There are
many questions that arise when someone hears that “blasting” will occur near
their property. They may include:
- Is this
something we should be scared of?
- Why does the
contractor need to blast?
- Will my
house be shaking continuously for weeks?
- What happens
before, during and after the blast?
We want provide answers to these questions below
but, first and foremost, we need you to know that blasting is safe.
Is
this something we should be scared of? No.
One way to lessen fears is by understanding the process.
The
ground where the Euclid Creek Tunnel is being constructed is comprised of
overburden (typically soil or, in our case clay) and bedrock. If you were
digging in your garden, you might use a small shovel to remove a small area. For our shafts, we use hydraulic machinery
because the volume of overburden we need to remove is much greater. Regardless of the technique, overburden is very easy to excavate, but
bedrock is a different story, and it leads to the second question:
Why
does the contractor need to blast? Bedrock,
compared to overburden, is much harder to remove for obvious reasons: it is solid
rock.
The
excavation of the bedrock is aided by controlled blasting inside a series of precisely spaced holes drilled
into the rock. In addition to the drill
holes, there are many other factors that an experienced explosives expert must
consider when planning each blast, including the amount of charge (explosive) and time
delay between when one charge goes off compared to the next one (nanoseconds). Rest assured, the old movies where a guy in
overalls lights a stick of dynamite, throws
it in a hole and yells, “Fire in the hole!”
are truly fiction are nowhere close to what actually happens.
That's
not entirely true. We do yell, "Fire in the hole" as you'll read
later. And we may wear overalls.
Will
my house be shaking for weeks? No.
You will hear the blasts, and it is likely that you will feel them, but
monitoring equipment will ensure they are maintained far below maximum
allowable limits. Here's how.
When a
blast goes off, more than 90 percent of
the energy is used to break up the rock. The remaining energy travels through the
ground and air as vibrations and noise.
Think of the vibrations traveling through the bedrock and overburden as
the ripples in a pond from dropping a pebble into the water. The most apparent
difference is that the vibrations from blasts cannot be seen; they can only be detected with high-tech equipment. But they likely
will be felt.
Just as the ripple in a pond lessens the further away from where the
pebble is dropped, so does the vibrations from the blast. The noise from the blast can be more alarming
than the actual vibrations themselves.
The
vibrations and noise are accurately measured with a series of seismographs placed
around the blast site. These
seismographs are placed, maintained and read by an independent third party, not
the contractor or the Sewer District.
These seismographs record the peak particle velocity from each blast
event. Various federal, state and local
municipalities and/or agencies have maximum allowable peak particle velocity
for blasting; these standards have been thoroughly tested and are determined to
be safe for nearby structures and utilities.
The Sewer District takes the federal
government’s maximum allowable peak particle velocity and cuts this level in
half, and this becomes the new maximum allowable value for our projects. After each blast event, the data on the
seismograph are read to ensure that these maximum
values were not reached. All Sewer District blasts will not exceed
50 percent of federal standards.
Each
individual’s response to a blast event is different. Someone busy doing housework or cutting the
grass will likely perceive much lower vibrations than someone else sitting at
the kitchen table with a cup of coffee.
People can sense very low levels of vibrations (just think of the
garbage truck picking up the trash), much lower than what would do damage to a
structure. Walking through a house,
closing windows, slamming the door or children running throughout a home will
stress individual building components more than a blast that is below the
maximum peak particle velocity.
Vibrations
that are accompanied by a noise will appear to be stronger than the same
vibration without the noise. Your senses
work together to give the body an impression for a certain experience; try
tasting your food while holding your nose, and again without, and the taste
will be very different.
What
happens before, during and after the blast? The blasting process
is extremely regulated, standardized for safety, and remains consistent each
and every time. Here’s what you can
expect.
There are strict
protocols used when handling the explosive material and in the blasting
preparations:
- The actual
explosive material will have a police escort at all times.
- A barricade
will be established around a preset blast area prior to the detonation of the
explosives.
- 5 long
air horn blasts will be sounded 5 minutes before the blast.
- 5 short
air horn blasts will be sounded 1 minute before the blast.
- “Fire in the
hole” will be yelled immediately prior to blast detonation.
After the blast, one long air horn blast will
be sounded for the all-clear.
The Sewer
District has used these techniques for decades both in residential and
non-residential areas and all of our contractors are experts in their fields.
- The
seismographs are placed strategically around the construction site to record
the peak particle velocity for the neighboring buildings and utilities.
- Sound-muffling
materials are placed above the bedrock to reduce noise while steel plates are
placed above and ensure the loose rock is contained.
- The entire
blasting process is monitored by police; access to the roads and walks will be
temporarily interrupted five minutes before the blast and not re-started until
the entire site is given an all-clear.
At a typical
shaft site, the Sewer District will carry out two blasts per week (the blast
itself lasting less than 15 seconds each); each blast helps break up to eight
feet of bedrock. That rock must then be removed before preparation for another
blast can occur. This process is repeated until the final depth required is
reached.
