were most appropriately accomplished, the cost will be demanding, and the work period be unavoidably lengthy. Moreover, the removal of soft and weak foundation accompanying the improvement requires the preparation of a yard to dispose of dredged spoil and careful measures against environment pollution.
Thus, the essential task was to develop a special breakwater with economical construction not needing foundation improvement. The outcome was a new type of breakwater, a dike designed to settle directly on soft ground (hereinafter referred to as soft ground dike), well-adapted to the local characteristics of Kumamoto Port. Practical application was successful as well.
The basic principle of the soft ground dike is an shown in Fig. 5. Against horizontal wave force to the breakwater, the dike obtains stability from adhesion between the dike bottom slab and clayey earth layer and from lateral resistance of piles driven down from the slab. This design greatly reduces the dike weight, consequently eliminating foundation improvement even in the case of soft ground, an epochmaking feature of this new development.
A survey was commenced in 1983 to establish the design method for soft ground dikes and to solve various relevant problems. With regard to wave action, hydraulics modeling experiments were carried out, and with regard to the stability and ground characteristics of the structure centrifugal loading experiments and static and dynamic laboratory experiments carried out. Furthermore, a field demonstration was executed from 1985 to 1988, which proved the practical applicability of the new type of breakwater.
3.1.1 Survey and Consideration Results 1) 2) 3) 4)
?@ Hydraulics characteristics
Unlike the conventional type of breakwater, the soft ground dike is installed directly on the clayey ground and has the wide slab at its bottom. It is necessary, therefore, to know the distribution of wave pressure on the dike and wave overtopping and transmission characteristics. Hence, after obtaining basic characteristics of the type with the inversed T-shape top in hydraulics experiments, further tests were conducted with inverted π-shape, permeable breakwaters possessing wave chamber consisting of double walls with a number of holes provided.
It turned out that the inverted T-shape soft ground dike exhibits a wave pressure distribution similar to that of Goda type, reducing vertical wave pressure to zero at the slab front footing whereas increasing wave overtopping in comparison with the ordinary type of breakwater. In the case of the invertedπ-shape, permeable settling dike, it was found that an appropriate wave pressure reduction rate provide the desired horizontal wave pressure distribution when the rate was given as the function of the wave chamber width, front and back wall openings. opening shape and dike top height. Fig. 6 shows an approach to the understanding of design wave force at the peak phase of horizontal wave force.
