Compaction Grouting for Seismic Retrofit of the North Torrey Pines Bridge

The North Torrey Pines Road Bridge in Del Mar, California was built in 1933 and is eligible for listing in the National Register of Historic Places. As a result of its outdated design and deterioration in a corrosive saline environment, the bridge was classified as structurally and seismically deficient and functionally obsolete. The historic significance of this bridge is important to the surrounding community and thus a seismic retrofit project was initiated with the goal of improving the expected seismic performance of the bridge while preserving its aesthetic and historic character. This paper provides a brief description of the overall retrofit design strategy, and detailed descriptions of the design of compaction grouting ground improvement to mitigate liquefaction and seismic slope instability hazards. Techniques used in the compaction grouting construction are presented, along with some particular construction challenges and solutions. Pre- and post-construction Standard Penetration Test data are compared and the improvements to the soil are discussed. The compaction grouting program was successful in achieving the ground improvement levels required by the design