Geotechnical Aspects of the Seismic Safety Assessment and Retrofit of the Knight Street Bridge

摘要

The Knight Street Bridge, which was built in 1974 over the Fraser River, connects the City of Vancouver to the City of Richmond via Mitchell Island. The bridge is approximately 1500 m long and includes three distinct structures: the south channel crossing, the north channel crossing, and the Marine Drive overpass. The subsurface conditions at the north and south bridge structures are very different. The north side is underlain at shallow depth by dense till-like soils and the south side is underlain by deltaic sediments comprising potentially liquefiable Fraser River sand and marine silt, which increase in thickness from north to south. The bridge abutments and the piers on the north side are supported on spread footings and the piers on the south side are supported on timber and steel pipe piles.The seismic performance of the bridge under the design earthquakes was assessed and a number of retrofit measures were recommended for the seismic upgrade of the bridge. Seismic soil-structure interaction and deformation analyses were conducted using the computer program FLAC to assess the seismic withstand of the bridge and to design and optimize retrofit measures. Recommended geotechnical retrofit measures included ground densification schemes at three abutments to improve potentially liquefiable Fraser River sand and addition of steel pipe piles at a pier adjacent to the riverbank. The seismic upgrade of the bridge was completed in November 2011. This paper presents a summary of the geotechnical assessment and retrofit design, and describes the key challenges faced in the assessment and design.