Microstructural interpretation on reliquefaction of saturated granular soils under cyclic loading

It is well known that the resistance to liquefaction of a saturated sand de creases sharply when it has been presheared, either cyclically or quasi sta tically, beyond a threshold value. The possible mechanism is discussed in l ight of recent findings on the microstructural anisotropy developed in pres hearing (induced anisotropy). A columnlike structure, through which applied stress is mainly transmitted, grows parallel to the major principal stress direction in the strain hardening process. Voids, randomly distributed at first, are also connected in series between the columnlike structures. The anisotropic structure can carry the increasing stress as long as the major stress is applied parallel to the elongation direction of the structure. Ho wever, it becomes extremely unstable when the major stress is rotated. The excess pore-water pressure increases markedly under undrained cyclic loadin g, particularly when the connected voids are stressed perpendicular to thei r elongation direction. This is the reason why once liquefied sand sharply loses liquefaction resistance in a subsequent reliquefaction faction.