The authors investigated the damage of pile foundations during the 2016 Kumamoto earthquake and carried out simulation analyses to estimate a cause of the damage. We show below conclusions provided by these studies.
It occurred that a building was inclined and became unusable continuously for the damage of pile foundations, though there was little damage of the superstructure by the earthquake. The damaged building had the following characteristics, those were that the kind of piles was pre-stressed concrete piles, and the building was built on the alluvial plain, and there were various damage patterns of piles. These events were common with the 2011 off the Pacific coast of Tohoku earthquake.
To analyze process reached to severe damage, a static analysis model consisted of both piles and ground springs considering nonlinearity was employed, and the lateral load calculated based on observed earthquake motion at pile head and ground displacement were given to this model. The results of our analyses showed that this analytical model was useful for evaluation of process reached to damage and estimation of seismic performance of pile foundations during severe earthquakes.
It was confirmed that the damage did not occur when only load caused by earthquake motion was given to piles, in contrast the results obtained by our analyses almost accords with the actual damage situation when ground displacement by the cause except the earthquake motion was added. Influence of movement of the fault or embankment was thought about as for the prime cause of added ground displacement, but it is necessary to investigate quantitatively in the future.
To secure the performance of piles and continuous use after severe earthquake of the building on particular ground condition like nearby the faults, the consideration for the ground displacement caused except the earthquake motion may be necessary even if it is relatively small building and is not the liquefaction ground.
Results obtained our analyses accorded mostly with tendencies of real damage, and this study showed that axial load and the ground displacement distribution have a strong influence on the damage of each pile. On the other hand slightly difference of each pile in damage situation appeared. In order to explain the difference, analysis conditions must be set individually even if existing in the same pile cap. Therefore for realization of the quantitative method, accumulation of data about actual damage, structural experiments or analyses are necessary.
Judging from obtained results, when seismic designed method based on the condition that design horizontal seismic intensity was decided to 0.6 without ground displacement was employed, it seems assumption on the safe side against severe earthquakes. However, the design using this method may not guarantee safety when ground displacement by the cause except the earthquake motion occur, in particular a close investigation for securing of shear capacity is required.
