| A series of blast-induced liquefaction experiments was performed in conjunction with ground-penetrating radar (GPR) surveys to determine the behavior of hydraulically emplaced soils at Treasure Island, an artificially filled structure in San Francisco Bay. Two PVC-cased boreholes, 9 m deep, were drilled adjacent to the blast zone, where approximately 7 m of loose, sandy fill overlie muddy bay sediment. The GPR system was deployed in crosshole mode, whereby two antennas were incrementally lowered down the closely spaced boreholes. High frequency GPR signals (100 MHz) were transmitted between the antennas 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 were fully saturated (i.e., all void space filled with water), we use the velocity tomograms 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 m/ns, 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 m/ns, which translates to an average void ratio of 0.664. The average void ratio reduction due to liquefaction was 10%. The ability of GPR to quantify the density state of soils in situ may yield a new method of assessing liquefaction potential due to earthquakes.
|
