Fluid pathways in the shearing zones of kaolin subjected to direct shear tests

The dilatant microstructure of kaolin sediments, formed during direct shear testing under drained and undrained conditions in normal and overconsolidated samples, was examined using freeze-dried specimens. The kaolin for testing was consolidated after being deposited in a column in 30% salt water. Following the behavior of volumetric strains by shear the overconsolidated kaolin is classified into three types: (1) the increasing-volume type, (2) the roughly non-changing-volume type, and (3) the decreasing-volume type. The microstructure in the sheared zone is characterized as follows: an oriented fabric parallels the shear direction with relatively small voids together with large equidimensional pores similar to channels parallel to the shear direction. This same microstructure appears in all three types of volume change behavior, both overconsolidated and normal consolidated conditions. The experimental results suggest that the dilatancy of the kaolin microstructure causes large pore-like channels and changes in packing of the grains. The large pores were probably formed by the high fluid pressure in the shear zone.