Laboratory and Field Hydraulic Conductivity of Three Compacted Paper Mill Sludges

Hydraulic conductivities of three compacted paper mill sludges were measured in various ways to assess their viability for use in barrier layers in landfill final covers. Compaction tests showed that the sludges have compaction curves similar to those for clays, albeit with lower maximum dry unit weights and higher optimum water contents. Hydraulic conductivities less than 1 × 10⁻⁹ m/s can be attained for these sludges at low effective stresses (<10 kPa) when compacted using standard Proctor energy if the molding water content is 50–100 percentage points greater than optimum water content. The lowest hydraulic conductivities were obtained in this range. At higher effective stresses (>20 kPa), hydraulic conductivities less than 1 × 10⁻⁹ m/s can be achieved at higher molding water contents. Field tests conducted on barrier layers constructed with two of the sludges showed that field hydraulic conductivities can be obtained that are similar to those measured on laboratory compacted specimens prepared at the same molding water content. Laboratory tests on large and small undisturbed specimens removed from the field showed that no scale dependence existed in the hydraulic conductivity of the field compacted sludge. Additional tests showed that freezing increased the hydraulic conductivity of two of the sludges, regardless of whether the sludges were permeated between freeze-thaw cycles or only after the last thaw. In contrast, for the third sludge, increases in hydraulic conductivity only occurred if the sludge was not permeated between freeze-thaw cycles. Significant shrinkage and cracking of the sludges occurred when they were dried, suggesting that barrier layers constructed with sludge should not be permitted to desiccate. Long-term tests showed that the hydraulic conductivity remains stable or decreases slowly if permeation is continued over an extended period of time.