In recent years, large inland earthquake damages have been reported in areas where active faults have not been certified, and active fault surveys in the plain have become an issue. The new clues are needed because there is a limit to the discovery of active faults that are not accompanied by ground displacement due to only tectonic relief. Even a slight accumulation of flexure, the thickness of the aquifer may change significantly due to vertical displacement due to active faults, and this is considered to be one of the factors that contribute to the formation of losing stream in the fan. In this study, the geophysical surveys of microtremor survey, two-dimensional microtremor array survey, and voltage difference method electrical survey were conducted at the losing stream site of the Koto River Fan in central Kagawa prefecture, and the hydrogeological structure was comprehensively analyzed. As a result, it was found that the large change in aquifer thickness is likely to reflect the flexural structure of a concealed active fault of the Busshozan fault （tentative name）. In this case, it shows that the losing stream in the fan is a clue to discover concealed active faults.

The pore size distribution within rock materials is an important geometric parameter to control fluid transport through pores and cracks. Measurement via mercury injection capillary pressure is a simple and popular method to obtain the pore size distribution with high replicability. However, the influence of the pressure generation rate on the results has not been investigated strictly. In the present study, pore size distribution, porosity, and permeability calculated from pore size data were investigated for four rock specimens at various pressure generation speeds. There was no significant difference between the results obtained from a continuous pressure generation mode and those from a pressure step increase mode. Even though the low porosity rocks show very low permeability, the volumetric ratio of large pores（>10 µm） was found to be high. The large pores detected by the mercury injection method are considered to reflect the rough surfaces of specimens that are disconnected from fluid flow pathways. The permeability predicted using the modified pore size distribution that disregards surface pores based on the percolation model and the hydraulic radius model is in agreement with the measured permeability. An increase in effective porosity with decreasing pressure generation rate was observed, and the largest effective pore diameter and the representative pore diameter increased with an increase in generation rate for sandstone. This result suggests that it takes some time for mercury to intrude fully into the small pores that are equivalent to intruded pressure. Therefore, lower pressure generation speed is recommended to measure small pore size for low permeable rocks.

Most accidents in public works projects, such as the road collapse during the construction of the Fukuoka City Subway Nanakuma Line, are caused by uncertainties in geological and geotechnical features. The influence that “the geological and geotechnical uncertainty” has on a project is called a “geological and geotechnical risk”. It is necessary to grasp the sources of a geological and geotechnical risk in advance to manage the risk appropriately. However, there are only a few examples that have considered risk sources from this perspective. This report is an analysis of the documents of “the re-evaluation of the public works projects.” First, the authors classified the sources of increase in construction cost and allocated an amount of money for each source. Furthermore, the stage at which the cost increased was divided into either design or construction. When the sources were assumed beforehand, they were divided into estrangement of geological “distribution” or “property.” As a result, the increase in cost because of geological and geotechnical sources was approximately 2 trillion yen. The source that required the most money was “the geological feature that was worse than the assumption.” The analysis revealed that “the estrangement of the geological and geotechnical distribution” appears for a design stage, and “the estrangement of the geological and geotechnical property” appears for a construction stage. Because geological and geotechnical properties present certain uncertainties, it is believed that the width of the property estrangement is larger than that of the distribution estrangement.