8
Borehole scoping conducted at the intersection between
entry No. 4 and cross-cut 25 showed no deformation in the
immediate roof up to a depth of 8.5 ft. Therefore, the mon-
itored roof displacement is likely due to roof strata separa-
tion occurring between 8.5 ft and 15 ft in the roof.
Pressure Change in the Instrumented Pillar
Visual observations indicate that the rock parting in coal
pillar ribs remained intact during the retreat of the pillars,
including those small pillars left (stumps) in the gob area
(see Figure 12a). This stability is attributed to the high
strength of the shale, which is measured at 5,556 psi, along
with the use of rib bolts to support the rock parting. As
the roof converges, the rock parting presses against the coal
seam in the coal pillar ribs. The pressure changes within the
coal seam were monitored by BPCs, as shown in Figure 16.
At the entry side of the instrumented pillar P3,
Figure 16 shows that BPC 1-30 unexpectedly stopped
functioning after the extraction of the first cut, No. 34, in
the instrumented pillar. Meanwhile, BPC 2-20 and BPC
3-10 continued to monitor pressure increases in the pillar
until they were cut by the continuous miner at cut No. 40.
Except for BPC 4-30, located at the cross-cut side of the
instrumented pillar, all other BPCs remained functional
until half of the pillar was mined.
Figure 16 also shows that the pressures monitored by
the BPCs at the entry side are greater than those at the
cross-cut side. This higher pressure at the entry side is due
to their proximity to the inby gob, which exposes them to
both front and side abutment loads. In contrast, the BPCs
on the cross-cut side are less affected by front abutment
pressures
Converting the measured pressures in the BPCs to the
corresponding stresses in the surrounding rock is a debat-
able issue. According to the Geokon manual for the BPC
used in this study, the pressure-to-stress conversion factor
for soft rocks ranges from 0.625 to 0.83 (Geokon, 2024).
Additionally, studies in coal indicate that this conversion
factor is typically less than 1.0, as noted in the Geokon
manual. Su et al. (1990) utilized calibrated finite element
models and calculated a conversion factor of 1.18 for
BPCs in coal at a setting pressure of 1,500 psi. The con-
servative estimation of the stability factor for the instru-
mented pillar involves applying a conversion factor of 1.18.
Consequently, the average change in induced stress within
the instrumented pillar was calculated and is illustrated in
Figure 17.
Figure 17 shows that, prior to retreating the instru-
mented pillar at cut No. 34, the average stress change in the Figure 15. Displacement of immediate roof at both entry
No. 4 and cross-cut 24
Figure 16. Pressure changes in the BPCs for the entry and
cross-cut sides
Figure 17. Calculated stress changes in the instrumented
pillar
Borehole scoping conducted at the intersection between
entry No. 4 and cross-cut 25 showed no deformation in the
immediate roof up to a depth of 8.5 ft. Therefore, the mon-
itored roof displacement is likely due to roof strata separa-
tion occurring between 8.5 ft and 15 ft in the roof.
Pressure Change in the Instrumented Pillar
Visual observations indicate that the rock parting in coal
pillar ribs remained intact during the retreat of the pillars,
including those small pillars left (stumps) in the gob area
(see Figure 12a). This stability is attributed to the high
strength of the shale, which is measured at 5,556 psi, along
with the use of rib bolts to support the rock parting. As
the roof converges, the rock parting presses against the coal
seam in the coal pillar ribs. The pressure changes within the
coal seam were monitored by BPCs, as shown in Figure 16.
At the entry side of the instrumented pillar P3,
Figure 16 shows that BPC 1-30 unexpectedly stopped
functioning after the extraction of the first cut, No. 34, in
the instrumented pillar. Meanwhile, BPC 2-20 and BPC
3-10 continued to monitor pressure increases in the pillar
until they were cut by the continuous miner at cut No. 40.
Except for BPC 4-30, located at the cross-cut side of the
instrumented pillar, all other BPCs remained functional
until half of the pillar was mined.
Figure 16 also shows that the pressures monitored by
the BPCs at the entry side are greater than those at the
cross-cut side. This higher pressure at the entry side is due
to their proximity to the inby gob, which exposes them to
both front and side abutment loads. In contrast, the BPCs
on the cross-cut side are less affected by front abutment
pressures
Converting the measured pressures in the BPCs to the
corresponding stresses in the surrounding rock is a debat-
able issue. According to the Geokon manual for the BPC
used in this study, the pressure-to-stress conversion factor
for soft rocks ranges from 0.625 to 0.83 (Geokon, 2024).
Additionally, studies in coal indicate that this conversion
factor is typically less than 1.0, as noted in the Geokon
manual. Su et al. (1990) utilized calibrated finite element
models and calculated a conversion factor of 1.18 for
BPCs in coal at a setting pressure of 1,500 psi. The con-
servative estimation of the stability factor for the instru-
mented pillar involves applying a conversion factor of 1.18.
Consequently, the average change in induced stress within
the instrumented pillar was calculated and is illustrated in
Figure 17.
Figure 17 shows that, prior to retreating the instru-
mented pillar at cut No. 34, the average stress change in the Figure 15. Displacement of immediate roof at both entry
No. 4 and cross-cut 24
Figure 16. Pressure changes in the BPCs for the entry and
cross-cut sides
Figure 17. Calculated stress changes in the instrumented
pillar