Effect of Backpressure Saturation on the Hydraulic Conductivity of GCLs: Fid-Bau Portal

Effect of Backpressure Saturation on the Hydraulic Conductivity of GCLs

Conzelmann, Joel / Scalia, Joseph / Shackelford, Charles D.

The sensitivity of laboratory measured hydraulic conductivity, k, to backpressure saturation is investigated for a fiber reinforced GCL containing natural sodium bentonite (Na-B GCL) and a non-reinforced GCL containing a polymer modified bentonite. Hydraulic conductivity tests were performed using a synthetic soil pore water (ionic strength, I = 4 mM, ratio of monovalent-to-divalent cations, RMD, = 0.22 mM^1/2), and a synthetic gold pregnant leach solution (PLS; I = 42 mM, RMD = 11 mM^1/2). Both GCLs were permeated by two methods under matching average effective stresses (27.6 kPa) and hydraulic gradients (160-200) following the ASTM D6766-12 default method (average pore water pressure = 524 kPa) and following ASTM D6766-12 but without backpressure saturation (average pore-water pressure = 6.9 kPa) or permeant interface devices. Both methods resulted in complete saturation of the GCL specimens by the termination of permeation, i.e., backpressure saturation was not necessary to attain saturation. Measured values of k were similar for both test methods for the GCL containing polymer-modified bentonite, but resulted in 50 to 100 times higher k for the Na-B GCL permeated without backpressure saturation relative to the ASTM D6766-12 default method. Flow through Na-B GCLs permeated without backpressure saturation (i.e., Na-B GCLs with higher k) was visually observed to occur through needle-punching fiber bundles.