EXPERIMENTAL-STUDY OF SOIL MICROFABRICS INDUCED BY ISOTROPIC STRESSES OF WETTING AND DRYING

The relationship between texture and microfabric under isotropic stresses generated by wetting and drying was examined. This can be achieved by observing and measuring the reorganization of microfabrics (plasmic fabrics and related distribution pattern, RDP). Microfabrics are produced in the laboratory by mixing montmorillonite with various proportions of sand and silt, and subjecting them to wetting and drying without confinement. Thin sections are cut to study their microfabrics. Results show broad and systematic variations in fabric with changes in texture. Fine matrix (silt plus clay in physical continuity) is developed in samples with low sand:silt ratio, and regardless of this ratio at high clay levels. Coarse porphyric and fine porphyric RDP are formed at low and high clay contents, respectively. Plasmic fabrics are represented mainly by insepic and weakly developed mosepic and masepic fabrics in the matrix in the form of small plasma separations with random orientation, and weak skelsepic plasma as embedded grain argillans. Coarse matrix (sand grains in physical continuity) dominates the sandy samples which are associated with relatively thick and strongly birefringent free grain argillans giving well-developed skelsepic fabric. The RDP follows 2 major paths: beginning with orthogranic (very low clay content) along the iunctic sequence at low clay:silt ratio and along the chlamydic sequence at high ratio. The regularities in fabric development are interpreted in the light of the dynamic properties and behavior of the active clay fraction in a clay-water system. The pedogenetic implications of the results are explored, and the term "erdic" fabric defined.