indicating a low potential for buckling failure. The following is
a list of general trends identified from the parametric analysis:

1. At low regional horizontal stresses (< 3.45 MPa), beam
buckling is highly unlikely in strong (high stiffness) rocks
with slenderness ratios of less than 90. In this case,
localized wedge failures due to gravity forces may be a
much more significant factor

2. At moderate regional horizontal stresses (6.9 to
13.8 MPa), beam buckling is likely to occur in most
limestone rocks with intermediate stiffness characteristics
and slenderness ratios of greater than 50.

3. At high regional horizontal stresses (> 27.6 MPa), beam
buckling is likely to occur under almost any strength
limestone rocks with slenderness ratios of greater than 30.

CONTROLLING HIGH HORIZONTAL STRESS
THROUGH REINFORCEMENT: A CASE STUDY

Roof bolting has become an accepted means of maintaining,
roof stability in some stone mines with high horizontal stress
conditions, In studying the role of reinforcement, the authors
have recognized two basic approaches: 1) support the broken
rock mass after the failed rock stabilizes, or 2) reinforce the roof
beam to prevent buckling failure from occurring. Different roof
support systems can be employed to achieve either of these
objectives. The design guidelines presented here stress the
second philosophy, i.c., prevention of roof beam buckling failure
from occurring

Available roof reinforcement systems consist of both active
and passive types. An active system attempts to pulls the roof
together by tensioning the bolt head, Conversely, a passive
system relies on the sag of the roof beam to actively load the roof
bolt, Two major considerations in deploying roof reinforcement
are 1) suspension, pinning, or tying wedges or prisms of rock to
a more competent member above or 2) reinforcing or beam
building by joining individual thinner beams together to increase
the effective beam thickness. In some cases where vertical
jointing is common, blocks thickened by reinforcement can form
the stable Voussoir Arch (Parker, 1996). Other important factors
are bolt spacing, length, and capacity, all obviously tied to local
stress, geologic, and mining considerations. The options
available to achieve the above design issues are many, The types
of products includes both mechanical and resin point anchor
bolts, full-beam active and passive bolts (combinations of
these two systems), and friction stabilizers, such as split-sets and
swellex bolts, which rely on radial and frictional forces for
anchorage. Because roof bolts in the underground stone industry
are generally used on a spot basis, the selection process is
complicated by the need to determine how, when, and where to
use roof reinforcement. The following section will outline a case
study where roof reinforcement was used to control high
horizontal stresses.

Site Conditions

In-depth field studies were conducted at an underground
operation in Pennsylvania mining the Loyalhanna Limestone.
The limestone averages 21 m (70 ft) in thickness and is overlain
by soft claystones and shales. Horizontal bedding planes are
generally not laterally persistent in the Loyalhanna Limestone;
therefore, it is sometimes necessary to create the roof line with
blasting techniques. Immediate roof beam thicknesses were
measured that ranged from 0.15 to 0.5 m (0.5 to 1.5 ft). Local
practice is to leave approximately 2 m (6 ft) of limestone in the
roof. This mine had experienced some problems with controlling
a zone of failed roof that had a distinct directional trend to it. It
had also periodically used roof support to control this ground
with varying degrees of success. During this study, four types of
support were used and their performance was evaluated: 1.59-cm
(5/8-in) mechanical point anchor bolts, 2.22-cm (7/8-in) fully
grouted resin bolts, 2.54-cm (1-in) torque-tensioned resin bolts,
and 3,49-cm (1-3/8-in) friction stabilizer bolts (Figure 7). All
bolts used in this study were 2.4 m (8 ft) in length.

MECHANICAL POINT ANCHOR FULLY GROUTED

Resin

U
Ay Mechanical
‘Shell
(anchor)

Compression

Movement

Root? f
tine piote the Piote
TORQUE TENSION FRICTON STABILIZER

al]—
Fost set Resin Rebor pas | Pe
- =|] =
an] | me
Radio!
-
Force
Strato -||>
Movement pas es
Slow set Resin | oa] |
wT
Root Plate Root
te tnreods A line Plate

Figure 7. - Four common roof support systems: mechanical
point-anchor, fully grouted resin, torque tension resin, and
friction stabilizer bolts.


while considerable numbers of point anchor and friction stabilizer
bolts failed. Bolt stability was directly related to load capacity.
In some high horizontal stress conditions, the 6 to 12 metric tons
(7 to 14 ton) of load capacity measured in the point anchor and
friction stabilizer bolts was not enough to withstand loads
generated by the deforming beams.

SUMMARY AND CONCLUSIONS

Roof beam behavior in underground stone mines with high
horizontal stress conditions has been characterized using data
gathered from the literature, mine visits, and field test sites.
These roof beam behavior characteristics are:

1. Roof beams subjected to high horizontal stresses will
deform or sag more than those only subjected to gravity
loading,

2. — Horizontal stresses are not dependent only on overburden.
Shallow underground stone mines can have a wide range
of horizontal stresses.

3. Horizontal stresses in western Pennsylvania, West
Virginia, southwestern Virginia, and eastern Kentucky can
be several times greater than the vertical stresses.

4. Observations of roof beam failures associated with high
horizontal stresses have been recognized in several
underground stone mines by the authors, as well as several
other investigators.

5. Measurements of principal horizontal stress at one
underground stone mine ranged from approximately 10 to
60 MPa (1.4 to 9 Ksi), with an average of approximately
25 MPa (3.6 Ksi) oriented in the ENE direction.

6. Buckling failure will occur once critical stress conditions
exist within roof beams subjected to high horizontal
stresses and may appear as tensile or shear failures

7. Critical stress conditions are influenced by beam
stiffness/strength and geometry.

8. At one underground stone mine, approximately $ cm
(2 in) of total roof sag preceded a total roof collapse. It
is assumed that roof layers began failing at approximately
one-half of this total, or 2 to 3. cm (0.8 to 1.1 in).

9, Elastic numerical models agreed with field data when
Euler’s stress-based failure criterion was applied. These
models were then used to examine the interaction of
unique roof beam geometries with far field applied stress
conditions.

10. Numerical models with roof bolts suggest that elastic
beam sag could be reduced to 2 cm (0.8 in) or less at
applied horizontal stress at or below 27.6 MPa (4 Ksi) and
slenderness ratios of less than 90.

A design technique for layered limestone roof rock is presented
based on the use of both stress- and deformation-based failure
criteria. The stress-based failure criterion uses the Euler formula
to evaluate the critical stress for buckling of beams of varying
stiffness/strength and geometry subjected to different horizontal
stresses, The stability of roof beams is also achieved using a
deformation-based failure criterion calibrated with field data.
The deformation-based failure criterion is used to evaluate the
role of rock reinforcement in stabilizing roof beams. Roof bolt
selection is based on bolt strength, length, and installation time
Conclusions drawn from this investigation are —

1. At low regional horizontal stresses (< 3.45 MPa), beam
buckling is highly unlikely in strong (high stiffness) rocks
with slenderness ratios of less than 90. In this case,
localized wedge failures due to gravity forces may
represent the more hazardous condition.

2. At moderate regional horizontal stresses (6.9 to
13.8 MPa), beam buckling or shearing is likely to occur in
most limestone rocks with intermediate stiffness
characteristics ands ratios of greater than 50.

3. At high regional horizontal stresses (>27.6 MPa), beam
buckling is likely to occur under almost any strength
limestone rocks with slenderness ratios of greater than 30,

4, Rock bolts act to reinforce the roof beams, increasing the
effective thickness of the beams. These thicker layers
produce roof beams that are more stable against buckling
failure.

5. Two important rock bolt selection criteria under high
horizontal stress conditions are bolt stiffness and load

capacity.

The design technique presented provides stone miners a method
for making stability assessments of roof beams in high horizontal
stress conditions. The consequences of widening rooms,
changing geology, and horizontal stresses can be examined.
Also, provisions have been made for considering the role of rock
reinforcement and selecting an appropriate roof bolt.

REFERENCES

Bickel, D_L., “Rock Stress Determinations From Overcoring -
An Overview”, U.S, Bureau of Mines Bulletin 694, 1993, pp, 58-
62.

Gale, W. J., “The Application of Stress Measurements to the
Optimization of Coal Mine Roadway Drivage in the Iawarra
Coal Measures,” Proceedings of the International Symposium on.
Rock Stress and Rock Stress Measurement, Stockholm, Sweden,
September 1986, pp. 551-560.

Emery, C.L., “In Situ Measurements Applied to Mine Desig
6" Symposium on Rock Mechanics, Rolla, MO, 1964, pp. 218-
230.


orientations match reasonably well with the regional stress field
found in other mines within this region (Figure 3)

Roof Failure Measurements

To understand better how these local horizontal stress
conditions affect roof behavior, measurements of roof sag and the
associated roof failure were made. The immediate roof began to
fail in the middle of July approximately 90 m (300 ft) from the
end of a previous directionally controlled roof fall. This original
failure took on the appearance of a series of low-angled shear
planes cutting or ripping the rock. The orientation of these
planes was perpendicular to the orientation of the horizontal
stress field as determined from previously discussed stress
mapping and stress monitoring activities. Seven sag monitors
were quickly placed along the projected failure trend. Two of
seven monitors used the 20-point sonic probe; the other five were
prototypes of a recently developed NIOSH monitor called the
Remote Monitoring Safety System (RMSS) (Iannacchione et al.,
1997).

Data collected from three of these monitors are shown in
Figure 10. Monitor No. 7 collected sag measurements for almost
70 days prior to total roof collapse. During this time, the roof
sagged in three distinct phases, The first phase was marked by
a slow but steady sag in the lower roof beam. At approximately
40 days there was a sudden increase in the sag of the beam. The
third phase indicated that the beam sag velocity lessened, but

ended in total roof failure. A total of approximately 5 cm (2 in)
of roof sag occurred prior to roof collapse. Data from monitor
No. 3 showed a much different trend. Unlike monitor No. 7, this
instrument was placed close to an existing failure; therefore,
significant beam sag could have already occurred. The area
began to “cut or rip” on July 26, rapidly extending the zone of
failed roof. The roof associated with monitor No. 3 went from.
stable to unstable in a matter of 5 hours.

Monitor No. 4 was purposely placed slightly away from the
main failure trend. The magnitude of sag measured from this
instrument was 1/10 the magnitude of the instruments within the
failure trend, However, these measurements did show that beam
bending and associated shearing extended significant lateral
distances on the order of 6 m (20 ft) from the fall’s edge and
12 m (40 f) from the center of the fall. This monitor also
showed that while sag was initiated in the lowest beam, beam
separations quickly moved much higher in the roof.

Rock Bolt Performance

Friction stabilizer bolts 2.4 m (8 ft) in length were installed in
portions of the directional roof failure trend. Unfortunately,
many of these bolts failed in the lower 1/3 of the bolt or at the
location of the first major beam separation. Some of the bolts
were stretched apart while others were sheared, indicating
considerable shifting along the beam separations. In either case,
these bolts were not of sufficient strength to resist the
considerable beam sag,

LT

Sr °
Monitor No.7 / N
4a} y, Prior root fall ~.
y N
g 3) we
g2 July 26 / Oct. 3
4 \ i
0 ——
o bays 70
‘Anchor depths
—03m
- 27m
—-- 51m
0.5 - Monitor No.4
4
o4 duly 26 3 Monitor No.3 7
g 03 | § oa
B02 fauyre [7 2 f
Es 1 } July 26
ot - oo
| 1s Oct. 3 |
0 = 0
° Days 120 ° Hours 2

Figure 10. - Roof behavior associated with the large roof fall caused by high horizontal stresses.


‘The characteristics of each of these supports are defined by
their load-bearing capacity, stiffness, and _postfailure
characteristics. These characteristics were measured by
conducting pullout tests (Figure 8) in the Greenbrier Limestone
at NIOSH’s Lake Lynn Underground Laboratory. The 1.59-cm
(5/8-in) mechanical point anchor bolts displayed a low level of
stiffness as the bolts elastically deformed approximately 0.5 cm
(02 in), attaining a load of approximately 10 metric tons
(II tons). At this point, the bolts stretch and the anchors began
to move or slip. These bolts displaced 14 cm (5.5 in) before
failing. The 2.22-cm (7/8-in) fully grouted bolts are much stiffer,
achieving approximately 22 metric tons (24 tons) of load at only
0.5 cm(0.15 in) of elastic deformation, primarily at the bolt head.
One of the fully grouted bolts failed at this point; the other
achieved over 4 cm (1.2 in) of plastic yield and gained an
additional 10 metric tons (11 tons). The 2.54-cm (1-in) fully
grouted torque-tension bolts were very stiff, achieving almost
35 metric tons (39 tons) of load at just over 0.5 cm (0.15 in) of
elastic deformation, Both torque-tension bolts displayed very
little plastic yield, displacing a total of only 1 cm (0.39 in) prior
to failing between 36 to 37 metric tons (39 to 41 tons), The
friction stabilizer bolts, ie., split-sets and swellex bolts, rely on
radial and frictional forces for anchorage. They are excellent at
stabilizing wedges because of their ability to provide immediate
resistance to deformation as a result of their high initial stiffness.
‘This unique anchorage, however, does not provide very high load
resistance, ranging between 4 and 12 metric tons (5 and 14 tons)
(Scott, 1989; and Stillborg, 1992). These bolting systems can
deform continuously for upwards of 5 cm (2 in) under constant
load.

Displacement, in

o5 1 15 45 5 88
“0
2 {o 2.54 em Torque Tension 48
330 ss 3
Eo] i ae
° ; - ‘em Fully Grou! or ”
220bhe 2.22 ly Gro uted Reb ne
& oh n £
|i a ;
Zo: ' 10 8
“2+ + 1.59 em Point Anchor
° °
o 1 2 ton Te

3
Displacement. cm

Figure 8. - Pull tests conducted on mechanical point anchor, fully
grouted, and torque-tension bolting systems in limestone roof at
the Lake Lynn Laboratory.

Measurements of Horizontal Stresses

Effective and efficient roof support systems require an
understanding of how high horizontal stress conditions control
the behavior of roof strata. An understanding of roof behavior
can be accomplished by observational and measurement
techniques used to analyze roof rock performance. In addition
to stress mapping, | | in situ stress measurements were conducted
with the hydraulic fracturing method at the underground study
site. The borehole where the tests occurred was examined with
a fiber-optic probe (Figure 9). The roof in this area is composed
of alternating sequences of limestone, claystone, and limey

sandstones. The first 1.8 m (6 ft) contains a massive limestone
with occasional bedding laminations. Between 1.8 and 2.7 m (6
and 9 ft) the first of numerous red claystone members occurs.
Above these red bed members is another competent limestone
unit. A relatively thick unit of red claystone is found between 4
and 5.2 m (13 and 17 ft). A massive calcareous sandstone unit
is found high in the hole (5.2 to 5.9 m or 17 to 19.5 ft).

C2 pea

Test No.8 erent

Depth, m

Test No 3&5
Test No-4
Test 34°

Test No.2

Test No.9

Test No.10

peed mT

Red shole bone “cate wee
Test Nott s
Legend
re = Bedding
x seasing ed shale Limestone

- ooo [anes sore

Figure 9. - Stress ellipses from six successful hydraulic
fracturing tests within the roof of a Loyalhanna Limestone
Mine.

Of the 11 tests, six produced enough data to estimate stress
field orientations and magnitudes. Measurements of principal
horizontal stress ranged from approximately 10 to 60 MPa (1.4
to 9 Ksi) with an average of approximately 25 MPa (3.6 Ksi). Of
the five unsuccessful tests, three were due to our inability to
propagate a fracture using pressures ot aimost 69 MPa (10 Ksi)

These are very high pressures and attest to the strong nature of
the rock and the high in situ stress conditions. The other two
unsuccessful tests were related to either problems with the
fracture tool or hydraulic packers.

Of the six successful tests, stress field orientations could be
determined from only four tests. Orientations were measured
from the direction of vertical propagation produced during
fracturing (Figure 9). All four vertical cracks propagated in the
N45 to 65E direction. With the exception of test No. 9, all stress
field values indicate that elevated horizontal stress conditions
occur within this portion of the mine. In the case of test No. |,
the stress field was found to be extremely high. The reasons for
this are still under investigation, but are undoubtedly related to
the limestone beam-bending process, the location of the softened
(fractured) zone within the immediate roof, the stress
concentrations around the mine opening, and the variations in
beam stiffness, i-e., stiffer layers store high levels of stress. The


LEcEND
Models without bolts.
© y= 3.45 MPO

+ o,= 69 Meo
eo, 13.8 MeO
Sieh ¢ Oye 275 uPa

Models with bolts
one 13.8 Pe
40, 276 Mea

Moximum Vertical Beam Sog, em

ot

© 10 20 30 40 50 60 70 80 90
Slenderness Ratio, L/t

Figure 11. - Maximum vertical beam sag for several beam
geometries and applied horizontal stress conditions with and
without roof bolts

To help examine the role played by rock reinforcement in
controlling critical roof beam sag, numerical simulations were
again used. This time, structural elements were placed in the
model to simulate rock bolts. All four supports discussed earlier
were evaluated. Material properties representative of these
supports were chosen to simulate the material properties
observed in the pull-out tests. Mechanical point anchor bolt
performance was simulated by attaching the two end-points of
the support elements to the finite difference grid. The two resin
bolts were glued to their adjacent elements by placing a softer
material with properties similar to resin grout between the
modeled limestone and steel reinforcement. The friction
stabilizer was modeled by removing the resin grout and allowing
the steel to become attached directly to the modeled limestone.

These four rock reinforcement systems were run under two
conditions presented previously where buckling failure was
predicted by both the critical stress and critical displacement
failure criteria. These two conditions represent a moderate
failure potential, where horizontal stress was 13.8 MPa (2 Ksi)
and the slenderness ratio was 45, and a very high failure
potential, where the horizontal stress was 27.6 MPa (4 Ksi) and
a slenderness ratio 90. All four bolts under both test conditions
reduced maximum vertical beam sag to below the 2.5 cm (1 in)
critical displacement failure criteria. That is approximately 1 cm
(0.4 in) for the moderate failure potential conditions and 2.1 cm
(0.8 in) for the very high potential failure conditions.

Selecting Effective Roof Reinforcement

Although the beams have been stabilized against buckling
through rock reinforcement, some of the simulated bolts in the
model were at or near their yield strength. Therefore it would be
inappropriate to say that any kind of bolt could stabilize the beam
under these conditions. In fact, our case study showed just the
opposite. This is a direct result of the unique manner in which
different bolts resist beam bending.

One way to evaluate the individual bolt performance is to
examine their load distribution. For example, a mechanical point-
anchor bolt compresses the beams between the anchor and the
plate (Figure 7). Therefore, as the bolts resist sag the load is
evenly distributed along the axis of the steel bar. The bar near
the plate or in the threaded anchor section can stretch and fail
when excessive loads are encountered. When this happens, the
entire bolt loses all reinforcement value. The resin bolts
distribute load in a very different fashion. Their resistance to sag
is concentrated along that section of the bar where the beam
separation is occurring. If the bar would happen to fail at this
location, the rest of the intact bolt would still provide some
resistance to separation at other locations in the bolted horizon.
The friction stabilizers are unique in that they can gain pullout
resistance in response to beam separation or lateral shifting.
However, loss of contact with the host rock along the bolt axis
can reduce its load-bearing capacity.

In addition to examining the influence of bolting on resisting
beam sag, numerical simulations were used to evaluate the
stability of the individual bolts, This was accomplished by
determining the ratio of the axial load to the yield strength of
each bolt at or near the beam’s critical stress conditions
(Figure 12). With a slenderness ratio of 45 and the horizontal
stress field of 13.8 MPa (2 Ksi), the point anchor bolts were
within 30% of achieving their maximum strength. Both fully
grouted and torque tension resin bolts had more than 60% of
their strength remaining, while one of the friction stabilizer bolts
was at its yield strength. It is assumed that some bolts may fail
when their yield strength is achieved and some additional yield
takes place. In the case of the point anchor and friction stabilizer
bolts, it may take additional sag to develop further plastic
deformation before failure occurs. When the slenderness ratio
was increased to 90 and the horizontal stress field was doubled
to 27.6 MPa (4 Ksi), a different set of stability conditions was
observed. The point anchor bolts were now at or close to yield.
The resin bolts still had at least one-half of their strength
remaining, while friction stabilizer bolts were in yield.

2 O, = 13.8 MPa

8

5 (TOS hin = 45

wal 5035 8 06s

o 3 2 o a

5 ini, Ana

a BBs s

e 2 2 +

§ NX 3 5 z On = 276 MPa

a *-@-- ~- 2 4--1/ = 90
0.29 049

Figure 12. - Profiles of the average ratio of axial load to yield
strength for four different bolt types placed under roof conditions
that could potentially cause buckling failure of the lower roof
beams.

The model results match well with field observations. These
observations indicate that no resin bolts failed in the field test,



Controlling Roof Beam Failures From High Horizontal Stresses in
Underground Stone Mines

A. T. lannacchione, Acting Deputy Laboratory Director
D. R. Dolinar, Mining Engineer
L. J. Prosser, Research Physical Scientist
T. E. Marshall, Engineering Technician
D. C. Oyler, Mechanical Engineer
C. S. Compton, Engineering Technician

National Institute for Occupational Safety and Health
Pittsburgh Research Laboratory
Pittsburgh, PA

ABSTRACT

The U.S., Australian, and United Kingdom coal and the
Canadian hard rock mining industries have long recognized the
significance of high horizontal stresses as a factor affecting the
stability of roof and rib conditions in underground mines.
Recently, a growing segment of the U.S. underground stone
mining industry has also begun to recognize that horizontal
stresses occur in some of its more than 90 mines. Considering
the typically high strength and massive nature of limestone, this
fact is a revelation in itself. High horizontal stresses produce
extensive and sudden rock failures and, in some cases, resulted
in injuries to mine workers. Through the years diverse control
strategies have been proposed and experimented with
Reorientation of mine entries to reduce stress concentrations
have proven successful and are widely accepted in practice.
Other solutions, like rock reinforcement, are poorly understood
and less accepted in practice. It is the purpose of this National
Institute for Occupational Safety and Health (NIOSH) study to
develop a better fundamental understanding of these ground
control strategies under high horizontal stress conditions through
a series of field and laboratory studies. To this end a design
technique is presented which provides stone miners with a
method for making stability assessments. The consequences of
widening rooms, changing geology and horizontal stresses, and
different rock bolts on roof beam failures are discussed.

INTRODUCTION

Excessive roof beam sag is considered the major cause of roof
failure in U.S. underground stone mines. Roof beams can sag in
response to gravity forces that pull subjacent beams away from
overlying stable beams. Generally, these roof beams are formed
in response to geologic characteristics, such as stratigraphic
bedding, and are found in many stone mining provinces. Roof
beams of varied thickness have been observed to span rooms
ranging from 7 to 16 m (23 to 53 ft), some without any roof
support. This can only occur if the beams are sufficiently
strong/stiff, massive, and thick. Unfortunately, blasting fractures

the immediate roof beam, adding to the frequency of roof
failures.

The strength aspects of this failure criterion is generally easily
met by most limestones. Typically, intact (solid) pieces of
limestone are very strong, often having compressive strengths of
70 to 210 MPa (10.1 to 30.4 thousand psi or Ksi) and tensile
strengths as high as 15 MPa (2.2 Ksi). Limestone has also been
found to have a high stiffness, with elastic modulus (E) ranges
from 13.8 to 41.4 GPa (2 to 6 million psi or Mpsi).

Not all limestone is massive; in fact, most limestone has
horizontal layering that forms beams typically ranging from less
than 0.15 to 0.6 m (0.5 to 2 ft) thick (Figure 1). Variations in
roof beam thickness are caused by horizontal bedding planes of
varying lengths and spacing. Some of these beams contain
different numbers of vertical and subvertical discontinuities
(vertical joints or fractures and subvertical crossbed planes).
These fractures allow local sections of the immediate roof or rib
to separate, forming prisms or wedges. These local wedge
instabilities can suddenly fail without warning and are especially
common in rib falls.

Figure 1 - Limestone mine portal showing a roof composed
of numerous flat lying beams separated by bedding planes
and ranging in thickness from 0.3 to 0.6 m (I to2 8). Itis
estimated that greater than 95% of all underground
stone mines have dips less than 10°


Iannacchione, A. T., T. P. Mucho, and L. J. Prosser,
“Underground Limestone: An Overview of Ground Conditions,”
Proc. of the Environment, Safety, and Health Forum (National
Stone Association), Nashville, Tennessee, Oct. 22-24, 1995, pp.
371-397

lannacchione, A. T. and L. J. Prosser, “Roof and Rib Hazard
Assessment for Underground Stone Mines,” SME Preprint 97-
113, SME Annual Meeting, Denver, CO, Feb. 24-27, 1997, 5 pp.

Iannacchione, A. T., L. J. Prosser, D. C. Oyler, D. R. Dolinar, T
E. Marshall, and C. S. Compton, “Preventing Accidents Caused
by Unrecognized Roof Beam Failures in Underground Stone
Mines,” National Occupational Injury Research Symposium,
Morgantown, WV, Oct. 14-16, 1997, 10 pp.

Mark, C. and T. P. Mucho, “Longwall Mine Design for Control
of Horizontal Stress,” U.S, Bureau of Mines Special Publication
01-94, New Technology for Longwall Ground Control, 1994, pp.
$3-76,

Molinda, G. M., K. A. Heasley, D. C. Oyler, and J. R. Jones,
“Effects of Horizontal Stress Related to Stream Valleys on the
Stability of Coal Mine Openings,” U.S. Bureau of Mines RI
9413, 1992, 26 pp

Mucho, T. P. and C. Mark, “Determining Horizontal Stress
Direction Using the Stress Mapping Technique,” 13 Conference
‘on Ground Control in Mining, Morgantown, WV, 1994, pp. 277-
289.

Parker, J. “Mining in a Lateral Stress Field at White Pine,”
Canadian Institute of Mining and Metallurgy Transactions,
Vol. LXIX, 1966, pp. 375-383.

Parker, J., “How to Design Better Mine Openings: Practical
Rock Mechanics for Miners,” Engineering and Mining Journal,
Part 5, December 1973, pp. 76-80

Parker, J., “Everybody Goes Underground Eventually”,
‘Aggregate Manager, June 1996, pp. 30-35

Petersen, G., Personal Communication, 1996.

Scott, J. J., “Roof Bolting - A Sophisticated Art,” Coal, August
1989, pp. 59-69.

Stillborg, B., “Rock and Cable Bolt Tensile Loading Across a
Joint,” Swedish Rock Mechanics Symposium, Stockholm,
Sweden, 1992.



‘The next bolt tried was a 1.59-cm (5/8-in) mechanical resin-
assisted point anchor bolt 2.4 m (8 ft) in length. These bolts
were similar in strength to the friction stabilizers, so they would
have probably suffered from a similar fate. However, poor
installation prevented this bolt from being placed in the high
horizontal stress locations. The mechanical resin-assisted
anchors were prone to spinners and could sometimes be pulled
from their holes by mechanical scalers. It is thought that the
limestone hole and the steel bar were both too smooth to allow
the resin to bond well between the bolt and the rock, while the
mechanical expansion shell seemed to have trouble anchoring into
the limestone.

At this point, a new directionally controlled failure began to
develop in another room. Mine management decided to try a
much stiffer support, the 2.54-cm (1-in) torque-tension resin bolt.
These bolts were quickly installed along the projected trend of
the new directional failure zone. The fast-set resin along the
upper bolt allowed the bolts to be tensioned to an estimated 5
metric tons (6 tons). This tensioning had the added benefit of
pulling some of the loose lower beams together. Six RMSS’s
and three hollow inclusion cells were installed in this area to
examine roof behavior. Additionally, a 10-m (30-ft) production
bench was extracted to within 14 m (45 ft) of the bolted entry.
Through the course of the 8-month monitoring period, stresses
in the lower beams have changed as the roof was first loaded then
unloaded as the bench extraction zone approached. Although the
roof has shown some signs of localized cracking between the bolt
heads, no beam collapses have occurred. Additionally, roof sag
monitors have shown that very little sag has occurred in the
bolted roof area. To this date, no torque tension resin bolts have
failed.

Recently, the mine began using 2.22-cm (7/8-in) fully grouted
resin bolts in new development areas. The reasons for this are
twofold. First, the 2.54-cm (1-in) torque tension bolts must be
manually inserted into the bolt holes, Because the bolts are very
heavy, long-term usage could cause injury to the operator
installing them. Second, because roof bolts are placed soon after
mining, minimal separations exist in the roof. Hence, there is
little need to tighten the beams together. To date, these bolts
have performed without failure in areas where high horizontal
stress conditions are thought to have existed

ROCK REINFORCEMENT PERFORMANCE AND.
SELECTION

After reviewing the case study data, the question now is why
some bolts performed better than others and what can we learn
from this study to help in making the best selection in the future.
In the case study, it seemed evident that stiffness and load
capacity were the controlling factors in successful rock
reinforcement performance. Clearly, rock reinforcement acted to
strengthen the immediate roof by increasing the effective beam
thickness. This enhanced reinforcement action was accomplished
in our field test by higher capacity bolts, but it could have also
been accomplished by closer bolt spacing. The critical point is
that the thinner beams are tied together and act like thicker

beams. Therefore, the critical stress needed to induce buckling
is increased, decreasing the potential for buckling

Although the concept of a stress-based failure criterion is
straightforward, its application in controlling high horizontal
stresses is not. After all, there is little evidence to suggest that
rock reinforcement can significantly change the stress conditions
within a roof beam, When rock reinforcement passes through
adjacent layers, it does little to change the stresses that would
have existed in the beam if the support were not there. This is
true until failure or softening takes place. What the support does
do is resist beam separation and reduce beam sag. Therefore, to
analyze rock reinforcement performance it is necessary to utilize
a deformation-based failure criterion.

Controlling Critical Roof Beam Sag
Through Roof Reinforcement

To achieve a deformation based failure criterion, a critical sag
magnitude needs to be established. It would be ideal to develop
this critical sag magnitude based on many observations under a
complete suite of geometric and strength characteristics.
Unfortunately, there are limited experience and data in this area.
Therefore, critical sag magnitudes need to be developed on a
case-by-case basis, In this case study, 5 cm (2 in) of total sag
preceded a major roof collapse. However, the lower beams
began to fail prior to the main beam roof failure. It seems
reasonable to assume that beam failure from buckling had begun
in the lower beam at one-half of the total sag, or approximately
2.5 cm (1 in). Let’s assume that this value is the critical sag
magnitude and use it to assess the performance of unsupported
and support roof beams.

In the parametric investigations discussed earlier, elastic beam
sag occurred in beams with different slenderness ratios and under
different horizontal stress conditions. The maximum vertical
beam sag for each condition was calculated and displayed in
Figure 11. Ifa match is made between conditions where a beam
has a high potential for buckling, as defined by the critical stress
failure criteria, and maximum vertical beam sag, then all
conditions that produced more than 2.54 cm (1.0 in) of sag
would represent a failure beam. This value correlates well with
the sag magnitudes measured in the test site, validating the
numerical model’s performance.


(30 ft) in height was excavated in the model to simulate a typical
development entry used by many mines operating in the northern
Appalachian Basin (Iannacchione, et al. 1995), The interface
boundaries were placed at different locations above the mined
rooms to simulate different roof beam thicknesses. A shear and
normal stiffness of 10 GPa (1.45 Mpsi) and an angle of shearing
resistance of 20° were assigned to each interface. Four different
far-field horizontal stress fields were applied to the models, 3.45,
6,9, 13.8 and 27.6 MPa (0.5, 1, 2, and 4 Ksi), a range consistent
with horizontal stress conditions found to exist in the northern
Appalachian Basin.

Cloy/Shole-Limestone
[Boundorinterfoce
3A = 5.628 Immediate Root Rock
t= 30 Wt = 225

Le 90 bit = 45

pocscsqcenrn

OS m 031m 0.46 m O61 m

Figure 5 - Dimensions of five beams used in parametric
investigations. At high horizontal stresses and stenderness
ratios, the beams began to sag, creating a gap in the modeled
roof.

The emphasis of these parametric investigations was to
examine various shaped beams under varying regional horizontal
stress conditions. In this study, five beam slenderness ratios were
examined: 5.6, 22.5, 30, 45, and 90 (Figure 5). At high
horizontal stress levels, the immediate roof beam began to sag,

inducing gaps in the modeled roof. The location, areal extent,
and thickness of these gaps are the most critical observational
data available to identify potential roof instabilities.

All material models were elastic to conform with Euler's
stress-based failure criteria and to simulate elastic-beam bending
phenomena. Euler’s stress-based failure criteria assumes that
beam failure is initiated when critical stress initiates beam
buckling. However, the elastic numerical model continues to
bend the beam until the unbalanced forces dissipate through the
finite difference grid through a process called relaxation. Failure
conditions are considered to be stress-based, relying on the
critical stress condition to define beam stability. Beam horizontal
stress conditions were calculated as the average stress along a
vertical plane cutting the center of the beam.

Plots of average maximum horizontal stress conditions for five
different beam geometries and four applied horizontal stress
conditions are shown in Figure 6. In this analysis, the critical
stress is calculated for a material with a stiffness of 27.6 GPa
(4 Mpsi) using a fixed-end condition. The fixed-end condition is
thought to more closely represent the end conditions found in
underground mines than the pinned-end condition.

oo Eulers equotion (£=27.6 GPo, fixed end)

2 FLAC Models
140

oe ss
é8ss8

é

Criticol Stress, MPo
Critical Stress, Ksi

8

oO 4 io
0 10 20 30 40 SG 60 70 80 90
Slenderness Ratio, L/t

Figure 6 - Potential stability to buckling failure of
differently shape roof beams with a varying
horizontal stress field, ranging from 3.45 to 27.6 Mpa.

This new analysis supplies a very instructive view of the
relationship between beam strength, beam geometry, and the
regional horizontal stress field. If two of the conditions are held
constant, the third can be altered to determine how it can
potentially influence the onset of buckling failure. For example,
if the mine is subjected to a 13.8 MPa (2 Ksi) horizontal stress
field and the beam has a stiffness of 27.6 GPa (4 Mpsi), then a
0.3 m (1 ft) thick beam in a room 13.7 m (45 ft) wide
(slenderness ratio=45) will plot in the unstable zone, indicating a
high potential for buckling failure. Conversely, if the beam is 0,6
m (2 ft) thick, the intersection point of the 13.8 MPa and the 22.5
slenderness ratio conditions place this beam in the stable zone,


LEGEND

Underground stone
mines wsted

Figure 3 - Location of U.S. underground stone mines visited
during this investigation and measurements of in situ stresses at
nearby mines and quarries (Bickel, 1993).

Although these horizontal stress trends appear to be consistent
over most of the eastern U.S., local variations can occur due to
geologic structures (faults, igneous dikes, etc.), stratigraphic
features (Parker, 1966), and topographic features (Molinda et.
al, 1992). For example, Parker (1966) found that horizontal
stress at the White Pine Copper Mine in the Upper Peninsula of
Michigan varied from an apparent small degree of tension to
13.8 MPa (2,000 psi) of compression. Closer to a fault, stresses
rose to 31 MPa(4.5 Ksi). In.a deep limestone mine in Kentucky,
Parker (1996) measured pressures in roof beams in excess of
96.6 MPa (14 Ksi). Therefore, local confirmation of stress field
magnitude and orientation are necessary because stresses can
change rapidly. This can be accomplished through measurement
or by mapping the orientation of failure patterns developed in
response to high horizontal stresses (stress mapping).

Stress mapping uses observational techniques to understand
the local horizontal stress conditions, If high horizontal stress
conditions exist, they will manifest themselves through several
characteristic failure patterns. These patterns have been noticed
and discussed by others (Parker, 1973; Gale 1986, and Mucho
and Mark, 1994). Many of the same conditions noted by these
researchers were observed by the authors at several underground
stone mines. The following conditions were particularly
noteworthy:

1. The strike of low-angle shear failure planes with
associated rock powder oriented perpendicular to
maximum horizontal stress direction. ;

2. Oval or elliptical roof failures oriented perpendicular to
maximum horizontal stress direction (Figure 4).

3. Thin delaminated stone layers separated from the
immediate roof and oriented perpendicular to maximum
horizontal stress direction.

4. Joints and shear-type faults oriented parallel to maximum
horizontal stress direction,

Figure 4. - If the roof is not reinforced properly with rock
bolts, the beams can progressively fail higher into the
roof until a stable roof beam is encountered. Here the

roof fall has assumed an elliptical shape.

HIGH HORIZONTAL STRESSES AND
ROOF BEAM BEHAVIOR

It has already been shown that the critical stress necessary to
induce buckling failure within an Euler beam is dependent on the
geometric characteristics and stiffness/strength of the beam.
Unfortunately, the Euler beam is an oversimplification of the
conditions found in most U.S. underground stone mines. For
example, limestone roof beams are neither pinned nor fixed in the
mechanical sense. More importantly, limestone roof strata
typically contain multiple layers. The writers have found that
many failures appear to originate in the lowest beams, or the
‘beams closest to the mine opening (see case study discussed later
in this report). This is probably due to the concentration of
stresses in the lower beam and constraints to bending in the upper
beams, and the fact that stress is highly concentrated in the lower
beam at room comers.

Parametric Studies of Beam Thickness, Beam Strength,
and Applied Horizontal Stress

Analyzing the complex interaction of multiple beams cannot
be accomplished with analytical tools. Therefore, numerical
models were employed to examine the interaction of far field
applied stress conditions, local beam stiffness characteristics, and
unique boundary properties. The finite difference code called
FLAC was utilized because it contains realistic capabilities to
model beam boundaries through the use of interfaces

A grid 150 elements square was constructed that defined a bed
of limestone 14 m (46 ft) in thickness and surrounded by weaker
and thicker clay/shale member. The limestone has a stiffness of
27.6 GPa (4 Mpsi) surrounded with a clay/shale with a stiffness
of 6.9 GPa (1 Mpsi). A room 13.7 m (45 ft) in width and 9.1 m


When high horizontal stresses are applied to the ends of thin
beams comprising the immediate roof, a buckling failure can
occur. The approximate value of the critical load inducing the
buckling failure can be determined by Euler’s formula (Eq. 1):

n?-El
Pa (Eq. 1)
where — P., = Critical load
E = Elastic Modulus
I= Moment of Inertia
L= Length of beam

The value of the critical stress, o,, corresponding to the
critical load, P,,, which induces buckling failure with respect to
different end conditions is determined as follows:

n?-E-t?

Do (4.2)

where

Critical stress

t= Thickness of beam

L, = Effective length of beam; for pinned end L, = 1*L
and fixed end L, = 0.5*L.

Using Euler’s formula for both pin- and fixed-ended beams
provides some general parameters related to stone roof beam
stability. The strength of beams with different geometries and
stiffnesses can be examined by plotting the critical stress at
different beam slenderness ratios and elastic moduli. The critical
stress for pinned and fixed-end beamsis calculated by multiplying,
the beam length by 1 and 0.5, respectively. Figure 2 shows this
relationship for three different elastic moduli: 13.8, 27.6, and
41.4 GPa (2, 4, and 6 Mpsi). The relative potential for buckling
failure can be determined by evaluating the position of the
calculated critical stress value. If the value plots below the
strength/stiffness line, the beam has a low potential for buckling.
Conversely, if the value plots avove this line, the beam has a high
potential for buckling,

As horizontal stresses are increased, roof beam deflection is
induced. This deflection causes a maximum displacement in the
center of a mined room and can be measured as roof beam sag
At the critical stress state, beam failure is thought to commence.
The manner in which failure occurs has become a subject of much
study. Roof beams can fail in shear as well as tensions. Multiple
low angle shear failures have been recognized at several
underground stone mines by the authors, as well as others
(Emery, 1964; Parker, 1996, and Petersen, 1996). These failure
surfaces are generally oriented perpendicular to the application
of the load. Also, some roof beams have been observed to crack
at their bottom center from the elevated tension levels in this
portion of the deflecting beam.

CIE = 13.6 cee KQJe = 27.6 oPo ZZJE = 41.4 GPO

o 80

a
3

Critical Stress, MP:
poe
8 6

Critical Stress, Ksi

°

°
0 10 20 30 40 50 60 70 80 90
Slenderness Ratio, L/t

Figure 2 - Critical stress conditions for beams of various geometries
and stiffnesses from Euler's formula.

These calculations clearly show that increasing the stiffness
and/or the thickness of these beams is expected to dramatically
increase their strength and hence the stability of the roof. Certain
types of rock bolts can act to join thin layers, enhancing roof
strength by increasing its flexural stiffness. Therefore, strategies
to increase roof beam strength can minimize roof beam failures
from moderate to high horizontal stresses and lessen the
exposure of underground stone miners to hazardous conditions.

STRESS CONDITIONS IN U.S. UNDERGROUND
STONE MINES:

Because vertical stress is a function of the overburden, most
underground stone mines are not deep enough to have high
vertical stress problems, However, excessive levels of vertical
stress are sometimes encountered in benching or multiple layer
mining operations where pillar sizing and pillar positioning could
produce local high stress concentrations, Unlike vertical stresses,
horizontal stresses are directly related to overburden removal or
tectonic forces especially in many near-surface conditions.

Inthe eastern U.S., it has been found that the magnitude of the
horizontal stresses often exceeds the vertical stresses by a factor
of at least two and can be much greater in shallow mines.
Additionally, the ratio between the primary and secondary
principal horizontal planes of stress ranges between 1.5 and 1.95
based on 26 measurements at 25 eastern U.S. coal mines (Mark
and Mucho, 1994). These same measurements show that the
orientation of the principal horizontal stress plane exhibits an
ENE to E-W orientation in the northern Appalachian Basin.
Because almost 20 underground stone mines operate within this
same geologic province, it is expected that they will experience
similar regional stress conditions. During 1995 and 1996, almost
one-half of the underground stone mines in the U_S. were visited
by the authors (Figure 3). High horizontal stress conditions were
noted in western Pennsylvania, eastern Kentucky, West Virginia,
and southwestern Virginia