I am the receptionist at the main office for Hills
Materials Company. We are located across the street from our Rapid City
Quarry. My days are full of phone calls, typing letters and quotes,
helping the lost tourist that wanders in looking for Mt. Rushmore or the
unemployed looking for a job. This office runs like a well oiled machine.
We’ve all worked together for so long, we can almost read each other’s minds.
Jake Smith is the blaster at Hills Materials Company and I
have to say I am more than a little envious of Jake’s job. How many of us would
love to blow up an entire side of a hill? Talk about a stress reliever! Come
to think of it, Jake is one of the calmest people I know.
Jake graduated from the South Dakota School of Mines &
Technology in 2002 in Rapid City with a bachelor’s degree in mining
engineering. He came to work at Hills Materials Company in 2002 as a lab
technician and became a foreman in 2004. He and his wife Kari have three
children, Keaton (7), Paul (3) and Olivia (4 mos.), they live in Hayward, south
of Rapid City.
Shannon. Jake, we all
want to know, do you get a charge out of your job?
Jake. Yeah, it is
pretty interesting as well as exciting. No two shots are the same and
figuring out how to best set a shot off is challenging. I always say it’s
hours of work for a few seconds of fun.
S. Do you have to wear
any special protective clothing?
J. No, I wear the
standard PPE (Steel Toe Boots, Hardhat, Safety Glasses). I only have to worry
about stray sparking, but that is not that high of a danger most of the time.
The chemicals I work with do not pose any extra danger, except that they
detonate, I guess.
S. Are you required to
be licensed with some kind of agency?
J. Hills Materials Co.
has an “Explosives License” with the ATF (Bureau of Alcohol, Tobacco and
Firearms). Any users under that license have had background checks and some
of the officers of the company have also had to be fingerprinted. We also
have a “Hazardous Materials License” with the US-DOT. We are required to have
this because I transport explosives over-the-road to our other quarries.
S. Is it difficult to
obtain this licensing?
J. It is difficult
because there are a lot of requirements you need to fill before a license will
be issued and then you are subject to inspections. Three separate agencies
inspect our explosives program and it can be difficult to please all three.
S.
Are you also licensed in
Wyoming?
J. Yes, we have a quarry
located in Wyoming and blasters are required to be licensed by the state.
South Dakota does not have the same licensing program and relies on the ATF.
S.
How do you know where to blast,
say, at a new pit location?
J. We set up an
exploration drill pattern to find where and the depth of the rock. After a
mine plan has been designed, then we decide where we want the first shot.
S.
What is involved in forming a
new pit?
J. In addition to what
I explained in the previous question, overburden is removed from the top of
the rock. Then the driller drills a tight pattern that will be shot upwards.
Since there is no free face except up, we have to push the rock upwards.
However, we control how much the rock goes upwards with the drill pattern,
delay pattern and explosive charge. Obviously there is more involved with
other areas to get a pit started, but this is the basics as it pertains to
blasting.
S.
How do you set up a blast and
how do you know how much to use?
J. First, we determine
where in the pit we want to blast. The driller drills a pattern using
standard practice. Then I figure out the delay pattern. I figure out the
approximate quantity needed. We load the shot using standard procedure and
load the powder column in the hole to a predetermined height. I’m not sure
how detailed I should go because I don’t want to confuse anyone. I look at a
lot of different variables when loading a shot; the rock structure,
irregularities (ie cracks) and other obstacles (such as crushing equipment).
S.
Once the blast is done, what
happens?
J.
First, I check to make sure
everything has “gone off”. Once everything is clear, I allow everyone to come
back into the pit and allow traffic to go on the highway, if the road is
blocked. Next, I check for any overhangs or unstable highwall and then let
the foreman and the loader operator know where they are. Finally, I talk with
the foreman and driller and determine where the next pattern is going to be
drilled so I can assess any dangers or concerns that might affect the blast.
S.
Where do you get the explosives?
J. Our explosives
supplier is out of Wyoming. There are no explosives suppliers in western SD.
I order what we need, based on our inventory, and they deliver the explosives
to us.
S.
What kind of explosives do you
use?
J.
We mostly use ANFO (Ammonia
Nitrate Fuel Oil). ANFO is the cheapest explosive
available on the market. It comes in a “prill” form; it looks much like what
many use as fertilizer on their lawns. Actually, Ammonia Nitrate is a
fertilizer. ANFO’s dubious distinction is that it was homemade ANFO that was
used in the Oklahoma City Federal Building bombing. We
also use “shock tube” type caps for downhole and surface. We also use cast
boosters with our downhole caps to “set off” each hole. The boosters contain
TNT, PETN and RDX. Most know what TNT is, but PETN and RDX are explosives
that were initially developed for the military and later adapted to industry
use. We do not use any Nytroglycerin or Dynamite type explosives due to
safety and cost.
S.
Do you get a lot of complaints
from neighbors of the quarry about the noise and/or shaking?
J.
At times we do get complaints
after a blast. Most people do not like to feel any vibration what-so-ever and
automatically assume that any vibration is causing damage to their property.
I have to address any complaint and explain to the caller that the vibration
they feel is not causing damage. It is hard, because I’m basically asking the
caller to trust someone they have never met. At times I do talk to the
complainer face to face if the need is required. I do setup a seismograph on
my shots to monitor vibration and make sure the vibration stays within
tolerable levels. This helps when I’m trying to explain about vibration
intensity.
When explaining
vibration, I tell people that there is a combination of effects that they
feel. One is, of course, ground vibration. The second is air blast
overpressure or just air blast. The ground vibration is the direct transfer
of the explosives detonating in the ground and the resulting seismic wave that
moves through the ground. Air blast is similar to that of a sonic boom or
thunder clap. A person could feel one or the other or even both. The
seismograph reads both the ground vibration (in in/sec) and air blast (in
decibels).
When I look at the
seismograph, I’m not only looking at the intensity of the vibration, but also
the frequency. The latest studies have shown that a building has a certain
harmonic signature. Most buildings (depending on the construction) have a
harmonic signature between 4-12 Hertz. Any vibration that has a frequency in
that same range will have a higher effect on the building. However, if you
increase the frequency of your vibration above the 12 Hz range, then you have
less effect on the building. I can best describe this concept by looking at
the relationship between a boat and waves. Lower frequency would be the low
slow moving waves and higher frequency is the fast choppy waves. A boat
moving through the low frequency will move up and down through the trough and
over the crest of each wave, causing the boat to rock with each wave. If the
waves are choppy and fast then the boat will actually bridge the gap between
the wave and thus the wave doesn’t affect the boat so much (see figure 1).
With this explanation in mind, I try to get the frequency vibration as high as
I can. I set my delay patterns as “quick” as I can because the quicker a shot
goes off the higher the frequency of the vibration. With the latest
technology in blasting caps and electronics, some mines delay between holes
only 2-3 milliseconds, I use conventional blasting and delay 17 ms. The 2-3
ms delay causes very high frequency vibration waves that have little effect on
buildings.
The concept of air blast
is very similar to a thunder clap. The explosive detonation causes a sudden
increase in air pressure in front of a shot. That air pressure radiates out
from the shot causing a loud bang just like a thunder clap. Normally the
farther away from a shot you are, the less you hear the sound. But certain
weather conditions can affect the intensity of the air blast.
The environment affects
air blast in the following ways. If the clouds are low or it is an overcast
day, then the air blast will be more intense. This is because the clouds act
as an insulator and prevent the sound waves from dissipating. They also will
reflect any sound waves back to the earth and people who would normally not be
affected by air blast might actually experience it. Also, weather,
temperature and wind can affect the intensity of an air blast. When a weather
phenomena called a “temperature inversion” occurs, the sudden change in
temperature at a certain altitude can reflect sounds wave back down to the
ground just like the clouds do. Wind can actually “carry” sound waves further
than normal. Or, if there is varying wind speed at different altitudes, the
wind can reflect sound waves back down to the surface.
I have to be aware of all
of these factors when I am detonating a pattern and talking to a person who
calls with concerns. All of our shots are small enough that we never approach
the limit on vibration, however, even a little vibration can cause a person to
be concerned. We do alleviate some of the concern by having a call list of
certain people who have regularly called when we blast. We now call the
people on the list to let them know ahead of time. This helps, because the
people feel that we are keeping their concerns in mind when we are blasting,
as well as eliminating the element of surprise when a blast goes off.
Overall, our biggest task is to educate people on what we are doing. When
they understand, then we have less problems with complaints.
Hills Materials Company was
the prime contractor on a City of Rapid City street project to mill and asphalt
overlay four downtown alleys. One of the four alleys is locally known as “Art
Alley”. Art Alley was originally started with graffiti and local artist
painting the back alley sides of the downtown buildings. Local artists have
turned many graffiti makers into artists in this alley. The alley receives a
lot of pedestrian traffic local and out of town tourists. 
