University District Pedestrian Bridge

Like many growing cities, Spokane, Washington is prioritizing pedestrian access through walkable corridors as it continues to develop and modernize its downtown neighborhoods. One of these neighborhoods is the University District, bordering downtown Spokane to the east, and home of the Spokane branches of Washington State University, Eastern Washington University, University of Washington and Gonzaga University.

A busy BNSF railway line cuts through the University District just south of the Washington State University campus, making pedestrian access from the local campuses to Sprague Avenue and points south very difficult. To solve this challenge and connect the academic institutions and two medical schools located in Spokane to business and medical community located on the south side of the BNSF corridor, city developers proposed an ambitious plan of building a pedestrian bridge over the railway and parallel road.

The University District Pedestrian Bridge was designed as an impressive 120-foot-tall, cable-stay bridge. With public plazas on both sides of the bridge approach and plans for a network of walking paths, the bridge was intended to be a centerpiece of the city’s overall development plans.

GeoEngineers began geotechnical and environmental work on this project in 2010 with a preliminary evaluation of soil and groundwater conditions at five possible landing locations. Project sites on both sides of the railway had a history of industrial use, and were found to contain varying amounts of lead, arsenic, petroleum and polycyclic aromatic hydrocarbons.

The environmental team carefully characterized the extent of contamination and then recommended a corrective action plan to bring the preferred landing sites into compliance with environmental standards. Geotechnical work included recommendations for the bridge’s foundations and construction, site preparation, earthwork, stormwater management and lab work.

Approach

• Early in the project, GeoEngineers performed a Phase 1 Environmental Site Assessment to identify potential environmental problems. Based on earlier investigations of properties in the area, the team called attention to likely lead contamination, unknown debris and residual petroleum from the remains of underground storage tanks on the sites of a former service station and trucking terminal.
• The GeoEngineers’ environmental team performed a Phase 2 ESA to further characterize the extent of environmental contamination and design a corrective action plan that would safely prepare the preferred landing site for development. The team drilled eight boreholes throughout the site to collect soil samples. Select samples were sent to an accredited lab where chemical analysis on the samples found lead, arsenic and polycyclic aromatic hydrocarbons.
• GeoEngineers’ environmental professionals designed a corrective action plan to remove or contain contaminated material at the site. The team recommended a hybrid approach based on the depth and location of impacted material. They recommended disposing of some portions of affected soil that could not be reused in the grading plan while safely capping other material in place.
• As the geotechnical engineer of record, GeoEngineers provided extensive recommendations for foundation design and construction, site preparation, earthwork, stormwater management and managed site investigations and lab work. The team recommended a combination of drilled shafts and spread footings to support the bridge structures. More economical shallow foundations were appropriate for the bridge abutments but weren’t feasible for the bridge’s central pier due to space constraints.
• GeoEngineers also performed a cultural resource assessment to predict the likelihood that sensitive archaeological artifacts could be uncovered during site excavation. Based on a literature review and analysis of the site’s development history, the team determined that additional testing for cultural artifacts wasn’t necessary.
• The team also gave recommendations for the design and construction of stormwater management/infiltration systems at both the north and south landings.
• During bridge construction, GeoEngineers’ staff were on site to monitor drilled shaft installation and the preparation of foundation grade for shallow spread footings, ensuring that contractors completed the work according to approved designs.

Results

Throughout the process, GeoEngineers worked closely with the City of Spokane, KPFF (design lead) and Parametrix (construction lead) to complete this important piece of infrastructure. The new bridge successfully links the University District to Sprague Avenue and will be a hub for future walking paths and additional development.