Compaction Grouting as Part of Seismic Retrofit of Two Bridges in British Columbia, Canada

This paper describes the successful applications of compaction grouting as part of the seismic retrofits of two major bridges across the Fraser River in British Columbia, Canada. At both sites, liquefaction of deep post-glacial river sediments consisting of loose fine sand with trace to some silt is the key issue that affects the seismic performance of the bridge. At Mission Bridge, the south approach piers are founded on timber pile groups. Analyses showed that liquefaction of a 2 m to 3 m thick sand layer beneath the pile toes at 17m depth would cause unacceptable settlements of the piers. Compaction grouting was consequently used to create an annular ring-shaped densified zone beneath the pile foundations. Grout casings were advanced at 9 degrees inclination to reach the loose zone beneath the pile toes, and grouting was conducted with target volume and limiting pressure established based on trials. At Knight Street Bridge, the south abutment is supported on spread footing founded on liquefiable sand. A horseshoe-shaped ground densification zone around the abutment embankment was designed to reduce liquefaction-induced displacements. Timber compaction piles were used for densification outside the bridge deck. Under the deck, because of the limited headroom (similar to 5 m), compaction grouting to 16 m depth was implemented. Cone penetration tests were conducted at both sites to confirm the effectiveness of compaction grouting densifications.