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

Excess pore water pressure
Figure 7. Time histories of accelerations recorded in Test C
Time histories of excess pore pressure ratio, ru, recorded at depths of 15cm and 35cm below the center of the foundation
are shown in Figures 8-10. Comparing these figures with the acceleration time histories indicates that acceleration
amplitudes attenuated due to excess pore pressure buildup since the chronological agreement between the maximum
excess pore pressure and the attenuation of acceleration amplitude is clearly seen from these figures. Maximum excess
pore pressure ratio (ru) was achieved during some initial cycles and remained almost unchanged within the shaking
period. The maximum excess pore pressure ratios in all tests, were almost the same however, the number of cycles
causing this maximum ru was different. The gravel drains increased the resistance against liquefaction and ru reached its
maximum within a larger number of cycles. A similar behavior was observed in test C with the compacted subsoil.
Compaction was able to increase liquefaction resistance more than gravel drains. During shaking, gravel drains were not
able to reduce the excess pore pressure considerably; and changes in the behavior of the remediated ground was
primarily a result of the stiffening effect of the gravel drains.
After the shaking excess pore water pressures at deeper locations started to dissipate, however they increased in
shallower deposits due to the upward movement of water from deeper strata and flows draining from the surrounding far
field soils; such a phenomenon was also observed by Liu and Dobry (1997) as well as during the 1995 Kobe earthquake,
where upward seepage was observed in Rokko Island an hour after the main event (Shibata et al., 1996). This migration
of water may reduce the strength of surface soils and generate "secondary" (or seepage induced) liquefaction, causing
large deformations or loss of bearing capacity (EERC, 1975; Yoshimi and Kuwabara, 1973).
Furthermore the excess pore water pressure ratios show that at any specific depth there was a moment after which, the
excess pore pressures started to dissipate faster. This is the initial period, where vertical dissipation had not had a chance
to get hold on the soil at that corresponding depth and only radial drainage was experienced at that depth.
After shaking the differences in dissipation rates of various tests were remarkable which indicates that gravel drains
accelerated the excess pore pressure dissipation after shaking, showing their effectiveness in non-dynamic cases i.e.
effectively mitigating secondary liquefaction due to the upward flowing water after earthquake. The deeper pore water
pressure used the full drain capacity and overlying deposits waited for the way to be clear. At shallower sections water
left through surface rather than the drain itself. Such phenomenon was also observed by Brennan and Madabhushi
(2002).