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Core samples to a depth of approximately 50 m and undisturbed soil samples were retrieved from eight locations in the Saga Plain, Japan, to investigate the effects of the depositional environment and post-depositional processes on the engineering properties of Quaternary clays in the Saga Plain. The engineering properties of these soil samples were measured, and the test results showed that the clays in the Saga Plain have a high void ratio, high water content, high compressibility, high sensitivity, and low stiffness. The one-dimensional compression curves plotted above the sedimentation compression line proposed by Burland, and the compression index was high, which indicated that the clays had been cemented to some degree. The clays have high strength sensitivity and high stress sensitivity, indicating that they have strong structures. It was considered that a considerable amount of volcanic ash from the four large volcanic eruptions that occurred during the deposition of these clays settled into the clays, resulting in a certain degree of cementation, which was attributed as the main driver of the strong structure in these clays. Comparisons of the NaCl concentration, pH, and oxidation-reduction potential (ORP) of the clay samples representing the initial depositional environment and the current ground environment showed that salt leaching occurred in the Ariake clay formation, and that the ground environment changed. When the NaCl concentration was less than approximately 2 g/L, the strength sensitivity of the Ariake clay samples with low-activity clay minerals significantly increased due to the decrease in the liquid limit and subsequent significant decrease in the remolded undrained shear strength. Therefore, salt leaching was probably a main cause of the high strength sensitivity of the clays in the Saga Plain. The depositional environment at some locations changed from an alkaline, reducing environment, to an acidic, oxidative environment, with the potential to change into a strongly acidic environment, which could have deteriorated the strength of the cement/lime-treated clays in the Saga Plain. [ABSTRACT FROM AUTHOR]

The zero load readings of cone resistance (qc) and sleeve friction (fs) measured by a piezocone (uCPT) shift with the ambient temperature. A method of correcting the effect of temperature on uCPT measurements in seabed sediments has been proposed for the case where there is no temperature sensor in the uCPT probe. This method is based on the assumption that the "actual" profile of fs of soft shallow seabed sediments linearly increases with depth, and a rate of increase of α = 0.2 kPa/m was obtained using measured ground temperatures and fs values in seabed sediments in Isahaya Bay, Japan. An "actual" fs profile can then be constructed using the measured fs value at the shallow surface and the value of α. Using the differences between the measured and the estimated "actual" fs profiles, the ground temperature profile can be obtained, and then the effect of temperature on the uCPT measurements can be corrected. The proposed method was used for temperature corrections on uCPT measurements in Isahaya Bay, Japan. The values of undrained shear strength (su) from the temperature-corrected uCPT measurements agree well with the laboratory measured values of su using the undisturbed soil samples. [ABSTRACT FROM AUTHOR]

In this study, settling tests were conducted to investigate the sedimentation and self-weight consolidation behavior of seafloor sediments from Isahaya Bay, Ariake Sea, Japan. During the tests, the density variations with depth and time were measured by a gamma-ray transmission radioisotope densitometer. The test results show that the settling process of the seafloor sediments can be classified into the flocculation stage, settling stage, and consolidation stage. The settling rate of the seafloor sediments in the settling stage is dependent on the temperature and initial water content, while the settling rate in the consolidation stage is independent of the temperature and initial water content. The density profile changes from a constant density profile to a linear density profile when the sedimentation process transitions to the self-weight consolidation process. The relations between the void ratio (e) and effective vertical stress (p') at very low pressures can be calculated from the measured density values, and this can be used for the analysis of the self-weight consolidation of seafloor sediments. For the seafloor sediments tested in this study, the undrained shear strength (su) values are almost the same when the density values are less than 1.14 g/cm3, and the su values increase linearly with an increase in density when the density values are in the range of 1.14–1.2 g/cm3. [ABSTRACT FROM AUTHOR]

Abstract This paper reviews the recent environmental deterioration in Ariake Sea, Japan, including an increased frequency of red tides and hypoxic waters and decreased fishery production. Analysis of the mechanisms of environmental deterioration suggests that it is possibly induced by the decrease in tidal flat area, decreases in the tide and tidal current and changes in the sediment environment. The Isahaya Bay reclamation project resulted in the loss of 1550 ha of tidal flats, and is one of the possible reasons for the decreases in the tide and tidal current. Therefore, some fishermen and researchers believe that opening the reclamation project dike's floodgates is an effective environmental restoration countermeasure for Ariake Sea. However, the central government decided not to open the floodgates at present due to strong opposition from local farmers, and some researchers believe that the influences of the Isahaya Bay reclamation project on the environmental changes outside of Isahaya Bay are minor. Several lawsuits regarding these relationships and the opening of the dike's floodgates are currently under dispute. To revive Ariake Sea as a sustainable ecosystem, other countermeasures for environmental restoration are suggested and discussed in this paper. Highlights • Ariake Sea has displayed signs of environmental deterioration in recent years. • Possible mechanism is decrease in tidal amplitude, tidal current and tidal flats. • The Isahaya Bay reclamation is one of the potential causes for environmental changes. • Dispute has arisen concerning opening the reclamation project dike's floodgates. • Countermeasures for reviving Ariake Sea as a sustainable ecosystem are suggested. [ABSTRACT FROM AUTHOR]