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

在日本西南诸岛广泛分布着珊瑚礁、琉球石灰岩等钙质沉积物。这些沉积物包括较强的胶结层和较软的未胶结物质。从岩土工程的角度来看，这种情况对这种沉积物上的地基性能是一个问题。钙质沉积物上的桩基宜采用足够厚度的较强胶结层作为桩基础的持力层，而非松散的非胶结层。在这项研究中，起源，沉积物的结构和其他因素参与这些沉积物的基础，并指出，有必要澄清非胶结材料的力学性能和了解桩的界面抗剪强度，以采用桩基础上的非胶结层包括琉球石灰岩层。对碳酸盐粉土、碳酸盐砂、琉球石灰岩碎砂和轻胶结碳酸盐砂进行了一系列三轴压缩试验。另一方面，对碳酸盐砂和琉球石灰岩砂进行了恒压和恒容直剪试验。结果表明，非胶结钙质沉积物在压应力作用下表现出较大的压缩性，在剪切作用下表现出明显的收缩体积应变。在这种可压碎的沉积物中，打入桩的低轴承载力是由于收缩体积变化引起的桩与未胶结的碳酸盐材料之间的径向有效应力减小而导致的。结果表明，非胶结钙质材料与钢的摩阻力可由定容试验结果来评价。注浆对提高打入桩的承载力是有效的。

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.