The optical determination of preferred orientation in clays and its application to the study of microstructure in consolidated kaolin. ІІ

The degree of preferred orientation and fabric homogeneity of a clay depend upon its method of preparation and subsequent history of loading. The degree of preferred orientation can be determined by measuring the birefringence ratio for a thin section of clay. When the fabric is homogeneous this value is shown to be independent of the area observed. For a fabric composed of packets containing oriented clay particles a representative value of birefringence ratio for the bulk clay may be obtained by measuring over an area large in comparison with the size of the individual packet. In order to study the development of preferred orientation in a kaolin subject to one-dimensional consolidation, birefringence ratio measurements have been obtained for specimens prepared by sedimentation with both a flocculating and deflocculating pore fluid. Significant differences were observed in the initial structure. When loaded beyond 0.1 ton / ft.2 the optical data indicated little difference in the preferred orientation of these clays. This was corroborated by the near identity of both compressibility and permeability at particular pressures beyond this level. The birefringence ratio measurements show that for flocculated kaolin prepared by sedimentation the development of intense orientation in this test is virtually complete at a stress level of 0-1 ton /ft.2. Microscopic observations of the initial structure of kaolin remoulded at different water contents and with different pore fluids revealed the presence of packets composed of parallel particles. The consolidation of these clays was studied and shown to be strongly influenced by the mobility of these packets. Reconsolidation parallel to the direction of preferred orientation does not materially alter the fabric for the range of stresses considered here.