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CMG Scientists Study Blast-Induced Liquefaction of Artificial Fill in San Francisco Bay and Determine Soil-Density Changes with Ground-Penetrating Radar
Treasure Island is an artificially filled, 717-acre structure in central San Francisco Bay, north of Yerba Buena Island (Figure 1). The island was constructed for the 1939 Golden Gate International Exposition by hydraulically pumping estuarine soil behind a perimeter rip-rap dike (Figure 2). This loosely packed sandy material experienced liquefaction during the 1989 Loma Prieta earthquake. Formerly a U.S. Navy base, Treasure Island was recently turned over to the City of San Francisco and is now home to a National Geotechnical Experimentation Site (NGES).
Walter Barnhardt, Rob Kayen, Diane Minasian, and Brad Carkin performed a series of GPR experiments to estimate the in-situ density of sandy soil, a critical parameter that is typically inferred from standard penetration testing (SPT) and conventional cone penetration testing (CPT).
If a large budget is available, the in-situ density can also be determined from laboratory analyses of frozen samples, or from neutron or gamma-ray density logging. Soil density state and texture have a first-order influence on the liquefaction susceptibility of soils.
Artificial liquefaction events were produced by the detonation of eight explosive charges, 0.5 kilograms each, which were arranged in a circular pattern around a group of test piles (Figure 3).
An array of PVC-cased boreholes, 9 m deep, was drilled adjacent to the blast zone, where approximately 7 m of loose, sandy fill overlie muddy bay sediment. The GPR system was deployed in cross-hole mode, whereby two antennas were incrementally lowered down the closely spaced boreholes.
Identical GPR surveys were collected in the sandy fill before and after blasting. Prior to liquefaction, the average GPR velocity was 0.0570 meters/nanosecond, which translates to an average void ratio of 0.738. Blasting clearly liquefied the soil within the tomographic plane, causing elevated pore pressures, sand boils, and settlement at the site. Average GPR velocity in post-liquefaction soil was 0.0597 meters/nanosecond, which translates to an average void ratio of 0.664. The average void ratio reduction due to liquefaction was 10%. These experiments demonstrate the ability of GPR to quantify the density state of soils in situ, and may yield a new method of assessing liquefaction potential due to earthquakes.
in this issue: Southern California
SF Bay Soil Liquifaction
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