Motion characteristics of compacted earth dams under small earthquake excitations in Taiwan

This paper summarizes the characteristics of small seismic motions of four major compacted earth dams in Taiwan. Seismic data of four major dams recorded in the past thirty years were collected and analyzed. In general, ground motion characteristics can be expressed by peak ground acceleration (PGA), velocity (PGV), displacement (PGD) and predominant frequency of each motion history. The different statistical correlations between the PGA, PGV and PGD have been derived. The correlations are very consistent for different dams. The PGA amplifications from dam base to dam crest were also derived and show that the larger the base PGA, the smaller the amplification ratio, a phenomenon called non-linear effect of geo-material. The fundamental frequency of each earth dam was identified by the ratio of Fourier and response spectra of the seismic records at the dam crest and base. It was found that the larger the base PGA, the lower the fundamental frequency of the earth dam. The fundamental frequencies of the earth dams in direction transverse to the dam axis range from 1.5Hz to 2.0 Hz. It is interesting to observe that the fundamental frequency does not correlate well with the dam height as expected in the 2D dynamic analysis of the earth dam. This may suggest a possibility that a 3D dynamic analysis is required to more reasonably capture the real seismic response of earth dam.