Design, Manufacture, and Microtunnel Construction Using Wet Retrieval of the 108-Inch Diameter Low Level Outlet at NYC’s Gilboa Dam

Schoharie Reservoir is part of the Catskill System, and one of three systems that supply water to the city of New York. Approximately 35% of the city’s average daily demand is provided by the Catskill System, of which about 25% is supplied by the Schoharie Reservoir. The construction of Gilboa Dam was completed in 1926 and is comprised of a 700-ft-long earth embankment with a concrete core wall, and a 1,324-ft-long cyclopean concrete spillway with a maximum height of 160 ft. Like many reservoirs in the northeast, the capacity of the release works became restricted due to the age of the mechanical equipment, silting at outlet locations, and changes in release needs since the time of original reservoir construction. The final design of the low level outlet (LLO) was completed in June of 2014 and the project was bid in late fall of 2014 followed by award to SRJV and notice to proceed for construction in June of 2015. The project is scheduled to be completed in June of 2020 and when completed will represent one of the largest microtunnel boring machine project installations with wet retrieval ever accomplished in the United States. The 2100 linear feet of 108-in diameter Permalok steel casing will be installed in the tunnel while maintaining normal operating conditions of the Schoharie Reservoir at all times. This paper places particular emphasis on the practical applications from the view point of the owner, engineer, and manufacturer. Areas of interest include planning and design of the tunnel, the selection of tunnel lining materials, the Permalok steel casing manufacturing process, and key tunnel planning, design, and construction elements.