In this study, on-line pseudo-dynamic response tests were conducted on the earthquake response characteristics of alternating layers of clay and sand to precisely investigate the effects of the degree of consolidation of clay on the vibration properties of the alternating layers. The following conclusions were derived from the tests.
The nonlinear time-history deformation characteristics of clay varied depending on its degree of consolidation.
The difference affected the degree of liquefaction of overlying layers, and the strain generated in the overlying layers was the smallest for under-consolidated clay, followed by normally consolidated clay and over consolidated clay in an ascending order.
When a clay layer underlies a liquefaction-prone layer, the clay layer attenuates earthquake motion for short period structures but amplifies the motion for long period structures.
The Michoacan Earthquake, which occurred in Mexico in 1985 (Mendoza et al., 1988), and the Loma Prieta Earthquake, which occurred in 1989 in the US along the San Francisco Bay (Yasuhara.,1999) (Earthquake Engineering Committee., 1990a-c), are typical examples of earthquakes in which soft clayey layers amplified the ground motion and worsened the overall degree of damage. During the latter earthquake, the recorded accelerations on soft ground layers were two to three times larger than those on hard ground (Earthquake Engineering Committee., 1990c). On the other hand, there have been cases in which soft layers acted like dampers and reduced damage, an example of which is the small amount of damage to the Imperial Hotel in Tokyo during the Great Kanto Earthquake (Fukutake., 2001). Another phenomenon that notably attenuates ground motion is liquefaction of sandy layers. Liquefied ground shows drastic drops in stiffness, impedes propagation of shear waves to up-per layers, and thus reduces the earthquake response acceleration.