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Chantry Flat Road Bridge at MM 2.95 and Retaining Wall Repair Off-System (2020 Bobcat Fire)


Natural Disaster Mitigation Project


The Chantry Flat Road Bridge at MM 2.95 is a 240-long six-span cast-in-place concrete deck bridge located in the ANF. The new bridge spans a segment of the mountain road destroyed by slope erosion stemming from the Bobcat Fire in 2020. The fire had destroyed the existing timber retaining structure supporting the outboard side of the roadway, which caused the erosion that destroyed a 200-foot-long segment of the roadway and prompted a full closure of Chantry Flat Road.

The new bridge acts as a viaduct structure that spans the eroded segment of the roadway. The viaduct structure design was chosen due to the steep slope and the loose and fractured bedrock at the project site. Geotechnical investigations revealed that construction of a typical soldier pile retaining wall may potentially destabilize the slope and would also pose significant safety challenges for construction workers. In addition, in order to achieve an adequate roadway width and slope setback for supporting the roadbed, the retaining wall would have to be located further downslope from the location of the original timber retaining structure that was destroyed. This would increase the required lengths of the soldier piles and the retaining wall would become very costly to construct. The viaduct structure alternative chosen for the final design allowed for all construction work to be performed from the roadway surface and access to the steep slope was completely avoided.