We found that the leakage from the Yashio dam reservoir which flowed along the network consisted of open fissures with high dip angles which were found in deeper areas, more than 500m in depth, via the investigation of the photos taken in the borehole or the tracer. Based on the boring core investigation and studies of the topography formation processes, we estimated that these open fissures with high dip angles were generated by hydrothermal alternation and large-scale mass rock creep at the inner wall of the caldera. At present, we are conducting the grouting at the bedrock of the bottom of the reservoir filled with water. We decided to try the grouting method, because we considered that it would be impossible to make a grout curtain at more than 500 m in depth, and we expected that the groundwater flow (velocity of fluid was about 1 cm/sec over(MAX about 4cm/sec )) would carry the grout material a long distance. We confirmed that this grouting method was very effective and that leakage was decreased by this grouting. In accordance with these studies, we are conducting the grouting at the bedrock of the bottom of the reservoir filled with water. In this paper, we will report on this situation of the grouting with an evaluation of the decrease in the leakage.

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The measurement for safety management of embankment dams of exterior deformation is one of the most important measuring items. The number of the completed dams, recently in Japan, has been gotten more and more, but the cost for the management and the operation of completed dams should be reduced. Therefore, the rationalization of the instrumentation for dams is strongly required. The global positioning system (GPS) is a system that can perform surveys quickly in relatively low cost. GPS has already been installed at many rockfill dams in Japan, as a measuring method of their exterior deformation. For the purpose of the effective utilization of GPS as a measuring method of exterior deformation of embankment dams, we study and propose a method of selecting important measuring points for exterior deformation.

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Concrete gravity dam frequently suffers thermal stress cracks, if measures are insufficient. There is possibility that these cracks affect its serviceability, and hence it is important to understand which factor is significant for the occurrence of the thermal stress cracks. For this purpose, parametric analysis of thermal stress of concrete gravity dam was conducted. Firstly, three-dimensional FEM thermal stress analysis was conducted, and validity of two-dimensional FEM thermal stress analysis was checked by comparing its result with the result of two-dimensional FEM thermal stress analysis. Two-dimensional FEM thermal stress analyses on various conditions are conducted, and the factor analysis exerted on thermal stress is carried out. Moreover, examinations of the effective measures against thermal stress crack control were also performed.

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