Name: Masaya Morita

Thesis title: Loss of the over-consolidated effect by liquefaction of sand

Supervisor: Hirofumi Toyota

The loose sands which liquefied once become dense with the drainage after that. 
Therefore, it is thought that the soil which liquefied becomes difficult to liquefy. 
However, the example to which the soil which liquefied in the past re-liquefied in fact has been reported. 
It can be said that it is pressing need to clarify the liquefaction characteristic at the time of the re-liquefaction in a saturation sands in order to use the information on liquefaction by the past earthquake beneficially. 
In considering the liquefaction strength, it is necessary to consider the stress history because the ground is actually considered to have a variety of stress history, 
However, the research on the re-liquefaction in consideration of a stress history is not nearly so far
The experiments were conducted to liquefaction strength ratio at the time of re-liquefaction., RL, using specimens with over-consolidated and shear stress history in the laboratory.
In addition, we have also examined changes in the shear wave velocity ,Vs, which is strongly related to the physical properties of the soil.
The results obtained from the study are summarized as follows:

1.  When normal consolidation sand has the small shearing-strain amplitude in the 1st liquefaction examination, a re-liquefaction strength ratio becomes large, but if shearing-strain amplitude becomes large, a re-liquefaction strength ratio will fall. 
2.  If a re-liquefaction strength ratio does not increase fault compaction sand (OCR=6) with shearing-strain amplitude and shearing-strain amplitude becomes large, an over-consolidated effect will be lost completely and a re-liquefaction strength ratio will fall remarkably. 
3.  Since it is not dependent on an over-consolidated history or shearing-strain history which is not repeated, it can not be used for the evaluation of liquefaction strength considering stress history. 
4.  Even if it is the same density, a liquefaction strength ratio changes with the method of preparing of the specimen
It seems that the engagement of sand particles and anisotropy is involved. 
The sand which liquefied also by the same density shows very small re-liquefaction strength.