Earlier this month, contractors installed 24 girders for the South 228th Street Bridge in central Kent, Washington. This is one of the final milestones in the City of Kent’s ongoing South 224th/228th Street Corridor project, which will provide a robust new east-west artery to connect Kent’s communities.
The 200-foot-long dual-span bridge will carry South 228th street over a set of Union Pacific Railroad tracks and the popular Interurban Trail. In addition to room for four or five lanes of traffic and dual sidewalks, the project includes bicycle and pedestrian connections to the trail below.
The City of Kent’s overall corridor plan has included a number of distinct and challenging projects, including the South 224th Street Extension, which earned GeoEngineers a National Finalist Gold Engineering Excellence Award from ACEC Washington in the transportation category last year. Associate Geotechnical Engineer Lyle Stone and his team overcame unexpected artesian conditions and other complications to help the city complete a new overpass over SR 167 and link South 224th Street with the new corridor, but the route still needed to cross this set of railroad tracks in Kent’s central warehouse district.
“The bridge over SR 167 already makes it much faster to get from my house on the East Hill to downtown Kent,” says Stefani Braicks. “And, once this railroad overpass is open we won’t have to worry about broken railroad crossing arms or slow trains, a real problem here for traffic flow.”
Geotechnical services for the South 228th Street Bridge included recommendations for foundations for the two-span concrete girder bridge, soil investigation, embankments, earthwork and seismic criteria. Settlement of the embankments was a particular concern, as the bridge embankments could put a nearby sewer utility at risk. The initial embankment design called for a lightweight, and expense, expanded polystyrene fill. To find the right balance between mitigating settlement and keeping costs acceptable for the city, Zack Simpson and Michelle Deng of the performance-based engineering unit performed Plaxis modeling. The results confirmed that a smaller amount of polystyrene fill could be used in the embankment, reducing costs and giving the City of Kent the confidence to move forward. This approach was confirmed in the field with a robust instrumentation program developed and implemented by Brett Larabee and Clinton Lindgren.
“GeoEngineers has been working with the City of Kent for 20-plus years,” explains Lyle. “In those years we’ve built significant trust and mutual respect. We were able to continue and build on that relationship with this job.”
When the 228th Street Bridge is completed later this year, it will be the final overpass needed for the South 224th/228th Street Corridor project, which will link Kent neighborhoods from East Hill to I-5 in this rapidly growing region.