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Structure Response
The response of structures to blasting vibrations is characterized in terms of
relative cracking potential in construction materials. We can pose this
another way and ask,
"Can we create strains in structure walls during blasting-induced shaking
greater than tensile failure strains required to
produce a new crack or extend an
existing crack for a given wall material?"
The next logical question follows.
"What are the expected wall strains that occur everyday in structures from
natural, environmental influences such
changes in temperature and humidity
and human activities?"
Further,
"How does the normal aging process and general use of construction
materials, imparting wear and tear, contribute to the
formation of defects and
cracks?
We need to ask these questions and establish scientific measurements to prove without a doubt there are many non-blasting influences that induce cracking in homes because they create far greater strains in construction components than those induced by blasting vibrations within safe criteria established to prevent cracking.
It
is important to make cracking potential comparisons within the weakest
materials in structures because they may fail at very low or threshold
levels of loading. These materials include interior drywall or gypsum
wallboard mounted in walls and ceilings, exterior stucco and mortar between
bricks or concrete block. The dynamic failure strains of these materials are
well known and established over years of testing under different strain
rates that can be slow (static) loading or fast (dynamic) loading. It
should be noted that strong materials, such as concrete, stone tiles, and
some wood and metal structural members, have far higher failure strain
thresholds and are expected to fail at high failure strains. Our interest is
with threshold materials. For instance, alleged blast structure damage claims
that involve
only cracking in concrete slabs without any interior drywall cracks, cannot be related to blasting. Think
about it.
Comparing blasting-induced structure motions with other normal and expected forces
Aimone-Martin Associates have developed a unique and very simple approach to structure instrumentation used as a public relations tool demonstrating to home owners blasting within industry-accepted safe vibration and air overpressure guidelines cannot possibly cause cracking or damage to their structure.
Special
instrumentation has been developed to make these comparisons and
scientifically prove safe blasting is protective of structures. Small,
wall-mounted velocity sensors record vibratory
motions of the gross (whole) structure and mid-walls.
Wall strains may be calculated from these blast-induced motions and compared
with strains generated by normal and expected forces such as wind loading, human activities,
as well as wall displacements from foundation soil movement in response to
seasonal moisture fluctuations. Wall shear and tensile strains may be compared
with strains required to crack wall materials and other construction
components to determine the likely cause of structure cracking.
The most successful instrument used to convince home owners blasting is not the cause of cracking are very small displacement gages, mounted across an existing wall crack used to record crack opening and closing. Crack gages are uniquely programmed to record both static and dynamic crack width changes from
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the slow, 24-hour wall material response to ambient environmental changes (temperature and humidity),
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long-term foundation movements from soil moisture changes,
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dynamic forces of wind and normal household activities (such as opening and closing doors and windows, simply walking through structures), and
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blasting.
With proper instrumentation, operating modes, and analysis, the effects of blasting on dynamic motions in structures and resulting tensile strains in walls may be compared with, and have been shown to be far less than, static and dynamic forces of weather, soil movements, and human activities. Notably,
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wall strains resulting from structure motions when blast-induced ground vibrations are within safe criteria are 1% to 50% of the strain required to cause cracking in drywall and mortar materials; hence blasting cannot contribute to cracking in these materials, and
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normal, everyday static forces always lead to large crack width displacements that can be over 100 times greater than crack displacements from blasting, even when ground vibrations from blasting are near the upper safe criteria limit.
Our successes
Aimone-Martin Associates, LLC are leaders in structure motion studies. We have refined instrumentation and analytical techniques first used by the U.S. Bureau of Mines in the 1970's, applying them to studies involving over 80 structures of various designs adjacent to mines, quarries, and construction blasting sites. Structure motion studies clearly demonstrate the effects of blasting on structures are negligible compared with other normal and expected forces that may readily lead to cracking.
The figures below represent static crack displacements over several days. The largest 24-hr crack width change is noted (horizontal dashed lines) in response to variations in weather. Shown below these diagrams are dynamic crack time histories representing the largest blast at the site (far right) and, in comparison, dynamic crack movements during two human-induced activities (top diagram), and during wind dusts against the wall containing the crack (bottom diagram)
In every case, these studies convince home owners, regulators, and permitting agencies that blasting within the U.S. Bureau of Mines recommended frequency-based ground vibrations safe criteria for modern structures cannot possibly result in structure cracking. Period.
To date, our studies have successfully prevented the closure of three quarries and four construction sites and have increased public awareness that blasting can be conducted safely and in a manner that is protective to off-site structures.
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