Durability of Boston Blue Clay in Waste Containment Applications

Abstract

An experimental investigation of the durability of natural and polymer-amended Boston Blue Clay (BBC) used in waste containment applications was conducted. Durability was assessed in terms of hydraulic conductivity and crack formation in response to freeze-thaw- and desiccation-induced stresses. The polymers tested were nonionic polysaccharide (guar gum) and cationic polyacrylamide (PAM) with aqueous concentrations ranging from 0.25 to 8.00 g/L. BBC proved resistant to shrinkage cracks from desiccation, but susceptible to ice lensing from the application of freeze-thaw cycles. The saturated hydraulic conductivity of natural BBC increased from 5×10−5 to 1×10−3 cm/s after five freeze-thaw cycles. The saturated hydraulic conductivity for guar- and PAM-amended BBC also increased in response to freeze-thaw cycling; however, both the initial and final values were nearly an order of magnitude lower than those of natural BBC. In terms of crack formation as identified with computed topography scans, extensive ice lensing was observed in both the unamended and guar-amended specimens, while the PAM-amended specimen exhibited no such behavior in response to freeze-thaw cycling. Crack formation from either desiccation or freeze-thaw cycling was quantified in terms of a crack area density (CAD) that decreased for both guar- and PAM-amended specimens, as compared with the natural BBC. In particular, the observed CAD values for natural and PAM-amended BBC subjected to freeze-thaw cycling were 69 and 8%, respectively. The observed CAD values for all BBC specimens subjected to desiccation stress were less than 3%. Overall, these results indicate that natural BBC may be unsuitable for waste containment applications where control measures are not implemented for frost penetration. Moreover, polymer amendment serves to improve the material by reducing crack formation and hydraulic conductivity.