Yokohama Bay Bridge is planned to be a Double-deck cable stayed bridge with the center span of 460 meters as shown in Fig. 1. Each deck carries 6 lanes of traffic. Tower foundations and side pier foundations are multi-column type. Each column is constructed by sinking a cylindrical open caisson of reinforced concrete which has the outside diameter of 7 meters. Nine caissons are a assembled for the tower foundation and 6 for the side pier foundation. Fig. 2 shows the structure of the tower foundation. The open caisson is set in place from the barge, which will become the footing when completed, and sunk through the alluvial clay stratum spreading from about 15 to 65 meters under the sea. Finally, it is Penetrated into the soft mudstone bed by 12 to 15 meters.
This paper describe the development of a new type of excavation machine for the caisson of 10 metere in diameter, which can excavate soft mudstone of 40 kg/cm in uniaxial compressive strength in the deep water.
Table 1 shows various types of large diameter boring machines manufactured and used in Japan at present. According to the actual performance record of these machines, the maximum drilling diameter by single-pass is 4.4 meters. In developing a new boring machine of the same type as those listed in Table 1, only the drilling diameter being 10 meters, various difficulties concerning machine weight, required Power, muck lift system and so on are expected to be encountered.
(figure in full paper)
Large diameter boring machines are generally used with other auxiliary equipments such as muck lift device and service cranes. In some cases, those auxiliary equipments become too large for the work on off-shore scaffoldings. Fig. 3 shows the relation between the drilling diameter and the total required power for boring. Estimating from Fig.),the total power required for boring of 10 meters in diameter is 4000 PS by single-drive and 2000 PS by multi-drive. These values are considered to be too large for the off-shore boring in a limitted work space. The value of the newly developed boom type underwater excavation machine is also plotted in Fig.) for comparison.
In the caisson sink procedure, full-face inner excavation at the cutting edge is necessary before loading the caisson head by hydraulic jacks upto 4000 tons. The machine for this excavation must have the capability to meet the requirement summarized as follows.
The machine body can go through the caisson whose inside diameter is 7 meters.
The maximum excavation diameter is 11 meters. That is, expansion of drilling from 7 to 11 meters is necessary.
Excavated rock is soft mudstone whose uniaxial compressive strength is about 40kg/cm2.
The maximum depth of the excavation surface is 100 meters under the water level.
The whole excavation work must be carried out on the barge whose area is 54 meters by 56 meters for tower foundations and 34 meters by 54 meters for side pier foundation.
