Challenging 72-Inch Dual Pipe Relocations through Nob Hill for San Diego County Water Authority

The Second San Diego Aqueduct is located within the San Diego County Water Authority’s (SDCWA) right of way that traverses the Scripps Ranch community, south of Miramar Hill. After crossing Interstate 15, the Second Aqueduct, which contains Pipelines 3, 4, and 4A, travels through a residential area, crosses Scripps Ranch Boulevard, and ascends a hill known as “Nob Hill.” Certain operating conditions within the Water Authority’s Second Aqueduct were predicted to produce transient events that could result in water column separation in the vicinity of the high point at Nob Hill. The transient event had the potential of damaging or rupturing Pipelines 3 and 4 and producing a catastrophic event for the Aqueduct system and the residential areas down-slope of the pipelines as they cross Nob Hill. This section of the Second Aqueduct contains the Water Authority’s Pipelines 3 and 4. Both are 72-inch diameter prestressed concrete cylinder pipe (PCCP), with Pipeline 4 having the addition of steel liners. To prevent a potential catastrophic event, SDCWA completed a project to “lower” both existing pipelines, thereby increasing the hydraulic gradient in the pipelines and reducing the potential for a damaging transient pressure event. This paper will discuss the difficulties in combining two, 72-inch diameter PCCP pipelines, along different elevations and slopes, into one 96-inch diameter steel pipeline, tunneled through Nob Hill. Several design and installation issues will be examined.