Compatibility Of Geosynthetic Clay Liners With Leachate From CCP Landfills

Experiments were conducted to evaluate whether coal combustion product?CCP?leachates adversely affect the hydraulic conductivity?K?of geosynthetic clay liners?GCLs?as an alternative material in lieu of a compacted clay liner?CCL?.Chemical properties of CCP leachates were compiled based on a nationwide survey of CCP disposal facilities.Five synthetic leachates were developed to represent a range of conditions encountered in CCP disposal facilities:typical CCP leachate,strongly divalent cation fly ash leachate,flue gas desulfurization?FGD?residual leachate,high ionic strength ash leachate,and trona ash leachate.Nine GCLs were tested:two Na-bentonite?NaB?GCLs,six bentonite-polymer?P-B?GCLs,and one bentonite polymer composite?BPC?.Hydraulic conductivity tests on GCLs were conducted on prehydrated?60-d on subgrade soil or DI water?and non-prehydrated GCL specimens using 4-inch?10.6 mm?flexible wall permeameters.Tests were run to hydraulic and chemical equilibrium at effective stresses of 20 kPa,and then higher effective stresses were applied up to 450 kPa.At 20 kPa,the GCLs with NaB had high hydraulic conductivity(>10^-6 m/s)to trona leachate and moderate to high hydraulic conductivity(10^-10 to 10^-7 m/s)to the other CCP leachates.Hydraulic conductivity was directly proportional to the ionic strength of the leachate and inversely proportional to the swell index of the bentonite when hydrated in leachate.The hydraulic conductivity of B-P GCLs with low polymer loading??1.2%? behaved similarly to NaB GCLs.However,the hydraulic conductivity of GCLs with higher polymer loading??1.9%?is controlled by the clogging of pores by polymer hydrogel.For this scenario,the hydraulic conductivity is not affected by the ionic strength of leachate,and not related to the swell of bentonite-polymer.Polymer elution was observed during the hydraulic conductivity tests,and increasing hydraulic conductivity was associated with cumulative polymer elution.The uniformity of polymer distribution in the GCL roll is important,and flow occurs through the defect area with relatively low polymer loading.Increasing the effective stress from 20 to 450 kPa caused the hydraulic conductivity to decrease by up to three orders of magnitude.At 450 kPa effective stress,the hydraulic conductivities of all tests were lower than 10^-9 m/s except for one test with Trona leachate.This observation indicates that high effective stress can minimize the effect of CCP chemistry on the hydraulic behavior of GCLs.GCLs hydrated under higher effective?e.g.,100 kPa or 450 kPa?stress induced similar or lower hydraulic conductivity than the GCLs hydrated at lower effective stress?20 kPa?then subjected to equivalent higher effective stress.This outcome indicates that filling the ash rapidly into the landfill will benefit the performance of the liner system.Subgrade hydration had no beneficial effect on hydraulic conductivity;however,prehydration with DI water can effectively protect the GCLs due to the large amount of immobile water retained in the bentonite.Interface shear strength of GCL and geomembrane?GM?was investigated on theNa-bentonite?NaB?and bentonite-polymer?B-P?GCLs.The GCL samples were sheared over a 60-mil textured HDPE geomembrane to a maximum displacement of 50 mm under normal stresses of 20,100,250 and 400 kPa.The B-P GCLs presented lower peak shear strength than the NaB GCLs under the same normal stress.Polymer extrusion was observed on the interface and is the likely reason for the decrease in interface shear strength.Both high polymer loading and high normal stress enhance the amount of polymer extrusion on the GCL-GM interface.The greatest amount of polymer extrusion was observed during the hydration stage as polymers swell and migrate onto the GCL-GM interface.However,changing the hydration liquid did not have a significant effect on the polymer extrusion during the hydration stage.Applying a thin layer of woven slit film inside the original nonwoven carrier geotextile helps containing polymer in the bentonite.After adding the woven slit film,the interface shear strength characteristics of B-P GCL are similar to that of NaB GCLs.