Site Blasting For Large Facility Causing Residential Foundation Issues?? 2

[ndalle03]

For several months now, blasting as been occurring twice a day for site work for a million + square foot facility. Recently, nearby (approximatley 1 mile or more) homeowners have noted foundation issues. It is starting to become a news story and homeowners are adamant that the foundation cracks are a result of the blasting. I've only had conversations and haven't seen the cracks in person.

I'm unaware of the type and/or size of blasting. Homeowners have told me that they can physically feel the "tremors" following the blasts. These homes are in northern Kentucky (negligible seismic activity).

Considering the blasts are occurring 1 mile away, is it likely that these structural cracks are related to the blasting? Seems unlikely to me, but I'm not too familiar with the effects of blasting. FYI, the typical foundation type in the region are poured in place concrete foundation walls with wood stud framing and brick veneer.

 

[MTNClimber]

At that distance its most likely not from the blasting. This can be proven by using seismographs to record the vibrations at their house and/or bringing some experts to examine the cracks after they have been discovered. Sure, maybe they can feel the blast but perceived vibrations are worse than they actually are. After feeling the vibrations, homeowners start to look around their houses for the first time and find existing cracks that they believe to be new. It's so common that we always recommend offering pre- and post-construction surveys to owners of structures within a certain distance of the site so we can say whether a crack was there prior to construction starting.

 

[miningman]

Let me guess. The chickens have stopped laying, the cows milk production has dropped, the car doesnt start as well as it used to, and the birds have stopped singing as well. Actually the bird one might be legitimate but everything else is pure BS.

I'd be surprised if the blasting contractor did not do a pre construction survey before the first shot was fired. This usually includes photographic evidence which destroys the legitimacy of 95% of claims. It takes a peak particle velocity of 50mm per second to open up new cracks in plaster and 300 mm /sec to open up fresh rock. I can keep vibrations down below 25 mm/ sec and I am probably not the most experienced powder monkey inn North America.

 

[ndalle03]

MTNCLimber - I was thinking the same thing! I'm a structural engineer so I look for cracks and such. A typical homeowner doesn't pay attention until it's significant; (or until they hear their neighbor noticed cracks following the blasts so they start looking themselves). However, the few folks that I've spoken to are certain that the cracks did not exist prior to October when the blasting started.

I heard a geologist was visiting some of the homes, so I'm trying to reach out to him to see if he took any readings.

Miningman - Given that you're a "powder monkey", what kind of blast is required to potentially cause foundation concerns 1 mile away??

 

[miningman]

ndalle, the answer to your question is "almost nuclear " There were some anecdotal stories from the early 1970s when we first took in the hole hammers with an 8 inch bit underground. Yeah unacceptable rock damage occurred but by the late 1970s when I was using 6 inch holes , problem solved. I did some blasting in the tar sands a few years ago using 36 inch diameter holes , each with 2 tonnes of hi strength slurry explosive. Apart from setting the oilsands on fire on one occasion when we overloaded one hole , no problems at all.

Its all to do with the weight of explosive per delay that is detonated instantaneously. And a word of warning , experience and knowledge of blasting is not a geologists normal area of expertise.

 

[fattdad]

the use of a seismograph seems critical to telling the whole story.

Blast design relates to the size of the explosive that is detonated with each delay. "One shot" is typically dozens of individual blasts sequenced to release the rock in an orderly fashion. The release of each delay is managed to minimize the release of energy in an efficient manner - to remove the most rock!

So, we'd have to know what type of rock (limestone/dolostone) likely, the weight per delay and the distance.

Irregular things happen. The ground is not isotropic! Often folks perform a test blast with an array of seismographs radiating out from the blast. That way, you can see the attenuation of energy at different orientations from the blast.

Peak particle velocity tells only part of the story. Frequency matters! So, at distance, some of the high frequency energy is filtered out and the low frequency remains.

Who knows?

f-d

ípapß gordo ainÆt no madre flaca!