The Journal of Explosives Engineering - November/December 2024 - 31

Figure 2. Concrete test block drilling and loading.
quarry rock mass. The block's dimensions were square to create
a perfect cubic yard block (0.77 cubic meter). An outer Lshaped
structure was created with 4,000 to 5,000 pounds per
square inch (psi) (275.79 to 344.74 bar) concrete and rebar
reinforcement 10 inches (in) (0.25 meters (m)) deep. The intent
was to provide rigidity to the mass as well as support the
direction of movement during the test blasting. The remainder
26 by 26 in (0.66 by 0.66 m) block was filled with a mixture of
3,000 psi (206.84 bar) concrete and polystyrene packing peanuts.
The polystyrene was mixed in at a ratio of 0.5 cubic feet
(0.014 cubic meter) per 50 pounds (lbs) (22.68 kilograms (kg))
bag of concrete as the blocks were formed. Every 6 in (0.15 m)
a layer of 3 in (0.08 m) thick polystyrene peanuts was placed.
This continued all the way up the form as the concrete was
poured. The idea was to create a semi-porous membrane with
seams similar to a highly layered rock formation. Though the
materials were poured in lifts the block remained a moderately
homogenous structure.
The blocks were cured for 27 days prior to testing. The
forms were pulled, and the blocks were drilled in accordance
with their projected designs. Each block was drilled with four
holes, as seen in figure 2. Three of the boreholes in each block
were used for installing pressure sensors at the same depth as
the black powder column with a diameter of 0.75 in (19.1
mm). The hole remaining in each block was to be loaded. The
first test block contained a 0.5 in (12.7 mm) loaded borehole
at a depth 29 in (0.74 m). The second test block received a
0.75 in (19.1 mm) loaded borehole at the same depth. The
purpose of the intended design was to ensure that the structure's
floor was resilient to prevent blowout during blasting.
Early on, it was critical to find a reasonable hole diameter that
would not create flyrock. The porosity of the system was not
fully known in the initial design, which later warranted the
need for a larger diameter hole. The sensor-loaded holes were
stemmed with a foam plug and then topped off with hydraulic
foam to ensure pressurization. The first loaded borehole
was stemmed using a concrete crack seal which performed
28
marginally well. The second loaded borehole was stemmed
using quick-drying hydraulic concrete, providing the necessary
confinement for the borehole.
Black powder was selected as the explosive and initiated
with a MasterDet electric blasting cap. Black powder is comprised
of potassium nitrate, sulfur, and charcoal though our
specific derivative utilized was sulfur free. The selection of
this product was intentional due to its extremely low critical
diameter as well as its ease of loading in a small hole. As a
low explosive, it deflagrates subsonically while providing high
gas pressurization levels. The intent was to remove the shock
wave which might impede the research results. Each of the
block's loaded holes were loaded with Triple 7 - FFFG Black
Powder to a stemming depth of between 6 and 8 in (0.15 and
0.20 m). Test 1 received 45.1 grams and Test 2 received 181.6
grams of black powder.
Eleven sensors were installed and connected to a data
acquisition system. Both a fiber optic and break wire system
were utilized within the loaded borehole to ensure that the
system was triggered. Flexible lead piezo electric ribbon sensors
were inserted into the sensor boreholes. A precision laser
movement device, pressure pencil probes, and seismographs
were installed exterior to the block to capture the relevant
timings described in more detail in the following paragraphs.
Lastly, a GoPro camera was installed to record the testing. The
site layout can be seen in figure 3.
A Synergy Data Acquisition System (DAQ) captured the
data from all pieces of equipment. The DAQ provides a unique
capability to capture both transient and real time data with
the ability to calibrate the instrumentation for the testing.
With this system a streaming visualization of the domain is
projected for continual analysis which ensures the highest levels
of accuracy. The DAQ was installed at less than 35 to 45
feet (ft) (10.67 to 13.72 m), limiting the distortion from cabling
and allowing for ease of connection for each test.
Two White Seismology Mini-Seis III seismographs, serial
numbers 7173 and 7174, with overpressure microphones and
The Journal of Explosives Engineering
November/December 2024
The Journal of Explosives Engineering - November/December 2024

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