Profiled High Risk - No Threat to Gilboa Low Level Outlet Microtunnel Design and Construction Project

摘要

Gilboa Dam, in upstate New York, impounds Schoharie Creek to form the Schoharie Reservoir. Constructed in 1929, it is the northernmost reservoir in the Catskill System of the New York City water supply system. 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 is constructing a Low-Level Outlet (LLO) to facilitate the ability to drain the reservoir to meet NYSDEC dam safety requirements and to provide an ability to make conservation releases. The project has been advanced through stages of risk evaluation from planning through engineering design and is now in the construction phase. The LLO consists of two 108-inch-diameter tunnels (a water leg and a land leg), ultimately terminating at a new valve chamber that will release water to the Schoharie Creek downstream. The two tunnels are approximately 2,160 linear feet combined, and are designed to be completed using trenchless microtunneling construction methods. When the water leg is mined into the receiving site at the bottom of the reservoir at 153 feet below normal pool water level, retrieval of the microtunnel boring machine (MTBM) will be completed via wet retrieval methods. This paper presents the project’s risk profile, and the risk management approach adopted in the delivery of the project.