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Tunnel Blasting Study

Tunnel Based PCB Collection System Using VARISTEM Stemming Plugs

International Society of Explosive Engineers (ISEE)

In an effort to recover Polychlorinated biphenyls (PCB's) that had leached into bedrock below the Hudson River in Upstate New York, a shaft and tunnel complex was designed that would allow installation of inclined and vertical wells into the zones of contamination to recover PCB's by gravity. A joint venture of Merco Inc., of Lebanon, NJ, and Obayashi Corporation, of San Francisco, CA, was awarded the contract for constructing the first Tunnel Drain Collection System in the world. Conventional drill and blast techniques were the most economical method to construct the project but due to the presence of PCB's and community concerns, innovative drill and blast practices were needed.

Community concern over the disruption from the project had driven strict limits on PPV and air overpressure. Fly rock was defined as any rock traveling beyond 10' (3.35m) of the limit of excavation. New York State Department of Environmental Conservation (NYDEC) had direct oversight on the project and any blast that exceeded these limits would immediately halt all blasting until NYSDEC approved a new plan, a very costly prospect for the contractor. Also impacting all blasting operations was the presence of PCB's in the work area. All workers had to be protected from contamination and strict procedures had to be put in place to prevent the migration of contaminates from the exclusion zone of work.

The contractor retained Gerard McAlinden Sr., of Jerry Gerard Ltd., to assist with blast design and compliance with the NYSDEC limitations. Mr. McAlinden proposed several innovative ideas to limit overpressure. Firstly, conventional blast design for shafts was abandoned. Traditional shaft blasts use large relief holes but those relief holes and the high powder factors associated with holes in close proximity contribute greatly to overpressure. He also suggested limiting the depth of each shaft round to 10' (3.35m) matching rock support patterns. The shaft diameter was 24' (7.31m) and by pulling smaller rounds the relief holes could be eliminated. The use of VariStem stemming plugs in all holes to help retain stemming and reduce overpressure was also recommended. Lastly, a shaft cover was designed to contain any fly rock to the shaft and mitigate overpressure.

When the shaft was completed and tunneling began, the contractor continued to use stemming plugs in tunnel rounds to help reduce air overpressure with great results. The Shaft and Tunnel complex was completed with 143 blasts, using 26,444# (11,995kg) of explosives and 10,485 detonators and all the blasting was done in conformance with the specified limits and requirements of NYSDEC.