Rehabilitation of the diversion tunnel outlet works at Cobble Mountain Reservoir Dam

Construction of Springfield Water and Sewer Commissions Cobble Mountain Reservoir Dam, located in Granville, Massachusetts, was completed in 1931. For the past 75 years the reservoir has served as the primary drinking water source for the City of Springfield, Massachusetts and its surrounding communities. In 2001, a rehabilitation project was planned for the diversion tunnel high-pressure oudet works facility, which included replacement of two existing 42-in × 30-in Larner-Johnson differential needle valves. The project also included rehabilitation of the two existing 40-in Escher Wyss rotary (ball) style guard valves. The oudet works operates under 200-ft of static head. The needle valves were replaced with 30-in diameter United States Bureau of Reclamation (USBR) style jet-flow gates, which discharge direcdy into the 1500-ft, long 12-ft diameter diversion tunnel. There were no isolation gates installed upstream of the guard valves in the original design of the dam and tunnel, so the valves could not be dewatered prior to disassembly. These conditions required the design of a unique non-entry mechanical plugging system, which allowed insertion of a 40-in diameter pneumatic bulkhead plug, from the gate chamber (in the dry), through the valve and into the upstream conduit. This was accomplished while the system was under 200-ft of static head. After the plug was inflated within the conduit, and mechanically restrained in the gate chamber, the guard valves could be dewatered and disassembled for rehabilitation in the dry. The new jet-flow gates and rehabilitated guard valves were put into service in August 2006. This paper describes the history, inspection, testing and evaluation of the existing valves and piping; project design objectives and details; construction techniques used; and the results derived from hydraulic testing of the new outlet works after commissioning.