Journal of the Japan Society of Engineering Geology

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In order to determine geochemical safety route for a high-standard highway in mining area, natural background of arsenic and heavy metals were evaluated, and geochemical risk assessment was performed.
The study area, including two candidates of the routes （A route and B route） were divided into 1km mesh, and one sample from soil layer B was collected from individual mesh. Analyzed elements were arsenic and heavy metals such as copper, zinc, lead and cadmium. Based on chemical composition of the sample, we carried out multivariate analysis to evaluate geochemical risks of arsenic and heavy metals. Correlation coefficient among copper, zinc and lead showed higher than 0.8. Those facts indicate that natural background of heavy metals strongly related into mineralization and ore mineral assemblages. Additionally mercury and arsenic showed high correlation with heavy metals, which was also coincide to natural mineralization. In contrast, manganese has no strong correlation with other heavy metals.
According to multivariate analysis, the 1^st principal factor indicated very high correlation to copper, zinc, lead, mercury and arsenic, and relatively strong relation with cadmium. Those facts suggest that the 1^st principal factor strongly related natural mineralization and it was well indicator to evaluate strength of mineralization.
Two types of distribution maps were created, one was simple concentration map for each element, and the other was factor score due to multivariate analysis. Based on distribution of arsenic and heavy metals and principal factor of the multivariate analysis, we could determine the geochemical safety route, here, the A route was adequate plan to build the high-standard highway. Base on risk assessment and estimation of total amount of excavated rocks, 7.8% of the total excavated rocks should have high risks of heavy metals, and then 7.4% of excavated rocks were recognized as high risk, which was very good corresponded to estimated value.
Geochemical risk assessment is useful assessment techniques to determine low risk route and to estimate total amount of excavated rocks which need treatment of arsenic and heavy metals.

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A new outcrop of active structure was observed at the construction site of Tohoku-Chuo Expressway through the Murayama city, Yamagata prefecture. Active structure running across the newly discovered outcrop is one of traces of the western marginal fault zone of Yamagata Basin, but it had not been described on active fault maps. We observed some outcrops which exposed folding or monoclinal flexure of middle to late Pleistocene deposits. From its continuation and cumulative deformation, we recognized active concealed thrust fault at the eastern margin of the Kawashimayama hill and newly called this fault “Ukinuma Fault”. Ukinuma Fault is concealed one without surface fault, but it has folding and monoclonal flexure. Ukinuma Fault's slip rate is estimated 0.45～0.55m/ky based on age of the terraces and vertical displacements of the fault inferred from hangingwall side's geomorphic profiles and footwall side's geological cross sections. It is important to research not only surface fault scarp but also a distribution and an array of geomorphic features and subsurface geological structure at concealed fault for disaster prevention.

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