Fly‐Ash Slurry Island: II. Construction in Hakucho Ohashi Project

In the Hakucho Ohashi (Muroran Bay Bridge) project, a man‐made island of 67 m diameter has been made by underwater placement of a self‐hardening slurry on a soft seabed. From October 1988, $53\text{,}600m^3$ of slurry, containing 46.1 Gg of fly ash, 22.2 Gg of volcanic ash, and 2.6 Gg of cement, was filled into a steel‐pile cofferdam during a three‐month period. During this filling work, great advantages of the new method over conventional methods were confirmed: (1) Reduced lateral earth pressure on the cofferdam walls and the resulting smaller bending moment and lateral displacements; and (2) higher strength of the island for subsequent construction work. In spite of low temperatures in winter, heat supplied from the slurry kept the fill temperature above 10 °C, so strength development was higher than expected. From these results, we can conclude that the underwater placing method is a great advance in man‐made island construction and contributes to the creative use of waste fly ash.