Blasting Near Dolloff Dam

Assurance from NHDES that the blasting is safe, despite Dolloff Dam being classified as high hazard.

Construction of the Pawtuckaway Ridge housing subdivision, off of Rt. 156, just north of the town line, will involve blasting near Dollof Dam, which the NH Department of Environmental Services considers a high-hazard dam.

A nearby resident inquired with the department about the safety of this blasting, and provided to the Nottingham Blog the following re-assuring response from John T. Poisson, P.E., Senior Project Engineer, Engineering & Construction Section, Dam Bureau, NH Department of Environmental Services.

The contractor, design engineer, and blasting subcontractor were contacted after you reached out to the Dam Bureau to raise your concerns about the potential impact blasting might have on the dam. The engineer, Beals Associates, and the subcontractor, Maine Drilling and Blasting, were responsive to the concerns raised by NHDES. Maine Drilling and Blasting performed a technical analysis to show that the blasting at the construction site was not anticipated to produce any measurable vibration at the dam. Furthermore, their staff also willingly agreed to install a seismograph at the dam to monitor induced vibrations for the initial rounds of blasting to be done at two primary sites within the subdivision. So far, the seismograph readings indicated no vibrations at the dam exceeding the minimum detectable threshold correlated with blasting operations. It is worth noting that this minimal threshold for detection (0.05) is lower than the allowable threshold (0.2) at which some level of damage might be expected occur at a building (e.g. cracks in foundations, sheetrock, etc.). Additional readings will be taken when blasting moves to a second site located approximately 1,300 ft from the dam (the current t site is over 2,500 ft away). Additional vibration monitoring is being undertaken at residences close to the construction site, to ensure the vibrations from blasting are kept below acceptable levels.

The blasting contractor, Maine Drilling and Blasting (MD&B), is responsible for the monitoring. The measuring device (seismograph) is installed each day they are blasting and retrieved at the end of the day for data download, recharging the batteries, and to prevent theft. The plan agreed to with MD&B was to take measurements at the dam for the initial rounds of blasting to confirm the contractor's expectations that vibrations would not be created at the dam by the blasting. So far so good on that front. When MD&B moves to the next blasting site, placing the operation approximately 1300' from the dam, the plan is to repeat this process of installing seismographs during the initial blasts to presumably confirm the lack of vibrations being produced at the dam. If vibrations are induced at the dam which approach or exceed levels of concern, then discussions with the contractor would ensue to make changes that satisfy both parties.

MD&B has been monitoring vibrations at residences significantly closer to the project site than the dam, as they had initially planned to do. I am unable to speak definitively about MD&B's process for deciding not to engage the NHDES Dam Bureau as part of this project. However, I suspect their analyses and subsequent monitoring showing that vibrations remain below tolerable thresholds for the nearby residences likely factored into the decision. Furthermore, their analyses for expected vibrations at the dam site indicated that no induced vibrations would be expected at the dam due to blasting. Based on this, I can only extend the benefit of doubt to MD&B that they concluded there was no compelling reason (in the absence of legal requirements) to notify us about their activities in the area. Fortunately, they have been more than willing to work with us to assuage our concerns.

The plan is for the seismograph data from the dam location to be shared with NHDES once it is all compiled.