

16. Fuller, P.G., and R. H. Cox. Rock Reinforcement Design Based
on Control of Joint Displacement—A New Concept. Paper in Proceed-
ings of Australian Tunnelling Conference (Sydney, 1978), pp. 28-35.

17. Dolinar, D. R,, and S. C. Tadolini. Entry Stabilization Utilizing
Rib Bolting Procedures. BuMines RI 9366, 1991, 13 pp.

18. Babcock, C. O. A Flexible Helical Rock Bolt. BuMines RI 8300,
1978, 26 pp.

19. Conway, J. P., S. M. Dar, J. H. Stears, and P. C. McWilliams,
Laboratory Studies of Yielding Rock Bolts. BuMines RI 8058, 1975,
40 pp.

20. Martin, E., F. Car, and G. Hendon. Strata Control Advances at
Jim Walter Resources, Mining Division. Paper in Proceedings of 7th
International Conference on Ground Control in Mining (Morgantown,
WV, Aug. 35, 1988). WV Univ, 1988, pp. 66-75.

21. O'Beimne, T., and J. Shepherd. The Failure of Coal Pillar Ribs
and Possible Methods of Control. Paper in Proceedings of the Fourth
Australia-New Zealand Conference on Geomechanics (Perth, May 1984).
Inst. Eng,, Aust. N.Z. Inst. Eng,, 1984, pp. 661-667.

22, Brady, B. T., and W. I. Duvall, Strengthening of Fractured Rock
Pillars by the Use of Small Radial Reinforcement Pressures. BuMines
RI-7755, 1973, 20 pp.

23. Castle, B. R, and J. J. Scott. Cable Slings—A Versatile ’Band-
Aid’ for Providing Safety in Underground Mining. Paper in Proceedings
of 1st AIME-SME International Conference on Stability in Underground
Mining (Vancouver, British Columbia, Canada, Aug. 16-18, 1982).
AIME, 1983, pp. 978-993.

2A. Dalzell, R. S., and E. A. Curth. Remedial and Strata Replacement
Techniques on Longwall Faces. Paper in Proceedings of 2nd AIME-
SME International Conference on Stability in Underground Mining
(Lexington, KY, Aug. 6-8, 1984). AIME, 1984, pp. 473-494.

25. Snodgrass, J. J., and D. A. Newman, An In Situ Technique for
the Assessment of Failure in Coal Pillars. Paper in Proceedings of 26th
U.S. Symposium on Rock Mechanics (Rapid City, SD, June 1985).
Balkema, 1985, pp. 1181-1188.

26. Lu, P. H. Determination of Ground Pressure Existing in Vis-
coelastic Rock Mass by Use of Hydraulic Borehole Pressure Cells.
Paper in Proceedings of the International Symposium on Weak Rock
(Tokyo, Japan, Sept. 1981). Balkema, v. 1, 1981, pp. 459-465.

27. Smith, W. C. Evaluation of Progressive Rib Failure in Thick Coal
Seams. Soc. Min. Eng. AIME preprint 89-174, 1989, 7 pp.

28. Hobbs, D. W. Scale Model Studies of Strata Movement Around
Mine Roadways. IV. Ribside Support. Int. J. Rock Mech. Min. Sci.,
v. 7, No. 2, 1970, pp. 183-192.

29. Kripakov, N. P., L. A. Beckett, D. A. Donato, and J. S, Durr.
Computer-Assisted Mine Design Procedures for Longwall Mining.
BuMines RI 9172, 1988, 38 pp.



steel bands, post and timbers, and chemical stabilization.
As shown in figure 6, with the exception of timbers and
chemical stabilization techniques, these methods are gen-
erally used in conjunction with bolting.

Research indicates that small radial forces applied to
the rib wall can have significant results in reducing the
progression of rib failure (22). Ideally, it is preferable to
apply confinement before rib failure has progressed to
the point where fractures have opened than to apply con-
finement after significant fracture separations have oc-
curred. As a general rule of thumb, support should be in-
stalled at the earliest possible time while most of the failed
material is in place and frictional contact along fractures
is maintained.

Friable coal ribs usually require liners or strapping
to hold "broken coal" in place. Steel mesh is the most
widely available liner material for use in coal mines and
comes in various mesh sizes. Figure 10 shows one strategy
for using wire mesh for rib control. Alternative materials
for meshing and strapping, such as polyethylene netting
and geotextiles, are more widely used in Europe and
Australia (22).

Cable bolting can be usefully applied to the outside of
unstable ribs. Wrapping cable around the pillar is another
method to provide active confinement to coal ribs when
larger slab formation is expected. Wrapping pillars with
cable has been effective in reducing high shear stress along
roof and rib fractures (23). Generally, cable bolting works
most effectively in combination with bolting, meshing,
and/or shotcreting to handle different sizes of "broken
coal." Wire rope can be used to reinforce large sections
of wire mesh, allowing the mesh to withstand greater pres-
sures from large slab and block displacements in large
areas of rib. This technique has been shown to prevent
face sloughing in thick coal seams (24).

Installing timbers and posts close to the rib to halt
sloughing coal is another method to contain loose coal and
provide some stability. Since active support is not applied,
confinement pressures do not develop with this method
until significant block and slab displacement occur in the
rib. Depending on timber-to-rib distances, considerable
block and slab movement may be necessary before stability
is achieved. In situations of oversized pillar dimensions,
these larger displacements may be more tolerable when
intact pillar core is adequate. Using wooden posts and
headers with bolts represents another variation that com-
bines the yieldable characteristics of wood with the an-
chorage capabilitics of bolts. This approach has been
successful in restraining large slab movements.

Grout, polyurethane, and foam injection techniques are
effective methods for the stabilization of fragmented strata,
particularly on longwall faces in thick coal seams. Brought

Figure 10.—Using wire mesh for rib control.

over from Europe, these methods have been successfully
tested in U.S. coal mines with highly fractured, and often
highly faulted, coal seams (24). Properties that make
chemical rock stabilization effective are (1) high expansion
to fill voids and cracks, (2) variable viscosity (low initial
viscosity to fill all cracks, followed by high viscosity once
cracks are filled), (3) good adherence to rock material and
support, and (4) plasticity to deform with rock movement.
In theory, rib stability can be increased with this method
by decreasing the number of active fracture surfaces. This
method works well in highly fractured, highly stressed coal
faces, particularly in thick-scam deep longwall mines.
Cheaper and more effective chemical and grout delivery
systems are needed to encourage more widespread use of
the procedure in gateroad coal pillar and rib stability
designs.



°C

ft

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT
degree Celsius in

foot

inch



30. Crouch, S. L., and C. Fairhurst. The Mechanics of Coal Mine
Bumps and the Interaction Between Coal Pillars, Mine Roof, and Floor
(contract 0101778, Univ. MN). BuMines OFR 53-73, 1973, 87 pp.

31, Beckett, L. A., and R. S. Madrid. MULSIM/BM—A Structural
Analysis Computer Program for Mine Design. BuMines IC 9168, 1988,
302 pp.

32. Bathe, K. J. ADINA, A Finite Element Program for Automat-
ic Cynamic Incremental Nonlinear Analysis. Mech. Eng. Dep., MIT,
Cambridge, MA, Rep. 82448-1, Sept. 1975 (rev. Dec. 1978), 435 pp.

33. Bathe, K. J, E. L. Wilson, and R. H. Iding, NONSAP, A
Structural Analysis Program for Static and Dynamic Response of Non-
linear Systems. Dep. Civ. Eng., Univ. CA, Berkeley, CA, Rep. UCSESM
74-3, 1974, 73 pp.

34. Dixon, J. D., J. A. Mahtab, and T. W. Smelser. Procedures for
Determining Support of Excavations in Highly Yielding Ground.
BuMines RI 8990, 1985, 19 pp.

35. Smith, W. C. Controlling Rib Stability in High Coal Seams. Soc.
Min. Eng. AIME, Annu. Trans., v. 288, 1990, pp. 1843-1848,


ee

Pe eae

Figure 14.Comparison of supported versus unsupported ribs using base-friction model. Photographs
taken at 10-sec intervals.

Eee - ;
oe ee tees
eee



IMPORTANT GEOLOGIC PARAMETERS TO RIB INSTABILITY

The lithologic and structural variation of roof, rib, and
floor strata in underground coal mines significantly con-
tributes to rib and other instabilities by affecting the gen-
eral stress field surrounding an entry. Geologic features
that have a strong bearing on local strata conditions in-
clude (1) distribution and thicknesses of weak and strong
rock strata; (2) joints and cleats; (3) anomalies, such as
slickensides and pots; (4) depositional features, such as
clay veins and sand pockets; (5) folds and faults; and
(6) depth and dip of the mined coal seam. The implica-
tions of these parameters on overall rib stability need to
be considered.

A primary characteristic of sedimentary rock is the reg-
ular or irregular sequences of strata of different lithol-
ogies with different strengths and thicknesses. With few
exceptions, strata thickness is often related to stability.
Thick, uniform roof and floor strata usually indicate rel-
atively stable ground conditions. The same holds true for
thick, homogenous coal seams of similar rank. A destabi-
lizing effect on coal pillars and ribs is the alternating coal
bands of hard and soft coal within thick coal seams; here
the interaction between strong and weak bands leads to
unfavorable stress concentrations within the coal that can
lead to extrusion of individual coal layers and differential
shearing between coal layers of different hardnesses (9).
When the extrusion is severe, inclined coal faces can inter-
rupt longwall mining, particularly in thick coal seams, and
cause roof falls due to increased spans.

Examining and testing cores extracted from the various
rock and coal units for physical and petrological properties
remain the most dependable means to distinguish between
weak and strong rock and to estimate strata thicknesses.
A valid approach to studying rib behavior is to compare
the strengths and possible interactions between different
coal members within the same coal seam. Also, data on
strength and stiffness comparisons of the roof and floor
rock to the coal seam should be gathered.

The behavior of jointed or cleated coal ribs is depend-
ent on the mechanical and geometrical properties of the
joints or cleats. Surface roughness, cohesion, angle of in-
ternal friction, and normal and shear stiffnesses are im-
portant mechanical properties of joints and cleats. Geo-
metrical properties that affect rib behavior are cleat and
joint orientation and spacing. Cleating often occurs in sets
of two: face cleats, which are the main cleats, and butt
cleats, which are oriented perpendicular to the face cleats.
Cleat spacing is usually indicative of the coal’s physical

properties and the premining load history on the coal
seam. Preexisting fractures and other joint traces may
have similar origins as cleating but generally do not exhibit
the same repetitious frequency of occurrence. As discuss-
ed in the previous section, mining-induced stresses may be
responsible for forming new fracture systems in coal.

Conducting direct shear testing of intact coal samples
is the best method to determine cleat and/or joint shear
characteristics of coal. However, problems of scale need
to be addressed when the orientation and distribution of
cleats and joints on laboratory samples are different than
those of the coal pillar. Joint surveys on the coal pillar
and vicinity should be conducted,

Geologic anomalies and depositional features often
tend to weaken the strata that contain them. Anomalies
such as kettle bottoms, slickensides, and fossils are usually
associated with convoluted, and sometimes very smooth,
low-friction joint surfaces that displace easily. Deposi-
tional features represent breaks in the lithology and often
consist of clay veins and sand pockets that often intersect
more than one stratum. Anomalies and depositional fea-
tures that represent planes of geologic weakness in or near
the coal seam can cause significant reduction in the shear
strength of the roof and pillar. However, detection tech-
nology for anomalies and depositional features has proved
unreliable.

Folds and faults, while not limited to, are generally
more pronounced along mountain belt areas such as the
Rocky and Appalachian Mountains. Representing primary
topologic features, folding and faulting of sedimentary
strata are partly responsible for the premining stress field
surrounding mine entries, including unusually high hori-
zontal stresses not due to mining (3). Fault zones inter-
secting mine entries are frequently a leading cause of mine
opening instability, resulting in massive roof falls, floor
heave, and pillar yielding. Topographical investigations,
such as remote sensing, have shown limited success in
identifying fault traces, as have general geologic studies of
the areas prior to mining.

Related to folding and faulting is the depth and dip of
the mine coal seam. The thickness of overburden above
a mine entry also greatly influences premining or virgin
stress fields. Currently, there is a dearth of information on
how seam dip affects coal rib stability other than those
effects due to increasing depth and a rotation of fracture
and cleavage planes from true vertical.



INTRODUCTION

Rib falls contribute to the instability of mine openings,
representing more than a nuisance from the standpoint of
overall mine safety. Between 1983 and 1989 there were
1,316 reported fatal and nonfatal rib-related injuries in
US. coal mines (J)? A review of Mine Safety and Health
Administration injury reports covering the 7-year period
characterizes rib instability as follows:

1. A high number of rib falls appear to occur naturally
for no apparent reason,

2. Almost half appear to be related to activity, e.g.,
drilling, bolting, continuous mining, and hanging brattice.

3. Only 5% to 10% appear to be bump related.

4. More than half of all rib falls involve large-sized
slabs and/or chunks that may cause back injury, shoulder
dislocations, smashed toes and fingers, etc.

5._A significant number of injuries result from avoid-
ing rib falls rather than from direct contact with rib-fall
material. This suggests that even a minor rib fall can con-
tribute to a major injury.

This U.S. Burcau of Mines report investigates methods
to improve rib stability as part of the Bureau’s goal to
improve the safety of miners. The sudden and detrimental
nature of rib falls suggests that while rib falls are not
entirely avoidable, a practical approach to understanding,
characterizing, and controlling weak rib conditions is need-
ed. This report will discuss the mechanics of rib failure,
coal mine geology as related to rib behavior, rib support,
rib monitoring, and physical and numerical modeling pro-
cedures for assessing rib stability.

MECHANICS OF RIB FAILURE

Successful efforts to curtail rib failure require a basic
knowledge of how weak ribs behave. Two general kinds
of rib failure patterns are blocky or plate-like (slabbing)
failure and brittle failure, as shown in figure 1. The terms

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

Brittle failure

Slab failure

Figure 1—Two basic types of rib failure.

blocky or plate-like and brittle failure, as used in this
paper, describe the degree of fracturing in the coal rib.
Lightly or moderately fractured coal results in blocky or
plate-like failure patterns, creating larger slabs or blocks
of coal that may or may not be cleat related. Heavily frac-
tured coal indicates a pronounced brittle failure pattern
and much smaller and finer loose material. The failure
mechanism for ribs is primarily influenced by several fac-
tors: (1) general and local stress distributions, (2) orienta-
tion and frequency of fractures and cleats, (3) shear resist-
ance of preexisting fractures and cleats, (4) coal strength,
and (5) geometry of mine opening.

BLOCKY OR PLATE-LIKE FAILURE

Blocky or plate-like failure is characterized by slabs or
chunks of coal sloughing off the rib along a particular cleat
or fracture surface. The presence of large pieces of mate-
rial and low fracture activity indicate the coal is strong
relative to current stress levels in the coal seam. As the
intact strength of the coal is increased or the stress level
is decreased, the propensity toward brittle failure is dimin-
ished, and failure becomes more likely along preexisting
planes of weakness. When cleats and fractures open, dis-
tinct blocks are formed and, depending on their orienta-
tion, position, size, and rate of displacement relative to the
mine opening, can significantly affect rib stability.

The orientation of the cleat system (dip and strike)
relative to the rib wall usually has an immediate effect on
stability, with the most dangerous orientation being verti-
cally dipping with the strike parallel to the rib wall. The
effect of cleat and entry orientations is illustrated in
figures 2 and 3. British researchers have shown that a



RIB STABILITY: PRACTICAL CONSIDERATIONS
TO OPTIMIZE RIB DESIGN

By W. C. Smith!

ABSTRACT

The U.S. Bureau of Mines examined previous research on rib stability in an effort to develop a
practical approach to understanding, characterizing, and controlling weak rib conditions in underground
coal mines. Because success in stabilizing ribs depends on a basic knowledge of how weak ribs behave,
the report reviews the mechanics of rib failure and the relationship of coal mine geology and pillar
constraint to rib instability. Strategies for choosing an effective method of rib support are considered,
and various rib support methods are discussed. Finally, the report documents techniques for monitoring
ribs and use of models to assess rib stability; such monitoring and modeling can also help determine the
most effective method for roof support.

‘Mining engineer, U.S. Bureau of Mines, Denver Research Center, Denver, CO.


rib model

Boundary-element model

LL&£éE

Typical mining sequence

Figure 15.—Numerical procedure for studying rib behavior.

Roof
Ly
Coal }
t
t + Floor + A
+ t
\ t t t t t t ;
sf t t t t ty

Figure 16.—Discrete model of mine opening for rib study.
Thick lines indicate roof bolts. Arrows Indicate displacement
direction.

CONCLUSION AND RECOMMENDATIONS

In conclusion, this report presents a practical view to-
ward the study of rib failure and discusses steps that can
be taken to reduce the incidence of failing ribs. Once the
potential for rib fall has been established, the mechanics
of rib behavior should be adequately identified. Other
contributing causes such as geology, material and fracture
properties, and stress anomalies should be evaluated and
determined. The design of remedial support strategies
should be based on the mode of expected rib failure from
past mine experience and recent data obtained from a
field ground control monitoring program. Physical and

numerical modeling can provide additional information on
rib behavior based on global and local parameter in-
teractions not easily quantified in field measurements.
Decisions for improving stability through secondary sup-
port can involve more strategy and understanding of the
problem than to simply install bolts when rib failure
occurs.

It is suggested that further studies be undertaken to—

1, Develop the methodology to match rock and support
deformability characteristics. Coal seam interactions and
support effects need further evaluation.


COMBINATION OF SLABBING AND BRITTLE
RIB FAILURES

In general, both slabbing and brittle failures are
increasingly likely in the same rib as mine conditions
(stress field distribution, mine depth, coal strength and
quality, etc.) change. The redistribution and alteration of
the mine stress field ahead of an advancing longwall may
cause high levels of mining-induced fracturing in the
periphery of coal mine pillars where slabbing and block
type failure once predominated (10-12). Figure 4 shows

a pillar subjected to slabbing and brittle failures. Note
the buckling behavior that occurs along the rib center-
line. Often dramatic changes in shape due to volumetric
changes in the coal indicate the onset of mining-induced
failures. Some damage to coal may have previously oc-
curred during entry development, causing yielding and
crushing along the outer pillar margins. In one coal mine,
substantial pillar fracturing was found to occur in 50% to
60% of the total pillar volume immediately after mining

(22).

Figure 4.—Rib experiencing brittle and slabbing failure.


IC

9323

BUREAU OF MINES
INFORMATION CIRCULAR/1992

Rib Stability: Practical Considerations
To Optimize Rib Design

By W. C. Smith

UNITED STATES DEPARTMENT OF THE INTERIOR


used in combination with mesh and pans to provide addi-
tional constraint on smaller sized pieces of coal.

The question arises, How long should the bolts be?
Progressive pillar failure theory estimates the yielding zone
around average-sized pillars to range from 10 to 30 ft.
The obvious answer is the bolts need only be long enough
to ensure stability in the opening. Creating an integral
shell of "broken" rib coal that adequately confines coal
fragments behind the rib would be one valid approach,
Unfortunately, no easy method is available to determine
how thick this shell should be. The usual approach is
strictly an exercise in trial and error until an effective bolt
length is found and rib stability occurs. Angle bolting
from rib to roof may be a good idea in fractured coal
under competent roof, which can serve as an anchoring
base for the "broken" rib.

Another question concerning bolts is, What kind of bolt
should be used? The answer is to use a bolt that interacts

Cleat erection

Cable or straps

Timber

Chemical stabilization

Figure 6.—Several support types that improve rib stability.

with the rock mass in such a way to develop forces con-
ducive to stabilizing the rock mass whether tensioned or
nontensioned bolts are used. In other words, the deform-
ability characteristics of the rock mass and bolts should be
matched. Stiff, stronger coal seams can tolerate stiffer
support, while a yielding coal mass requires yieldable sup-
port. Otherwise, failure will arise in the support or rock
mass before stability is achieved.

Progressive fracturing of the coal rib represents an
unstable condition that is controllable in one of two ways:
(1) reduce local stresses, or (2) increase confinement
stresses. Because of high levels of frictional energy often
associated with this type of failure, the desired approach
to improve rib and pillar stability is to use yieldable sup-
port, which reduces local stresses by allowing small, con-
trollable displacements while still maintaining adequate
confinement pressure to the rib.

Some underground coal mines that have a history of rib
rolls frequently experience rib support failures, such as
bolt heads “popping off" conventional resin and expansion-
anchor-type bolts. Yieldable bolt designs that have been.
utilized in such mines appear to stabilize the coal seam
and ribs more effectively by controlling displacements to
reduce stress buildup. Important considerations before
designing yieldable bolts are to estimate the magnitude of
the rib and pillar stresses and instability to be encounter-
ed, and then the maximum allowable deformation that can

key

‘Maximum bolt head yield distance
D_ Supported rock yield cistance

Figure 7.—Compatibility of support with expected rib de-
formation. Top, Yieldable bolt matches deformability character-
istics of rib; bottom, shift bolt is not compatible with deformation
and bolt head "pops off."


PILLAR CONSTRAINT

Vv Hard roof. v v

Bureau researchers demonstrated that an important
factor contributing to the mode of failure is pillar con-
straint (74). Pillar constraint was shown to occur in West-
ern underground coal mines because of sharp differences
in roof, rib, and floor physical characteristics, specifically
Young’s modulus; Poisson’s ratio; and the coefficient of
sliding friction of roof, rib, and floor rock. Figure 5 shows
how a stiff roof and ductile floor can affect rib displace-
ments, Generally speaking, strong roof and floor rock en-
hances rib confinement pressures near the roof and floor;
weak, ductile roof and floor rock reduces rib confinement
pressures near the roof and floor. As was previously dis-
cussed, even the presence of hard and soft banding in the
coal seam causes similar stress anomalies within the coal.

The effect of constraint was demonstrated on a simu-
lated cleat system. A Bureau study showed that the be-
havior of an entire pillar was dependent on whether the
cleats were open or closed (15). As with pillars, whether
a cleat system is open or closed influences how load is
transferred through the rock mass. Open cleating indi-
cates a loss of constraint and load transfer, and is gen-
erally a sign of a progressing failure condition. In coal

Figure 5.—Effect of stiff roof and ductile floor on rib dis-
placements,

ribs, open cleating may indicate that slab or block forma-
tion is underway and sloughage may be imminent. Closed
cleats or fractures without displacement and/or any new
fracturing suggests a stable pillar and rib condition in
which the pillar is not yielding.

SUPPORT STRATEGIES

Whether to use roof support—and what type and
amount of roof support is necessary—is usually a subjective
decision based on site-specific factors. Some factors sig-
nificantly affect support selection and strategy:

1. The magnitude, location, and direction of the high-
stress regions in the pillar and vicinity,

2. The orientation of the cleat system and other geo-
logic structures in the coal in relation to the stress field.

3. The joint and coal physical properties compared with
the support characteristics. 7

4, The depth and extent of the fracture zone in the pil-
lar to be stabilized and the height of coal seam.

5. The sizes, shapes, and pattern of the failed rib mate-
tial to be immobilized and retained.

6. The rate and magnitude of allowable rib failure and
life expectancy of the mine opening.

The mechanics of rib failure, as previously discussed,
has a strong bearing on support selection and strategy, as
does economics, The challenge is to sufficiently support
the rib during the useful life of the mine in a practical
manner. Installing rigid steel sets may adequately main-
tain rib stability in many mines, but such installations are

impractical in temporary coal mine workings such as gate-
roads near a longwall panel. Figure 6 shows practical rib
support ideas for coal mines. In the following sections, rib
support alternatives for different rib conditions in under-
ground coal mines will be presented.

RIB BOLTING

In coal ribs where slabbing or plate-like rib failures
dominate, bolting will improve rib integrity. Securely
anchored bolts must effectively increase the frictional
forces between slabs to restrict horizontal and vertical
movements, ¢.g., toppling, buckling, and sliding (16). This
assumes that the shear properties of the bolts are high
enough to prevent bolt failure, regardless of the orienta-
tion of rib cleat or fractures.

Bureau research has developed an approach to design-
ing a bolting pattern based on average slab widths and dis-
tribution along the rib centerline (17). The method allows
for variability of bolt spacing to account for changes in
slab size and overlap. In most cases, depending on slab
configuration, one or two rows of bolts are sufficient to
secure slabs in place. Unless highly yielding coal is en-
countered, resin-grouted or expansion-anchor bolts can be


Mining-induced
fracturing

Floor

A. Stable rib condition
8 Unstable rib condition

Figure 2.Effect of cleat orientation on rib stability.

20 T T T T
o 0.82-in-diameter
s 15 fiberglass bolt { |
o “7
s 1-in-diameter an
Stok “7 4
= type 40 steel
bolt
sr - KEY 7
“7 © Yield point
-" A 1 1 1
0 o2 04 06 08 10 12

DEFORMATION, in

Figure 3.—Effect of changing entry orientation on rib con-
ns.

dramatic increase in both face and roof instabilities in
longwall coal mines occurs if the angle between the strike
of the cleat and the face line is less than 30° (2).
Chemical changes due to weathering of coal and clay
stringers found in coal, particularly along cleat surfaces,
may decrease coal integrity. Rescarch in this area indi-
cates significant weathering effects to coal at temperatures
from 25° to 80° C, particularly under high humidity (3).
Most underground coal mines fall within the lower end of
that temperature range and often experience high humidity
in certain areas of mine workings. Chemical changes in
some clay partings found in coal may cause swelling nor-
mal to the bedding or cleat direction and contribute to

splitting as a result of high tensile stress concentrations
(4). While certainly not a leading cause of rib instabil-
ity in most mines, weathering effects of coal should be
considered a possible cause of rib instability in specific in-
stances when clay stringers are present.

Once a potentially dangerous slab and block condition
develops, rib instability does not occur until "loose coal”
moves uncontrollably into the mine opening, The danger
from such a rib fall is based on the height of the fall, the
volume of material that falls, the suddenness of the fall,
and the overall effect on the stability of the entire opening.
Coal slab or block movement may be a reaction to stress
relaxation caused by overburden or mining-induced load-
ing, or to body forces due to gravity sufficient to influence
slab and block rotations along the pillar margin. “Loose”
slabs or blocks subject to little or no pillar confinement
may exhibit planar sliding and toppling behaviors charac-
teristic of rock slopes (5).

BRITTLE FAILURE

Brittle failure of coal induces micro- and macro-
fracturing, causing the coal to become almost granular as
the fracturing process intensifies. The size of coal frag-
ments depends on the degree of fracturing, cleat and/or
preexisting fracture pattern, and the height of the mine
opening. Brittle failure has been well documented in lit-
erature and has been shown to occur at low levels of con-
finement in weak coal and at loads as little as 30% of the
uniaxial compressive strength (6-7). Brittle failure can
cause greater rib dilation than larger slab and block
failures as fractures open and the fracture intensity in-
creases. As fracturing intensifies, coal strengths decrease.
The decrease in strength can range from 50% to 80% of
ultimate strength in soft rock (8).

Canadian researchers describe brittle failure in coal
from the Rocky Mountain coal belt as pseudoplastic flow
behavior known as extrusion deformation, and attribute
microfracturing in the coal to interactions between repe-
titious hard and soft banding found in most bituminous
coals. Interaction between individual bands of different
hardnesses within the coal seam may respond to mining-
induced changes in the stress field by promoting high
shear stresses and subsequent failure within the coal, caus-
ing a highly fractured rib (9). The reduced coal strength
leads to a weakened rib with flow characteristics as frac-
tured coal moves into the entry. This behavior is charac-
teristic of low-rank coals found throughout the Rocky
Mountain coal belt and not of higher rank coals found
elsewhere (9).


CONTENTS

Page
Abstract .........08 . 1
Introduction ....... 2
Mechanics of rib failure . 2
Blocky or plate-like failure 2
Brittle failure 3
Combination of slabbing and brittle rib failures 4
Important geologic parameters to rib instability . . 5
Pillar constraint . . 6
Support strategies . 6
Rib bolting. ... 6
Other loose rock confinement methods 8
Test site monitoring techniques . 10
Rib model studies 11
Physical models . . 2
Numerical models 12
Conclusion and recommendations beeen eee . 14
References . 2... 6... e cece cece eee eee eee eee . 15
1, Two basic types of rib failure 2
2. Effect of cleat orientation on rib stability... 3
3. Effect of changing entry orientation on rib conditions. os 3
4. Rib experiencing brittle and slabbing failure ....... . 4
5. Effect of stiff roof and ductile floor on rib displacements .. . 6
6. Several support types that improve rib stability 7
7. Compatibility of support with expected rib deformation . 7
8. Rib support types designed for yielding rib ........ 8
9. Pull-test results of fully grouted steel and fiberglass bolts 8
10. Using wire mesh for rib control . 9
11. Technique for quantitatively monitoring rib profile .. . 10
12. Drilling horizontally in coal to install instrumentation 10
13. Example test plan for longwall study cr
14. Comparison of supported versus unsupported ribs using base-friction model B
15. Numerical procedure for studying rib behavior 14
16. Discrete model of mine opening for rib study 14



TEST SITE MONITORING TECHNIQUES

Rib monitoring techniques should be compatible with
the types of rib failures expected. Movement of coal into
the mine opening is a primary concern and should be
monitored if instability is expected. Two regions of rib
that may require routine monitoring are the rib profile and
the immediate thickness of coal that comprises the rib.

Monitoring the rib profile provides input on the nature
of the failure from roof to floor. One simple method to
detect changes in the rib profile is by applying rock dust,
A drawback of this method is that it is strictly visual and
does not provide information on the rate of movement
or degree of sloughage. Photographic methods, however,
work well as visual aids for qualitative determination of rib
deterioration.

A radical change in the rib profile often is a precursor
to a burst. To detect potentially dangerous rib conditions
in Australian mines, monitoring rods are erected alongside
the rib and horizontal rib movements are routinely mon-
itored at different elevations on the rod (27). This method
of rib monitoring using a vertical line of reference, as
shown in figure 11, can be helpful in evaluating rib re-
sponse during face advancement.

Another practical reason for monitoring rib behavior
is to determine the extent and depth of the yield rib zone
so that the proper length dowels or bolts can be installed.
As shown in figure 12, installing horizontal multipoint

KEY
Ad displacement

Figure 11.—Technique for quantitatively monitoring rib profile.
(MPBX = multipoint borehole extensometer).

Figure 12.—Drilling horizontally In coal to install instrumentation.


stress, and geometric parameters affect rib behavior.
Modeling is one method to study these interactions.

Regardless of the model method used to study rib in-
stability, a practical approach is to (1) define the problem
of rib instability, (2) identify important parameters and
processes to be studied, (3) determine the appropriate
model based on the expected rib failure, (4) verify and
validate the results with field or laboratory data, and (5) if
significant, incorporate the results into rib and pillar
design.

PHYSICAL MODELS

Limiting the number of parameters in the model is one
technique that reduces model complexity while saving time
and money. This approach can be most usefully applied
in physical modeling, in which the simplicity or complexity
of the model is directly related to the number of worker-
hours needed for model preparation and execution. Often
loading core or small block samples can generate useful
information. For instance, cores or blocks of coal can be
prepared and tested to investigate the effects of various
confinement techniques, such as bolting pattern and mesh,
on coal strength and deformation. Bureau research has
developed a rib bolt spacing formula based on triaxial test
results on rock cores with preconfigured bolt patterns (17).
The base-friction modeling method has been used to com-
pare supported ribs with unsupported ribs, as shown in
figure 14 (27). Simple hands-on rib modeling techniques
are useful in developing and testing concepts prior to
conducting large-scale field tests or running more complex
models.

Physical models requiring similitude between what is
modeled and the measured field conditions present greater
challenges in terms of time and expense. Usually such
models have been reserved for global mine control design
rather than for local problems such as rib instability. Re-
searchers in England have applied dimensionless analysis
to evaluate ribside pack-width effects and rib behavior
under different loading conditions (28).

NUMERICAL MODELS

Normally, complex-parameter interactions are more
efficiently modeled using numerical techniques. A valid
numerical procedure for determining progressive rib
failure considers changes in the mine-wide static load dis-
tributions imposed on underground mine workings dur-
ing various stages of the mining cycle (29). A useful ap-
proach is shown in figure 15. Initially, a three-dimensional
displacement-discontinuity boundary-element model (30),
such as MULSIM (37), is used to determine global stress
distributions above mine workings during any stage of
mining. Predictions about pillar and ground control per-
formance are provided by the output of this first program.
The next step is to zoom in on a smaller scale portion of
the larger mine layout model, such as the region of in-
terest around a single pillar or a slice along a pillar edge.
In the second step, loading conditions predicted by the
boundary-element code are input into a pseudoelastic,
finite-element model such as ADINA (32) to observe post-
yield behavior patterns of the pillar or rib in greater detail.
Establishing boundary conditions in this way can be useful
when using other model types.

Supported rib effects under yieldable pillar conditions
have been studied using NONSAP finite-element code
(33), which is similar to ADINA. Findings from that study
showed that by using rib support that complies with the
deformability characteristics of the rock mass, e.g., coal,
the resistance needed by the support to achieve stability
can be relatively small (34).

When slab or block failures are expected, discrete-
element models offer appropriate solutions to rib failure.
These programs discretize the rib into discontinuous slabs
and blocks that interact, and the programs generate dif-
ferent outcomes depending on material and fracture sur-
face characteristics of the blocks. The discrete model
(fig. 16), which takes support elements into account, has
been used to test different support strategies for varying
rib conditions (35).



Library of Congress Cataloging in Publication Data:

Smith, W. C. (William Cyrille), 1955-
Rib stability : practical considerations to optimize rib design / by W.C. Smith.

p. cm. — (Information circular, 9323)
Includes bibliographical references (p. 15).
1, Ground control (Mining). -2. Coal mines and mining—Safety measures.

3. Coal—Geology. 4. Structural stability. I. Title. II. Series: Information circular
(United States. Bureau of Mines); 9323.

TN295.U4  [TN288] 622 s—dce20 [622’.28] 92-9092 CIP



Information Circular 9323

Rib Stability: Practical Considerations
To Optimize Rib Design

By W. C. Smith

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

BUREAU OF MINES
TS Ary, Director


be expected and tolerated in the support. It should be
remembered that the plastic deformation of conventional
steel expansion-anchor bolts is limited to only a couple of
inches and that any rib dilation greater than that around
the bolt head would most likely result in premature bolt
failure. The importance of matching the deformability
characteristics of the support to that of the rib is illus-
trated in figure 7. Figure 8 shows other promising tech-
niques for supporting yielding coal ribs.

Two yieldable bolt designs that require special fabrica-
tion are helical and smooth-bore-die yieldable bolts. The
flexible helical rock bolt (18) appears to support load well
in a yieldable coal rib but is difficult to install, often
requiring an oversized hole. Another yieldable bolt tested
by the Bureau consists of a standard expansion-anchor bolt
with a yieldable collar or smooth-bore die that deforms
along the threaded portion of the bolt head (19). This
bolt design is used extensively in deep coal mines in the
Republic of South Africa to prevent coal bumps and shows
promise for many U.S. coal mines with similar problems.

A less expensive and perhaps equally effective approach
to designing a yieldable bolt is to convert a standard resin-
grouted or expansion-anchor bolt into a more yieldable
bolt design by inserting yieldable structures between the
bolt head and plate, e.g., wooden blocks, sections of stcel
tubing, or polyvinyl chloride (PVC). One coal mine suc-
cessfully controlled violent rib failures by installing angle
bolts with split-steel tubes between the bolt heads and
plates (20).

Other materials to replace steel bolts are available, such
as wooden dowels, fiberglass, and PVC bolts. An advan-
tage of wood, fiberglass, and PVC bolts is that they can be

Hollow bolt

Helical bolt

Fiberglass bolt

Hl

Figure 8.—Rib support types designed for yielding rib.

mined through with a cutter or shearer during pillar ex-
traction without serious damage to mine machinery and
personnel. Wooden dowels have been successfully tried in
many coal mines, especially in forepoling procedures for
pre-reinforcement of roof and rib in room-and-pillar min-
ing. Fiberglass bolts, used in an underground borate mine,
but not in U.S. coal mines, have yield characteristics that
may benefit failing coal ribs. Figure 9 shows contrasting
deformation differences between fully grouted steel and
fiberglass bolts with similar yield strengths. Australian
coal mines have successfully applied fiberglass and PVC
bolting to rib support (27).

OTHER LOOSE ROCK CONFINEMENT
METHODS

In addition to bolting, other methods available for the
confinement of loose rib rock include mesh, cable slings,

7

v4

/

Improved rib stability

Any” £
“4
10°
Coal pillars
NORTH LEGEND
t \ Face cleat orientation
0 30 60
es |
Scale, ft

Figure 9,—Pull-test results of fully grouted steel and fiberglass
bolts.


LEGEND
--@ Extensometers

—<= Borehole pressure cells

A> Cleat direction

Figure 13.—Example test plan for longwall study.

borehole extensometers (MPBX) into the rib is one way
to gauge horizontal rib movement at varying depths into
the pillar. Geophysical methods offer another alternative
for measuring the rib pillar yield zone to detect changes in
coal integrity. One such method that shows promise is
capable of obtaining sonic velocity profiles through. the
coal seam and may be used to detect potentially dangerous
rib conditions (25). Another technique to detect the de-
grce of rib fracture is an air-injection method that relates
the rate of air leakage to fracture intensity (12).
Gathering baseline information on general load trans-
fer and pillar yield characteristics along gateroads in

underground longwall coal mines would also help quantify
rib failure. Such comprehensive field investigations and
instrumentation have been implemented by the Bureau to
monitor ground control around longwall panels (26). One
such plan is shown in figure 13. Notice the MPBX in-
stalled along the rib periphery and pillar corner. Strat-
egies for installing arrays of borehole pressure cells and
MPBX are aimed primarily at testing pillar performance
during mining. This supplemental information represents
a foundation for understanding the "driving forces" behind
failing ribs.

RIB MODEL STUDIES

A low-risk procedure for evaluating support needs and
behavior of supported rock masses around underground

mine openings is the modeling of in situ conditions. As
mentioned previously, the interaction of many structural,