Pardee Tunnel Inspection and Condition Assessment

The Pardee Tunnel is a 2-mile long concrete-lined tunnel with an 8-foot diameter horseshoe cross-section. It was constructed between 1926 and 1929. The tunnel excavation was initially supported by timber sets and timber lagging. The Pardee Tunnel is critical to East Bay Municipal Utility District (EBMUD) raw water system. It provides 90 percent of EBMUD’s water supply. The last manned inspection occurred in 1962. A manned inspected was attempted in 1982 but was aborted due to excessive ground water intrusion. However, without a redundant conveyance tunnel, dewatering the tunnel for a manned inspection is difficult due to operational constraints. In order to reduce outage costs and water supply concerns, a remotely operated vehicle (ROV) was launched through the reservoir outlet tower to inspect the tunnel during the winter of 2018 rather than a manned inspection. Prior to launching the ROV, a team of 10 divers removed three tons of unforeseen debris blocking both access points into the tunnel. The ROV was equipped with 3D multi-beam sonar, inertial navigation system, 2D multi-beam imaging sonar, tell-tails, a hydro-phone, and a dye injection system. Flows were reduced to approximately one foot per second during the inspection. A comprehensive decontamination plan was developed to ensure invasive species were not introduced during the inspection. The data collected formed the basis for the condition assessment which concluded that the structural integrity of the tunnel and liner is good with no significant defects. The condition assessment report recommended continuing seepage monitoring and regular inspection by manned entry or ROV. Conceptual design drawings were developed to improve future tunnel access and facilitate planned or emergency repairs.