8
• If the hole was not drilled wet, hollow insertion rods
are used to clean the hole with water, and the hole is
dried prior to installation.
• To prepare the HI Cell, the pieces of the instrument
are gathered, the exterior surface of the HI Cell is
sanded to increase the bond strength of the cell and
the epoxy, the two-part epoxy is mixed, and the hol-
low interior of the cell is filled with the epoxy.
• Once the cell is filled with epoxy, the pusher tube is
inserted part way into the cell as shown in Figure 11,
and small, easily deformable metal pins are inserted
into the body to keep the pusher in place.
• The wooden dowel is affixed to the end of the pusher
tube and the HI Cell is ready for insertion.
• The same setting rods used to clean the hole are now
used for the insertion of the HI Cell.
• The HI Cell is gently pushed to the back of the hole
and once resistance is met, a final strong push is used
to break the metal pins, force the pusher into the
hollow body, and squeeze the two-part epoxy out of
the hollow body filling the annulus of the borehole.
• At this point, insertion is complete. The insertion
rods are tied off to roof bolts and left in place over-
night for the epoxy to set.
• The following day, the insertion rods are removed,
and the HI Cell is connected to the datalogger.
Borehole Pressure Cells
The Borehole Pressure Cell (BPC) is designed to detect
changes in rock stress and is intended for placement within
a borehole through grouting. The BPC (Panek and Stock,
1964) is crafted from two steel plates, seamlessly welded
along their edges. These sheets are shaped into a unique
“dog bone” design, similar to a borehole flatjack. The void
between the sheets is then filled with hydraulic fluid to the
desired setting pressure. Stainless steel tubing then links the
BPC to a pressure gauge and/or pressure transducer located
outside the borehole (see Figure 14).
Typically, BPCs are prepped and pre-cast for installa-
tion such that they arrive at the field site as shown at the top
of Figure 14 (Miller and Sporcic, 1964)—that is, complete
with stainless-steel, high-pressure tubing, pressure sensing
assembly, and pressure transducer. If desired, the BPC can
also be post-grouted following installation. Installation of
the BPC at the specified field site is as follows:
• A 2-¼” nominal diameter hole is drilled into the rib
horizontally to the specified depth.
• The hole is cleaned by the continued rotation of the
auger steel in the hole.
• Secondary cleaning of the hole is completed using
chimney sweep brushes or cleanout tool if necessary.
• The BPC is then inserted into the hole perpendicular
to the desired stress orientation using insertion rods
with a BPC specific adapter.
• After insertion, if the BPC is pre-grouted, the BPC is
then pressurized to the desired setting pressure (typi-
cally 1.1 times the overburden depth in psi).
• If the BPC is post-grouted, grout is poured in the
hole and left to set based on the manufacturer’s rec-
ommendations. Upon setting, the BPC is then pres-
surized to the final setting pressure.
• Once the setting pressure is achieved, excess tubing
is coiled and hung appropriately off a rib or roof bolt
plate or spad nail.
Moving beyond the measurement of the inherent
stresses within the rock itself, it is also important to under-
stand the support loading characteristics as well. Support
load instrumentation attempts to measure the forces on
ground support structures such as roof bolts and standing
support in the mine.
SUPPORT LOAD INSTRUMENTATION
Roof and Rib Bolt Load Cell
Load cells installed on roof and rib bolts allow the load
applied to the ungrounted length of the bolt to be measured.
The GEOKON Model 3000 Load Cell (See Figure 15)
utilizes strain gauges in Wheatstone bridge configuration
to measure load forces and is currently used by NIOSH
researchers. The steel body construction with a pass-
through feature allows for installation on a variety of cable
bolts, mechanical bolts, rib bolts or other anchored bolt
mechanisms. The long-term and active mining load forces
applied to roof and rib bolts are typically monitored using
such load cells (Gearhart et al., 2017). NIOSH currently
uses GEOKON Model 3000 (GEOKON, 2020) load cells
Figure 14. Example of an uncast BPC (bottom) and a
pre-cast BPC pressure sensing assembly ready for field
installation (top)
• If the hole was not drilled wet, hollow insertion rods
are used to clean the hole with water, and the hole is
dried prior to installation.
• To prepare the HI Cell, the pieces of the instrument
are gathered, the exterior surface of the HI Cell is
sanded to increase the bond strength of the cell and
the epoxy, the two-part epoxy is mixed, and the hol-
low interior of the cell is filled with the epoxy.
• Once the cell is filled with epoxy, the pusher tube is
inserted part way into the cell as shown in Figure 11,
and small, easily deformable metal pins are inserted
into the body to keep the pusher in place.
• The wooden dowel is affixed to the end of the pusher
tube and the HI Cell is ready for insertion.
• The same setting rods used to clean the hole are now
used for the insertion of the HI Cell.
• The HI Cell is gently pushed to the back of the hole
and once resistance is met, a final strong push is used
to break the metal pins, force the pusher into the
hollow body, and squeeze the two-part epoxy out of
the hollow body filling the annulus of the borehole.
• At this point, insertion is complete. The insertion
rods are tied off to roof bolts and left in place over-
night for the epoxy to set.
• The following day, the insertion rods are removed,
and the HI Cell is connected to the datalogger.
Borehole Pressure Cells
The Borehole Pressure Cell (BPC) is designed to detect
changes in rock stress and is intended for placement within
a borehole through grouting. The BPC (Panek and Stock,
1964) is crafted from two steel plates, seamlessly welded
along their edges. These sheets are shaped into a unique
“dog bone” design, similar to a borehole flatjack. The void
between the sheets is then filled with hydraulic fluid to the
desired setting pressure. Stainless steel tubing then links the
BPC to a pressure gauge and/or pressure transducer located
outside the borehole (see Figure 14).
Typically, BPCs are prepped and pre-cast for installa-
tion such that they arrive at the field site as shown at the top
of Figure 14 (Miller and Sporcic, 1964)—that is, complete
with stainless-steel, high-pressure tubing, pressure sensing
assembly, and pressure transducer. If desired, the BPC can
also be post-grouted following installation. Installation of
the BPC at the specified field site is as follows:
• A 2-¼” nominal diameter hole is drilled into the rib
horizontally to the specified depth.
• The hole is cleaned by the continued rotation of the
auger steel in the hole.
• Secondary cleaning of the hole is completed using
chimney sweep brushes or cleanout tool if necessary.
• The BPC is then inserted into the hole perpendicular
to the desired stress orientation using insertion rods
with a BPC specific adapter.
• After insertion, if the BPC is pre-grouted, the BPC is
then pressurized to the desired setting pressure (typi-
cally 1.1 times the overburden depth in psi).
• If the BPC is post-grouted, grout is poured in the
hole and left to set based on the manufacturer’s rec-
ommendations. Upon setting, the BPC is then pres-
surized to the final setting pressure.
• Once the setting pressure is achieved, excess tubing
is coiled and hung appropriately off a rib or roof bolt
plate or spad nail.
Moving beyond the measurement of the inherent
stresses within the rock itself, it is also important to under-
stand the support loading characteristics as well. Support
load instrumentation attempts to measure the forces on
ground support structures such as roof bolts and standing
support in the mine.
SUPPORT LOAD INSTRUMENTATION
Roof and Rib Bolt Load Cell
Load cells installed on roof and rib bolts allow the load
applied to the ungrounted length of the bolt to be measured.
The GEOKON Model 3000 Load Cell (See Figure 15)
utilizes strain gauges in Wheatstone bridge configuration
to measure load forces and is currently used by NIOSH
researchers. The steel body construction with a pass-
through feature allows for installation on a variety of cable
bolts, mechanical bolts, rib bolts or other anchored bolt
mechanisms. The long-term and active mining load forces
applied to roof and rib bolts are typically monitored using
such load cells (Gearhart et al., 2017). NIOSH currently
uses GEOKON Model 3000 (GEOKON, 2020) load cells
Figure 14. Example of an uncast BPC (bottom) and a
pre-cast BPC pressure sensing assembly ready for field
installation (top)