applied to such regions as well.
3.3 High-density Air-compressed Transport Method
3.3.1 Development of Method
In dredging and reclaiming Kumamoto Port, it is critical to consider the followoing conditions peculiar to Kumamoto:
−The sea area under construction and its periphery constitute one of Japan,s leading green laver farming grounds, and therefore careful consideration must be given to waste water from dredging and reclamation to prevent turbidity;
−The current disposal yard is not capacious enough to receive the estimated amount of dredged spoil;
−Social conditions needing early putting into service of reclaimed land are matured;
−The entire work including dredging, reclamation and accompanying work must be economical.
Conventional work methods, however, did not meet these requirements. Thus, there was a need for the development of a new method whereby spoil could be transported with the low water content maintained. Hence, 4th District Port Construction Bureau began surveys in 1985 for the purpose of developing the screw feeder method mentioned later in this paper. As a result, the applicability of this method was confirmed in 1987 and 1988 in field tests in the practical sea area using experimental equipment. The high-density air-compressed transport method has been put into practical service at Kumamoto Port in the form of work craft to transport spoil under air pressure, prepared by scaling up and sophisticating the experimental equipment with the debris removing system and work progress control instruments provided.
Capable of transporting spoil as is in high density without adding water thereto, this method features: ?@ that waste water does not occur, thus eliminating waste water treatment facilities: ?A that spoil volumetric change can be minimized, thus possibly minimizing the capacity of reclaimed land; and ?B that the resulting land dries sooner than otherwise, thus permitting early service of the land.
3.3.2 General Description of the High-density Air Compressed Transport Process
This is as diagrammed in Fig. 18. Earth dredged by a grab dredger is carried by a hopper barge to the air-compressed transport barge moored at the pier front. The earth, unloaded by the backhoe into the hopper on the barge, is then transported under air pressure through the pipeline.
The air-compressed transport method applied to Kumamoto is basically classified into the screw feeder type (4th District Port Construction Bureau type) and the tank type. Fig. 19 shows the conceptual diagram of the tank type of air-compressed transport system. In the tank method soft mud is placed into two tanks, wherefrom the mud is alternately discharged with compressed air. Although this system has been in use on only a limited scale for a number of applications, such as lakes, marshes and dams, the system employed for Kumamoto Port is developed by scaling up and further systematizing such a precedent. Fig. 20 shows the conceptual diagram of the screw feeder type of air-compressed transport system. This system features continuous feed whereas the tank method transports mud in batch. Earth unloaded by the backhoe into the hopper is first screened, debris and other inclusions removed. After being stir in the mud tank, the mud is discharged via the screw feeder into the pipeline, then sent to the transport piping. The compressed air fed through the piping inlet forms a plug flow consisting of alternate layers of mud and air, which flow is then compelled by virtue of air expansion, and finally discharged from the outlet. The pressure of air fed via the air feed nozzle is maximally 7 kgf/cm2. The diameter of the pipe used ranges from φ300 to 350 mm and the length is 1 km.
