Canyon Power Tunnel Inspection

Jacobs Associates was retained by the San Francisco Public Utilities Association (SFPUC) to provide tunnel inspection services for the 55,132-foot-long (16,804 m) Canyon Power Tunnel. The tunnel was built in 1965 through granite. The tunnel contractor used drill-and-blast construction methods, leaving a very uneven invert with a lot of rock debris. Accordingly, this 21st century inspection was very slow and difficult as the inspection team walked through knee-to-waist-deep cold water with a rocky, ankle-twisting prone invert. Use of a walking stick was a must.

Normal inspection procedures were followed for both advance and recording phases. The advance team stationed the tunnel walls, allowing the recording team to locate the features to be documented. The advance team also checked and scaled any potential loose rock or liner in the crown that could be a falling hazard. The recording team consisted of a photographer, videographer, recorder, measurers, and safety personnel carrying first aid kits and air monitors. When an inspection was completed, the team returned created CADD maps of the tunnel for use by the owner for future repairs or inspections.

The Canyon Power Tunnel was unwatered for the inspection, so Jacobs Associates also oversaw the repair of the existing adit plug. Cracks had formed through the plug, which was leaking a few hundred gallons per minute. With the tunnel unwatered, the team injected cement grout into the visible cracks. They also found washed-out pockets of deteriorated concrete, which they removed with hand spades and replaced with a structurally reinforced mortar patch.