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

Figure 22. Excess pore water pressure ratio isopiestic lines from numerical simulation of Test C.
The isopiestic lines of the excess pore water pressure ratios, obtained from the numerical simulations, are depicted in
Figures 20-22, and can be compared with their counterparts in Figures 11-13. Higher excess pore pressure ratios in test O
and lower ones in tests G and C are the effects of the ground remediation. The numerical code applies the overburden
pressure uniformly all over the surface, that's why the contours are not the same shape as the actual ones in Figures 11-
13, especially in tests O and C. However, in corresponding depths the computed excess pore water pressure ratios agreed
very well with the actual ones in test O but in tests G and C they were much less than the real values. In addition the
numerical code was not able to simulate the dilative response and the consequent negative excess pore pressure beneath
the foundation in test C. Besides the pore pressure reduction capability of the gravel drains was expressed by the
numerical simulation.
CONCLUSIONS
A series of 1g shaking table tests were carried out to evaluate the performance of two common ground improvement
techniques, gravel drains and sand compacted piles. For comparison a test with no improvement was also performed to
compare the behaviors. Following the experimental work, these techniques were modeled with a finite element code.
The experiments presented that the improvement provided by the dynamic compaction method not only reduces the
excess pore pressure, but also the stiffer compacted sand piles provided higher overall foundation shear strength and
bearing capacity, preventing settlements better than gravel drains. Settlement was mainly due to migration of underlying
foundation soil towards the free field (lateral spreading) in the tests with sand compacted piles. However with the gravel
drains, the settlement was considerably raised due to the loss of shear strength and punching attained by the shaking
process. Since gravel is a frictional material possessing negligible cohesion, confining pressure applied by the soil is of
paramount importance. Sufficient vertical stress or confining pressure might be required to engage the full reinforcing
effect of the gravel drains. This confinement can be obtained with the weight of the structure and method of installation.
The installation process should embed the drains tightly within the soil matrix, while preventing mixing of the in-situ
soft soil with the drain material. Such contamination not only compromises the strength of the columns, but also reduces
their drainage capacity. Furthermore it was observed that the intensity of shaking was enough to produce liquefaction,
however the excess pore pressure ratio never reached 100% under the center and edge of the foundation due to the static
driving shear stress.
In general, the test results suggest that compacted sand columns as stiffer elements are likely to be more viable solutions
in mitigating liquefaction where the only possible mitigation benefit is from the stiffening stress concentration criterion.