Performance of Foundation for Uncemented Carbonate Sea Bed Sediments in the Southwest Islands

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Calcareous sediments such as coral reefs and Ryukyu Limestone are widely distributed in the Southwest Islands, Japan. These sediments include stronger cemented layer and softer uncemented material. This situation is problem for performance of foundation on such sediments from viewpoint of geotechnical engineering. Stronger cemented layers with sufficient thickness are used to be support layer for pile foundation on calcareous sediments expected loose uncemented sediments. In this study, the origin, structure of deposition and other factors involved in the foundation of these sediments are reviewed, and it is pointed out that it is necessary to clarify the mechanical properties of uncemented materials and understand the interface shear resistance on pile in order to employ a pile foundation on uncemented layers included in Ryukyu Limestone layer. A series of triaxial compression tests were carried out on carbonate silt, carbonate sand, Ryukyu Limestone crushed sand and lightly cemented carbonate sand. On the other hand, direct shear tests are performed on both carbonate sand and Ryukyu Limestone sand under the constant pressure test and the constant volume test. It is found that uncemented calcareous sediments exhabit large compressibility under compressive stress and also significant contractive volume strain when sheared. In such crushable sediments low shaft capacity of driven pile results from the decrease in the radial effective stress acting between the pile and the uncemented carbonate material due to contractive volume change. It is shown that skin friction for uncemented calcareous material to steel is evaluated from the test result of constant volume test. It is also suggested that grout is effective for the development of the shaft capacity of driven pile.