Chihiro Tsukada

Effects of cementation failure and densification induced by liquefaction on reliquefaction

Hirofumi Toyota

In the off the Pacific coast of Tohoku Earthquake, it has been reported that there are differences in damage caused by liquefaction and reliquefaction in reclaimed grounds because of effects of aging. According to damage investigation reports, liquefaction was observed not only new points but also the same points where liquefaction had occurred in the past, indicating reliquefaction. However, when the ground has once liquefied, it is difficult to liquefy again because the ground becomes dense through drainage of pore water. Thus, the mechanism of reliquefaction has not been elucidated. Previous study has reported that liquefaction strength and shear modulus increase with adding cement to reproduce aging phenomenon. Furthermore, other study indicated that reliquefaction strength varies with the degree of liquefaction history.
The research purpose is to identify major mechanical properties correlated with liquefaction strength using aging reproducing samples, which are prepared by cement addition to sand. For this purpose, we conducted the mechanical property tests with various strain levels: Shear strength tests are for large strain level (εs=1.0%~), bender element (BE) tests are for small strain level (about εs=0.001%), and local small strain (LSS) tests are for medium strain level (εs=0.001%~1.0%). The results obtained from those three tests were compared with liquefaction strength.
Findings are shown below.
・Densification induced by liquefaction is different by the degree of liquefaction: The relative density increased by 3.0% in the case of a small liquefaction history and 7.5% in the case of a large liquefaction history. However, liquefaction strength does not increase with those densifications. Therefore, it was confirmed that reliquefaction is not related to density but other factors.
・Shear strength is related to both effects of cementation failure and densification.
・Initial shear modulus increases because of cementation between soil particles. Degree of cementation failure depends on degree of liquefaction history.
・The decreasing trend with strain in secant shear modulus of medium strain level depends on degree of liquefaction history.
・Secant shear modulus around 0.005% shear strain is most correlated with liquefaction strength.