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January 27, 2020

Best in state: Gold award
Future Value to the Engineering Profession

HWA GeoSciences

Photo provided by HWA GeoSciences
The design team for Everett’s Grand Avenue Bridge faced poor soils and a steep bluff.

Project: Grand Avenue utility and pedestrian bridge
Client: City of Everett

The Northwest Everett neighborhood surrounding Grand Avenue Park sits along the brow of an 80-foot-high bluff with a commanding view of the Everett waterfront.

When it rains, however, residents’ basements would often flood with raw sewage due to the lack of a combined sewer overflow outfall in the area. To fix the problem, a complex utility bridge would need to be built (which also included pedestrian access) to extend over the top of the bluff and railroad right-of-way below, ending on the west side of West Marine View Drive.

While designing a bridge to convey peak stormwater flows away from the neighborhood and still maintaining ADA requirements for the pedestrian element would be difficult, addressing the geotechnical issues involved would be equally challenging.

At the top of the design team’s list of concerns was the unstable nature of the steep bluff near the eastern bridge abutment and the poor-quality fill — vestiges from land-modifying practices by operators of the area’s historic timber mills long ago — in the vicinity of the proposed western abutment.

At the top of the slope, HWA’s design of two drilled shaft foundation elements to support the eastern bridge abutment was critical. These 6-foot-diameter, 60-foot-deep drilled shafts were strategically spaced, at the top of the slope, to allow the two 36-inch CSO pipes to thread between massive elements and enter at the base of the bridge structure.

HWA designed the shafts to limit future slope instability that might reduce the foundations’ vertical and lateral capacity. To further reduce the potential for slope instability, HWA specified the installation of a wire mesh retaining system across the slope. The wire mesh was anchored to the slope surface with 30-foot-long anchors to prevent the near-surface slope instability observed in the vicinity of the bridge.

The design for the western bridge abutment foundations possessed different challenges. Poor-quality fill and compressible soils required that the abutment be supported on deep foundations. However, the presence of potentially liquefiable soils that were expected to undergo lateral spreading as a result of an earthquake would exert sizable and damaging loads onto the proposed foundation elements.

Rather than trying to resist these large lateral spreading loads with the structure or mitigate the onset of liquefaction with the use of expensive ground improvement, HWA recommended an innovative cofferdam structure that would result in the removal of potentially liquefiable soils.

With the new bridge structure now in place, peak stormwater flows are directed away from the neighborhood.


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