This manuscript presents the background and editorial history of the special issue on geo-disasters and their countermeasures of Japanese Geotechnical Journal, which contains the technical papers selected from the Proceeding of the 63rd Symposium on Geotechnical Engineering “Research, Survey and Case Report on Geo-disasters and Their Countermeasures”, as the foreword of this special issue.

Numerical simulation tools based on cellular automata and multi-agent systems have been developed to evaluate the arrival hazard of simultaneous sediment flows over wide areas during heavy rainfall. This paper discusses the validity and applicability of the developed tool by conducting a parametric study using simple slope models and a replicated analysis of an actual landslide disaster during heavy rainfall. A case study is also presented in which the sediment arrival hazard is evaluated at several canyon outlet points.

Strong and resilient levees have been required to be developed because of many levee failures caused by heavy rainfall in recent years. The authors have proposed a shell-type capillary barrier (CB) soil system which consists of crushed shell particles overlain by sand layer constructed in the shallow depth of the back slope, to reduce rainwater infiltration into the back slope and soil erosion during flood overtopping. In order to successfully implement the CB soil system into the river bank, the long-term stability of the shell-type CB soil system in a field condition should be well verified. In this study, a test embankment protected by the CB soil system is constructed in the field, then the rainfall and the soil water content both in the CB soil system and the embankment are measured for 1 year and 3 months. It is found that the shell-type CB soil system reduces effectively the rainfall infiltration into embankment and, even after the soil water breaks through the barrier, recovers its function quickly. Also is found that the soil water breaking through the barrier and infiltrating into the lower layer drains downward along the slope into the toe of slope, resulting that the soil water does not infiltrate into the embankment nor increase the soil water content, within the embankment.

In recent years, sediment-related disasters have become more frequent and large-scale, and the adaptability of social systems and disaster prevention/mitigation technologies related to sediment-related disasters to dangerous local residents have become an issue. This paper summarizes the evacuation of residents and safe land use/ land management from the perspective of laws and lawsuits related to sediment-related disasters, which is one of the social systems. Self-help, mutual help, and public help are important for residents to evacuate. Land risks are often resolved through consensus building among land owners, land managers, land developers, real estate brokers, and government officials. Enactment and revision of laws/regulations and contracts effectively prevent frequent sediment-related disasters. Lawsuits /insurance/guarantee/subsidy systems help the victims as a post-disaster process. The author proposes an evaluation method of land risk other than the sediment-related disaster hazard map based on the sediment-related disaster prevention law as a disaster mitigation technology.

Recently, surface collapse of cut slopes often occurs during heavy rains. To prevent this, it is necessary to understand the weathering of the ground and improve the durability of the cut slope. In this study, the applicability of the proposed weathering evaluation method was confirmed based on the results of elastic wave exploration that have continued for 40 to 50 years on cut slopes of expressways. As a result, the thickness of the weathered zone tends to be underestimated when the rock crack develops to the deep part. It was shown that this method can roughly evaluate the layer thickness and stability of the weathered zone, and it can be used as an index for judging management priority.

Seepage failure caused by water flow under high hydraulic gradients of soil structures and natural ground is a mechanical phenomenon that can lead to serious geo-disasters. In this study, a numerical method is proposed to analyze the seepage failure phenomenon in saturated soils. Material Point Method (MPM) is used to simulate large deformation of saturated soil, and Finite Difference Method (FDM) with the Darcy-Brinkman equation is used for incompressible fluid flow incorporating seepage flow in soil. In the FDM framework, advection terms are dealt with the Rational Constrained Interpolation Profile (RCIP) scheme, and a coupled method is developed considering the interaction forces between the MPM and FDM. The developed method was first validated through a one-dimensional seepage and boiling analysis comparing with the theoretical result. Then, a seepage failure analysis around a sheet pile was performed, and the boiling phenomenon could be represented.

Wavelet analysis is a method for producing microtopographic maps from aerial laser scanning data. In this study, we investigated the setting of the scale factor (s) and a visualization method for the wavelet coefficients to extract geological features (exposed rocks, boulders, and small cliffs) that may function as a source of falling rocks. Grid data with a grid size of 0.5 m were used for the study, which were generated from aerial laser survey data and the results of the survey. A correlation between the specific height difference in the range of 1.0–3.0 m and the positive wavelet coefficient was determined. Consequently, based on these results and those of a study on simulated terrain, a visualization method for the wavelet coefficients was proposed. Herein, setting s = 1.0 and 0.5, it was shown that s = 0.5 was appropriate for identifying geological features that may be considered as sources of rockfall.

Extreme rainfall events have been increasing in recent years. Scour is becoming more frequent. It is important to continuously measure the natural frequency, which is one of the stability evaluation indexes of pier foundations, during rising water and to evaluate the progression of scour. Therefore, in this paper, large-scale model tests were conducted to investigate the progression of scouring. The relationship between the degree of scour and natural frequencies was analyzed using a method based on microtremors continuously estimating natural frequencies. In the case of simulated side scour, the natural frequencies showed a linear decrease with the progress of scour. On the other hand, in the case of bottom scour, the natural frequency decreased significantly after the bottom excavation rate (the ratio of scour to pier width in the downstream direction) exceeded 20% to 25%, as the pier deformation and tilt increased toward the side where scouring occurred. When the temporary recovery process of the ground was simulated, the natural frequency showed a recovery trend but remained at about 70% of the initial value.

In this study, we summarize the history of flood disaster occurrence in the Kowakubi, Kushoda, Kariwano and Omagari areas of the Omono river basin in Akita Prefecture, Japan, and use it to predict future large-scale disasters from the viewpoint of time-related disaster prevention. The focal areas of the investigation were selected based on analyses of documents on early modern history and on the history of disasters and lives of the people of these areas. The stratum was examined using a handy geoslicer. Water content and particle size tests, microscopic observations, and radiocarbon dating were conducted on the collected cores and carbides. In each area, the striped layers that might have been deposited by past floods were also identified.

The temporal restoration of the damaged embankment does not fully consider the performance of the damaged embankment, leaving room for further study in determining the need for temporal restoration and the restoration specifications. In this study, the full-scale model of the railway embankment was subjected to rainfall test to evaluate the performance of the damaged embankment empirically. In addition, loading test were conducted at each collapse stage to evaluate the stability of the damaged embankment. The experimental results showed that when the collapse of the embankment was limited to a specific range, the collapse did not progress against loading equivalent to that of a train. The results also suggest that the stability analysis model can reproduce a slip surface of the collapsed size up to the mid-slope of the embankment by using saturated-unsaturated seepage flow analysis to evaluate the water level and degree of saturation in the embankment and to take them into account.

The authors introduced our own weather station in Takasu-cho, Fukui city from July, 2015 and have measured local rainfall continuously to analyze the landslide risk. After evaluating the landslide risk based on locally measured rainfall data, the landslide risk in Takasu-cho has increased significantly during the "Heavy rains of July 2018 (heavy rain in western Japan)", "Heavy rain of July 29, 2021" and “Heavy rain from August 4th to 5th, 2022”. While the “heavy rain of July 2018” was relatively heavy and lasted for a long time, the “heavy rain of July 29, 2021” and “Heavy rain from August 4th to 5th, 2022” are a typical case for localized torrential rainfall in a short period of time. In this paper, the landslide risks were evaluated by plotting snake curves based on locally measured rainfall data for these two completely different types of rainfall. In addition, the methodology to initiate and take early warning/evacuation actions (drive "evacuation switch" and "my timeline"). using not only disaster prevention weather information and evacuation information announced by the government but also landslide risk evaluated by locally measured rainfall data was investigated in detail.

In recent years, measurement devices and monitoring systems involving information and communications technology have been developed and used for slope monitoring in Japan. Although it is possible to measure slope monitoring data during slope failure, systems to determine slope anomalies are also required. In this study, anomalies were detected and verified from slope monitoring data using linear regression model based on machine learning. In this method, the data was predicted from time series of the slope surface strain by eight sensors installed in a slope failure experiment using a centrifuge, and verified with the measured data. For evaluation of the slope monitoring data, the machine learning method used slope monitoring data from normal state during slope stability as reference. When a pattern of data different from the normal state was detected, the slope was considered to be unstable and an alert was issued. Consequently, from the time series change in the number of anomalies detected by the eight installed surface strain sensors, it was confirmed that the anomalies on the slope can be detected before the collapse of the slope.

So many stone walls of Kumamoto castle were seriously damaged during 2016 Kumamoto Earthquake. Many places of these damaged stone walls were rapped with past reconstructed place, therefore it seems that these damaged stone wall place have a close relationship with its ground structure. From the viewpoint of cultural property protection, boring surveys that involve destruction are limited to areas necessary for Kumamoto castle restoration work, and the ground structure within Kumamoto castle area was partially clarified. In this study, microtremor surveys were carried out at points where boring data were insufficient. Then, by supplementing the boring data with newly stratum divisions obtained by these microtremor analysis, we created the three-dimensional ground map that can display the old topography of Kumamoto Castle Ruins. As a result, it was suggested that the Aso pyroclastic flow deposit layer, the thickness of the embankment and the degree of depth from the ground surface to the hard andesite layer affected the earthquake damage of stone walls of Kumamoto castle.

Muddy rocks such as mudstone and shale are prone to degradation over time, as symbolized by the phenomenon of slaking, which can cause problems in the construction and maintenance of cut slope surfaces. Therefore, it is important to predict and evaluate the progress of deterioration of muddy rocks in advance, and to implement countermeasures to ensure the long-term stability of cut slopes. In this study, in-situ investigation, laboratory tests, and X-ray diffraction analysis were conducted on mudstone, which has been a problem for construction of cut slopes due to its age-related deterioration. The results suggest areas of high potential for degradation in correlations such as “ soil particle density ρ_s - effective porosity”, “natural water content w_n - water absorption Q ”, “degree of weathering w_d - natural water content w_n ”, “natural water content w_n - effective porosity”, “natural water content w_n - Q ”, “water absorption Q - uniaxial compression strength q_u ”, etc., suggest areas with a high possibility of degradation.