EXPLOSIVE OPERATIONS FOR CONSTRUCTING A TUNNEL-BASED PCB COLLECTION SYSTEM

摘要

In an effort to recover Polychlorinated biphenyls (PCBs) that had leached intornbedrock below the Hudson River in Upstate New York, a shaft and tunnel complex wasrndesigned that would allow installation of inclined and vertical wells into the zones ofrncontamination to recover PCBs by gravity. A joint venture of Merco Inc., of Lebanon,rnNJ, and Obayashi Corporation, of San Francisco, CA, was awarded the contract forrnconstructing the first Tunnel Drain Collection System in the world. Conventional drillrnand blast techniques were the most economical method to construct the project but duernto the presence of PCBs and community concerns, innovative drill and blast practicesrnwere needed.rnCommunity concern over the disruption from the project had driven strict limitsrnon PPV and air overpressure. Fly rock was defined as any rock traveling beyondrn10' (3.35m) of the limit of excavation. New York State Department of EnvironmentalrnConservation (NYDEC) had direct oversight on the project and any blast that exceededrnthese limits would immediately halt all blasting until NYSDEC approved a new plan, arnvery costly prospect for the contractor. Also impacting all blasting operations was thernpresence of PCBs in the work area. All workers had to be protected from contaminationrnand strict procedures had to be put in place to prevent the migration of contaminatesrnfrom the exclusion zone of work.rnThe contractor retained Gerard McAlinden Sr., of Jerry Gerard Ltd., to assist withrnblast design and compliance with the NYSDEC limitations. Mr. McAlinden proposedrnseveral innovative ideas to limit overpressure. Firstly, conventional blast design forrnshafts was abandoned. Traditional shaft blasts use large relief holes but those reliefrnholes and the high powder factors associated with holes in close proximity contributerngreatly to overpressure. He also suggested limiting the depth of each shaft round torn10' (3.35m) matching rock support patterns. The shaft diameter was 24' (7.31m) andrnby pulling smaller rounds the relief holes could be eliminated. The use of Vari Stemrnstemming plugs in all holes to help retain stemming and reduce overpressure was alsornrecommended. Lastly, a shaft cover was designed to contain any fly rock to the shaftrnand mitigate overpressure.rnWhen the shaft was completed and tunneling began, the contractor continuedrnto use stemming plugs in tunnel rounds to help reduce air overpressure with greatrnresults. The Shaft and Tunnel complex was completed with 143 blasts, using 26,444#rn(11,995kg) of explosives and 10,485 detonators and all the blasting was done in conformancernwith the specified limits and requirements of NYSDEC.