Loading...
HomeMy WebLinkAboutSD-22-10 - Supplemental - 0500 Old Farm Road (40) 26 Smith Pl., Ste. 2, Cambridge, MA 02138 | 617.714.5784 | www.BrierleyAssociates.com March 14, 2022 O’Brien Brothers Agency 1855 Williston Road South Burlington, VT 05403 Attention: Mr. Andrew Gill, Director of Development Blasting Impacts Assessment Rock Excavation at Borrow Pit Area Hillside at O’Brien Farm Phase 2 South Burlington, VT This letter presents our assessment of the blasting impacts from rock excavation by drill and blast methods at the Borrow Pit area which is a part of the Hillside at O’Brien Farm Phase 2 project in South Burlington, Vermont. The purpose for our work was to assess the area on site where drilling and blasting will be required for bedrock excavation and to assess the potential impacts from the blasting work on the existing residential and commercial buildings and people in the area. Bedrock excavation by drill and blast methods will be completed by Maine Drilling and Blasting (MD&B) of Argyle, NY. Background We have reviewed the following background information to complete our assessment: • Blasting Plan for Hillside @ O’Brien Farm – Borrow Pit by Maine Drilling & Blasting, Inc., dated March 10, 2022. • State of Vermont Department of Environmental Conservation – Waste Management Prevention Division, 2014 Best Management Practices (BMPs) for Blasting to Avoid Environmental Contamination. • State of Vermont Blasting Plan Guidance Document. • NFPA 495 Explosives Materials Code, 2013 Edition. As part of our evaluation, Mr. Jay Perkins of Brierley Associates has corresponded on the proposed rock excavation and abutter concerns with Andrew Gill at O’Brien Brothers Agency, and has discussed the submitted Blasting Plan with the Project Manager for MD&B. The site is approximately 5.5 acres of farming landscape and slightly wooded area located at the west side of Kimball Avenue across from #70 Kimball Ave. Site grades are relatively level across the area. The purpose for the borrow pit is to mine and excavate approximately 70,000 cubic yards of bedrock for processing crushed stone as a construction material for the Hillside Phase 2 - Eastview residential development. The maximum depth of rock excavation at the pit area will be approximately 25’ below the existing grade. The Blasting Contractor has estimated the blasting duration for the borrow pit area to be 75 days assuming two drill crews completing 5 to 10 blast rounds per week throughout the project duration. Hillside at O’Brien Farm Phase 2 – Borrow Pit Blasting Impacts Assessment March 14, 2022 Page 2 of 6 Site Safety and Mitigation Given the proximity of the blasting activities to the surrounding residents, site safety is paramount to the successful completion of the project. The following provides a list of site safety measures and security that MD&B will implement during blasting: • Worker: Conduct daily safety meetings. • Blast Area Security: Designated Access Points, Safe Areas and Sentry Locations (see Blast Plan). • Charged Hole Area: Posted, guarded and barricaded. • Warning Signals: Three (5 minutes), two (1 minute), one (all clear). • Kimball Ave.: Vehicle and pedestrian traffic paused briefly as needed. • Blasted Rock Control (flyrock): Use of matting and blast rock berms. • Blast Monitoring: Two seismographs to measure ground and air vibrations as per ISEE Guidelines. • Misfires: Wait, reconnect and reshoot, reprime, washout with water or air, recover, displace (drill and shoot adjacent hole), and disposal to separate magazine, as per IME Safety Library Publication 17. Blast Designs MD&B has provided a blast design in their submitted Blasting Plan for the borrow pit area. For pit blasting approximately 70,000 cy of rock with a cut of 20’ that has been designed and will be used on this project. The pit blast design includes drilling a 3.5” diameter hole at a burden and spacing of 8’x8’ for pit blasting. The number of holes per blast round will range from approximately 20 up to 40 with an average of approximately 30 resulting in approximately 1,400 cy (neat lines) of rock excavation per blast round. Explosives to be used will be MD&B’s 1966 bulk Emulsion Blend of Ammonium Nitrate and Fuel Oil (ANFO) using a Dyno Nobel Cast Booster as a primer. Dyno Nobel Blastex Emulsion with a cast booster primer will also be used for supplemental explosives and for much smaller blast rounds. The initiation and trunkline system will be the Dyno Nobel NONEL (nonelectric) delay detonators. Timing diagrams show 25 msec delays between holes and 42 msec delays between rows. For the submitted blasting design, a corresponding Powder Factor (PF) will be approximately 1.3 lbs./cy. The maximum explosive charge weight per delay will be as high as 52 lbs. and will be dependent on the distance to the nearest structure. This blast design has incorporated features that should minimize blasted rock Oversize and the need for rehandling blasted materials, and limit Overbreak at subgrades and lateral blast lines and the need for additional backfill material placement by the General Contractor. Anticipated Closest Residences to Blasting The following is a list of the existing residential buildings located nearest to the blast areas, and their approximate distance from nearest blast to that structure: • #70 Kimball Ave. (Immigration Building): 334’ • #102 Kimball Ave. (Warehouse): 484’ Hillside at O’Brien Farm Phase 2 – Borrow Pit Blasting Impacts Assessment March 14, 2022 Page 3 of 6 • #102 Kimball Ave. (Tennis Store): 496’ • #200 Old Farm Road residence: 854’ Brierley has reviewed the Project blast plan and blast design with these structures in mind. Specific plans are in place for all blasting of structures with criteria that we believe will keep the project impacts below design standards, and will not cause damage to these closest structures, or any other neighboring structures. One of the first steps in the blasting process is to protect the abutting homeowners and the contractor, and to help prevent or mitigate potential property damage claims by completing pre-blast condition surveys of structures near the blast area. Upon acceptance from the property owners, these surveys will be performed at the interior and exterior of all structures to a radial distance of at least 500’ from the closest blasting, which is twice the distance generally used and required by the NFPA 495 Code. Pre-blast surveys will be completed by MD&B using photographs, videos and written and oral documentation. All pre-blast surveys will be conducted prior to the commencement of the project. Potential Blasting Impacts on Buildings The primary concerns with blasting close to buildings and their occupants are ground vibrations, air blast overpressures, flyrock, noise and dust, all of which can be controlled with experienced Blasters and Engineers applying modern techniques to blasting designs and mitigation measures. As a part of the project outreach, we have included a “Blasting and Vibration Q&A” handout to be provided to the abutting residents. This two-page handout provides a basic description of blasting and explanation on the impacts of blasting to buildings and their occupants. These impacts are also briefly discussed below: Ground Vibrations: Seismographs measure ground vibrations in Peak Particle Velocity (PPV) recorded in units of inches per sec (in./sec), given standards of damage are based on the ground particle velocity. The PPV from each blast round will be monitored using the seismograph set up immediately adjacent to the nearest residence or structure of concern. In the interest of minimizing blasting impacts and annoyance levels to occupants of the buildings Nearby, the PPV from ground vibrations is designed to be below 1.0 in./sec, which is one-half the industry standard safe limits line recommended by the U.S. Bureau of Mines (USBM RI 8507, 1980). These limits are half of the studied and documented safe limits for preventing cosmetic damage (hairline cracking, or the extension of existing hairline cracks) to residential structures. It should be noted that vibrations levels generally attenuate at a rate of one-third for every time the distance is doubled. Setting the blast design standard at 50% of the proven safe industry-standard level is being done at the request of the Project owner and Brierly not because the proven limit is in any way unsafe or known to cause damage to structures. This design threshold was developed to reduce the vibrations perceived by Project neighbors and to ensure that neighbors to the project are respected and that all reasonable efforts to minimize impacts are taken. These reduced standards have a cost, which is that more blasts will be necessary to achieve the ledge removal required by the Project and the blasting will therefore take longer. However, our belief is that the 1.0 in./sec standard achieves a good balance between efficiency of process and reduction of impacts. Any further reductions in blast design vibration limits could Hillside at O’Brien Farm Phase 2 – Borrow Pit Blasting Impacts Assessment March 14, 2022 Page 4 of 6 greatly extend the Project duration, and because vibrations are perceptible at 0.02 in./sec, would be unlikely to reduce the physical perception of neighbors to a point where the blasting was not felt. Air Blast Overpressures: Blasting air overpressure, recorded in units of pounds per square inch, (psi) is also measured with seismographs on a Linear Peak scale and is converted and reported as the sound equivalent in decibels (dBL). Air blast overpressures produces an atmospheric pressure wave transmitted from the blast outward into the surrounding area. Air blasts at nearby residences will be kept below a limit of 133 dBL, or 0.014 psi, which is an industry standard limit recommended by the U.S. Bureau of Mines to minimize the probability of both human annoyance and potential damage to windows. This pressure is equivalent to a wind gust of 27 mph. Actual breakage of some older windows would not be expected until air blast overpressures reach 150 dBL (0.095 psi) or approximately seven (7) times higher pressure than the USBM safe limit. This is equivalent to a wind gust of 80 mph. Our experience has shown that if ground vibrations are kept below recommended safe limits, the air blast overpressures from blasting are not a threat to damage buildings. Flyrock: In general, flyrock, or the uncontrolled ejection of blasted rock, can come from either the face of the blast or the top of the blast, both of which can be controlled by proper blast designs. To reduce the potential for flyrock, the Blasting Plan has provided the following controls: • Placement of blasting mats to fully cover the blast area for every blast. • Drillers logs will be kept for all blast holes drilled, documenting open joints, seams, and other anomalies; and the logs will be reviewed by the blaster prior to each blast. • Observation of each blast round detonated so that small problems can be detected and corrected. Noise and Dust: A detailed noise evaluation and impact has been completed and will be reviewed on a separate document. Dust produced from the drilling and blasting activities will be controlled using water or other fluid placed at its source, and drill machines equipped with dust collectors. Potential Blasting Impacts on People People can perceive vibrations from blasting at significantly lower levels than might cause cosmetic damage to buildings. Blast designs have been provided to generate vibration levels that should reduce the impact to people. Vibrations may be noticeable and therefore may result in complaints, at PPVs as low as 0.02 to 0.06 in/sec. This is equivalent to the vibrations generated from a heavy truck traveling down a bumpy road, and vibrations of this magnitude should be expected for neighboring residences Experience has shown that blasting operations in locations such as the Project will be felt by neighbors and that the impacts will feel significant, even if they are well below the design standard. There is no correlation between the design standard that is studied and documented to be safe for structures, and the human experience of vibration in their home. The Project Owner is keenly aware of this, having experience from the Phase I blasting operations. It is impossible to set a design standard for blasting that is imperceptible as the blasts would be too small to make meaningful progress, but this Project has set its design standard at 50% of the levels we know to cause damage, well below what is required. We believe that this Hillside at O’Brien Farm Phase 2 – Borrow Pit Blasting Impacts Assessment March 14, 2022 Page 5 of 6 reduction in allowable vibration is meaningful, will greatly reduce the perception of vibrations for neighbors, and will eliminate the risk of damage to surrounding structures. Conclusions In summary, it is our opinion that proposed Blasting Plan submitted by MD&B for rock excavation at the borrow pit site provides the safety controls and mitigation measures and design that should not adversely impact structures in the area or cause excessive human annoyance. The primary concerns have been discussed and addressed in the Blasting Plan. The following provides a brief summary of our assessment: 1. A Blasting and Vibration Q&A handout (attached) will be provided to the residents to offer a basic understanding of blasting and impacts of blasting. 2. All the necessary site safety and security measures will be implemented for the duration of the blasting activities. 3. To minimize potential damage claims pre-blast condition surveys will be conducted at the interior/exterior of all buildings within a 500’ radius of the blast area. 4. A blast design has been provided to minimize blasted rock Oversize, and to limit Overbreak. 5. Blasting at the pit area will start at a location furthest away from the nearest buildings. 6. At least two seismographs will be used during each blast to monitor ground and air vibrations. 7. To minimize human annoyance, blast induced vibrations as recorded at the nearest buildings will be kept to less than 1.0 in./sec which is one-half the industry standard limit. 8. Blast rounds have been designed to maintain air blast overpressures below 133 dBL (0.014 psi) safe limit as per USBM standard. This is equivalent to a wind gust of 27 mph. Actual breakage of some older windows would not be expected until air blast overpressures reach 150 dBL (0.095 psi) or approximately seven (7) times higher pressure than the USBM safe limit. 9. When ground vibrations are kept below recommended safe limits, the air blast overpressures from blasting are not a threat to damage buildings. 10. The blast is typically felt by the overpressure more than heard by the noise. 11. Residents in the area may feel vibrations, and be slightly disturbed by them, even though the vibration levels are well below the safe limit to cause cosmetic damage. 12. Flyrock will be controlled by proper blast designs, placement of blast mats, review of driller’s logs and observation of each blast round. 13. Dust generated from drilling and blasting will be controlled by placement of water in the blast area and drill machines equipped with dust collectors. Hillside at O’Brien Farm Phase 2 – Borrow Pit Blasting Impacts Assessment March 14, 2022 Page 6 of 6 We hope the above assessment has addressed any concerns regarding the proposed methods for bedrock removal at the O’Brien Farm Phase 2 borrow pit. If you have any questions or require additional information, please feel free to contact me. Sincerely yours, BRIERLEY ASSOCIATES Jay R. Perkins, P.E. Geotechnical Engineer Attachment: Blasting and Vibrations Q&A Handout