HomeMy WebLinkAboutSD-22-10 - Supplemental - 0500 Old Farm Road (41)For more information, contact Andrew Gill: 802-658-5000, andrew@obrienbrothersvt.com
Blasting is an important part of the O’Brien Farm – Eastview project. Construction for the building involves bedrock excavation by drill and blast methods below the ground surface. The challenge is to balance the effective fracturing of the rock with the control of impacts from the blast. The following provides questions and answers for the residents of the surrounding area on the art of Blasting and the science of Vibrations. We encourage residents to continue to ask questions, raise concerns, and share ideas. The developer, O’Brien Brothers Agency and the Contractor, are constantly seeking new and innovative ways to reduce impacts and improve construction procedures and blasting methods in ways that should benefit the environment and residents.
A blast is a controlled event that places a designed amount of explosive energy into rock to fragment for excavation. Noise and vibration from the blast is energy that was not consumed in the fragmentation of the rock. Designing a blast is about balancing the need to adequately fragment the bedrock with the need to minimize environmental impacts.
A blast begins with the design by the blasting contractor (Maine Drillng & Blasting) and the Blasting Consultant (Brierley Associates). A plan of the drill hole pattern is developed that identifies the size of the holes, the number of holes, spacing between the holes, the size of the burden, and the sequence in which the holes detonate in the blast. The drill pattern is then laid out on the ground and measured to ensure accurate spacing and depth of holes. The drill machine then drills a 3.5” diameter column into the rock. When drilling is complete, each hole is loaded with a specific weight of explosive. The first thing down the hole is a non-electric detonator inserted into the explosive at the bottom of the hole. The detonator sets off the explosive in the borehole. The explosives are transported to the site each day by a secure magazine truck on the day of a planned blast. Bags of bulk emulsion or boxes of emulsion cartridges each 2.5” diameter by 16” long are loaded from the truck into a portable magazine. The hole is not filled to the top with explosives; space is left at the top which is referred to as the collar. The collar at the top of each borehole is then filled with crushed stone stemming material. The purpose of the stemming is to contain the explosive energy during detonation. All explosives not used are removed from the site at the end of each day. Each hole is detonated separately. A blast is actually a series of small detona- tions as each hole is detonated. Each hole is detonated with at least 8 millisec- ond delay between them. A typical blast would be not more than 1 second in duration.
What the blasting contractor does to minimize nuisance of blasting effects:
⇒ Designs all shots with millisecond delays between holes to reduce vibration.
⇒ Notifies abutting residents in advance.
⇒ Monitors ground and air vibration levels at the nearest occupied locations.
There are two types of energy produced from a blast:
1. air overpressure
2. ground vibration
The detonation of an explosive produces a very rapid and dramatic increase in volume due to the conversion of the explosive from a solid to a gaseous state. When this occurs within the confines of a borehole, the bedrock in the area imme- diately adjacent to the explosive product is crushed. As the energy from the deto- nation radiates outward from the borehole, the bedrock between the boreholes becomes fragmented and is displaced, while the bedrock at the perimeter is fractured.
Energy not used in the fracturing and displacement of the bedrock dissipates in the form of ground vibrations, sound, and air concussion. This energy attenuates rapidly from the blast site due to geometric spreading and natural damping. The rate at which ground vibrations attenuate or decrease with increased distance from a blast depends on a variety of conditions, including the type and condi tion of the bedrock being blasted, any depth and composition of the earth covering deposits (soil), and the general topography.
Air overpressure effects are influenced by the prevailing weather conditions. On a still day, air overpressure travels in the opposite direction of ground vibration. However , wind direction has an impact on air overpressure and can focus it in one direction. Whether conditions do not change the intensity of the air concussion, whether only influences how that energy is distributed.
Ground vibrations and air overpressures from a blast are controlled by controlling the amount of explosives activated within any single delay within the blast. The amount of explosives detonated is controlled by the size and depth of the
hole and the timing of detonation of each hole.
O’Brien Farm - Eastview Project: Blasting and Vibrations Q&A
What is a blast?
We’re working on it…..
How does the energy of the blast move?
How are blasts controlled?
Blasting and Vibration
For more information, contact Andrew Gill: 802-658-5000, andrew@obrienbrothersvt.com
Because there are two types of energy from blasting, blast monitors or seismographs need two different instruments to measure them: one, a geophone, to measure the seismic waves caused by ground vibration and a microphone to measure the pressure waves travelling through the air caused by air overpressure. Both the geophone and microphone are connected to the seismograph. The International Society of Explosives Engineers (ISEE) Performance Specification for Blasting Seismographs 2017 has established guidelines for monitoring vibrations and air overpressure and is followed by the Contractor. Brierley has established the limits for both air and ground vibrations provided in the Contract Documents.
Air overpressure is determined by measuring the peak pressure level. The ground vibration is determined by measuring the vibration velocity.
The peak particle velocity (PPV) limit for ground vibration resulting from blasting operations for the project is a maximum of 1.0 in/s and is a function of the frequency. This is one-half of the industry standard to address potential human annoyance. The limit for air overpressure resulting from blasting operations for the project is 133 dBL. Air overpressure is measured as decibels in the Linear or unweighted mode.
The human body can detect vibrations at very low levels. Humans can start to feel vibrations at levels of 0.02 - 0.05 in/s. This threshold of human perception of vibration is well below the levels at which cosmetic damage occurs to structures and below the limits set by USBM RI 8507 1980. How people perceive vibrations will depend on what they're doing. People who are sitting or lying down would be more perceptive to vibrations than those taking part in an activity. The determination of whether a vibration is annoying is dependent on the individual. Vibration from blasting can be perceived as annoying as it is unexpected unlike the vibrations that we regularly experience in our homes, such as when we walk up and down stairs or open and close doors. Door slamming can produce vibrations have up to 0.5 in/s.
When blasting, the weather conditions, such as high humidity or the presence of cloud cover, can cause the levels of overpressure and noise to seem more severe than there would be on a day when the humidity is low and there is a lack of cloud cover.
Structural Response to Ground Vibrations
Peak Particle Velocity
Ground Vibration Effect
in/s
24.0 Microcracks start developing in rock
6.0 Lower limit often set for concrete
3.0 Cracks start developing in plaster
2.0 Industry Standard Limit
1.0 Project Contract limit
0.5 Ground Vibrations Annoying to humans
0.02 Ground vibrations become perceptible to humans
in/s = inches per second
Structural Response to Air Vibrations
Over-
pre ssure Air Blast Effect
dBL
180 Conventional structures severely damaged
170 Most windows break
150 Some older windows may break
133 Contract Limit
124 Some windows could rattle
dBL = decibels, linear (different from noise)
Blasting and Weather
Contract Blasting Requirements and Restrictions
⇒ Provide an independent qualified Blasting Consultant.
⇒ Blasting operations M-F, 7 AM – 5 PM.
⇒ Provide audible signals to warn of impending blasts.
⇒ Not more than 5-10 blasts in one week.
⇒ Ground and air vibration limits set at 1.0 in/s and 133 dBL, respectively.
⇒ Monitor each blast at nearest occupied locations.
⇒ Post blast reports posted on project website.
Measuring Vibrations
How do humans perceive vibrations?
O’Brien Farm - Eastview Project - Blasting and Vibration Q&A