文档介绍：alluvial clay layer (Ac~)in a prescribed period of m at the maximum under the airspace restrictions.
time. The number of drains driven into the ground was as
On the other hand, the restricted ground surface many as 4 millions, their length adding up to a dis-
around the airport set a limit to the height of drain tance of about 80,000 h. At the peak time of the
machines that could be used in this area. As a result, ground improvement works, about 50 units, or 70%
the maximum depth of ground improvement was
of the drain machines available in the country,
limited to AP -28 m. This means that the entire
ground could not be improved, hence a certain grouped in Haneda (). Accordingly, the settle-
amount of residual settlement was unavoidable. ment of the ground was brought to an end in six
Therefore, in constructing the new C-Runway in this months to one year, which otherwise would take
area, it was necessary to provide allowances against 1,000 years. About 50% of allocated budget of the
the possible residual settlement. project was invested in the ground improvement
work
Ground improvement by vertical drain
Different ground improvement works carried out on
the site were: plastic board drains (PBD), small-
caliber fabripacked sand drains (PD) with a diame-
ter of 12 cm, sand drains (SD) with a diameter of 50
cm, and sand drains partially sheathed with geotex-
tile (FPD) with a diameter of 50 cm. These types of
improvements were conducted individually or in
combination, depending on the ground condition that
varied from place to place. shows the ground
improvement method used in the TIA project.
In the sites of Stage-I and Stage-II only Acl layer A forest of drain machines
was improved mainly by plastic board drains. This
type of drain is easy to drive into the ground and also a) Plastic board drains
cost-effective. The thickness of the Acl layer was To reduce the residual settlement of the gr