Investigation and Rehabilitation of Prestressed Concrete Cylinder Penstock - Black Butte Hydroelectric Project

The Black Butte Hydroelectric Project consists of a penstock and powerhouse addition constructed in 1987-88 to the pre-existing Corps of Engineers' Black Butte Dam and Reservoir, located on Stony Creek in Tehama County, California. The penstock includes an intake structure, a 51O-foot-long tunnel lined with prestressed concrete cylinder pipe (PCCP), and a 567-foot-Iong cut-and-cover section of PCCP. The powerhouse contains a single 6.2 MW turbine generator unit in a semi-outdoor reinforced-concrete structure, operating under a maximum head of about 100 feet. Other Project features include a switchyard, re-regulation dam, irrigation canal intake, access road and transmission line. The general Project location and site arrangement are shown in Figures I and 2. On September 23, 1990, the City of Santa Clara's Director of Electric Utility ordered the Black Butte Hydroelectric Project shut down because of the potential for failure of the prestressed concrete cylinder pipe (PCCP) penstock. The City notified FERC of this action by letter on that date, pursuant to 18 CFR Ch. I, Section 12.IO(a)(l). The letter states, in part: ". . . the design does not provide for differential settlement between the Penstock and the powerhouse. The Penstock bridges fill adjacent to the powerhouse. We are concerned that leaks could develop in the liner plate of the Penstock, eventually leading to corrosion of the prestressed reinforcing wire and shortening the life of the Penstock ... " The Black Butte Penstock was specified to be prestressed concrete cylinder pipe (PCCP), with every pipe joint welded except in one location where a sleeve-type flexible coupling was installed. The welding of the joint system throughout the penstock is an unusual application for PCCP, which is meant to have articulated joints because of its low longitudinal strength. (See Figure 3, which shows a typical penstock joint detail.) Even small differential movement of the rigid PCCP structure at Black Butte would result in cracking of the core and mortar coating. Cracking of the mortar coating, which protects the prestressing wires with its highly alkaline environment, will expose the wires to the surrounding groundwater environment and lower the pH. This condition will allow corrosion of the wires and result in eventual failure of the pipe. An example of a recent PCCP pipe failure is shown in Photograph I, which shows the inspection of the failure site of the Mojave Siphon in California on March 3, 1994.