A Large-Scale Seismic Test (LSST) Program has been conducted at Hualien, Taiwan, to obtain earthquake-induced soil-structure interaction (SSI) data in a stiff soil site environment. Forced vibration tests of the Hualien 1/4-scale containment SSI test model were conducted in October, 1992 before backfill (without embedment) and in February, 1993 after backfill (with embedment) for the purpose of defining basic dynamic characteristics of the soil-structure system. As results, peak frequencies were obtained as 4.1 and 4.3 Hz with twin peaks for both NS and EW horizontal excitation before backfill and as 6.1 and 6.3 Hz for NS and EW excitation respectively after backfill and orthogonal components were rather large. This results suggested existence of principal axes which were different from excitation directions. They were estimated 34 degree before backfill and 30 degree after backfill counterclockwise from NS direction, respectively. After transformed to the principal axes, each resonance curve had single peak and the orthogonal component became negligible. Based on this transformed results, the basic dynamic characteristics of the soil-structure system were obtained, namely, natural frequencies and damping factors. Soil impedances were also obtained by the method minimizing error function defined newly.
