Atlas Geotechnical provided means-and-methods engineering and field engineering support for installing 110 high-capacity rock socket foundation that support the 5-foot thick concrete floor of the new valve chamber at Gilboa Dam in upstate New York. Eleven of the rock sockets are tension elements with 30 high-tension steel strands locked off at 980 kips. Matcon drilled the 12-inch to 20-inch diameter foundations using a crane-mounted reverse air circulation rig capable of making several feet per minute. Alas Geotechnical managed the quality assurance program that included logging drilling progress, verifying inclinations and depths, monitoring pressure grouting needed to block artesian water, and collecting video inspections of each socket.
Gilboa Dam retains Schoharie Reservoir, the northernmost of 19 structures that supply drinking water to New York City. Constructed in 1927, the 120-foot high dam is a gravity masonry structure that was designed solely for drinking water and had no low-level outlet works. Flooding and overtopping during Hurricane Irene in 2011 caused mandatory evacuation orders of the towns of Middleburgh and Schoharie. The valve chamber project is part of a retrofit program that includes an intake tower in the reservoir, twin bored tunnels around the abutment, and a new outlet works into Schoharie Creek. Once in service, the outlet works allow the reservoir to be drawn down to create flood control storage capacity before severe rainfall events.