9
pillar was about 442 psi. Using the tributary area method
to assess pillar loading, the calculated development vertical
stress in the instrumented pillar was found to be 2,021 psi.
Therefore, the total vertical stress in the instrumented pillar
prior to the commencement of mining at cut No. 34 was
approximately 2,021 psi +442 psi =2,463 psi
Assuming an in-situ coal seam strength of 900 psi
and applying the Mark-Bieniawski pillar strength formula
(Mark et al., 1997), the instrumented pillar’s strength was
determined to be 3,565 psi. Therefore, the instrumented
pillar stability factor for development loading is 1.76. For
retreat loading, the stability factor decreased to 1.45. The
high intact coal seam strength (3,747 psi) and the stable
pillar (pillar stability factor above 1.3) provided a stable
foundation for the rock parting, which explains why rib
brow was not formed in the instrumented pillar.
Load Change in the Instrumented Pillar
Figure 18 shows the changes in load on the instrumented
rib bolts installed on both the entry side and the cross-cut
side of the instrumented pillar. As observed in the results
from the BPCs, the load cells mounted on the rib bolts
on the entry side continued to monitor the load until the
continuous miner advanced to cut No. 40. In contrast, the
load cells on the rib bolts of the cross-cut side remained
operational and continued to be monitored until half of the
pillar was mined
Before retreating the instrumented pillar, Figure 18
shows that a small load was induced in the ungrouped
length of the rib bolts due to rib dilation. This load ranged
from 530 lbs. to 3,178 lbs., which represents approximately
2% to 12% of the yield capacity rib bolts (see Figure 9).
This explains the insignificant rib sloughing observed at
this stage of mining (see Figure 13b).
Figure 18 shows that the instrumented rib bolts sus-
tained a load of up to 12,000 lbs., which is approximately
50% of their yield capacity, during the retreat of the instru-
mented pillar. This indicates that the first 24 in. (unsup-
ported length) of the rib maintained its integrity throughout
the mining process. As a result, the formation of a rib brow
in the instrumented pillar was effectively prevented
Rib Displacement
Before retreating the instrumented pillar, no rib displace-
ments were recorded by the MPBXs on either the entry
side or the cross-cut side. Figure 19 shows the monitored
rib displacement on both sides of the instrumented pillar.
Slight rib displacements were detected when the continu-
ous miner operated at cuts No. 38 and 39. However, for
an unexplained reason, the MPBXs on the cross-cut side
did not record any further rib displacement during the pil-
lar retreat. In contrast, at cut No. 40, a sudden increase in
rib displacement was observed on the entry side, followed
by a continuous displacement of up to 3 in. as approxi-
mately half of the instrumented pillar was mined. The small
rib displacement of about 0.04 in. monitored before pillar
retreating began further supports why a rib brow did not
form in the instrumented pillar.
DISCUSSIONS
The question to be addressed in this study is: Why did a
rib brow not form in the instrumented pillar, despite the
high overburden depth and a rib composition that would
typically make it susceptible to rib brow formation? To
explore this, both visual observation of the instrumented
pillar and analysis of its instrument data are essential. Key
factors to consider include the combination of pillar load-
ing, the geology and geometry of the rib, and the density
of rib support.
The monitoring data and visual observations con-
firm that there is no potential for rib brows to form at the
instrumented pillar. This can be explained by the following
factors:
1. A relatively strong, thin rock parting approxi-
mately 2-ft thick, with a compressive strength of
5,556 psi, supported by fully grouted rib bolts.
2. A relatively strong coal seam, measuring between
3.2-ft and 4-ft thick, lies directly beneath the rock
parting. With a compressive strength of 3,747 psi,
this coal seam serves as a stable foundation for the
overlying rock parting. Figure 18. Changes in rib bolt load for the entry and cross-
cut sides
pillar was about 442 psi. Using the tributary area method
to assess pillar loading, the calculated development vertical
stress in the instrumented pillar was found to be 2,021 psi.
Therefore, the total vertical stress in the instrumented pillar
prior to the commencement of mining at cut No. 34 was
approximately 2,021 psi +442 psi =2,463 psi
Assuming an in-situ coal seam strength of 900 psi
and applying the Mark-Bieniawski pillar strength formula
(Mark et al., 1997), the instrumented pillar’s strength was
determined to be 3,565 psi. Therefore, the instrumented
pillar stability factor for development loading is 1.76. For
retreat loading, the stability factor decreased to 1.45. The
high intact coal seam strength (3,747 psi) and the stable
pillar (pillar stability factor above 1.3) provided a stable
foundation for the rock parting, which explains why rib
brow was not formed in the instrumented pillar.
Load Change in the Instrumented Pillar
Figure 18 shows the changes in load on the instrumented
rib bolts installed on both the entry side and the cross-cut
side of the instrumented pillar. As observed in the results
from the BPCs, the load cells mounted on the rib bolts
on the entry side continued to monitor the load until the
continuous miner advanced to cut No. 40. In contrast, the
load cells on the rib bolts of the cross-cut side remained
operational and continued to be monitored until half of the
pillar was mined
Before retreating the instrumented pillar, Figure 18
shows that a small load was induced in the ungrouped
length of the rib bolts due to rib dilation. This load ranged
from 530 lbs. to 3,178 lbs., which represents approximately
2% to 12% of the yield capacity rib bolts (see Figure 9).
This explains the insignificant rib sloughing observed at
this stage of mining (see Figure 13b).
Figure 18 shows that the instrumented rib bolts sus-
tained a load of up to 12,000 lbs., which is approximately
50% of their yield capacity, during the retreat of the instru-
mented pillar. This indicates that the first 24 in. (unsup-
ported length) of the rib maintained its integrity throughout
the mining process. As a result, the formation of a rib brow
in the instrumented pillar was effectively prevented
Rib Displacement
Before retreating the instrumented pillar, no rib displace-
ments were recorded by the MPBXs on either the entry
side or the cross-cut side. Figure 19 shows the monitored
rib displacement on both sides of the instrumented pillar.
Slight rib displacements were detected when the continu-
ous miner operated at cuts No. 38 and 39. However, for
an unexplained reason, the MPBXs on the cross-cut side
did not record any further rib displacement during the pil-
lar retreat. In contrast, at cut No. 40, a sudden increase in
rib displacement was observed on the entry side, followed
by a continuous displacement of up to 3 in. as approxi-
mately half of the instrumented pillar was mined. The small
rib displacement of about 0.04 in. monitored before pillar
retreating began further supports why a rib brow did not
form in the instrumented pillar.
DISCUSSIONS
The question to be addressed in this study is: Why did a
rib brow not form in the instrumented pillar, despite the
high overburden depth and a rib composition that would
typically make it susceptible to rib brow formation? To
explore this, both visual observation of the instrumented
pillar and analysis of its instrument data are essential. Key
factors to consider include the combination of pillar load-
ing, the geology and geometry of the rib, and the density
of rib support.
The monitoring data and visual observations con-
firm that there is no potential for rib brows to form at the
instrumented pillar. This can be explained by the following
factors:
1. A relatively strong, thin rock parting approxi-
mately 2-ft thick, with a compressive strength of
5,556 psi, supported by fully grouted rib bolts.
2. A relatively strong coal seam, measuring between
3.2-ft and 4-ft thick, lies directly beneath the rock
parting. With a compressive strength of 3,747 psi,
this coal seam serves as a stable foundation for the
overlying rock parting. Figure 18. Changes in rib bolt load for the entry and cross-
cut sides