Microfabric effects in frozen clays in relation to geotechnical parameters

The thermodynamic conditions within frozen ground at temperatures just below 0 degreesC cause continuing translocation of water, ice and the displacement of mineral skeleton. The consideration of a wide range of micro-fabric transformations after freezing provides useful clues to the understanding the mechanism of void channel formation, which leads to increased permeability and changes in the shear resistance of soils. Microstructure observations, frost heave, water intake, permeability and shear strength measurements revealed the dynamic nature of cryogenic alterations. This paper deals with the effects of freeze-thaw on microstructure of clayey soils and soil-bentonite mixtures in relation to changes of the permeability and the shear strength. The SEM analysis was employed to study directly cryogenic alterations within clayey soils particularly close to the shearing plane and after permeability tests. The results show that the modifications of fabric are progressive and occur before their transition to the frozen stage and in the negative temperature range. Aggregates of fabric created by ice lensing are generally stable. A very significant increase of the permeability was observed after freezing and thawing. In this study, the magnitude of changes measured was in the range between two and three orders the original value. The shear strength parameters of unfrozen samples and frozen close to the freezing front do not vary significantly. The temperature effects on the shear strength are mainly reflected in cohesion. (C) 2002 Elsevier Science B.V All rights reserved.