Garco was awarded the $20.1M Spokane River East Trent Bridge Replacement contract for the Washington State Department of Transportation (WSDOT) in January of 2020. This contract included removal and replacement of the oldest bridge in the state transportation department’s inventory, the SR 290 Trent Avenue crossing over the Spokane River. Constructed in 1910, the existing 513-foot five-span concrete arch bridge was replaced with a new four-span precast concrete girder bridge extending 522.5 feet.
With the new bridge located directly on the existing alignment, the project required development of extensive in-water access for demolition and construction. Garco used an innovative approach for access and containment of the demolition debris by placing streambed gravels and shot rock into the river, blocking approximately half of the river prism each season. Garco engaged a third-party engineer to perform hydrologic analysis, hydraulic modeling, and scour analysis, and worked extensively with WSDOT, Washington Department of Fish & Wildlife (WDFW), and other permitting agencies to prepare and gain approval of the proposed in-water access plan. Garco used these access pads along with two driven steel sheet pile cofferdams for demolition, 847 LF of 8-ft and 10-ft diameter drilled shaft construction, concrete construction of the bridge columns and pier caps, and girder erection. The pads were removed prior to the end of the in-water work window each season to provide for high flows during spring runoff.
Another difficult feature of the project was the design and construction of an intricate vertical earth shoring wall to protect an adjacent brick building over 100 years old. The building was located only a few feet from the excavation line for one bridge abutment, and required an excavation depth of 32 feet. Garco developed a plan utilizing overlapping drilled concrete secant shafts with embedded wide-flange beams, two rows of strand tie-back anchors up to 65 feet long, and horizontal steel walers to address this challenge. Garco worked with the building owner to secure a subsurface easement for the tiebacks, and our shoring designer and subcontractor to finalize the details, and successfully perform all work with no damage or settlement
of the structure.
The in-water work restrictions and interaction with river levels greatly affected the contract duration. Permitting conditions only allowed six months of in-water work each year, and normal river conditions precluded an additional month. Despite these conditions, Garco was able to plan and schedule our work to complete all demolition, foundation work, and abutment and pier construction to allow the two center spans of girders to be set in a total of 11 months, including installing and removing the access pads each season. On a project with an original duration of 42 months based on working days, Garco completed the facility in 27 months of actual work, or 15 months ahead of schedule.
As the prime contractor, Garco self-performed 43% of the work; the remaining scopes of work were performed by valued trade partners under Garco’s management.