South Ferry Station, located beneath Battery Park at the tip of Manhattan, was originally built in 1905. Today, this facility is used by more than six million people a year. To add additional tracks and also remedy other deficiencies to meet modern standards, the Metropolitan Transportation Authority Capital Construction Company initiated a $400M improvement program. The construction work, being accomplished under a design-build contract awarded to the joint venture of Schiavone/Granite-Halmar, included a new cut-and-cover subway tunnel. Moretrench designed and installed the dewatering system that facilitated underpinning of, and excavation below, two existing subway tubes to allow tunnel construction.
Ejector Well System Selection
The subsurface profile consisted of 20 feet of fill overlying a stratum of silty sand to sandy silt as deep as 65 feet below ground surface. Beneath this, a variable thickness of weathered rock overlaid competent, unweathered rock. Groundwater was at approximately 10 feet below grade. Since drawdown was required to top of competent rock, Moretrench utilized an ejector well system. Ejectors are typically used when the groundwater must be lowered more than 15 feet, i.e. more than a single-stage wellpoint system is capable of, and the hydraulic conductivity of the soil is such that vacuum application is warranted to improve drainage.
Installation and Monitoring
Mud rotary drilling techniques were used to install approximately 106 ejector wells required for the project. The wells were specifically extended into the permeable weathered rock to drain the less permeable soil layer above, and connected to a central pumping station via dedicated pressure and return piping. A scaffolding system was erected to support the piping over the 500-foot long work area.
To verify that the system was performing as required, an extensive array of piezometers was installed and monitored. Over 11 months of operation, the system produced approximately 70 gallons per minute, which was discharged into the local sewer system.
Digging up History
During excavation above the water table, prior to installation of the ejector system, a 45-foot long section of a 17th century fortification wall was uncovered. Described as “the oldest piece of a fortification known to exist in Manhattan,” this very significant historical find prompted an archeological investigation that was conducted concurrent with the ongoing construction program. While this resulted in logistical and scheduling difficulties for Moretrench, installation of the dewatering system and the required drawdown was nevertheless accomplished without impacting the overall project schedule.