Construction & Contractors/nearby quarry blasting

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Question
A new car dealership was recently built approx 1000' from our home.  The blasting went on each weekday for one year. With each blast we could feel the movement inside our home which is on a crawlspace type foundation. I did not realize the extent of the blasting until I walked up to the site to see that turned out to be a rock quarry that is enormous in size! It appears that the constructors blasted rock from this quarry in order to elevate their building site.  My question for you is could blasts of this type, at this distance, and over this period of time possibly cause damage to my home?  I have noticed some new cracks in new places since the blasting and am now absolutely disappointed. I'm unsure where to begin except to get an opinion from someone who knows about blasting to find out if these new cracks in the brick could possibly be related to the blasting. - Thank your time!

Answer
Although it is definitely possible for blasting to cause cracking in homes, it is also very common for those who notice blasting vibrations in and around their homes to then notice cracking that they never had cause to notice before.  Humans can detect blasting vibrations down to 0.1 or 0.2 inches per second, but this is almost never associated with damage -- even cosmetic damage -- to structures.  The possibility for damage to structures has to do with the magnitude of the strain (deflection or deformation) induced within the structure.  Larger deformations occur not only with higher particle velocities traveling from the blast to the ground around the structure, but also if the individual episodes are prolonged (more than 1-2 sec) and if the ground motions are low-frequency so that the structure can more readily "go along" with the ground motions.  However, at 0.1-0.2 inches per second at 1,000 ft the ground motions are usually so small that the possibility of damaging strains in a structure is extremely small.  The same thing can normally be said of the air blast overpressure effect (the structure movements induced by the air displaced by the blast).  In fact, over the course of a year, the strains induced by environmental factors such as temperature changes, wind, door slams and normal wear and tear, and water intrusion can be more than blasting vibrations.  Structures can tolerate larger vibrations than the threshold of detection by people, as it turns out, and if you felt that level of vibration every day for a year you would probably have become seriously concerned a long time ago.

If the blasting company has been blasting each day for a year just to build a car dealership, this suggests that the individual blasts were probably not very large.  My guess is that the cracks you are now seeing were very likely there before, or have occurred due to environmental factors.  However, any responsible blasting program in an urbanized area should have included ground and airblast vibration monitoring.  Blasting companies usually monitor just so they can improve the effectiveness of their blasts without increasing vibrations, although they would most likely monitorcloser in.  It is theoretically possible to calculate what the vibrations would have been in your more immediate area.

I get a lot of these kinds of questions and they usually would not have occurred if the blasting company or the project Owner had arranged for pre-blast surveys.  It is amazing what those surveys show, in terms of structure condition prior to the start of blasting.  They may have felt that 1,000 ft is too far for any likelihood of damage, but you might want to find out if there were pre-blast surveys of houses closer in, and talk with those homeowners.  And definitely ask for the blasting vibration data.  If there was no monitoring then there is some uncertainty about the vibration magnitudes at you house.

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Robert Cummings, P.E.

Expertise

I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.

Experience

30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.

Organizations
Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.

Publications
Mining Engineering, AEG Bulletin.

Education/Credentials
BS and MS Geological Engineering

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