Keeping Water and Traffic Flowing during Replacement of a Critical Regional Pipeline

Helix Water District's 48-inch Flume project consists of replacing an existing concrete cylinder pipeline (CCP) with a steel pipeline, in order to meet new higher pressure requirements. The project is part of the East County Regional Treated Water Improvement Program (ECRTWIP), partnered with the San Diego County Water Authority (CWA). The ECRTWIP included the Los Coches Pump Station Upgrade, which increases the hydraulic gradeline in the Flume pipeline. A 3,300 foot, 42-inch diameter CCP segment of the existing flume pipeline had insufficient pressure capacity for the increased hydraulic gradeline. The pump station upgrade and improvements to the Flume pipeline will enable Helix Water District to allocate an additional 10 mgd from the District's R.M. Levy Water Treatment Plant for delivery to neighboring Otay Water District and Padre Dam Municipal Water District facilities. Design and construction of the 48-inch Flume Pipeline needed to be completed concurrently with the Los Coches Pump Station Project. The design phase included an alignment study, preliminary and final design completed within an aggressive schedule. Shutdown sequencing and scheduling was particularly important since the existing pipeline is critical to the distribution of potable water to the region. The longest the pipeline could be taken out of service was 30 hours during periods of cool weather. The Contractor worked closely with the construction management and design team to develop a construction schedule that met the completion date criterion. Several shutdowns were conducted, which enabled a shorter duration for the final shutdown and connection. Construction of the pipeline along a new alignment would require blasting or more expensive rock breaking techniques for the hard granite rock in the area. To avoid blasting in close proximity to high pressure gas pipelines located in close proximity to the available alignment options, the new pipeline was placed in the original trench occupied by the existing pipeline. Since loss of use of the pipeline for more than 30 hours would result in severe operational difficulties and supply shortages for the region, a 32-inch diameter HDPE pipeline was designed to handle flow during construction. Challenges for the above-ground HDPE pipeline included control of movement due to internal pressure changes and thermal expansion and contraction.