OHL USA, engaged in a joint venture, excavated a 9,440-foot-long tunnel under Anchorage Channel for a new water transmission main – siphon – running between Staten Island and Brooklyn. The 12.5-foot diameter EPB TBM bored 115 feet below the water table and 59 feet below the New York Harbor bed through mixed-face soil conditions. The crews injected polymer to maintain a smooth flow of muck and consistent earth pressure, while pre-mixed single component grout was used for backfilling the annular gap. The tunnel, housing a 72-inch-diameter welded steel pipeline, has a 12-foot outer diameter while its precast concrete, bolted and gasketed liner has an inner diameter of 10.3 feet. The scope of work also included shaft excavation and micro-tunneling. OHL North America company, Judlau Contracting, Inc., used the cut-and-cover method to install the transmission mains in Brooklyn and Staten Island, 2,690 ft. and 1,250 ft. long, respectively. Judlau also constructed the chlorination building, and the installed the siphon pipe in the tunnel.
Project challenges included Hurricane Sandy—a 300-foot-long, 110-ton TBM had progressed 1,600 feet toward Brooklyn when its operations were suspended on Oct. 28, 2012 in advance of the approaching superstorm. Sandy flooded the waterfront project site in Staten Island, topping over protective concrete barriers that reached 3 feet above the 100-year flood level and submerging the TBM. The crews pumped the water out and refurbished the TBM, a process that was performed in the tunnel under 4.5 bars of water pressure.
When changes in geology from marine to glacial began to slow progress of tunneling, OHL added four 50-ton auxiliary cylinders to the propulsion system of the TBM to increase its thrust. A hyperbaric intervention was also initiated to change the TBM’s cutting tools within a pocket of glacial soils. Support of excavations below the water table was accomplished using secant piles. The team also installed and maintained geotechnical monitoring instrumentation and developed plans for protection of existing structures that included structural reinforcement and ground improvements to minimize movements of sensitive structures and control ground water to protect excavations.
Once the new siphon became operational, two old water mains that were located at a much shallower depth were removed, enabling U.S. Army Corps of Engineers to dredge and deepen the channel.
Tunneling with EPB TBM 115 feet below the water table
Hyperbaric intervention to repair the TBM and change the cutting tool
Changes in geology, with exceptionally hard boulders and excessive water ingress