Internal erosion of geosynthetic clay liners under high hydraulic heads

Abstract

A geosynthetic clay liner is a material composed of a thin layer of bentonite sandwiched between two layers of geotextiles. Low hydraulic conductivity of the bentonite in GCLs makes them more impermeable. Geosynthetic clay liners are primarily preferred as environmental protection barriers in cover systems with their low hydraulic conductivity, low cost and easiness of installment. When the level of water on the GCL increases after the installment of a GCL over a subgrade, the hydraulic head on the GCL also increases. Under high hydraulic heads, there is the possibility for the bentonite component to migrate out of the GCL which causes high increase in hydraulic conductivity. This process which causes for the GCL to lose its hydraulic barrier capability is called internal erosion. Flexible wall triaxial permeability tests were performed on different types of geosynthetic clay liners placed over bases with uniform voids under high hydraulic heads up to 50 m in order to investigate internal erosion. The bases with uniform voids represent rounded coarse uniform gravel subgrades beneath the GCL samples. The effects of the geotextile component (woven or nonwoven) that was in contact with the base with voids, bentonite component (sodium bentonite or calcium bentonite), manufacturing type (needlepunched or not) of the GCL samples on internal erosion were investigated. Triaxial permeability tests showed that the performance of the GCL against internal erosion where the nonwoven geotextile component was in contact with the base with voids was better than that of the woven geotextile component when the tensile strengths of the geotextiles were almost the same. However, the performance of the GCL against internal erosion where the woven geotextile component was in contact with the base with voids was the best among all of the GCL types when the tensile strength of the woven geotextile was relatively higher than that of the nonwoven geotextile. The performance of the GCL manufactured by needlepunching the nonwoven geotextile component through the bentonite to the woven geotextile component against internal erosion was found to be better than that of the GCL manufactured by bonding the woven and nonwoven geotextiles to the bentonite without needlepunching. According to the test results, type of the bentonite used in the GCL samples does not play an important role against internal erosion. Mass per unit area and moisture content of the bentonite component of the GCLs were also measured. Low mass per unit area and high moisture content obtained from the zones of the bentonite where the GCL sample faced the voids of the bases are indications of high amount of bentonite migration and internal erosion.