Design and Construction of Large Diameter Pipelines at a Major Southern California Water Treatment Facility

The Metropolitan Water District of Southern California (MWD) is retrofitting the Robert A. Skinner Water Treatment Plant (WTP) with new pretreatment consisting of raw water ozonation and expansion of their existing washwater reclamation system. This is a very large, fast track project that must meet a prescribed implementation schedule to maintain water production from this important 630 mgd treatment facility serving Southern California. The project includes furnishing and installation of piping up to 144-inch diameter. In order to meet critical milestones, MWD separated most of the large diameter piping (over 36-inch diameter) into a separate pre-purchase package. This required careful consideration and delineation of limits of responsibility to accommodate the expedited schedule and to result in a complete and functional installation. There were also several tie-ins to existing large diameter pipe that required careful coordination, including protecting the material integrity of the existing piping and limiting the period of time the plant would be removed from service. The routing of 5,600 feet of large diameter steel pipe, including 3,150 feet of 144-inch pipe, through both new and existing portions of the 630 mgd water treatment plant posed challenges to the design and construction of this major upgrade to this important facility. Special issues for the pipelines on this project included: making connections to existing pre-stressed concrete cylinder pipelines without causing immediate or long-term damage to the pre-stressed concrete cylinder pipe, consideration of the structural integrity of existing structures immediately adjacent to deep pipeline construction, maintaining pipeline grades that allow safe access for routine maintenance inspections during the pipeline's service, complexities associated with the fabrication and delivery of 12-foot diameter pipe and fittings. This paper will review the purpose of the project, and discuss the phased construction and other special issues confronted during design and construction of the project's large diameter pipelines. Design and construction solutions developed to deal with these issues will also be discussed.