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Rondout-West Branch Tunnel and Shafts


 


HAKS provided inspection services as part of the team managing this $239 million tunnel and shaft rehabilitation project for the New York City Department of Environmental Protection.

This project involved preliminary work required to repair leaks at two locations on the Rondout-West Branch Tunnel, a major component of the Delaware Water System which, along with the Croton and Catskill Water Systems, supply water to more than 8 million residents of New York City and to communities in Orange, Putnam, Ulster and Westchester counties. The Delaware Water System, with its four separate reservoirs, supplies some 50 percent of New York City’s annual water demand. Its water flows through the 45-mile-long Rondout-West Branch Tunnel, a concrete-lined, 13.5-foot-diameter pressurized conduit in rock 400 to 1,500 feet underground that connects the Rondout and West Branch Reservoirs.

The Rondout-West Branch Tunnel was constructed between 1937 and 1944 with a series of vertical shafts along the route. One of these, Shaft No. 6, is a dewatering shaft and contains the original pumping equipment. The shaft was last dewatered in 1957 for maintenance work and flooded again when the work ended two years later. The contract, for which work began in 2007, provided for installing an updated pump station in the shaft to dewater the Delaware Aqueduct so that tunnel repairs could be made, and for improved access to ventilation and access shafts.

Initial and follow-up diving operations were completed in Shaft 6 to access the bottom of the shaft, inspect mechanical and structural components and measure for new equipment and structural comp onents. Divers utilized a diving bell and a saturation system to work 700 feet deep, the equivalent of 20 atmospheres. Divers worked in the shaft and lived in a special habitat at the surface that maintained these pressures. They carried helmet cameras that transmitted video images to the surface where inspectors monitored their work, captured and recorded the transmitted data and communicated with the divers.

Data were analyzed to assess the pressure interface condition between the aqueduct and shaft bottom, and the condition of existing piping, valves and pump. Pump station upgrades were designed based on the findings. Inspection results indicated, however, that high pressures at the station entrance required a bulkhead, after which the pump station installation continued.