CS33A - Concurrent Session 33A: Rubber J-Bulb Seals Used for High Head Bulkhead on Corroded Surface

Abstract Description: Located in Yosemite National Park, the O’Shaughnessy Dam was originally constructed in 1923 and provides drinking water to the entire bay area surrounding San Francisco, CA. The facility is owned and operated by the San Francisco Public Utilities Commission (SFPUC). The multiple outlets from the dam are primarily controlled by mechanical slide gates at the upstream edge of the conduits. The slide gates were part of the original installation and cannot be serviced or replaced except by draining the reservoir. Original dewatering/service bulkheads for the conduits have never been successfully deployed, and would not provide a seal that would provide a safe work area. In preparation for work on the 100-year-old slide gates, SFPUC implemented a progressive design-build project for design, fabrication, and testing of new dewatering bulkheads to be installed at over 250 feet of head. Besides the high head and significant hydrostatic forces, numerous challenges existed, including multiple bulkheads placed down a single confined slot through the dam. The existing conduits included cast iron thimbles that included the intended sealing surface. The thimbles were known to be significantly corroded with iron tubercles and associated surface pitting. The use of rubber seals is not well documented under high head conditions and against non-uniform surfaces.

This paper (presentation) describes the process used to design the bulkhead seals, conduct laboratory tests to verify seal parameters, and field testing of bulkheads into the 100-year-old dam. Details that will be discussed include seal profile and hardness, bearing/spacer bars, seal stem and bulb deflection and confinement, clamp bar compression to reduce leakage, flow required to initiate the seal, bearing surface preparation, and other details impacting seal effectiveness. The resulting design was successfully fabricated and field tested, and was proven to provide a manageably small amount of leakage under 250 feet of hydrostatic head even when sealing against a highly corroded surface.