Gilboa Low-Level Outlet Microtunneling Construction Complete

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Project update by Babs Marquis, CCM

McMillen Jacobs Associates as a trenchless subconsultant to AECOM is providing construction management/resident engineering services for construction of a low-level outlet (LLO) tunnel to enable the New York City Department of Environmental Protection (DEP) to meet dam safety and conservation release objectives for Schoharie Reservoir. It is part of DEP’s reconstruction program to extend the Gilboa Dam’s service life to meet current New York State Department of Environment Conservation dam safety guidelines.

Water leg cofferdam structure with fiberglass soft eye

Gilboa Dam, in Upstate New York, impounds Schoharie Creek to form the Schoharie Reservoir. This is the northernmost reservoir in the Catskill System of the New York City water supply and yields approximately 15% of the water supply for the five boroughs.

Southland/Renda Joint Venture, the general contractor, deployed a Herrenknecht AVN2200B Microtunnel Boring Machine (MTBM) to excavate the approximately 2,176-foot (663 m) “potentially gassy” LLO tunnel. This MTBM is the first of its kind manufactured by Herrenknecht with explosion-proof electrical components and tunnel support accessories designed to mine in a “gassy” environment.

The large-diameter (108-inch [2,745 mm]) LLO tunnel is divided into a land leg tunnel and a water leg tunnel, with the 185-foot-deep (56 m) LLO Gate Shaft serving as the jacking shaft for both tunnel legs. The shaft was excavated by drill-and-blast.

The 1,206-foot-long (395 m x 2,745 mm) land leg was excavated by the MTBM through rock. Initially, it was planned that, for the land leg tunnel receiving site (located at the valve chamber), the MTBM would pass through the valve chamber portal with the grout plug located within a soil nailed wall with ground stabilization. However, four soil nails were encroaching into the tunnel horizon, presenting obstruction hazards to the advancement of the MTBM as it neared the final reach of the land leg tunnel. To mitigate the risk of obstruction and damage to the MTBM, a 12-foot (3.7 m) horseshoe-shaped tunnel was hand-mined for 80 feet (24 m) centered on the alignment to remove the drifted soil nails within the tunnel horizon. The MTBM then completed excavation of the land leg through the hand-mined tunnel to the valve chamber portal platform downstream of the dam at Schoharie Creek.

Gilboa LLO tunnel alignment

The 950-foot-long (290 m) water leg tunnel was excavated by the MTBM through mixed face—starting in rock, then through till, then lake bottom sediments, and finishing in tremied low-strength concrete at the approach to the intake structure. The MTBM holed through the 12-foot (3.7 m) square fiber glass soft-eye target in January 2019 into a 35-foot-diameter (11 m) cofferdam. It is presently seated in the cofferdam approximately 155 feet (47 m) under the reservoir pending completion of utility removal and contact grouting before wet retrieval.

The LLO shaft and tunnel construction began in July 2015 and is winding up in spring 2019.