Geotechnical and Lifeline Aspects of the October 17,1989 Loma Prieta Earthquake in San Francisco

Soil liquefaction in San Francisco caused by the 1989 Lorna Prieta earthquake occurred at the same locations that liquefaction was observed after the 1906 earthquake. The four main areas of San Francisco affected by soil liquefaction in 1989 and 1906 are the Marina, Foot of Market, South of Market,and Mission Creek districts. Liquefaction effects involved subsidence and loss of bearing of shallow foundations, with differential settlement, racking, and tilting of two to four-story timber structures. In the Marina, deformation was evident on virtually all streets. The pattern of cracking and surface ruptures in the streets did not show a coherent sense of displacement or lateral movement in a preferred direction. Strong ground shaking in the Marina was the principal cause of building damage. The most severe structural damage was concentrated at apartment buildings with mul t"iple garages at ground level. Lateral spreading was observed in the northwestern corner of the Marina Green and docks area, with 0.6 m of horizontal movement toward the bay. One of the most distinctive features of the earthquake was the concentration of permanent ground movements, sand boils, pipeline and surface structure damage at specific locations of the city underlain by Recent Bay Mud. Amplification of bedrock motions through the deposits of soft clay and silt contributed to strong shaking and damage at the surface. Preliminary reconnaissance indicates that the underground infrastructure influenced the pattern of soil and street displacement and may have affected the potential for soil liquefaction in certain locations. Severe differential settlements occurred in the Marina and South of Market areas, where liquefaction and consolidation of loose sands occurred next to pile-supported storm drain and sewage facilities. In the Mission Creek district, the presence of the Bay Area Rapid Transit (BART) system seems to have influenced the pattern of soil liquefaction. Damage in water distribution piping was located primarily in areas of strong ground shaking, liquefaction, and permanent soil displacements. The heavy concentration of MWSS damage in the Marina underscores the importance of site response in the performance of pipeline networks. The recognition that pipeline