Tuesday, June 4, 2013

Behind the blast: A closer look at the sights, sounds and safety of blasting for ECT

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.

Although there are many regulations and references that our contractor utilizes in blasting, below is a link that talks about the federal government’s maximum peak particle velocity standards:  http://www.arblast.osmre.gov/downloads/USBM/RI%208057%20Blasting%20Vibration%201989.pdf.

Questions about the blasting process can be directed to Ryan Sullivan at sullivanr@neorsd.org.