Masaaki Katagiri

Key Issues in the Application of Vertical Drains for Sea Reclamation Using Extremely Soft Clay Slurry

Kitakyushu Airport is located on a man-made island, and opened on March 16, 2006. The man-made island is located on soft marine clay deposits and the island itself was reclaimed from the sea by extremely soft clay slurry with high water content. The application of a variety of ground improvement technologies was the key component of the project from the geotechnical point of view. During the construction of seawalls surrounding the man-made island, ground improvement technologies were absolutely necessary to maintain their stability. In the reclamation phase, the acceleration of consolidation to minimize the uneven residual settlement was of prime importance. When dredged clay slurry with high water content is discharged into a pond, suspended soil particles settle loosely on the sea bottom. The dredged clay layer thus created subsequently settles, largely due to its own weight in the long term. To create a reliable foundation ground for airport facilities, the placement of good- quality fill material over the dredged clay layer and the vertical drainage to accelerate the consolidation of the layer are necessary. For the design of vertical drains in such a soft reclaimed clay ground, the accurate evaluations of the profile of reclaimed clay layer and of consolidation parameters covering a wide stress range are necessary. Based on the authors’ experience, the key issues for sea reclamation projects are pointed out along with the flow of construction. Then, taking the Kitakyushu Airport project as an example, the authors describe the ground improvement employed, the prediction method of reclaimed soil profile, determination of consolidation parameters of reclaimed clay layer, and the results of observation.