Nondestructive Characterization of Liquefaction Features Using Ground Penetrating Radar

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9412666 Rix The January 17, 1994 Northridge Earthquake (M=6.6) caused liquefaction-induced ground failures in the San Fernando valley, Simi valley, and Redondo Beach areas. Of particular interest were the liquefaction features (sand boils and ground fissures) observed in the abandoned reservoir ("debris basin") upstream of the Lower San Fernando Dam. Free-field accelerations as large as 0.90 g (horizontal) and 0.60 g (vertical) were observed at California Strong Motion Instrumentation Program (CSMIP) stations in the vicinity of the dam. Although sand boils and fissures offer valuable surface evidence of liquefaction, they reveal little about the subsurface nature of the liquefaction. This project uses ground penetrating radar (GPR) tests to image the observed liquefaction features nondestructively. GPR uses electromagnetic waves to image the subsurface, resulting in a representation of the site stratigraphy that is similar to a seismic reflection cross section. The objective is to perform GPR tests along several lines to image the surface along which lateral spreads developed, and along several closely spaced lines to generate a three-dimensional image of a sand boil. It is anticipated that detailed subsurface images of these liquefaction features will be useful in planning subsequent site investigations. The project uses a global positioning system (GPS) receiver to measure the topography of the area of the ground penetrating radar measurements. These GPS measurements allow the radar data to be corrected for topographic effects. ***