Crossing That Bridge Later: Installing a Critical Water Pipeline over an Existing Bridge without Modifications

The Diablo Water District (district) is located in the northeastern part of Contra Costa County (county), in Northern California. One of the largest areas of anticipated growth for the District is on Bethel Island, a 5.6 square-mile island in the San Francisco Bay Delta area that is expected to grow from its current population of 2,200 residents to over 7,000 residents by 2040. In 2009, the County rebuilt the single, aging bridge that connects Bethel Island to the mainland with a new 900 ft long concrete bridge. As part of the planning for the bridge, the county allowed the district to reserve space within the bridge structure for construction of a future water pipeline with one restriction—no modifications would be allowed to the bridge during future water line installation. Growth within the district is driven and funded by development; because development was not active on the island in 2009, the district could not support the full predesign of a future water pipeline and could only give conservative guidance to the county in preparation for a future pipeline. In 2013, the district began working with a land developer to plan for a new 500-unit development on the island. The development was to be connected to the District’s main distribution system via a new 18-inch water pipeline over the Bethel Island Bridge. This paper discusses the design conditions considered for the pipeline: thermal expansion and contraction of the pipeline, differential seismic displacements between the pipeline and bridge, ground settlement at the bridge abutments, pipeline stability and flexibility, and overall constructability—all of which had to comply with the county’s requirement of “no bridge modifications”. The paper concludes with some lessons learned during construction.