Figure 3 compares SSG to a plate load test. In both cases, a force P is applied to the soil via a plate or ring. The soil deflects an amount d, which is proportional to the foot geometry, Young s modulus, and Poisson s ratio of the soil. The soil stiffness, as measured by SSG, also relates to shear modulus, void ratio, and density. Figure 4 presents the basic relationship.
Large forces are necessary to produce enough deflection to measure.
SSG uses technology borrowed from the defense industry to measure very small deflections, allowing much smaller loads. SSG does not measure the deflection resulting from the SSG weight. Rather, SSG vibrates, producing small changes in P that produce small deflections. To filter out the deflections resulting from equipment operating nearby, SSG measures over a frequency range. Figure 5 is a schematic of SSG showing the major internal components.