Masayuki Nagai

Study on Cementation Change of Sand Induced by Liquefaction

Hirofumi Toyota

In the 2011 off the Pacific coast of Tohoku Earthquake, a lot of liquefaction and reliquefaction occurred because of the long duration of strong ground motion and the effects of aftershocks. According to the damage reports, repeated liquefaction phenomena (referred to as reliquefaction) were confirmed at the same point in which liquefaction occurred in the past. In some areas, liquefaction was observed in smaller vibrations than in the past. However, reliquefaction is difficult to occur because once liquefied ground becomes dense with drainage. Those mechanisms have not been completely clarified yet.
This study focuses on aging effects on liquefaction strength. The purpose of the study is to compare the liquefaction characteristics between liquefaction and reliquefaction using the specimens reproducing aging effects, and to reveal the mechanism of reliquefaction. The specimens reproducing the aging effects were created by addition of cement to the specimens based on the previous studies. In this study. Then, variation of cementation in the specimen during liquefaction was investigated using sand and cement mixed sand (cement content 0.3, 0.5, 1.0%). Saturated cyclic undrained triaxial tests (liquefaction tests), bender element tests (BE tests), and local strain measurement tests (LSS tests) were conducted to estimate mechanical properties of cement mixed sand．In the previous study, the first liquefaction was defied when 5% double axial strain was generated (referred to as large liquefaction degree). In this study, the first liquefaction was defined when p’=0 kPa was achieved (referred to as small liquefaction degree). Then effects of liquefaction degree were investigated comparing the results between from small liquefaction degree and from large liquefaction degree.
The following results were obtained from the viewpoint of cementation changes induced by liquefaction using specimens reproducing the aging effects.
1. In the test with large liquefaction degree, reliquefaction resistance was smaller than liquefaction resistance.
2. In the test with small liquefaction degree, reliquefaction resistance was smaller than liquefaction resistance in sand. However, reliquefaction resistance was larger than liquefaction resistance in cement mixed sands.
3. By applying a liquefaction history to the specimens, the shear modulus tended to decrease as the cement content increased.
From the results described above, the liquefaction degree is also important for estimating reliquefaction.