Kaolinite-water mixtures, containing from 23 percent to 33 percent water, were subjected to compression plastometer tests and their plasticity was studied by means of the calculated strain-stress functions. X-ray diagrams taken of the compressed samples showed that an oriented texture developed during the compression, i.e., the basal planes of the flaky clay particles were arranged parallel to a plane normal to the direction of the applied stress. For the purpose of discussion the concept of the “plastic unit cell” is introduced. The “plastic unit cell” is a cubic volume element, the faces of which are perpendicular to the three principal stresses. The size of the “plastic unit cell” is very small as compared with the size of the macroscopic sample but large as compared with the size of one clay particle. It is the smallest unit for all considerations where macroscopic strain and stress distributions are involved. For all considerations involving the correlation between the characteristic strain-stress function of the material, the motions of the clay particles and water, and the forces present between the particles, the “plastic unit cell” is the largest unit.

The clay particle in itself is rigid and, therefore, it cannot undergo the same deformation as does the “plastic unit cell”. All of the clay particles present in a “plastic unit cell” together can go through a series of motions (translations and rotations) which, in effect, simulate the deformation of the “plastic unit cell”. In the case of the compression test, the translatory and rotatory movements of the clay particles lead to the development of the oriented texture.