Large-Diameter Microtunneling and MTBM Wet Retrieval at the Gilboa Dam Reconstruction Project: Fid-Bau Portal

Large-Diameter Microtunneling and MTBM Wet Retrieval at the Gilboa Dam Reconstruction Project

Marquis, B. / Vickers, J. / Gerlach, J. / Diamante, J.

Gilboa Dam, located in upstate New York, impounds Schoharie Creek and forms the Schoharie Reservoir, the northernmost reservoir in the Catskill System of the New York City water supply. To ensure the dam’s continued long-term performance and reliability, the New York City Department of Environmental Protection (NYCDEP) embarked on a reconstruction program to extend the dam’s service life under the current New York State Department of Environment Conservation (NYSDEC) dam safety guidelines. NYCDEP will construct a low-level outlet (LLO) to facilitate the ability to drain the reservoir and meet proposed conservation releases. The LLO consists of two 9-foot diameter tunnels (a water leg and a land leg), ultimately terminating at a new chamber that will release water to the Schoharie Creek downstream. The two tunnels are approximately 2,160 linear feet combined, and will be completed using trenchless microtunneling construction methods, with microtunnel boring machine (MTBM) wet retrieval when the water leg is mined into the receiving site at the reservoir’s bottom. This paper reviews the overall project and then focuses on selected trenchless construction methods for large-diameter microtunneling, MTBM wet retrieval, and underwater construction.