8
SUMMARY
Coal ribs play a crucial role in maintaining the stability of
underground coal mines. Rib bolting systems are widely
employed for their effectiveness in enhancing rib stabil-
ity and averting dangerous collapses. Inadequate rib bolt
design can lead to instability, posing serious safety risks.
Researchers are trying to develop an engineering-based
approach for more reliable rib bolting designs. Pull-out
tests, measuring the force required to extract a bolt, are
invaluable for evaluating bolt anchorage and bond strength,
aiding in rib bolting designs.
This study involved conducting 73 in-situ pull-out tests
on rib bolts, situated across different coal mines in the east-
ern US. The tests spanned various coal seams and focused
on grade 60 bolts with a tensile strength of 414 MPa, mainly
No. 5 bolts with a 15.875 mm diameter. Three anchorage
types were examined: Short Encapsulation (SEPT), fully
grouted (FGPT), and partially grouted (PGPT) bolts, with
varying anchorage lengths.
To assess grout anchorage capacity while preventing
steel rebar yielding, SEPTs with a fixed 0.305 m anchor-
age length were conducted, Results from SEPTs in vari-
ous mines indicated diverse load-displacement patterns.
Notably, No. 5 bolts exhibited a wide range of peak loads,
while No. 6 bolts were more consistent. Unexpectedly,
some SEPTs experienced steel yielding. To investigate these
issues, controlled SEPTs were performed at the NIOSH
Safety Research Mine, testing two bolt sizes and two rota-
tion speeds (170 rpm and 780 rpm). The controlled SEPTs
illustrate how rotational speed during bolt installation pro-
foundly influences SEPT anchorage capacity, with higher
speeds resulting in reduced anchorage capacity.
Pull-out tests were conducted for fully grouted bolts
(FGPT) in four mines, with rigorous adherence to safety
protocols to assess their resistance to fracture. Like the
PGPTs, the anchoring strength of tested bolts, as deter-
mined by FGPT, depend on the yielding of the steel rebar,
irrespective of the rotation speed employed during the bolt
installation.
STUDY LIMITATIONS
In this study, pull-out tests were carried out on rib bolts
within several eastern coal mines. The research focused on
the performance of Grade 60 steel rebars, specifically No. 5
and No. 6. The conclusions derived from the experimental
results are limited to the mechanical properties of the coal
seams tested, and the properties of the bolts that were used.
A more comprehensive study would have to be conducted
to make any conclusions on the overall contribution of rib
bolts to entry stability. It’s important to note that this study
does also does not account for potential external factors like
bolt corrosion, timedependent deterioration of the ribs, or
bolt grade.
The quality of grouted bolts is not solely determined by
a single parameter. Rather, it depends on a complex inter-
action of various factors. These factors include resin speed,
applied thrust and torque, as well as the mixing time.
Excessive mixing can have a detrimental effect on anchorage
quality, as illustrated in Figure 5b. Further experiments are
required to establish the rotation speed and the mixing time
thresholds beyond the manufacturer’s recommendations
that can adversely affect the quality of fully grouted bolts.
ACKNOWLEDGMENT
The authors would like to thank Mr. Todd Minoski for pre-
paring the data collection system, Mr. Craig Compton for
preparing the pull-out test gear, Mr. Mark Mazzella, Mr.
Timothy Matthews, Mr. Charlie Matthews, Mr. Charles
Warren, and Mr. Mark Zuspan, whose support was crucial
during the execution of the pull-out tests.
DISCLAIMER
The findings and conclusions in this study are those of the
authors and do not necessarily represent the official posi-
tion of the National Institute for Occupational Safety
and Health (NIOSH), Centers for Disease Control and
Prevention (CDC). Mention of any company or product
does not constitute endorsement by NIOSH.
The authors from the Missouri University of Science
and Technology acknowledge the financial support from
Table 3. Key features of PGPTs for bolts with rebar diameter of 15.875 mm
Mine Coal seam
Number of
tests
Bolt length
(m)
Average 1st
peak load
(kN)
1st peak
Load range
(kN)
Average 1st peak
displacement (mm)
B Pittsburgh 3 1.219 94 89-96 10
C Pittsburgh 2 1.219 85 84-87 6
D Kellioka 6 1.219 95 81-106 7
E Pocahontas No. 3 2 1.219 n/a n/a n/a
2 1.524 110 n/a 41
SUMMARY
Coal ribs play a crucial role in maintaining the stability of
underground coal mines. Rib bolting systems are widely
employed for their effectiveness in enhancing rib stabil-
ity and averting dangerous collapses. Inadequate rib bolt
design can lead to instability, posing serious safety risks.
Researchers are trying to develop an engineering-based
approach for more reliable rib bolting designs. Pull-out
tests, measuring the force required to extract a bolt, are
invaluable for evaluating bolt anchorage and bond strength,
aiding in rib bolting designs.
This study involved conducting 73 in-situ pull-out tests
on rib bolts, situated across different coal mines in the east-
ern US. The tests spanned various coal seams and focused
on grade 60 bolts with a tensile strength of 414 MPa, mainly
No. 5 bolts with a 15.875 mm diameter. Three anchorage
types were examined: Short Encapsulation (SEPT), fully
grouted (FGPT), and partially grouted (PGPT) bolts, with
varying anchorage lengths.
To assess grout anchorage capacity while preventing
steel rebar yielding, SEPTs with a fixed 0.305 m anchor-
age length were conducted, Results from SEPTs in vari-
ous mines indicated diverse load-displacement patterns.
Notably, No. 5 bolts exhibited a wide range of peak loads,
while No. 6 bolts were more consistent. Unexpectedly,
some SEPTs experienced steel yielding. To investigate these
issues, controlled SEPTs were performed at the NIOSH
Safety Research Mine, testing two bolt sizes and two rota-
tion speeds (170 rpm and 780 rpm). The controlled SEPTs
illustrate how rotational speed during bolt installation pro-
foundly influences SEPT anchorage capacity, with higher
speeds resulting in reduced anchorage capacity.
Pull-out tests were conducted for fully grouted bolts
(FGPT) in four mines, with rigorous adherence to safety
protocols to assess their resistance to fracture. Like the
PGPTs, the anchoring strength of tested bolts, as deter-
mined by FGPT, depend on the yielding of the steel rebar,
irrespective of the rotation speed employed during the bolt
installation.
STUDY LIMITATIONS
In this study, pull-out tests were carried out on rib bolts
within several eastern coal mines. The research focused on
the performance of Grade 60 steel rebars, specifically No. 5
and No. 6. The conclusions derived from the experimental
results are limited to the mechanical properties of the coal
seams tested, and the properties of the bolts that were used.
A more comprehensive study would have to be conducted
to make any conclusions on the overall contribution of rib
bolts to entry stability. It’s important to note that this study
does also does not account for potential external factors like
bolt corrosion, timedependent deterioration of the ribs, or
bolt grade.
The quality of grouted bolts is not solely determined by
a single parameter. Rather, it depends on a complex inter-
action of various factors. These factors include resin speed,
applied thrust and torque, as well as the mixing time.
Excessive mixing can have a detrimental effect on anchorage
quality, as illustrated in Figure 5b. Further experiments are
required to establish the rotation speed and the mixing time
thresholds beyond the manufacturer’s recommendations
that can adversely affect the quality of fully grouted bolts.
ACKNOWLEDGMENT
The authors would like to thank Mr. Todd Minoski for pre-
paring the data collection system, Mr. Craig Compton for
preparing the pull-out test gear, Mr. Mark Mazzella, Mr.
Timothy Matthews, Mr. Charlie Matthews, Mr. Charles
Warren, and Mr. Mark Zuspan, whose support was crucial
during the execution of the pull-out tests.
DISCLAIMER
The findings and conclusions in this study are those of the
authors and do not necessarily represent the official posi-
tion of the National Institute for Occupational Safety
and Health (NIOSH), Centers for Disease Control and
Prevention (CDC). Mention of any company or product
does not constitute endorsement by NIOSH.
The authors from the Missouri University of Science
and Technology acknowledge the financial support from
Table 3. Key features of PGPTs for bolts with rebar diameter of 15.875 mm
Mine Coal seam
Number of
tests
Bolt length
(m)
Average 1st
peak load
(kN)
1st peak
Load range
(kN)
Average 1st peak
displacement (mm)
B Pittsburgh 3 1.219 94 89-96 10
C Pittsburgh 2 1.219 85 84-87 6
D Kellioka 6 1.219 95 81-106 7
E Pocahontas No. 3 2 1.219 n/a n/a n/a
2 1.524 110 n/a 41