South 180th Street Grade Separation | Tukwila, Washington

With increased Green River Valley traffic, railroad crossings were causing traffic headaches. WSP designed an underpass for the unique soil conditions. 


  • Washington, USA


  • City of Tukwila, WA

With urban sprawl and increased vehicular traffic, the railroad crossings through the Green River Valley south of Seattle were creating headaches for drivers. Commuters, truckers, and shoppers would wait while mile-long trains passed 60 times every day.

Being a major artery, South 180th Street was an optimal site for unsnarling traffic. But it was not ideal for construction of the required underpass due to weak soil conditions, high water table, wetlands, and space limitations due to adjacent developments.

WSP's designers carefully examined the challenges and arrived at a number of innovative solutions, including using a cement deep soil mixing seal. This added concrete slurry to native soils, which strengthened them to prevent water penetration of the underpass. This method was untried as a permanent bottom seal on transportation projects in the United States. However, knowledge of the technique for port and harbor projects convinced WSP's designers to incorporate the method into the design. The solution saved more than $5 million over conventional tremie seal methods and resulted in construction that required virtually no dewatering despite having to excavate 20 feet below groundwater level.