Background / 背景
The 1964 Niigata and Prince William Sound earthquakes raised concerns about liquefaction hazards. However, 2011 Tohoku Earthquake, Japan, and 2010-2011 Christchurch series of Earthquakes, New Zealand, capsized the prevailing researches by putting forward concerns of repeated liquefactions and resistance to initial liquefaction even if soil has shaking histories by small tremors which did not cause liquefaction. Multiple sites had been reliquefied in Japan and New Zealand causing non-repairable damages to infrastructure and properties. Although, reliquefaction was observed in alluvial deposits and reclaimed lands but damage and extent of reliquefaction hazards in Great East Japan earthquake and Christchurch earthquake are alarming.
1964年新潟地震，Prince William Sound地震が液状化被害で代表的な地震であった．しかしながら，2011年東北地方太平洋沖地震，2010－2011年のカンタベリー地震では，複数回繰り返して液状化が発生したことや，液状化が発生しないような小さな振動履歴による液状化強度への影響など，これまでの一般論（一度液状化した地盤は液状化が発生しにくい）を覆すような現象が確認された．これらの地震では，公共インフラや家屋に修復不可能な甚大な被害を与え，特に沖積層や埋立地では液状化が繰り返し確認された．
Liquefaction in 2011 Tohoku Earthquake, Japan
Figure 1
1-G Shake Table / 1G振動台実験
The current research will put a light on the behaviour of sandy soils in model testing apparatus to analyze the effects of pre-shaking (foreshocks) on liquefaction as well as reliquefaction resistance. Since University of Tokyo Geotechnical Engineering Laboratory does not have the facility of Centrifuge Testing Apparatus; hence, 1 – G Shake Table facility is utilized (See figure 1 and Video 1).
砂材料の加振履歴の影響ならびに再液状化強度に関して，モデル試験による検討を行っている．モデル試験では，図１に示すような１ｇ場の振動台実験装置を用いる．
Video 1
Test Results / 試験結果
It has been observed that the effects of pre-shakes not only affect the liquefaction resistance of soil but also the repeated liquefaction resistance. The shear strain development in pre-shaking is peculiar and inferred as another governing factor in controlling the liquefaction and reliquefaction resistance of sandy soils (See figure 2, number of cycles means the liquefaction and reliquefaction resistance and figure 3 represents the formation of model in soil box on the shake table).
実験より，加振履歴は単なる液状化強度だけでなく，複数回の液状化に対する抵抗にも影響することが分かった．加振履歴によるせん断ひずみの発達が，複数回を含む地盤の液状化強度を支配すると推測された．
Figure 2
Figure 3
More Details...
1)Iqbal, M.S., Koseki, J., 2017. Shaking History Effect on Reliquefaction of Sandy Soils in Model Test, Japan Society of Civil Engineers.
