The Journal of Explosives Engineering - November/December 2024 - 32

Figure 3. A. Test Setup Diagram & B. Field Test Setup.
geophones, were positioned inline in front of the block approximately
perpendicular to the face. During the first test the
seismographs were placed at 18 and 26 ft (5.48 and 7.92 m)
from the face. The second test resulted in a much closer setup
distance at 8 and 16 ft (2.44 and 4.88 m). The geophones
were placed upon a bare rock surface and covered with sandbags
with microphones directly above.
Two pencil probe transducers, Model 137B23B, were
placed on stands in front of the face at the same distances as
the Mini-Seis Microphones at 90˚ from the face. The probes
were placed at 23.5 in (0.6 m) above the ground for both tests
and within the charge column height.
The two Mini-Seis III seismographs were connected via a
jumper wire in order to ensure the data was recorded on the
same time basis as they were not connected to the DAQ. Utilizing
the pencil probe instrumentation timing we were able
to determine the pressure wave arrival at the microphones
which in turn made it possible to calculate the speed of sound
through the air during the blast with the clearer air overpressure
waveforms from the seismograph microphones.
A laser sensor that measures surface change was employed
to detect the initial face movement for each blast. The laser
produces a change in voltage when the distance to its target
changes. The voltage change is recorded in the DAQ. This
method is more accurate than a high-speed camera when
working with a small-scale model.
Three flexible lead piezo electric ribbon sensors (FDT1028K)
were utilized for each test. These sensors were attached
to an audio wire and connected to the DAQ to capture the
precise timing of interactions inside of a pressurized borehole.
The flexible lead piezo electric ribbon sensor is commonly used
in vibration monitoring, movement monitoring, and capturing
even minor external forces that could be exerted inside a
borehole. Testing of the sensors showed that even the slightest
touch of a finger on the sensors would change voltage.
The sensors were used to capture the time when a pressure
change occurred inside the boreholes. The sensors proved to
be highly valuable at capturing the precise time of arrival inside
the borehole, but the magnitude of the signal is not calibrated
to a pressure and therefore not utilized in the analysis.
Two systems were utilized to ensure the triggering of the
system to include both a fiber optic and break wire setup.
Fiber optic instrumentation was installed in the blast hole to
record each instance of optical impulse within the explosive
column. This highly accurate methodology provides a much
faster response back to the DAQ than other break wire methods.
The fiber optic signal was used as the time the borehole
initiated. As a failsafe, an additional break wire was taped
to the blasting cap that triggered the DAQ when it broke. It
was noted that the response difference of the break wire was
much longer than the fiber optic system.
Lastly, a GoPro Hero Black 7 was utilized to capture the
system, which recorded 30 frames per second (fps) at high
resolution. The use of the camera was invaluable for capturing
the sequence of events that occurred during the test blast.
The research setup goal was to capture three major items
for analysis:
(1) The laser system to capture the distinct initial movement
of the face
(2) The piezoelectric sensors to capture the time at which
pressurization occurred through the rock mass
(3) Timing of the air overpressure as it reached the seismographs
and pencil probes.
The test blocks were drilled, loaded, and fired on 28 June
of 2023 on sunny, low humidity, and low wind day. This gave
the optimal conditions to capture the data while minimizing
the potential for atmospheric effects on the system.
November/December 2024
The Journal of Explosives Engineering
29
The Journal of Explosives Engineering - November/December 2024

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