In order to design embankment dams to resist earthquakes, it is essential to evaluate appropriately earthquake loading and shear strength for use in the seismic stability analysis. Seismic force acting on embankment dams is computed by using acceleration records of actual earthquakes. On the basis of response computations to different input motions having various frequency characteristics, a relationship for the induced maximum acceleration with the frequency characteristics of the ground motion is established. Undrained monotonic and cyclic strength of compacted rockfill and earth-rock mixture are tested and compared to drained strength. The shear strength for use in the stability analysis is discussed based on the experimental results.

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Observed accelerations at embankment dam during earthquakes are compared with calculation results. It is clarified that damping dissipation influences considerably the calculation. Deformation characteristics of rockfill materials are investigated by cyclic triaxial and torsional simple shear tests. Shear moduli are obtained in wide shear strain range including minute strain and examined for the effect of several factors such as confining pressure, void ratio, grading of materials, consolidation stress ratio and loading speed. The comparison of shear moduli from two testing methods are carried out.
Shear modulus is also calculated from shear wave velocity at in-situ geophysical exploration and compared with the results of laboratory tests. Both results are well coincides with each other.

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The existing Tsengwen Dam is located in a seismically active region in the southwest part of Taiwan. The dam is a 133-meter-high zoned earthfill and rock embankment with total volume of over 9 million m3. Crest length is 400m. The gross capacity of the reservoir is 700 million m3. During 1974-75, the Bureau of Reclamation, United States Department of the Interior carried out a special dynamic analysis using the newly developed technology to reconfirm the safety of the dam for normal reservior water elevation of 225. In 1986, 13 years after the completion of the dam construction, the authors had an opportunity to reassess the safety of the dam against the maximum credible earthquake, of which ground peak acceleration at the dam site was 0.45g, under a revised new normal reservior water elevation of 230. For the purpose of reaching more sophisticated and accurate results on the assessment of the Tsengwen Dam during earthquakes than the USBR's previous study, the authors conducted earthquake risk analyses, field investigations, dynamic laboratory tests, dynamic response analyses and safety evaluation of the dam embankment using state-of-the-art technical methods and evaluation procedures at that time. In this paper, the authors shows significant topics and results obtained from the safety assessment of the existing Tsengwen dam.

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