Shinya Iwasaki

Comparison of liquefaction resistance between triaxial tests and shaking table tests considering sand particle orientation

Hirofumi Toyota

Once loose sandy ground was liquefied, it subsequently became dense with drainage of pore water. Therefore, liquefied ground is considered to be less susceptible to liquefaction. In reality, however, reliquefaction has been frequently reported in the previously liquefied ground. There are some cases, in which reliquefaction occurs under the smaller seismic motion than the previous motions. For example, reliquefaction occurred several times during 2010-2011 New Zealand Earthquakes. Therefore, it is true that liquefaction is likely to occur again. There exist both cases reported, in which liquefaction resistance increases and decreases after the liquefaction. The clarification of reliquefaction characteristics is important issue to be incorporated in the liquefaction hazard assessments.

Based on the above-mentioned objectives, liquefaction resistance was investigated from the viewpoint of particle orientation. In addition to the triaxial tests, the model shaking table tests were conducted to reproduce similar seismic conditions to the actual ground. In the shaking table test, the liquefaction resistance was smallest at a particle orientation of 90 degrees and largest at 0 degrees. Moreover, the particle orientation tends to change 90 degrees in original 0 degrees deposition after the liquefaction. This indicates that liquefaction history changes the ground to be vulnerable to liquefaction, indicating week sedimentary structure. In addition, the particle orientation was measured using the undisturbed samples extracted from the Shinano riverbed in Chuo-ku, Niigata City. The particle orientation of the liquefied layers tended to be concentrated in 90 degree-direction, which was generally consistent with the results of the shake table tests. This result indicates that occurrence of particle orientation induced by liquefaction is an important factor affecting the reliquefaction resistance. Similar changes in particle orientation due to liquefaction were observed in the triaxial liquefaction tests. However, the liquefaction resistance was the greatest in the 90 degree- particle orientation, which was reverse trend with the model shaking table tests. This may be induced by the difference in test conditions, especially, direction of cyclic shear stress. The above results indicate that the liquefaction tests should be conducted under the cyclic stress conditions close to those actually applied in the ground.