Page 36

2016WinterShotcreteEMag

2015 Outstanding Underground Project Northern Boulevard Crossing Tunnel CQ039 By Frank E. Townsend III The Northern Boulevard Crossing tunnel is a crucial link for the East Side Access Program linking Long Island Rail Road trains to Grand Central Station, New York City. It is a 125 ft (38 m) long sequentially excavated (SEM) tunnel. The tunnel is situated approximately 55 ft (17 m) below the groundwater table and was mined through glacial deposits. The tunnel alignment also crossed beneath a pile-supported, elevated railway line; a six-lane street; and an active below-grade subway structure (refer to Fig. 1 and 2). Contaminated plumes in the area also dictated the installation of a protective frozen arch above the tunnel alignment, extending to bedrock for complete groundwater cutoff. The freezing of the ground costs the New York Metropolitan Transportation Authority (MTA) $11,000 per day. Every day the liner completion could be accelerated, the more savings to the MTA. A value-engineered approach was given to the MTA to shotcrete this liner in lieu of traditional cast-in-place. This offered both savings in time and construction cost. Without the shotcrete alternative approach, an elaborate, costly, and time-consuming tunnel form system would have to be engineered, delivered, and assembled in the tunnel for traditional formed cast-in-place concrete. The contractor estimates that close to 2 months were saved using the shotcrete alternative. Superior Gunite’s scope was to expeditiously shoot the structural liner of this tunnel to then allow quickly unfreezing the ground and transfer the load from the aboveground structures. Coordination with the contractor allowed Superior Gunite crews to use the same scaffolding system used by the lathers installing the reinforcing bars, again saving time and money (refer to Fig. 3 and 4). Challenges Preparation of a plan and logistics were critical to the success of the project. Through a very tight relationship with our concrete supplier, Ferrara, we mapped out trucking delivery routes to mitigate the New York City traffic to avoid lost time and waiting times on trucks. Ferrara had a Quality Control representative on site to work through any possible quality control and address issues on the spot, which fortunately were minimal. Due to the site constraints, laydown area was limited. We installed our primary and backup pumps inline so if we had mechanical issues we could easily swap the line and deal with the pump on the off-shift. This system proved important, as three times we had to divert to an Fig. 1: East Side Access overall project scope Fig. 2: CQ39 Excavation alternate pump. Because the pumps were inline, 34 Shotcrete • Winter 2016


2016WinterShotcreteEMag
To see the actual publication please follow the link above