Abstract
　Non-homogeneity in sandy ground due to the existence of clay layers is not uncommon. Cracks may be caused by earthquake forces in weak regions within the clay layers, Shaking table tests are performed to clarify the mechanism of sand boiling in such non-homogeneous sandy ground. Dynamic movement of sand particles and water is confirmed by measuring hydrodynamic pressure. A water layer is formed directly under the clay layer during liquefaction producing a water interface that separates the sandy ground. The weight of sand particles that settle on the clay layer causes a pore water pressure gradient around the cracks of the clay layer. As a result, the water directly under the clay layer, together with sand particles, penetrates upward resulting in the so-called sand boiling phenomenon. The water interface thus produced exerts a major influence on the behavior of the ground.

Key words: layered system, liquefaction, pore pressure, sand boiling, soil dynamics.

Budi Wibawa* *Graduate School of Science and Technology, Niigata University,
Hideo Ohkawa**
Takashi Okuma**
**Faculty of Engineering, Niigata University.