South 224th St. Extension and SR 167 Bridge

Kent, Washington is a vibrant and growing city located in the congested Tacoma/Seattle corridor. The city developed around three geographic regions: The Valley, East Hill and West Hill neighborhoods. The Valley is home to Kent’s downtown and commercial warehouse districts, while many residents live on the hills flanking the valley. State Route 167, a busy multi-lane freeway, runs the length of the valley and separates the East Hill neighborhood from the Valley.

For years, geographic and infrastructure limitations have made travel to and from Kent’s East Hill neighborhood difficult. Residents of East Hill must go out of their way to use the only two primary east-west routes across SR 167 into downtown, South 212th St. or South 240th St. Neither could handle the traffic load.

To address these concerns, the City of Kent began an expansive multi-phase effort to complete a new South 224th/228th Street Corridor that would link the East and West Hills through the Valley and downtown Kent. Planning has been ongoing for the corridor for more than 20 years, but the technical challenges of spanning a densely populated valley and freeway were daunting.

GeoEngineers helped the city overcome key challenges in a critical phase of the corridor—a bridge and expansion of South 224th Street across SR 167. The project included a new road alignment at the intersection of two future arterials, surface improvements, wetland mitigation, a stormwater detention pond and a new culvert at nearby Garrison Creek.

When complete, the South 224th/228th Street Corridor project will provide a robust new three-lane artery that connects Kent’s communities and supports community growth in this rapidly expanding region.

Approach

Originally, the City of Kent hired GeoEngineers to perform field investigations, deliver geotechnical recommendations and observe construction, but the firm’s role expanded during the project. As the project encountered unforeseen challenges, GeoEngineers was ready to meet the client’s needs by expanding its scope to include new services and pursuing creative solutions along the way.

Tackling Drilled-Shaft Foundations in Artesian Conditions

The geotechnical investigation encountered unexpected challenges while drilling on the east side of SR 167 to gather data for the bridge approach. Boreholes lost stability and couldn’t continue past a certain depth. The team realized they were likely hitting pressurized water from a previously unknown artesian aquifer. Additional investigations later confirmed this suspicion, measuring strong artesian pressures.

Artesian aquifers are enclosed underground pockets of water under positive pressure. Drilled shafts in such a region can become artesian wells—with pressurized water entering the borehole and even sometimes gushing up to the surface and eroding the sides of the casings, as it did at one of the drill team’s boreholes.

The bridge design called for drilled-shaft foundations, but artesian water pressure within the shaft could create voids in concrete or even cause the base of the shaft to heave during excavation, reducing its effectiveness. Geotechnical engineers also feared that instability of the drilled shafts could cause nearby soil to slough, potentially endangering the adjacent infrastructure of SR 167.

Choosing a new site for the bridge was prohibitively expensive, so to address these critical issues, GeoEngineers recommended a multi-faceted strategy to successfully install drilled shafts to support the east abutment of the bridge over SR 167. The strategy included temporary construction dewatering, maintaining a head of fluid in drilled shafts and using a continuous casing drilling method.

Garrison Creek Culvert and Habitat Improvements

As part of the expansion of South 218th St. on the east side of SR 167, the City of Kent needed to replace an aging timber bridge over Garrison Creek. The new culvert had to adhere to Washington Department of Fish and Wildlife (WDFW) water crossing design guidelines. Fortunately, GeoEngineers has experience working within WDFW guidelines and designing fish-friendly culverts that allow critical fish species to access upstream spawning and rearing habitat. The team’s regulatory and permitting experience helped smooth the process. GeoEngineers’ team recommended a robust 24-foot, pile-supported arched culvert with an expanded flood plain and fish-friendly features.

Evaluating Environmental Contamination at a Former Slag Disposal Site

The City of Kent asked GeoEngineers to provide geotechnical recommendations for a stormwater detention pond near the new intersection of 88th Ave South and South 218th St., the eastern-most section of the new corridor. This site was formerly used for steel slag disposal, and city officials wanted to evaluate the property for possible environmental contamination before the construction could go forward. GeoEngineers stepped in to perform a document review—sorting through available public data and Ecology records.

Results

GeoEngineers overcame highly variable soil conditions, an artesian aquifer, environmental conditions at a stormwater detention site and a rapidly evolving project scope to complete this critical transportation project for the City of Kent. The South 224th/228th Street Corridor will provide a robust new three-lane artery connecting Kent’s East and West Hills through the Valley. In addition to making daily travel easier for Kent residents, the new corridor is expected to improve business opportunities for both new and existing businesses along the right of way and improve fish passage up Garrison Creek.