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The hood opened before pile driving

Driving a pile with the hood closed

Fig. 1 Covered pile driver to minimi2e driving noise (courtesy of the Japanese Association for Steel Pipę Piles)

In Japan, the use of pneumatic caissons began after the great Kanto earthąuake of 1923. To build the Bandai bridge consisting of concrete arches in Niigata around 1930, pneumatic caissons were installed for its foundations. In 1964, Showa Ohashi bridge supported by Steel pipę piles fell down during the Niigata earthąuake, whereas the Bandai bridge did not. Because of this, caissons were highly regarded.

The caissons used for the Minato Ohashi bridge in Osaka are 40 by 40 meters in plan and 35 meters in height. In sinking caissons, pipę piles were driven in double rows with deep wells installed inside the interior row. The groundwater lowered thus madę it possible to reduce the air pressure inside the caisson.

An electrical shovel which can rotate 360 degrees works on rails attached to the ceil-ing of the working chamber.

Fig. 3 shows the cross-section of a pneumatic caisson used for the foundation for blast furnaces for the Ohgishima Steel mili in the Bay of Tokyo. It was 55 meters in depth and 31 by 31 meters in plan. The method of eon-struction is the same as that of the bridge in Osaka. By means of the deep well method, water was pumped out at a ratę of 2.5 cubic meters per minutę, thus successfully reducing the water pressure by 2 atmospheres.

In this country, caissons consisting of Steel pipę piles have been used, in which pipę piles are driven in circular or elliptical or rec-tangular forms with all the heads connected so that they act as a caisson. Fig. 4 shows an example. First Steel pipę piles are driven, and then pumping starts from the inside. After pumping, reinforcement bars are placed, and concrete is poured to construct the connecting slab. Concrete for the bridge pier is next poured on its top. This method ensures safety and the construction Schedule reąuired. It has also been used for foundations of some Steel mili blast furnaces.

Many improvements have been madę in the open-cut method. One of them is called the "Strut Preload" method. After the second stage excavation is completed, the second stage struts are jacked to press the sheet piles outward to such an extent that a space is created between the sheet piles and the foundation soil at the bottom of the excava-tion. Then the third stage excavation is carried out, and the same procedurę repeated until the excavation reaches the reąuired depth. According to this method, a fairly large working space is madę available, the strength of struts is improved, and removal

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