Mechanism of Settlement Generation

Clay contains a surprising amount of water. Approximately 70 percent of the soft alluvial clay (clay in the alluvial formation) is water. Even the hard Pleistocene clay (clay in the Pleistocene formation) is 40 percent water.

The weight of the artificial island presses the water out of the clay layers of the seabed, and causes the land to settle. By comparison, pressing a wet sponge pushes the water out of it. The volume of the sponge decreases in relation to the volume of the water pushed out of the sponge. This is the mechanism of the settlement of the artificial islands. The volume of the clay of the seabed decreases in relation to the volume of the water pressed out of the formation in the seabed by the weight of the airport land. Water can be pressed out of a sponge quickly. However, the clay does not allow water to pass through easily. The settlement takes a very long time on the order of tens or hundreds of years. This phenomenon is called "consolidation" in soil property science. The clay is gets harder and harder as the water is pushed out of the clay little by little. The ground settlement stops when the clay hardens sufficiently so that it can support the weight of the land.

The settlement of the seabed plus the sand compression make up the total settlement under the artificial islands of KIX.

How the clay settles down

The Airport Land is Very Heavy.

Clay petrifies when kept in the ground for hundreds of thousand of years. The Pleistocene layers have hardened sufficiently so that traditional reclamation along the seashore would not press the layers down so much. However, Kansai International Airport 's land is offshore reclaimed land that required tons of sand for reclamation. The pre-construction survey showed us that the weight of the reclamation sand that supports the Kansai International Airport land would exceed the limit that the clay layers could support and generate a certain degree of settlement.

Weight Comparison of Reclamation Lands