The Mid-Coast Extension of the University of California San Diego Blue Line adds 11 miles of light rail to the San Diego Metropolitan Transit System, from downtown San Diego to the university. The extension provides an alternative to congested freeways and connects the corridor with areas served by the existing trolley system.
At a cost over $2 billion, the extension is one of the largest infrastructure projects in the San Diego area. The project scope encompassed 11 miles of new double tracks, eight bridge crossings, more than 4 miles of aerial viaduct structures, five at-grade stations, and four aerial stations. The project also included 1170 new parking spaces at five of the new stations.
For the viaduct constructed within the median of Genesee Avenue, the original design called for another building material , but the project team opted to instead 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. The techniques used to construct this viaduct will influence future designs for projects that need to minimize traffic disruptions.
The Genesee Viaduct is the result of a collaborative process in which team members used innovative design techniques, limited construction costs, and optimized the schedule. It was critical to maintain local traffic and minimize the project’s 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,” explains Vladimir Kanevskiy, PE, engineering manager, WSP USA.
Precast concrete girders were fabricated off site and transported to the construction site. There, the girders were spliced together with a CIP closure pour for continuity under the final loading condition. The precast concrete girder layout was designed to limit girder length and keep the girder weight under 100 tons. This girder design reduced hauling costs and prevented the need for self-propelled modular transporters.
The viaduct design used three frame types with varied construction methods, girder assembly, and splicing operations. Nine precast concrete girder frames consisted of precast concrete U-girders spliced by one-stage or two-stage prestressing. Girders for spans over intersections were spliced in a staging area away from traffic. The entire spliced segment was lifted and placed on temporary shoring towers on either side of the intersection. Then a second prestressing tendon was used to splice the remaining girders that did not cross traffic while they were supported on shoring towers. Once all individual girders were spliced, a second stage of prestressing was performed to connect all the segments and create continuity between expansion joints. The remaining four frames were spliced and prestressed in one stage.
The longest span in the viaduct was the 225-ft-long crossing over La Jolla Village Drive. This span was too long to be spliced at a staging area and lifted on temporary shoring towers in one piece. Instead, a hybrid precast concrete/CIP superstructure was used to reduce the spliced length over the intersection and still make use of precast concrete girder construction in adjacent spans. Short, CIP cantilevered spans were built on both sides of the intersection integral with the columns in the shape of a hammerhead. The CIP segments were connected to precast concrete girder segments by prestressing and through closure pours.
The viaduct serves two aerial side-platform stations, each within a single structural frame. The side platforms are supported by a series of evenly spaced transverse beams connected to superstructure girders. Given the complexity of aerial station construction and the presence of multiple transverse beams that require integral connections to the superstructure, the design team decided to use CIP box-girder construction for the entirety of the frames carrying the aerial stations. Single-stage post-tensioning was performed at all station frames.
Three superstructure types were used to address the different types of loading and frame construction. The cross section at precast concrete girder frames consists of two PCI 96-in.-deep U-girders connected with a 9-in.-thick CIP deck. 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 US/Mexico border to University City and is considered a great addition to the region’s public transit system.