Effect of Wet-Dry Cycling on Swelling and Hydraulic Conductivity of GCLs

Atterberg limits, free swell, and hydraulic conductivity tests were conducted to assess how wet-dry cycling affects the plasticity and swell of bentonite, and the hydraulic conductivity of geosynthetic clay liners (GCLs) hydrated with deionized (DI) water (pH 6.5), tap water (pH 6.8), and 0.0125-M CaCl2 solution (pH 6.2). The plasticity of bentonite hydrated with DI water increased during each wetting cycle, whereas the plasticity of bentonite hydrated with tap water and CaCl2 decreased during each wetting cycle. Wet-dry cycling in DI water and tap water had little effect on swelling of the bentonite, even after seven wet-dry cycles. However, swelling decreased dramatically after two wetting cycles with CaCl2 solution. Hydraulic conductivity of GCL specimens remained low during the first four wetting cycles (∼1 × 10−9 cm s). However, within five to eight cycles, the hydraulic conductivity of all specimens permeated with the 0.0125-M CaCl2 solution increased dramatically, to as high as 7.6 × 10−6 cm s. The hydraulic conductivity increased because cracks, formed during desiccation, did not fully heal when the bentonite rehydrated. In contrast, a specimen continuously permeated for 10 months with the 0.0125-M CaCl2 solution had low hydraulic conductivity (∼1 × 10−9 cm s), even after eight pore volumes of flow.