Seismic Behavior of Gravel Drains and Compacted Sand Piles using ...

Experimental results
Acceleration time histories
Figure 4. Boundary conditions used in the numerical modeling (EERC, 1997)
The acceleration responses of the models are shown in Figures 5-7. A very clear reduction of acceleration occurred after
the second cycle in test O (Figure 6) which indicates severe liquefaction and softening of the soil particularly at the
positions of A1 and A2. This kind of softening also happened in test G (Figure 6) after the seventh cycle. The presence
of gravel drains delayed the softening of the soil; however it didn’t mitigate it completely. The larger amplitudes of
acceleration response in test G implies that the seismic shaking was transferred, with some amplification from the base
of the deposit up to the footing by the composite ground of sand and gravel drains. The loss of strength in test G was
larger and faster than that in test C (Figure 7). At corresponding locations, attenuations were smaller and delayed in test
C comparing to tests G and O. This can be attributed to the reinforcing effect of the compacted sand piles. In general,
throughout shaking, model test C behaved in a stiffer manner and the cyclic mobility induced softening occurred
gradually after eight cycles of shaking in shallower depths of A1 and A2. Compacted sand piles due to their dilative
characteristic appeared to be stronger and degradation of their strength was not very significant. The spikier and an
overall stiffer response of the compacted sand piles exhibit a more pronounced cyclic-mobility behavior of the stratum.
This cyclic mobility behavior explains why the accelerations reduce at a later time than those of the unremediated
ground.