Most of the previous studies on how shear history affected the re-liquefaction characteristics only considered one direction under plane strain; few studies examined the shear history considering the multidirectional seismic motions.
To investigate the re-liquefaction characteristics of sand subjected to a multidirectional shear history, we developed bi-directional shear test equipment that could conduct simple shear in multiple directions. We chose loading equipment with an electromagnetic clutch, of which a gap on the loading axis is minimize, so that it could perform fine repetition and stopping operations in a precise manner. Further, this apparatus hardly showed backlash when the loading direction was reversed, which enabled liquefaction tests to be conducted precisely.
To quantitatively evaluate the effect of various types of shear history on the re-liquefaction characteristics of sand, we calculated an index based on dissipated energy during shear loading from the relation between the stress ratio and the strain. This index was found to show a rational relation to the re-liquefaction characteristics. This energy index can be divided into two types of component. The one enhanced the re-liquefaction resistance of sand and the other reduced. We found that the number of repetitions of shear loading which causes re-liquefaction can be estimated based on the relation between these two types of components obtained from the shear history.