Seismic Retrofit of a Riverbank Pier Near Deep Scour Using Drag Struts

September 26, 2024

Agassiz-Rosedale Bridge, built in 1956, carries two lanes of Highway 9 over the Fraser River in BC. The seismic upgrade of the bridge was faced with two issues. The approach spans are founded on short H-piles that lack the capacity to carry additional loads from the upgrade. Strengthening of foundations by driving steel pipe piles and connecting them at pile cap level was proposed as the most cost-effective solution. However, the driveability and pile capacity of open and closedended piles in challenging soil conditions and the impact of pile driving on the existing structure became significant uncertainties. A field investigation and a test pile program with bridge monitoring during pile driving were undertaken to reduce the uncertainties and make design recommendations.

A second significant issue is the seismic performance of the south riverbank pier founded on a caisson with short H-piles. There is a very deep scour hole immediately adjacent to the pier. The pier was predicted to fail under seismic loading due to unbalanced seismic earth pressures caused by uneven backfills. Retrofitting the pier by filling up the scour hole or strengthening the pier by driving new large-diameter piles or installing anchors were not pursued due to uncertainties in their effectiveness, constructability, and environmental impact.

A drag strut solution was selected as a viable option because of its effectiveness, constructability, and minimal environmental impact. This option was to install anchor piles at the adjacent approach pier on the land side and install drag struts to transfer the seismic loads from the riverbank pier to the anchor piles. The anchor piles would also carry additional seismic loads at the approach pier. This solution posed significant challenges during design and construction due to its highly complex seismic soil-structure interaction, and the impact on the existing bridge structure. This paper describes the design and construction aspects of the seismic retrofit.

Thavaraj, T., I. Luk, J. Williams and J. Foote. 2024. “Seismic Retrofit of a Riverbank Pier Near Deep Scour Using Drag Struts,” in Proceedings of GeoMontreal 2024. Resilient Geosciences for Future Generations, 15-18 September 2024, Montreal, QC.