Kansai international airport is built on a soft seabed in deep waters.��For this reason, one of our major technical objectives is to develop measures against ground settlement.
　

Many sand piles - called sand drains - are driven into the soft holocene clay layer just beneath the seabed. Sand drains squeeze water from the holocene clay layer, thus accelerating settlement and promoting the consolidation of the layer.��As a result, the settlement of the holocene clay layer is completed during the reclamation work..

It is difficult to improve the pleistocene clay layers in terms of technology and financing. For this reason, the natural pleistocene clay layers subside after the airport is opened. We will sustain the functions of other facilities through careful maintenance.��For example, a jack-up system will be incorporated into the building pillars to control the differential settlement and maintain the level of buildings.

Reclaimed soil improvement work will be carried out on the surface of the reclaimed layer in order to prevent settlement of the reclaimed layer itself.

By linking advanced GPS and two-dimensional measuring systems to a computer , we can conduct efficient and precise reclamation work and minimize differential settlement.

We have been taking measures against settlement at each step of the 2nd-phase construction project, from planning, design and execution, to maintenance, to sustain the airport facilities.

In 2nd-phase Airport Island settlement prediction, we have fully utilized the observed settlement over the 10-odd years of the 1st-phase Airport Island.��We have also analyzed the results of the 400-m-deep boring exploration and state-of-the-art soil test method, and have asked a committee made up of specialists in geotechnical engineering to assess these results.��Based on these efforts, we predict that the mean amount of seabed settlement from commencement of reclamation work till semi-completion of subsidence （about 50 years after the airport's opening） will be about 18 m.

This prediction method has been attained using the latest technology.
However, settlement is a phenomenon that takes place over a long period of time.��For this reason, settlement for the 2nd-phase Airport Island will be observed with a fully arranged organization.��The observation results will also be reflected in reclamation work and maintenance.

The following, though it may be a bit technical, is an introduction to the settlement prediction method for the 2nd-phase Airport Island, whose construction work has already commenced.

The pleistocene layer comprises many layers of sand and clay, and can be classified into upper and lower parts based on deposition age and differences in properties, such as clay hardness.
The upper pleistocene layer, about 200 m thick, consists of 10 sand layers with 9 clay layers in between.��These clay layers contain much water.��The sand layers that sandwich the clay layers squeeze out the water in the clay layers, due to the reclamation soil of the 2nd-phase Airport Island. This causes settlement. The water drains to the island periphery through the sand layers.��The velocity of settlement is largely dependent on the speed of water drainage out to the island periphery through the sand layers.��When the sand layer is not present in part, the sand layer does not function as the passage of water, causing considerable settlement of the clay above and below .【Image : Settlement of Upper Pleistocene Layer】

The lower Pleistocene Layer is about 300 m thick. The age of its deposition is even older than that of the upper Pleistocene Layer.��Moreover, the lower Pleistocene Layer is hard enough to sustain the reclamation soil of the 2nd-phase Airport Island.��We thus believe that significant subsidence will not take place.

There are 3 key points in predicting such settlement of Pleistocene clay layers:

1)Accurate Measurement of upper Pleistocene clays compressibility

2)Clarification of drainage property of the upper Pleistocene sand layers

3)Appropriate assessment of lower Pleistocene consolidation　

・Deep boring
In order to identify the properties of the Pleistocene clay on the three key points accurately, it is necessary to look into the layer configuration by means of exploratory boring. It is also necessary to take undisturbed clay samples and conducting laboratory tests.
Prior to the 1st-phase Airport Island construction project, the Ministry of Transport carried out as many as 65 boring explorations from 1977 to 1982.��Of these, two were 400-m class; others included 100-m- and 200-m- class explorations*.��In addition, in 1994 and 1995, prior to the 2nd-phase Airport Island construction project, four 400-m-class boring explorations were conducted in an effort to identify the properties of the upper layers of the lower Pleistocene layer.

* In general, boring depth was less than 100 m.��There were only a few boring explorations beyond 400 m in the Osaka Bay area.��Many years of experience and careful work are required to sample clay deep in the seabed while maintaining its integrity.

The settlement prediction method for the 2nd-phase construction project is based on a great stock of data and findings far exceeding that of the 1st-phase construction project. This prediction method has been attained using the latest technology.
Prior to commencing 2nd-phase reclamation work, we installed 2 new derricks for observing the settlement of the Pleistocene clay layers and pore water pressures.��In addition, we installed as many as 37 settlement gauges on the seabed.��Thus, we are equipped with an elaborate observation system.
We will apply an observational construction control system in order to verify the validity of the settlement prediction method and incorporate the results into the reclamation work.