Overview
In an effort to provide daily water to New York City, the New York City Department of Environmental Protection retained Southland/Renda Joint Venture to rehabilitate the Gilboa Dam and replace the existing output facility including the installation of a new tunneling system 150 feet below the Schoharie Reservoir. The four-phased project scope included the construction of an intake structure.

Scope of Work
Ballard provided underwater construction services, deep-diving requiring decompression chambers, and hot water dive suits for this project.

Innovative Solutions
Phase One included assembling the crane barge, setting a four-point anchor spread, and dredging the area where the intake structure will be placed. Dredging was conducted using a Toyo Dredge pump capable of dredging in deep water with jet nozzles and crane assistance. Ballard’s welders also assembled the cofferdam that was placed in Phase Two.

Phase Two of the project required the installation of the thirty-foot diameter cofferdam that formed the outside walls of the structure. Ballard divers assisted in placing the structure in a water depth of 160 feet. The high elevation reservoir required 180-foot dive tables, as well as Ballard’s in-house decompression chambers. Ballard’s divers also utilized hot water dive suits to protect the divers from hypothermia from the cold water. After placement, Ballard divers tremie poured concrete around the outside and the inside was filled with sand.

Phase Three involves both the reconstruction of the dam and the construction of a Low-Level Outlet (LLO). The Gilboa LLO tunnel project consists of a 1,188-foot long, nine-foot ID “Land Leg” tunnel constructed between the gate shaft and the tunnel portal, and a 930-foot long, nine-foot ID “Water Leg” tunnel constructed between the gate shaft and the intake structure utilizing a Micro Tunnel Boring Machine (MTBM). MTBM tooling support was performed via pressurized interventions by Ballard’s compressed-air workers. Once the MTBM passed through the cofferdam, Ballard divers retrieved the machine. The picture above captures the success of one of the largest MTBM wet retrieval efforts ever accomplished in the United States.

Results
Phase Four completed construction by enclosing the structure. It began with Ballard divers installing an elbow to connect the MTBM pipe to the surface inside the intake structure. Finally, Ballard divers guided the lowering of the top of the intake structure into place and completed the connection to the cofferdam walls that formed the perimeter of the structure.