Effects of freezing and thawing on the hydraulic conductivity of paper mill sludges used as landfill covers

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Abstract
A major concern in the design of landfill covers and liners that use compacted clays as the hydraulic barrier is freezing and thawing. Paper mill sludges have been used in landfill covers to subtitute for clays as the hydraulic barrier. In this study, laboratory-compacted paper sludges have been subjected to one-dimensional and three-dimensional laboratory freezing and thawing cycles. Freezing and thawing increased the hydraulic conductivity of the paper sludges about one to two orders of magnitude. To determine why freezing and thawing cause an increase in hydraulic conductivity, an evaluation of the effects of freezing and thawing on the macrostucture of paper sludges was conducted. Frozen thin sections of paper sludges were prepared after freeze–thaw cycling and were compared to frozen thin sections of a compacted clay. Macrostructure analysis of the paper sludge and clay thin sections was conducted by using back lighting to reveal the details of ice structure. Analysis of the clay thin sections revealed ice lenses and shrinkage cracks. Ice lenses and shrinkage cracks in sludge thin sections were difficult to determine when the same procedure for compacted clays was used. Key words: freeze, paper sludge, landfill, permeability, macrostructure, thin sections.

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