A series of static horizontal load tests and shaking table tests was conducted on a model piled raft and a model pile group at 1-g gravitational field. The behaviours of the model foundations under shaking at their natural frequencies were intensively investigated and compared with their behaviours under static horizontal loading. Principle findings from the experiments in this study are summarized as follows: The horizontal stiffness and the horizontal resistance of the piled raft in the static horizontal load test were larger than that of the pile group. Under seismic loading, the horizontal stiffness of the piled raft and the pile group were almost the same. 1. The pile resistance in the piled raft in the horizontal load test was larger than that in the pile group, due to the increase in the stiffness and the strength of the soil beneath the raft caused by a vertical load transfer from the raft base to the soil. Under seismic loading, the pile resistance in the pile group was the same as that in the pile group. 2. The inclination of the piled raft in the horizontal load test was reduced compared to the pile group, indicating a contribution of the raft to suppress the inclination. Under seismic loading, the raft did not contribute effectively to reduce the inclination of the piled raft. 3. The distributions of the bending moments and the shear forces along the pile shaft were similar in each pile in the static horizontal load test. Under seismic loading, the bending moments and the shear forces of the piles fronted to the direction of displacement at that moment were increased very much compared to the back piles. 4. The magnitude of the horizontal acceleration of the raft of the piled raft was about 2 times that of the pile group under seismic loading at near the natural frequencies of the piled raft and the pile group. Nevertheless, the bending moments and the shear forces of the piles in the piled raft were smaller than those in the pile group. This means that risk of structural failure of the piled raft is reduced compared to the pile group. Many of the above findings in this study are conflicting with the results from Horikoshi et al. (2003). They concluded that the behaviour of the piled raft during seismic loading is comparable with the behaviour under static horizontal loading. In the centrifuge model tests by Horikoshi et al. (2003), the frequency of the input motion was about a half of the natural frequency of the model foundation. Further experimental and analytical investigations on the behaviour of pile foundations under seismic loading at their natural frequencies will be required to establish a seismic design of pile foundations.
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