Slurry Wall Excavations for a Starter Trench and Receiving Shaft of a New York City Siphon Tunnel Project

A rectangular starter trench measuring 220 feet long by 23 feet wide and a 24-foot diameter, 12-sided receiving shaft were built in order to accommodate launching and removal, respectively, of an earth pressure balance (EPB) tunnel boring machine (TBM) which was used to construct a subaqueous water supply tunnel in New York City. For the TBM starter trench located in Staten Island, the 4-foot wide slurry walls were excavated with a clam shell and a hydromill to a depth of approximately 90 feet through fill and glacial soils and into competent rock at the western portion of the excavation, and to 107 feet into decomposed bedrock at the eastern portion. During slurry trench excavation, the work was closely monitored by performing time-controlled slurry tests and examining muck conditions from a slurry separation plant. Also, vertical and horizontal deviations of the slurry trenches, rebar cage placement, and the concrete pour were closely monitored and inspected to maintain construction within the design tolerances. For the TBM receiving shaft in Brooklyn, the 3-foot wide slurry walls were excavated to a depth of approximately 155 feet mostly with a hydromill. Difficult ground conditions due to uncontrolled fill including wood timber and concrete debris, and glacial sand with boulders were overcome with intermittent use of a clam shell and large chisel. The rebar cages of the primary panels were fabricated in a “concave up” shape and the secondary panels were constructed in a rectangular shape. Together they formed the circular shaft shape. Due to the slurry wall depth, three sections of the rebar cages were spliced while inserting into the slurry trench. The TBM break-in and break-out portion of the slurry walls were reinforced with fiber-glass rebar.