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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

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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).

location map for Treasure Island
Figure 1. San Francisco Bay region. Treasure Island is northeast of San Francisco, in central San Francisco Bay. Star labeled "USGS" marks location of Western Region headquarters in Menlo Park.

airphotos of Treasure Island—1937 and 1989
Figure 2. Aerial views of Treasure Island (A) during construction in 1937, and (B) at the time of the Loma Prieta Earthquake in 1989. The test site is located at 9th and H Streets.
In collaboration with researchers from the University of California San Diego and Brigham Young University, members of the Western Region Coastal and Marine Geology Team are developing a new, non-destructive technique that uses ground-penetrating radar (GPR) to determine changes in soil density due to liquefaction.

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.

setting up the GPR system
Figure 3. Rob Kayen and Diane Minasian, deep in the muck at Treasure Island, handle the transmitter and receiver antennas for a cross-hole GPR survey. The blast charges were placed in a circular pattern between the steel piles and boreholes and were buried approximately 3.5 m below the ground surface.
High-frequency GPR signals (100 MHz) were transmitted between the boreholes along a large number of pathways of known distance, and one-way travel times were measured with picosecond-level (10-12 s) precision. Tomographic analysis of these data determined the velocity structure of soils in a process similar to a medical CAT scan. GPR velocity is very sensitive to changes in volumetric water content and, because the soils are fully saturated (all void space is filled with water), the velocity tomograms are used to estimate porosity or void ratio.

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.

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