Blast-Induced Liquefaction and Determination of Soil-Density Changes with Ground-Penetrating Radar, Treasure Island, CA
The equations  and , above, were used to map porosity and void ratio in the soil prior to, and following, blast-induced soil liquefaction. Prior to liquefaction, radar velocity in the central portion of the soil column ranged between 0.054 and 0.060 m/ns, and an average velocity of 0.057 m/ns (see Box 1 at right). These velocities translate into void ratios ranging from 0.846 to 0.647, and an average void ratio of 0.738. Generally, there is a zone of low to intermediate void ratios in the central portion of the image and a low void ratio zone in the upper left region (see Box 2). A locally high void ratio on the right side of the plane is seen at depths of 2.5-2.75 meters, and also in the lower left corner.
After blasting, GPR velocities had risen considerably throughout the tomogram. The velocity range for the post-liquefaction soil ranged from 0.056 to 0.064 m/ns, and an average velocity of 0.060 m/ns. The void ratios associated with these velocities range from 0.554 to 0.770, and an average of 0.664. By comparing the tomograms, it can be seen that almost the entire tomographic plane underwent some level of densification and reduction of void ratio during liquefaction.
Subtraction of the post-liquefaction void ratios from the initial values produces a difference tomogram (Box 3). On average, the soil experienced a densification (reduction in void space) of e = 0.074. The range of void ratio change spans from -0.066 to 0.172. That is, the entirety of the tomographic plane densified, with the exception of a narrow zone on the left side. That zone apparently loosened during the liquefaction event, or formed a void when sand was ejected to the surface. The average volumetric strain due to void ratio reduction was 4.2%. Given that the estimated thickness of the liquefied layer at the site was 4 meters, this strain would result in 17.0 cm of settlement.
The observed surface settlement at the test site is an independent measure of the volumetric strain. Maximum settlements of 16.8, 17.8, and 20.7 cm were recorded along three transects across the site, and are remarkably similar to the estimated 17-cm settlement derived from radar tomograms.