Hydrated area of a bentonite layer encapsulated between two geomembranes

Hydrated bentonite has a low shear strength, which may adversely impact the stability of structures incorporating geosynthetic clay liners (GCLs). Accordingly, in a growing number of GCL applications, the configuration is such that the bentonite layer is encapsulated between two geomembranes in order to reduce the potential for bentonite hydration. This paper considers an encapsulated bentonite layer formed using GM-GCL panels (i.e. panels consisting of a bentonite layer adhered to a carrier geomembrane). The panels are joined by overlapping at the edges and are overlain by a welded geomembrane. Water can migrate from the underlying soil into the bentonite of the overlaps, flow in the bentonite, and migrate laterally in the bentonite between the two geomembranes. Water is driven from the soil to the bentonite by a head difference that results in great part from the suction at the hydration front. This paper presents an analytical method to evaluate the extent of the hydrated area of the bentonite layer as a function of: time, the initial and hydrated moisture content of the bentonite, the hydraulic conductivity of the bentonite, the overlap width, the distance between overlaps, and the head difference. It is important to know the hydrated area for stability calculations. Numerical applications show that, for typical values of the parameters, it takes many decades to hydrate a significant fraction of the bentonite layer area. The analyses presented in this paper also show that, for typical landfill applications, the hydrated area due to leakage through defects in the upper geomembrane is negligible with respect to the hydrated area resulting from water migrating through the overlaps, assuming that the upper geomembrane is installed using good construction quality assurance practices. Uncertainties associated with the methodology presented herein are discussed, and guidance is provided on evaluation of the shear strength of the encapsulated bentonite layer as a function of the shear strength of the unhydrated bentonite, the shear strength of the hydrated bentonite, and the hydrated area.