Schoharie Reservoir Low Level Outlet

Completed in 2024 by Southland Contracting, the Schoharie Reservoir Low Level Outlet Project was a joint venture between Southland Contracting, Inc. and Oscar Renda Contracting, Inc. Southland’s scope of work included construction of microtunnel and deep shafts. The MTBM excavated tunnel is located in Gilboa, New York. Excavation primarily consisted of: 2,210 ft of 9.5 ft ID microtunnel with Permalok casing, 46 ft-diameter, 187 ft-deep Gate Shaft excavated by drill and blast. Land leg tunnel: 1,200 ft of 9.5 ft-diameter tunnel between the Gate Shaft and a Portal. The Water leg tunnel: 1,010ft of 9.5 ft-diameter tunnel with underwater retrieval in the Schoharie Reservoir. A portion of the land leg excavated with SEM (Sequential Excavation Methods) near the portal The tunnel was designed for 150 ft of hydrostatic pressure; bulkheads were designed to prevent uncontrollable emptying of the reservoir. For construction, a new MTBM (microtunnel boring machine) was required for Class 1 Division 2, the first of its kind, and be capable of hyperbaric interventions as well as wet retrieval. The project was constructed in both hard fractured rock and soft ground with cobbles. The rock was primarily sandstone with average compressive strengths of 27,000PSI and very abrasive; cerchar abrasivity index of greater than 6.5 in areas. Portions of mixed face and soft ground in both tunnels with a large quantity of cobbles and boulders embedded within the soil mass. Boulders greater than 10 feet in diameter and numerous smaller boulders and cobbles were encountered. The tunnel was excavated with water pressure up to 5 bar with underwater retrieval from bottom of reservoir at depth of 160 feet. The shaft was supported with liner plate and ribs though the overburden and 10-ft rock-bolts on a 5×5 diamond pattern with mesh and shotcrete in the rock portion of the shaft. The pre-excavation grouting program-controlled groundwater along with a robust pumping system. Both hydraulic and mechanical dredging was used to excavate approximately 10,000 cy of material from the bottom of the reservoir and a 35 ft diameter cofferdam installed at the bottom of the reservoir to retrieve the MTBM and make the underwater connection for the intake. Approximately 5000 cubic yards of tremie concrete were placed around and within the intake to secure it in place. Divers welded the intake to the tunnel pipe and positioned a stainless-steel grating around the 14 ft diameter intake. Additional site work includes the construction of the outlet works near Schoharie Creek, creek restoration that was damaged by hurricane Irene, a Roller Compacted berm adjacent to the existing spillway and construction of a new public overlook adjacent to the dam. Instrumentation and Controls as well as security measures will ensure the supply and safety of this important piece of NYC’s drinking water for generations to come. A significant restoration and landscaping package finalizes the work on this contract. Southland Contracting and Oscar Renda self-performed all major work on the project as described above as well as the large diameter mechanical piping and valves, 78 inch knife gate and fixed cone valves as well as the 10 ft roller gates in the Gate Shaft. Due to the remote location, an onsite batch plant was used for all the concrete. The separation plant for tunneling operations was enclosed from the weather.