Several studies have shown that freeze-thaw causes changes in the hydraulic conductivity of compacted clays. Cracks formed by ice lensing and shrinkage cause the hydraulic conductivity to increase. In this paper, changes in hydraulic conductivity are related to changes in morphology. Photographs of thin sections of frozen specimens show that ice lenses form in compacted clay during freezing in a closed system. Photographs also show that similar ice structures are obtained for one-and three-dimensional freezing, which explains why similar hydraulic conductivities are obtained for both conditions. The photographs also show that a significant network of cracks forms in a single cycle of freezethaw. With additional cycles, new ice lenses are created and thus the hydraulic conductivity continues to increase. However, after about three cycles the number of new ice lenses becomes negligible and hence further changes in hydraulic conductivity cease. The temperature gradient and state of stress affect morphology and hydraulic conductivity of compacted clays subjected to freeze-thaw. At larger temperature gradients, more ice lenses form and hence the hydraulic conductivity increases. In contrast, application of overburden pressure inhibits the formation of ice lenses and reduces the size of the cracks remaining when lenses thaw. As a result, the hydraulic conductivity is reduced.Plusieurs etudes ont demontre que le gel-degel produit des changements dans la conductivite hydraulique des argiles compacties. Les fissures causees par la formation de lentilles de glace et par le retrait produisent une augmentation de la conductivite hydraulique. Dans cet article, les changements de conductivite hydraulique sont relies aux changements de morphologie. Des photographies de sections minces de specimens geles montrent que les lentilles de glace se forment dans l'argile compactee durant le gel en systkme ferme. Des photographies montrent egalement que des structures de glace similaires sont obtenues lors d'un gel B deux ou trois dimensions, ce qui explique que des conductivites hydrauliques similaires soient obtenues dans les deux conditions. Les photographies montrent Cgalement qu'un rkseau significatif de fissures se forme au cours d'un seul cycle de gel-degel. Avec des cycles additionnels, de nouvelles lentilles de glace sont creees et la conductivitC hydraulique continue alors a augmenter. Cependant, aprks environ trois cycles, le nombre de lentilles de glace nouvelles devient negligeable, et par consequent, les changements subsequents de conductivite hydraulique cessent. Le gradient de temperature et l'eta; de contrainte influencent la morphologie et la conductivite hydraulique des argiles compacties soumises au gel-digel. A des gradients de temperature plus forts, d'autres lentilles de glace se forment, et ainsi la conductivite hydraulique augmente. A l'opposk, l'application d'une pression susjacente combat la formation de lentilles de glace et reduit la dimension des fissures qui demeurent lorsque les lentilles fondent. 11 s'en...

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Several laboratory studies have shown that the hydraulic conductivity of compacted clay may increase up to three orders of magnitude when subjected to freeze-thaw. In this paper, methods to freeze and thaw specimens of compacted clay are reviewed and compared. Methods to measure the hydraulic conductivity of the specimens are also reviewed. Only naturally formed clay soils are considered; soil-bentonite mixtures and other amended soils are not included. A review of testing conditions present during freeze-thaw and their effect on hydraulic conductivity is also included. Testing conditions that are addressed include availability of an external supply of water (closed vs. open system), dimensionality of freezing (one-dimensional vs. three-dimensional), rate of freezing, ultimate temperature, number of freeze-thaw cycles, and state of stress. The rate of freezing, number of freeze-thaw cycles, and state of stress appear to have the largest effect on hydraulic conductivity. The effect of sampling disturbance on the hydraulic conductivity of compacted clay subjected to freeze-thaw is also presented. Specimens removed in Shelby tubes may be disturbed during sampling and extrusion. As a result, the effects of freeze-thaw can be masked. Collecting block specimens of thawed clay or taking core specimens of frozen clay are suggested as alternative procedures. A method to collect block specimens is presented.

Results of a laboratory study are presented that describe hydraulic and mechanical properties of a compacted compost derived from a mixture of municipal solid waste and treatment plant sludge. The objective of the study was to determine if compacted compost has properties desirable of a hydraulic barrier used in liners and final covers of landfills. Ordinarily, the hydraulic barrier is constructed with compacted clay and/ or a geomembrane. Laboratory tests were performed to determine the particle size distribution, compaction characteristics, hydraulic conductivity and shear strength of the compacted compost. Tests have also been conducted to evaluate: (1) the resistance of compost to changes caused by desiccation and freeze-thaw, (2) the effects of extended permeation and (3) the concentration of contaminants leached during permeation. The results of the study show that compost can be compacted into a dense mass with low hydraulic conductivity (2 x 10-10 m s-1). It is also more resistant to increases in hydraulic conductivity caused by desiccation and freeze-thaw than compacted clay. Compacted compost also has greater shear strength than compacted clay and therefore is likely to remain stable on typical landfill slopes. However, contaminants considered hazardous, such as heavy metals, were leached from the compost at levels exceeding United States drinking water standards. The results suggest that the compost used in this study has hydraulic and mechanical properties desirable of a hydraulic barrier and further detailed study of its use is warranted.