Liquefaction and soil failure during 1994 Northridge earthquake

The 1994 Northridge, Calif., earthquake caused widespread permanent ground deformation on the gently sloping alluvial fan surface of the San Fernando Valley. The ground cracks and distributed deformation damaged both pipeline s and surface structures. To evaluate the mechanism of soil failure, detail ed subsurface investigations were conducted at four sites. Three sites are underlain by saturated sandy silts with low standard penetration test and c one penetration test values. These soils are similar to those that liquefie d during the 1971 San Fernando earthquake, and are shown by widely used emp irical relationships to be susceptible to liquefaction. The remaining site is underlain by saturated clay whose undrained shear strength is approximat ely half the value of the earthquake-induced shear stress at this location. This study demonstrates that the heterogeneous nature of alluvial fan sedi ments in combination with variations in the ground-water table can be respo nsible for complex patterns of permanent ground deformation. It may also he lp to explain some of the spatial variability of strong ground motion obser ved during the 1994 earthquake.