5
LC-5, and LC-6 were installed in the cross-cut side (cross-
cut No. 24) of the instrumented pillar.
Borehole pressure cells (BPCs) were installed at vari-
ous depths into the instrumented pillar to monitor the
induced vertical stress. The setting pressure for the BPCs
was at about 1,100 psi, corresponding to the overburden
depth at the study site. BPC-1-30, BPC-2-20, and BPC-3-
10 were placed at depths of 30 ft, 20 ft, and 10 ft, respec-
tively, within the rib of the instrumented pillar at the entry
side. Similarly, BPC-4-30, BPC-5-20, and BPC-6-10 were
installed at the same depths in the rib of the pillar at the
cross-cut side.
Multi-point borehole extensometers (MPBXs) were
grouted into the instrumented pillar to monitor the hori-
zontal rib displacements. Each MPBX comprises four
anchor points. The deepest anchor point, positioned 30 ft
into the pillar, is located where no horizontal rib displace-
ment is anticipated. The other three anchor points are set
at varying depths. The naming convention for the MPBXs
includes the MPBX ID followed by the depths of the four
anchor points, measured in feet from the coal rib. For
example, “MPBX-1-8-14-20-30” refers to an MPBX with
ID 1 and anchor points at depths of 8, 14, 20, and 30 ft.
Three MPBXs were installed on both the entry side and
the cross-cut side. As shown in Figure 8, the three MPBXs
on each side are placed close to one another. Therefore, the
MPBXs can be combined to show the rib displacement at
different depths, although they were installed in different
boreholes.
The anchorage capacity of rib bolts installed in the rock
parting at the study site was evaluated using short encap-
sulation pull-out tests (SEPTs) with a 12-in grout length.
Two 5-ft-long No. 6, Grade 60 rib bolts were installed in
the rock parting of the rib opposite the instrumented rib in
entry No. 4. Figure 9 shows the load-displacement curves
for these rib bolts. The bolts were unloaded at approxi-
mately 0.4-in displacement, as the applied load approached
the yield load of the rebar, which is 26,500 lbs.
To further assess the anchorage capacity of rib bolts used
to mount load cells, five additional SEPTs were conducted
on rib bolts installed in the coal seam of the rib opposite the
instrumented rib in Entry No. 4. Figure 10 shows the load-
displacement curves for these tests, indicating that all but
one test approached the yield load of the rebar.
PILLAR RETREAT PLAN
Figure 11 illustrates the pillar retreat sequence, retreat dates,
and overburden contours at the study site. The pillar retreat
Figure 9. Load displacement curves for a rib bolts SEPT
conducted in a rock parting
Figure 10. Load displacement curves for a rib bolts SEPT
conducted in a coal seam
LC-5, and LC-6 were installed in the cross-cut side (cross-
cut No. 24) of the instrumented pillar.
Borehole pressure cells (BPCs) were installed at vari-
ous depths into the instrumented pillar to monitor the
induced vertical stress. The setting pressure for the BPCs
was at about 1,100 psi, corresponding to the overburden
depth at the study site. BPC-1-30, BPC-2-20, and BPC-3-
10 were placed at depths of 30 ft, 20 ft, and 10 ft, respec-
tively, within the rib of the instrumented pillar at the entry
side. Similarly, BPC-4-30, BPC-5-20, and BPC-6-10 were
installed at the same depths in the rib of the pillar at the
cross-cut side.
Multi-point borehole extensometers (MPBXs) were
grouted into the instrumented pillar to monitor the hori-
zontal rib displacements. Each MPBX comprises four
anchor points. The deepest anchor point, positioned 30 ft
into the pillar, is located where no horizontal rib displace-
ment is anticipated. The other three anchor points are set
at varying depths. The naming convention for the MPBXs
includes the MPBX ID followed by the depths of the four
anchor points, measured in feet from the coal rib. For
example, “MPBX-1-8-14-20-30” refers to an MPBX with
ID 1 and anchor points at depths of 8, 14, 20, and 30 ft.
Three MPBXs were installed on both the entry side and
the cross-cut side. As shown in Figure 8, the three MPBXs
on each side are placed close to one another. Therefore, the
MPBXs can be combined to show the rib displacement at
different depths, although they were installed in different
boreholes.
The anchorage capacity of rib bolts installed in the rock
parting at the study site was evaluated using short encap-
sulation pull-out tests (SEPTs) with a 12-in grout length.
Two 5-ft-long No. 6, Grade 60 rib bolts were installed in
the rock parting of the rib opposite the instrumented rib in
entry No. 4. Figure 9 shows the load-displacement curves
for these rib bolts. The bolts were unloaded at approxi-
mately 0.4-in displacement, as the applied load approached
the yield load of the rebar, which is 26,500 lbs.
To further assess the anchorage capacity of rib bolts used
to mount load cells, five additional SEPTs were conducted
on rib bolts installed in the coal seam of the rib opposite the
instrumented rib in Entry No. 4. Figure 10 shows the load-
displacement curves for these tests, indicating that all but
one test approached the yield load of the rebar.
PILLAR RETREAT PLAN
Figure 11 illustrates the pillar retreat sequence, retreat dates,
and overburden contours at the study site. The pillar retreat
Figure 9. Load displacement curves for a rib bolts SEPT
conducted in a rock parting
Figure 10. Load displacement curves for a rib bolts SEPT
conducted in a coal seam