P061

Immersed Tunnels in Japan :Recent Technological Trends

A0138 Keiichi Akimoto Kinki Regional Development Bereau, Ministry of land, Infrastructure and Transport

A0139 Youichi Hashidate Development Construction Department, Cabinet Office, Okinawa General Bereau

A0140 Hitoshi Kitayama Kobe Ports and Airports Engineering Investigation Office, Kinki Regional Development Bureau, Ministry of Land, Infrastructure and Transport

A0141 Kentaro Kumagai Kobe Ports and Airports Engineering Investigation Office, Kinki Regional Development Bureau, Ministry of Land, Infrastructure and Transport

1.Introduction

In this paper, the overall structure of an immersed

tunnel, the structure of sunken elements and technologies for

structural joints and other elements are discussed, and development

of immersed tunnels in Japan are reviewed.

2.Overall Structure of the Immersed Tunnel

It has been the popular constructing method for many immersed tunnels

in Japan that caissons are placed at both ends of the immersed tunnel.

Recently a new technology was developed to counter the subsidence

of the vertical shaft. It decreases the subsidence of the vertical shaft by using

the less weight vertical shaft separated from the ventilation

equipment. At the construction of the Yumeshima Tunnel, which is

being constructed in Osaka Bay area, the latest constructing method

in which an underwater tunnel directly connected to a land tunnel

was adopted.

3.Structure of Submerged Element

RC structures have been adopted for submerged elements in most cases

in Japan. For Sakishima Tunnel in Osaka Bay, which was put into

service in 1997, these steel shells were utilized as structural elements

for the first time in Japan and integrated into a submerged steel-composite

structure (commonly referred to as 'sandwich structure').

At Minatojima Tunnel in Kobe Port, which was put into service in 1999,

this structure was improved further, and its structure was designed as a full

sandwich structure in which the upper floor plate, the side wall plate and

the outer and inner shell plates for a separation wall were

evaluated as a structural elements. This enables us to cut off

construction costs and shorten the construction period further

compared to the open sandwich structure. For the Naha Port Immersed

Tunnel, also, a variation of this full sandwich structure (in which

all members including its lower floor plate are incorporated into it)

is being designed, due to the fact that concrete will have to be

cast underwater.

4.Joint Structures of the Submerged Element

(1) Joint Structure In Japan, flexible joints based on rubber gaskets

and connection cables are generally used to deal mainly with

subsidence caused by compressed viscous soil. Recently, however,

gaskets' capacity, in response to this movement is being improved

by making them larger. Furthermore, to deal with an opening in

such a joint generated by earthquakes, steel bellows joints are

used in the Naha Port Immersed Tunnel.

(2) Terminal Joint Structure Recently in Japan, the terminal block

method and the V-block method (see Fig.2), which require virtually

no underwater work by divers, have been generally adopted. In the

V-block construction method, peg-shaped blocks are inserted into a

terminal joint, and the joint is closed by its own weight and the

pressure difference between the upper side and the lower side of a

block.