DESCRIPTION: The Portland Bureau of Transportation (PBOT) plans to replace a cantilever half bridge on North Willamette Blvd. and a rubble fill retaining wall on SW Broadway Drive with soldier pile retaining walls to support the roads. The geotechnical objectives for the projects included analysis and design of soldier pile retaining walls on steep slopes, evaluation of the impact of future slope instability on the retaining walls and retained roadway, and development of construction recommendations. Project challenges included wall design on steep slopes with a documented history of slope instability, limited access to the steep slopes, and a narrow right-of-way limits. In addition, North Willamette Blvd. is a major arterial with high traffic speed and volumes.
SERVICE: Foundation Engineering provided exploration permitting, a work plan, and a traffic control plan. The exploratory drilling including conventional borings through the bridge deck and adjacent to an existing retaining wall. Hand-auger borings were used to investigate the near surface soil where it was too steep for conventional drilling. Foundation Engineering worked closely with PBOT engineering to provide geotechnical engineering in support of their structural and civil engineering for the walls. Recommendations were developed for static and seismic lateral earth pressures for the soldier pile walls and an MSE wall option for SW Broadway. To mitigate the potential effect of future instability at North Willamette Blvd., we designed the soldier pile length to accommodate potential future slope failure and wall repair including additional wall lagging and tie-back anchors. For SW Broadway, we provided an allowable increase in the soldier pile axial and lateral loads to accommodate future residential construction.