Stress, shear deformation and micromorphological clay orientation: A synthesis of various concepts

Microscopic oriented clay bodies known as plasma separations to soil micromorphologists show various degrees of symmetry and regularity in their patterns. They could be regarded as sensitive and reliable indicators of stress-strain regime of a soil. Empirical and experimental evidence signifies the importance of stress, specifically that generated by in situ swelling, in the genesis of plasmic fabrics. Hitherto, most work in this field is confined to geometric characterization, with limited attempts to dynamic interpretations of their origin. This paper enlists appropriate concepts in soil mechanics, structural geology and agricultural engineering to interpret the possible stress-strain components that rearrange fabric units into plasma separations. Some pertinent concepts, including physico-chemical behaviour of the clay-water system, shrink-swell phenomenon, stress-strain, deformation and failure of soil, are employed to explain the origin of plasmic fabrics by mechanical principles. A conceptual model is suggested to synthesize the conditions, changes and consequences related to the formation of plasmic fabrics. Strongly developed plasma separations in natural soils are attributed to the establishment of an anisotropic state of stress generated by in situ swelling, in conjunction with the passive pressure of lateral confinement and vertical overburden. The difference between the maximum and minimum principal stresses produces a deviatoric stress that can, at certain stages of a shrink-swell cycle, exceed the shear strength of the soil. The extra stress is relieved plastically by shear deformation which is manifested as the alignment of platy particles in domain configuration by rotation and translation along the shear planes. Soil mainly fails in compressive (plastic) mode along a large number of small shear planes which form the plasma separations. The formation of different types of sepic plasmic fabrics is discussed in relation to the stress-strain regimes of soils. Copyright © 1990 Published by Elsevier B.V.