Cyclic torsional simple shear (CTSS) tests and cyclic triaxial (CTX) tests were carried out to investigate the dynamic deformation characteristics of sands and rockfill materials. It was found that the shear modulus and damping ratio can be expressed as a function of shear strain, void ratio, and confining stress. Also the shear modulus in CTSS tests is larger than in CTX tests because of the influence of the intermediate principal stress. When the shear strain is increased, the shear modulus (G) and damping ratio (h) of the rockfill materials were altered at smaller strains than in sands. Key words : sands, rockfill materials, torsional simple shear, shear modulus, damping ratio.

The deformation characteristics of rockfill materials at very small strains were investigated by comparing the results of monotonic and cyclic loading laboratory tests with geophysical P- and S-wave logging data from the field. Using a precision linear variable differential transformer for displacement, the elastic moduli of rockfill materials at very small strains were measured in monotonic and cyclic loading triaxial tests. The laboratory test results agreed well with the field results. The shear moduli of rockfill materials from both a monotonic loading torsional simple shear test and a cyclic loading torsional simple shear test also showed good correspondence. Furthermore, the shear modulus predicted from the in situ shear wave tests in rockfill dams corresponded reasonably well with the modulus in the large-scale triaxial tests in the laboratory. Key words : deformation characteristics, embankment dams, rockfill materials, laboratory test, in situ test.

During the Iwate-Miyagi Nairiku Earthquake (M7.2) of June 14, 2008, seismic motions with a maximum acceleration of 1,024 cm/s2 in the stream direction were recorded at the foundation bedrock of Aratozawa Dam, a rockfill dam located approximately 16 km from the epicenter. However, the maximum response acceleration in the same direction near the center of the dam crest was 525 cm/s2, and the acceleration amplification ratio of the dam body was far lower than that normally considered for a rockfill dam. Furthermore, it was measured that the crest settled down 19.8 cm after the earthquake. In this study, the dynamic properties of the embankment materials were identified using reproduction analysis of past earthquakes, and the recorded behaviors of the dam body during the mentioned strong earthquake were simulated. The generating mechanism of the peculiar earthquake behavior was investigated based on the results of the earthquake response analysis. Furthermore, in order to understand the deformation mechanism, sliding stability analysis and cumulative damage analysis were performed. According to the results, the residual deformation of the dam body after the strong earthquake is inferred to be caused by the shaking settlement of the embankment materials.

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