CENTRIFUGE TESTS OF IMPULSIVE VERTICAL ACCELERATION GENERATED BY FOUNDATION UPLIFT DURING STRONG SHAKING

Dynamic centrifuge tests were performed using soil-footing-superstructure models to investigate the effects of the roughness of the soil-footing interface on the uplift and the resulting vertical acceleration of the footings during strong shaking. Four footing models, each of which had a smooth or rough surface and was embedded or not embedded in soil, were subjected to strong shaking. The horizontal shearing and the vertical compressive forces on the base of each footing, as well as the earth pressure and the wall friction forces on the active/passive sides of each footing, were elaborately measured with newly developed 2D load cells. It was shown that (1) the collision impulse of the uplifted footing against the ground induced an extremely high vertical acceleration of the structure, which was much larger than could be induced by the vertical movement of the gravitational center of the structure; (2) the vertical acceleration caused by the collision impulse increased with the induced rotation angle of the footing; (3) if the footing was not embedded in the ground, the rotation angle and the resulting vertical acceleration tended to be larger for the rough footing than for the smooth footing, probably because the larger horizontal sliding of the smooth footing was able to reduce the footing rotation; and (4) if the footing was embedded, by contrast, the rotation angle and the resulting vertical acceleration tended to be smaller for the rough footing than for the smooth footing, probably because the larger wall friction that developed on the passive side in the rough footing was able to reduce the footing rotation.