7
measurements, taken at 60° intervals in the plane orthogo-
nal to the borehole, facilitate the analysis of changes in the
biaxial stress field surrounding the sensor (see Figure 12).
Refer to the VW 4350 Manual for additional descrip-
tion, specifications, procedures, and theory of operation
(GEOKON, 2019). The following installation procedures
primarily relate to practices and experience at NIOSH.
• Drill a 2.5” diameter borehole, typically horizontal
or subhorizontal into pillar at the required depth.
• Ensuring the hole is clean and free of debris is cruital
for setting orientation in place.
• Downward sloping hole provides ease of grouting
and ensure full encapsulation of the sensor.
• Grout can be pumped in or poured depending on
the slope.
• High-strength and fine-grained expansive grout is
recommended.
• Attach a thin steel wire to the anchor release hook
on the sensor.
• Partially fill the hole with grout, then orient the sen-
sor where gauge 1 is vertical and proceed to push the
sensor to the back of the hole maintaining orienta-
tion marked on insertion rods.
• Once the sensor is at the back of the hole, pull the
anchor release cable while keeping the sensor orien-
tied correctly. This will lock the orientation as the
insertion rods are removed and the grout sets up.
• In some instances, the hole is too large for the ori-
entation anchor to set, and this can be overcome by
swiftly pulling back on insertion rods, freeing them
from the oriented sensor.
• Allow up to 28 days of curing time for reliable instru-
ment measurements.
Hollow Inclusion (HI) Cell
The Hollow Inclusion (HI) Cell (see Figure 13), an inno-
vation by the Commonwealth Scientific and Industrial
Research Organization (CSIRO), was designed for the task
of measuring three-dimensional stress in rock and concrete
substrates (Duncan-Fama and Pender, 1980). Over the
years, its efficacy has been demonstrated in assessing stress
and strain dynamics within concrete tunnels and various
infrastructural elements.
The highly sensitive strain gauges are bonded to the
rock in a novel mechanical process using temperature-spe-
cific glue packs, allowing the gauges to become attached to
the rock in a reasonable amount of time. The sensor can
be left in-situ for long-term stress change measurement
(Gearhart et al., 2017) or can be over-cored to see the strain
change to determine a stress profile of that section of rock
(Earth Sciences, 2020).
The HI Cell requires several on-site preparation steps
before it is suitable for installation. The following will
address those steps along with the installation procedures.
• A 2-½” nominal diameter hole is drilled into the roof
at the specified angle and to the specified depth.
• The hole can be drilled either wet or dry depending
on the bits being used.
• At the end of the 2-½» diameter hole, a finishing bit
is used to drill a 1-½» diameter hole of an appropri-
ate length for the HI Cell to be installed into.
Figure 12. Cross-section rendering of the GEOKON
Model 4350 Biaxial Stressmeter depicting the location and
orientation of strain gauges within the instrument
Figure 13. Hollow Inclusion (HI) Cell depicting the cell
itself, plunger, wooden dowel, and a pre-selected length of
cable
measurements, taken at 60° intervals in the plane orthogo-
nal to the borehole, facilitate the analysis of changes in the
biaxial stress field surrounding the sensor (see Figure 12).
Refer to the VW 4350 Manual for additional descrip-
tion, specifications, procedures, and theory of operation
(GEOKON, 2019). The following installation procedures
primarily relate to practices and experience at NIOSH.
• Drill a 2.5” diameter borehole, typically horizontal
or subhorizontal into pillar at the required depth.
• Ensuring the hole is clean and free of debris is cruital
for setting orientation in place.
• Downward sloping hole provides ease of grouting
and ensure full encapsulation of the sensor.
• Grout can be pumped in or poured depending on
the slope.
• High-strength and fine-grained expansive grout is
recommended.
• Attach a thin steel wire to the anchor release hook
on the sensor.
• Partially fill the hole with grout, then orient the sen-
sor where gauge 1 is vertical and proceed to push the
sensor to the back of the hole maintaining orienta-
tion marked on insertion rods.
• Once the sensor is at the back of the hole, pull the
anchor release cable while keeping the sensor orien-
tied correctly. This will lock the orientation as the
insertion rods are removed and the grout sets up.
• In some instances, the hole is too large for the ori-
entation anchor to set, and this can be overcome by
swiftly pulling back on insertion rods, freeing them
from the oriented sensor.
• Allow up to 28 days of curing time for reliable instru-
ment measurements.
Hollow Inclusion (HI) Cell
The Hollow Inclusion (HI) Cell (see Figure 13), an inno-
vation by the Commonwealth Scientific and Industrial
Research Organization (CSIRO), was designed for the task
of measuring three-dimensional stress in rock and concrete
substrates (Duncan-Fama and Pender, 1980). Over the
years, its efficacy has been demonstrated in assessing stress
and strain dynamics within concrete tunnels and various
infrastructural elements.
The highly sensitive strain gauges are bonded to the
rock in a novel mechanical process using temperature-spe-
cific glue packs, allowing the gauges to become attached to
the rock in a reasonable amount of time. The sensor can
be left in-situ for long-term stress change measurement
(Gearhart et al., 2017) or can be over-cored to see the strain
change to determine a stress profile of that section of rock
(Earth Sciences, 2020).
The HI Cell requires several on-site preparation steps
before it is suitable for installation. The following will
address those steps along with the installation procedures.
• A 2-½” nominal diameter hole is drilled into the roof
at the specified angle and to the specified depth.
• The hole can be drilled either wet or dry depending
on the bits being used.
• At the end of the 2-½» diameter hole, a finishing bit
is used to drill a 1-½» diameter hole of an appropri-
ate length for the HI Cell to be installed into.
Figure 12. Cross-section rendering of the GEOKON
Model 4350 Biaxial Stressmeter depicting the location and
orientation of strain gauges within the instrument
Figure 13. Hollow Inclusion (HI) Cell depicting the cell
itself, plunger, wooden dowel, and a pre-selected length of
cable