As one of San Diego’s most important transportation projects, the Mid-Coast Extension of the University of California San Diego (UCSD) Blue Line extends the San Diego Metropolitan Transit System with 11 miles of double tracks. This light-rail addition goes from downtown San Diego to the campus of UCSD. It provides an alternative to congested freeways and connects the corridor with areas served by the existing trolley system.
At a cost of more than $2 billion, the extension is one of the largest infrastructure projects in the San Diego area. In addition to the new tracks, this project encompassed eight bright crossings, several miles of aerial viaduct structures, five at-grade stations, and four aerial stations. At five of these stations, an additional 1170 parking spaces were added.
For the viaduct constructed within the median of Genesee Avenue, the original design called for another building material, but the project team opted to splice precast concrete girders on site. The resulting viaduct is the first curved, spliced precast concrete U-girder light-rail transit bridge in Southern California. By constructing this viaduct, the project team has provided new techniques for future infrastructure projects that can be useful in minimizing traffic disruptions. The Genesee Viaduct is the result of a collaborative process in which team members used innovative design techniques, limited construction costs, and an optimized schedule. It was critical to maintain local traffic and minimize the impact on the surrounding community, and these priorities were the primary drivers as stakeholders selected the structure type and construction methods. Only nighttime closures of major intersections on Genesee Avenue were considered.
Precast concrete U-girder construction was selected for most of the viaduct to minimize falsework. “The falsework design would have to be continuous, which means the major intersections at which the viaduct crossed would be completely cut off for the duration of the construction through each intersection,” says Vladimir Kanevskiy, PE, engineering manager, WSP USA.
Precast concrete girders were fabricated off site and transported to the construction site. There, the precast concrete girders were spliced together with a cast-in-place (CIP) closure pour for continuity under the final loading condition. This girder layout reduced hauling costs, limited the girder weight to under 100 tons, and removed the need for specialized hauling equipment.
Three frame types of assorted construction techniques, girder assembly, and splicing operations were used on the viaduct design. Nine precast concrete girder frames consisted of precast concrete U-girders (Caltrans “bathtub” girders) spliced by one- or two-stage post-tensioning. Girders for spans over intersections were spliced in a staging area away from traffic. On both sides of the intersection sat temporary shoring towers, where the spliced segment was lifted and placed. To splice the girder segments on the shoring towers that did not cross traffic, a second post-tensioning tendon was used. A second stage of post-tensioning was performed after all individual girders were spliced, which connected all the segments. This technique created continuity between the expansion joints. The four frames that were left were spliced and tensioned in one stage.
Crossing over La Jolla Village Drive sits the longer span part of the viaduct, which was 225-ft-long. Due to its length, it could not be lifted and placed in one piece onto temporary shoring towers. The project team used a hybrid precast concrete/CIP superstructure, which reduced the spliced length while still using the girder construction in adjacent spans. The CIP girder segments were connected to precast concrete girder segments by post-tensioning and closure pours.
Two aerial side-platform stations – each station within a single structural frame – are served as the Genesee Viaduct. Transverse beams connected to superstructure girders support the side platforms. Single-stage post-tensioning was performed at all station frames.
Three superstructure types addressed the different types of loading and frame construction. Two 96-in.-deep U-girders connected with a 9-in.-thick CIP deck were used as the cross section at the precast concrete girder frames. Superstructure depth is constant in all precast concrete girder frames.
The Mid-Coast Extension is recognized as the most important transportation improvement project in San Diego for expanding capacity and accommodating future travel demands in the region. The extension provides a direct link from the United States/Mexico border to University City and is a great addition to the region’s public transit system.