Change in microstructure of kaolin in consolidation and undrained shear

Abstract

Changes of the microstructure of kaolin during undrained shear are reported. Digital image analysis of both optical and scanning electron micrographs was employed. The preferred orientation, which was originally horizontal, remained such during shear, until the failure plane cut through this laminar structure. The sample anisotropy was initially moderate, and it appeared to increase and later to decrease with strain. The difference in orientation of the individual structural elements seemed to decrease initially and later to increase, this being the complementary trend. The average sizes of all of the structual elements mapped in both types of microscope varied only slightly with strain. These results suggest a new hypothesis that the particles or domains tend to bed down into the horizontal during the early stages of shear, and then they tend to be disrupted slightly during later stages. However, in some samples the structural features identified by optical microscopy became slightly larger with strain increase, whereas in others they became smaller. Thus the effect of strain on microstructure was not the same for all the samples, which were originally similar. This suggests that there may be an element of instability in the microstructural response to strain.