<p>Seismic damages have been observed on concrete piles after recent large earthquakes. Most, pile damages have been found in such a visible change in superstructures as tilting. If no change is observed in a superstructure, it is difficult to know the damage status of piles without excavation survey. Since the survey takes time and cost, it is desirable to judge whether the survey is necessary and/or to specify which piles are to be surveyed in advance. Under this background, detecting methods of the pile damage based on seismic records of the superstructure have been studied. However, they have not been verified through actually damaged piles, and mutual influence between seismic response of the superstructure and damage to the piles has not been clarified. </p><p>To investigate damage mechanism of the piles, a series of shaking table tests were conducted in this study using "E-Defense" facility. A 3 × 2 reinforced concrete (RC) pile group was installed in a model ground consisting of two soil layers of dry and slightly wet sand with different relative densities. The pile group and the model ground were excited by the E-Defense shaking table. The employed input motions were multiple simulated seismic motions, observed seismic records and sine waves with different acceleration amplitude. The pile damage was evaluated from rebar strain measurements with strain gauges, while seismic behavior of the superstructure was measured by accelerometers and laser displacement transducers.</p><p>Inertial force of the superstructure acted on the pile heads as lateral loads, and bending deformation was generated along the pile shafts. Furthermore, rocking motion of the superstructure caused axial load variation in the piles. As a result, multiple cracks and rebar yielding were generated along the pile shafts. Focusing on the axial load variation, the examination yielded the following damage characteristics of the piles.</p><p>1. Near the heads of the edge piles, strain due to axial load variation was generated simultaneously with that due to bending in the same direction on the side facing the structure's center. Along the deeper parts of the edge piles, the same tendency occurs on the side facing the free-field. As a result, these sides become susceptible to damages such as rebar yielding and concrete crushing. Therefore, it is considered efficient to focus on these sides when investigating the pile damage after an earthquake.</p><p>2. The shaft rigidity becomes smaller in the pull-out piles than that in the push-in piles due to damages such as concrete cracking and rebar yielding. The rotational center of the rocking motion then moves horizontally from the bottom center of the structure toward the push-in piles. As a result, the pile cap is temporarily uplifted in conjunction with the rocking motion and responds vertically at twice the frequency of the horizontal response of the superstructure. By observing this phenomenon during an earthquake, it may be possible to detect the pile damage.</p>
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