This paper describes a study to establish ventilation in a sewage outfall tunnel in Boston, Massachusetts. During construction of the outfall tunnel, ventilation was achieved by means of an auxiliary forcing duct and multiple, high-pressure fans. At the end of the main tunnel are small diameter tunnels connecting to risers that each terminate at a diffuser head on the sea floor. All services, including ventilation and power were removed from the tunnel prior to the final step of removing safety plugs from the diffuser tunnels. Oxygen at the end of tunnel reduced to below 10%. The plug removal work was to be accomplished by personnel under oxygen apparatus. Work commenced in 1999 to remove the temporary plugs. After three plugs were successfully removed, a problem developed with the workers breathing apparatus resulting in fatalities. After the fatalities it was mandated by OSHA that active ventilation be re-established in the tunnel. This paper describes the options considered in establishing the tunnel ventilation system. The system chosen consisted of using an ocean barge to connect a caisson to one of the first three risers that had the temporary plug removed. Seawater was pumped out of the caisson and a ventilation pipe was installed to fit over the manhole cover on the diffuser head. The ventilation pipe was connected to an exhaust fan built on the barge. Ventilation was established on July 13, 2000. Measurements confirmed that the airflow predicted by modeling was within 3%. Acceptable oxygen levels were established in the tunnel within 16 hours and all temporary plugs were removed within five days.
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