OSHA weighs safety variance for New Hampshire sewer tunnel project
(UI) - The Occupational Safety and Health Administration (OSHA) is considering a request to allow alternative decompression procedures during construction of a major sewer tunnel project in Manchester, N.H., reflecting the increasing use of modern tunnel boring technologies on underground infrastructure projects.
Obayashi–Jay Dee Joint Venture has applied for a permanent variance from portions of OSHA's compressed-air construction standard for work associated with the Cemetery Brook Drain Tunnel Project. OSHA has issued an interim order allowing the contractor to proceed while the agency evaluates the request.
The variance would apply to periodic hyperbaric interventions required during operation of an earth pressure balance tunnel boring machine (TBM). During these activities, specially trained workers enter a pressurized chamber at the front of the machine to inspect, maintain or repair cutterhead components used in tunnel excavation.
According to OSHA, the project is expected to involve worker exposures to compressed-air environments of up to 37 pounds per square inch gauge (psig), remaining within the maximum pressure limits allowed under existing regulations.
The contractor is seeking approval to use alternative decompression procedures based on the 1992 French Decompression Tables rather than decompression schedules specified in OSHA's current standard. The proposed procedures incorporate staged decompression and, when appropriate, oxygen-assisted decompression protocols that OSHA said have been used on numerous recent tunnel projects across the U.S.
In its preliminary findings, OSHA said it believes the proposed alternative methods would provide worker protections that are at least as effective as those required under existing regulations. The agency noted that similar variances have previously been granted for major tunnel projects in Washington, D.C., Cleveland, Dallas, Alexandria, Va., Nassau County, N.Y., and other locations.
OSHA also cited research indicating that the alternative decompression procedures may reduce the occurrence of decompression illness, commonly known as "the bends," compared with older decompression methods.
The Cemetery Brook Drain Tunnel Project will use a modern earth pressure balance TBM that limits worker exposure to compressed-air environments by isolating pressure to the machine's excavation chamber rather than pressurizing larger sections of the tunnel. OSHA said such advances have significantly reduced both the number of workers exposed to hyperbaric conditions and the duration of those exposures compared with older compressed-air construction methods.
The agency is accepting public comments on the variance application through June 29 before issuing a final decision.
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