As in Typhoon No. 19 (Typhoon Hagibis) in East Japan in 2019, flood-induced damage on levees is expected to become more frequent and severe due to climate change and other factors. For such damage mitigation, resilient levees against overtopping are needed. One of the methods to realize this is to create a self-supporting structure in the core of levees using double steel sheet pile walls. However, the resistant mechanisms of such reinforced levees against overtopping from the start to the ultimate state have not been clarified, and design methods have not been established yet. Therefore, we identify the resistant mechanisms through a series of physical model tests on the reinforced levees subjected to overtopping by changing the steel sheet pile specifications (embedment depth and stiffness). The influence of the apparent wall height considering the scouring on the protected side on the deformation and stability of the double steel sheet pile walls is discussed. The test results and findings are expected to contribute to determining the limit states of double steel sheet pile walls in the levees against overtopping.

The safety surveys of large-scale embankments have been carried out in order to promote earthquake resistance of residential embankments. Furthermore, with the enforcement of the Embankment Regulation Law, earthquake safety surveys will be conducted on embankments other than residential areas. The safety of embankments during earthquakes is evaluated using a safety factor based on the "two-dimensional slice method." However, this method has a problem in that when the slip surface has a non-circular shape, the safety factor varies depending on the position of the center point of the moment calculation, which is said to be "arbitrary". This study revealed that the cause of this problem is that the two-dimensional slice method uses the same assumption of static determination as the simple method, so the balance of forces does not hold. Next, from a practical point of view, we proposed the approximate arc method as a method for objectively determining the position of the center point for moment calculation in the two-dimensional slice method. The validity of the approximate arc method was examined by comparing the safety factor determined by the simple Janbu method and the non-arc Spencer method.

Although coastal flooding hazard of sea level rise due to global warming has been evaluated for a long time, this evaluation does not consider land subsidence, which can further increase such hazard. Here we preliminarily evaluated the current and future the ratio of relative sea level rise (RSL) defined as the sum of sea level rise and absolute land subsidence at monitoring sites of reclaimed lands in Tokyo Bay and Osaka Bay using the settlement prediction methods. The results demonstrated that the RSL ratio was estimated as approximately 20 times at one of reclaimed lands of Tokyo Bay for 1980–2020, while the ratios at Osaka Bay were approximately 120 and 3 times for 2000–2020 and 2020–2150, respectively. Our analyses suggested future challenges for land subsidence as improving prediction accuracy of ex-post evaluation model, developing areal monitoring methodology, and collecting past long-term monitoring data by local government in the whole of Japan.

Earth structures such as road, railway, and river embankment have been evaluated for safety based on design guidelines and stability predictions through accurate construction managements. Subsequently, localized and concentrated heavy rains have induced so me damages for earth structures. This study measured the compressibility and unconfined compressive strength for unsaturated silty soil subjected to suction incline, which the suction incline is produced into axial direction in the specimen using relative humidity control. The obtained experimental results are summarized as following: the measured unconfined compressive strengths for unsaturated silty soil with and without suction incline control verify that the suction incline produced in the specimen is s ignificant factor for large reduction of shear resistances. It is required to develop water content distribution and volume change with unconfined compressive strength for unsaturated soils.