Frequently Asked Questions About Blasting
Why do I feel the blasting?
Most of the energy from a blast is used to break rock, but some energy will travel from the blast site in the form of ground waves and airwaves. Each of these can cause your house to vibrate or shake slightly. Humans are very sensitive to all vibrations. It is possible that you will feel or hear your house vibrate from the blasting, even at very low levels. (Source: International Society of Explosives Engineers)
It is important to note that WVB complies with vibration limits established prior to the onset of blasting to ensure nearby structures are not damaged. WVB continuously monitors blasting vibration levels to prevent building damage and also conducts pre-blasting and post-blasting structure surveys of structures within 500 feet of blasting.
Why do some blasts feel stronger than others?
How a blast feels depends on ground waves or airwaves that reach you or your house. These are influenced by the type of blast, the distance from the blast and the amount of explosives.
Your location on the property also affects your perception of the blast. If you are outside a house, you will tend to feel the ground vibrations in your feet and legs. Inside a house, you sense the structure and objects responding to the vibrations. You may also hear objects rattle. This is why you and your neighbors may feel or describe blast vibrations differently. (Source: International Society of Explosives Engineers)
Why do the tunnel blasts seem different from surface blasts?
Surface blasting primarily consists of a single detonation lasting a fraction of a second, compared with tunnel blasting, which involves a series of smaller detonations that last 10-15 seconds. The effect is similar to a single loud clap of thunder versus a long rolling, rumbling thunder.
Why does it feel like my house is vibrating more than before when there is a tunnel blast?
Early in the site excavation, ground vibrations associated with surface blasting were the most noticeable effect of blasting. However, as construction has moved from vertical excavation to horizontal excavation into the tunnel, the most noticeable effect is air over-blast, or airwaves from tunnel blasting. As stated above, the longer duration of the blast makes the vibrations more noticeable, even though the amount of blasting material used is typically much less. Also, tunnel blasting is being done at times when more people are home, so more people are likely to notice.
Regardless of the type of blast, strict vibration and air over-blast limits have been set for this project to mitigate potential structural damage, and monitoring devices have been placed throughout the project area to ensure that those conservative limits are not being exceeded.
As blasting moves further into the tunnel, will noise and vibration become less noticeable?
Yes. As blasting advances into the tunnel, noise and vibration will decrease because blasts will be further from homes and other structures, and the airwaves will be more contained. Also, the type of rock being excavated will change. The early tunnel blasts involved limestone, which is brittle. Tunnel work is now moving under the limestone into more pliable shale, which will help dampen the noise and vibrations.
How and where are blasts being monitored to ensure no damage occurs to homes or other structures?
Very conservative vibration limits have been set for this project in order to protect structures from damage. More than a dozen blasting seismographs have been placed around the construction site and surrounding properties to ensure those limits are not exceeded. The closest monitor is located approximately 50 feet from the south tunnel portal opening and is situated between the blast location and the nearest structure. Monitors at that closest location will record the highest vibration levels. As sound waves and vibrations move away from the blast location they decrease in intensity.
Seismic readings and monitoring thus far indicate that vibrations and air over-blast levels have been well within the legal limits set by the Department of Reclamation, Division of Explosives and Blasting, so as to not cause damage to surrounding structures. For this project the developer is only blasting at about 25% of those legal limits.
How does a seismograph work?
A blasting seismograph measures and records the ground waves and airwaves from a blast. The information is reported as waveforms, also known as time history records. Time histories show how the strength (amplitude) of the waves varies over time. Amplitudes are reported as particle velocity (inches per second) for ground waves and pressure (pounds per square inch) or decibels for airwaves.
Another important characteristic of the time history is frequency. Frequency is the number of complete waves that pass by in one second. It is reported in cycles per second or hertz. Both amplitude and frequency are needed to describe the motion from ground waves and airwaves.
The blasting seismograph information is used to show compliance with regulations or specified limits and to evaluate blast design performance. Most important, it verifies that the ground waves and air vibrations are within standards set to protect structures. (Source: International Society of Explosives Engineers)
Why is there blasting in Indiana, where no tunnel is being excavated?
Blasting is needed to cut through the limestone hilltops in order to level the roadway for the Indiana approach to the East End Bridge.
How long will blasting operations continue?
Blasting, which began in July 2013, is expected to continue for approximately three years.
Are there any restrictions as to the time of day that blasting is permitted?
Blasting operations are permitted between a half-hour before sunrise until a half-hour after sunset. Surface blasting is not permitted from 6 a.m.-noon on Sundays. Efforts are being made to conduct surface blasting at times that will minimally affect traffic. Tunnel operations will be 24 hours a day Monday – Saturday.
What should I do if I believe I have potential damage or other concerns associated with blasting or other construction activities?
If you have questions or concerns, please contact:
Angela L. Nichols
Office: (812) 920-0586
(For more information about blasting in general, please visit explosives.org, a website created by the International Society of Explosives Engineers for the express purpose of explaining blasting issues to homeowners and other affected parties.)