7
limiting stress transfer to the active mining zone. A bore-
hole roof scoping confirmed that there was no separation in
the rock strata in the immediate roof outby the pillar line.
Additionally, the rib conditions were generally good, show-
ing little to no sloughing (see Figure 12c). However, some
sloughing was noted at the ribs in areas where the pillar was
being actively mined, particularly starting from the coal/
rock parting interface (see Figure 12b).
Visual observations of rib performance at the instru-
mented pillar indicate that rib brows were generally not
formed. However, rib brows were observed at a few pillar
corners outby the pillar line, where timber supports were
subsequently used to stabilize them.
Figure 13 shows photos of the cross-cut side of the
instrumented pillar under different loading conditions.
Slight changes in the rib condition are noticeable between
Figure 13a, taken one week after the instrument installa-
tion, and Figure 13b, taken a few hours before mining the
instrumented pillar.
The monitoring effort lasted approximately six months,
allowing for a comprehensive observation of the rib per-
formance of the instrumented pillar under various loading
conditions. All recorded instrument data remained steady,
except in the last three days when the continuous miner
began retreating from the final pillar located between cross-
cuts 25 and 26. For instance, as shown in Figure 14, the
average pressure recorded by the BPCs within the instru-
mented pillar remained stable at about 1,000 psi. However,
as the continuous miner retreated to cut No. 68—situated
in the pillar between cross-cuts 25 and 26 (see Figure 11)—
the average pressure began to rise, ultimately reaching
about 2,000 psi when the continuous miner advanced to
cut No. 50 in the instrumented pillar. Consequently, the
analysis of the instrumented pillar data will focus only on
the last three days of the monitoring period.
Roof Convergence
Figure 15 shows the roof convergence observed at entry
No. 4 and cross-cut 24 during pillar retreat operations.
Due to a malfunction in the roof extensometer (RX-2)
at cross-cut 24, no roof convergence data was recorded at
that location. In contrast, the roof extensometer (RX-1) at
entry No. 4 indicated a slight convergence of the immedi-
ate roof—less than 0.02 in—attributed to front abutment
loading resulting from the retreat of the last inby pillars
between cross-cuts 25 and 26.
Once the continuous miner began cutting into pillar
P1, the roof convergence is slightly increased to 0.02 in.
A slight increase in roof convergence to about 0.05 in.
occurred as the continuous miner advanced into the
instrumented pillar P3, followed by an exponential rise in
convergence, reaching 0.7 in. as nearly half of the instru-
mented pillar was mined.
Figure 13. Photos taken at the cross-cut side of instrumented
pillar P3
Figure 14. Average induced pressure change in the
instrumented pillar P3
limiting stress transfer to the active mining zone. A bore-
hole roof scoping confirmed that there was no separation in
the rock strata in the immediate roof outby the pillar line.
Additionally, the rib conditions were generally good, show-
ing little to no sloughing (see Figure 12c). However, some
sloughing was noted at the ribs in areas where the pillar was
being actively mined, particularly starting from the coal/
rock parting interface (see Figure 12b).
Visual observations of rib performance at the instru-
mented pillar indicate that rib brows were generally not
formed. However, rib brows were observed at a few pillar
corners outby the pillar line, where timber supports were
subsequently used to stabilize them.
Figure 13 shows photos of the cross-cut side of the
instrumented pillar under different loading conditions.
Slight changes in the rib condition are noticeable between
Figure 13a, taken one week after the instrument installa-
tion, and Figure 13b, taken a few hours before mining the
instrumented pillar.
The monitoring effort lasted approximately six months,
allowing for a comprehensive observation of the rib per-
formance of the instrumented pillar under various loading
conditions. All recorded instrument data remained steady,
except in the last three days when the continuous miner
began retreating from the final pillar located between cross-
cuts 25 and 26. For instance, as shown in Figure 14, the
average pressure recorded by the BPCs within the instru-
mented pillar remained stable at about 1,000 psi. However,
as the continuous miner retreated to cut No. 68—situated
in the pillar between cross-cuts 25 and 26 (see Figure 11)—
the average pressure began to rise, ultimately reaching
about 2,000 psi when the continuous miner advanced to
cut No. 50 in the instrumented pillar. Consequently, the
analysis of the instrumented pillar data will focus only on
the last three days of the monitoring period.
Roof Convergence
Figure 15 shows the roof convergence observed at entry
No. 4 and cross-cut 24 during pillar retreat operations.
Due to a malfunction in the roof extensometer (RX-2)
at cross-cut 24, no roof convergence data was recorded at
that location. In contrast, the roof extensometer (RX-1) at
entry No. 4 indicated a slight convergence of the immedi-
ate roof—less than 0.02 in—attributed to front abutment
loading resulting from the retreat of the last inby pillars
between cross-cuts 25 and 26.
Once the continuous miner began cutting into pillar
P1, the roof convergence is slightly increased to 0.02 in.
A slight increase in roof convergence to about 0.05 in.
occurred as the continuous miner advanced into the
instrumented pillar P3, followed by an exponential rise in
convergence, reaching 0.7 in. as nearly half of the instru-
mented pillar was mined.
Figure 13. Photos taken at the cross-cut side of instrumented
pillar P3
Figure 14. Average induced pressure change in the
instrumented pillar P3