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

insitu soil improvement by grouting and chemical stabilization and using of relief wells such as gravel or rock drains for
the control of undesirable pore water pressure. Although these types of mitigation techniques are developed, the
effectiveness of these methods are not well defined and understood (Das, 1983).
All mitigation techniques which are frequently employed to reduce large deformations and subsidence of buildings are
based on the following philosophies:
Reducing the build up of pore water pressure by means of quick drainage of water during and immediately after the
earthquake.
Improving shear deformability of the soil skeleton to prevent large cyclic deformation during the earthquake.
Reinforcing the soil skeleton, which in turn can reduce both shear strain and generation of excess pore water pressure
and increases the soil strength.
One of the widely used mitigation methods is using gravel drains. The possible benefits of gravel drains are densification
of surrounding non-cohesive soil, dissipation of excess pore water pressure and re-distribution of earthquake-induced or
pre-existing stresses (due to introduction of the stiffer columns). When dealing with non-plastic silty soils, only the third
benefit can be expected primarily to mitigate liquefaction (Baez, 1995). The gravel drain technique is ideally suited for
improving soft silts and clays, and loose silty sands. The level of improvement depends on the soil type, installation
technique, relative spacing of the drains, and drain diameter. Crushed stones made of recycled concrete from torn-down
apartment buildings and complexes are suitable alternatives to be used as drain materials (Orense et al., 2003). Gravel
drains operate by providing preferential drainage paths which enable accumulated pore pressures to dissipate ideally
before the surrounding soil reaches a state of initial liquefaction (Brennan and Madabhushi, 2002). One of the first
studies on gravel drains as liquefaction remediation is that done by Seed and Booker (1977). Since this was published,
drains have been subjected to real earthquakes, such as Koshiro-Oki (Sonu et al., 1993), Northridge (Boulanger et al.,
1998) and Kobe (Yasuda et al., 1996). Several shortcomings of gravel drains have been reported in the literature. A
collected experience suggests that while drains can certainly provide a solution, settlement can still occur to an
unsatisfactory degree (Brennan and Madabhushi, 2002).
Regarding aforesaid remarks, in the current study some aspects of the effectiveness of gravel drains and compacted sand
piles in mitigating excess pore water pressure and reducing the subsidence of buildings has been studied using 1g
shaking table tests and following that the experimental results are compared with a numerical method which incorporates
an improved procedure of that used by Seed and Booker (1977).
PHYSICAL MODELING
A series of shaking table tests were conducted on model gravel drains and compacted sand piles. Figure 1 shows a three
dimensional view of the model. Models were constructed in a transparent plexiglass container of 180cm long, 45cm wide
and 70cm high. The bottom of the container was covered by a fine screen mesh so that the saturation process could be
performed by percolating water gradually and uniformly from the bottom of the soil box.