Ryotaro MATSUMOTO

Development of liquefaction testing method considering principal stress direction

Hirofumi Toyota

A reversal trend of liquefaction strength was obtained in specimens with particle orientations from the triaxial liquefaction test and the shaking table test. This difference will be induced from the different loading method for each test. Cyclic loading tests considering the relations between particle orientation and principal stress direction are necessary to precisely evaluate the effects of anisotropy. The triaxial test can provide only vertical or horizontal direction for principal stress. Therefore, particle orientation of the specimen should be varied in various directions. Then, a hollow cylinder torsional shear apparatus was used to develop liquefaction tests set at a certain principal direction using the specimen having the same particle orientation.

Following four test conditions were used because four external forces can be applied to the specimen: loading at a constant strain rate, constant principal stress direction, constant intermediate principal stress coefficient, and constant mean principal stress. Control for constant hollow external pressure is much easier than that for constant mean principal stress. Therefore, comparative study was conducted in the control between constant hollow external pressure and constant mean principal stress. Toyoura sand was used for the experiment.

The following findings were obtained:
1. Liquefaction tests, which can control the principal stress direction and the intermediate principal stress coefficient, were successfully developed using the hollow torsional shear apparatus.
2. Smoother and less fluctuated results were obtained from the control for constant hollow external pressure rather than that for constant mean principal stress.
3. Monotonic loading test was conducted under a simple shear condition, which is considered to be the closest to actual ground condition. The result indicates that intermediate principal stress coefficient converges to 0.25 with shearing.
4. It is inferred that the greater the principal stress direction is, the weaker the undrained strength becomes.