Climate change is expected to cause more frequent and severe water-related disasters. Although dams effectively control floods in watersheds, the number of suitable dam sites in Japan is limited. Therefore, there is a growing need to increase the flood control capacity of existing dams; thus, the need for dam rehabilitation is increasing. Among possible measures, dam heightening is an effective means of dam rehabilitation. This study examines the Shin-Katsurazawa Dam, which is a case of coaxial heightening of a concrete gravity dam. (1) The mechanical properties of the existing dam concrete, which is 67 years old, were determined based on the results of material tests of core specimens. The validity of the test results was confirmed through a reproduction analysis of the dam during an earthquake. (2) It was clarified that concrete placement at relatively high elevations should ideally be planned when the reservoir water level is relatively low if the reservoir is to be operated all year round. (3) The seismic behavior of the dam after heightening was reproduced, and the physical properties of the existing dam were identified. Based on the results, the mechanical properties of the heightened dam concrete did not reach those of the existing dam concrete due to the young age of the material. (4) After heightening, the Shin-Katsurazawa Dam was predicted to have a greater external stability in terms of sliding at the dam base than the existing dam.

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