| This research deals with the shear strength, slope stability and consolidation behavior of paper mill sludges used as hydraulic barrier material in landfill cover systems. Experimental studies consisting of laboratory and field testing, as well as analytical studies for several case histories were performed. In addition, an extensive literature review was done to summarize the major findings of previous researchers working with paper mill sludge landfill covers. The laboratory testing program was developed to assess the geotechnical properties, especially initial or unconsolidated undrained shear strength and consolidation behavior, of three paper mill sludges. The field testing program was centered around the in-situ undrained shear strength measurement of paper mill sludge landfill covers using a hand-held field vane. The laboratory and field results showed that paper sludge undrained shear strength is low and comparable to soft clays, peats, highly organic soils, and dredged sediments. The undrained shear strength decreases with increasing water content, and the undrained shear strength increases with increasing organic content. Also, field vane shear strength values were correlated to laboratory test results to provide a field vane correlation or calibration factor for paper mill sludges. Settlement back-analyses focusing on secondary compression evaluated the consolidation behavior of three full-scale landfill covers. These results showed that paper sludge landfill covers can exhibit varying secondary compression rates, i.e., non-constant Cα/Cc, which can potentially contribute a considerable amount of settlement beyond primary compression and should be accounted for in design. Also, slope stability back-analyses were performed on three slope failure case histories to determine the in-situ undrained shear strength and interface friction angles (between paper sludge and textured geomembrane) at failure. Finally, design and construction strategies incorporating the results of this research were presented in an effort to provide a rational approach for dealing with the unique properties of paper sludge. In general, the governing design considerations for paper sludge landfill covers are hydraulic conductivity, slope stability and total settlement. |
