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One of the prediction method of the long-term settlement of soft clay is to predict it by defining the linear slope of the latter half of the s-log t curve, as the rate of secondary consolidation. This method has some disadvantages, because the slope of the straight line is depending on how to set the origin of elapsed logarithmic time, and how to determine the initial thickness of soil layer, and so on. The author carried out a series of laboratory experiments using oedometer apparatus against some kinds of highly organic soil and one silty clay which were sampled in Japan. From the experimental results, the author proposed a prediction method for long-term settlement of highly organic soil based on the isotache theory using natural strain which can be used conveniently by eliminating previous problems. To confirm the applicability of the proposed prediction method, some field measurement data were used, constructed in Japan. They were good correlations, and the applicability of proposed method was proved in this study.

Coal had been used as fuel for steam locomotives, and coal cinders (coal waste) had been buried underground for disposal, many years. In recent years, redevelopment projects around train stations have been planned for the land where coal cinders was once disposed of. In the redevelopment projects, buried coal cinders is increasingly being excavated, and the disposal and treatment of the excavated coal cinders are becoming an issue for developers. Excavated coal cinders must be treated and disposed of as industrial waste, not as excavated soil. In other words, even though the soil is excavated from the ground, it cannot, in principle, be used as backfill material to its original location (railroad sites) and is disposed of as industrial waste at a high disposal cost. Moreover, the high cost of disposal, along with the shortage of final disposal sites, has led to an increase in the project cost of redevelopment projects. For railroad operating companies such as East Japan Railways Company, the increase in development costs poses a major problem for the implementation of their projects in Japan. Therefore, the purpose of this study is to reuse coal cinders dumped underground with the backfill material. Laboratory solidification experiments were conducted to determine the initial water content, the cement added ratio, etc., as design parameters for the strength of backfill materials. From the experimental results, the strength characteristics of the improved soil mixed with coal cinders also changed as the various mixing ratios were varied-the possibility of applying the improved soil containing coal cinders as backfill materials were clarified.

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