Permeability and internal erosion of a GCL beneath coarse gravel

ABSTRACT: Experimental results from flow tests to assess the hydraulic performance of a geosynthetic clay liner (GCL) when subjected to extrusion beneath coarse (50 mm) gravel are reported. Although the permittivity was slightly higher then when loaded uniformly, it remained very low (6.3 × 10⁻¹⁰ s⁻¹) when the GCL was placed directly beneath coarse gravel and subjected to 250 kPa of overburden pressure. In some cases, hydraulic failure from internal erosion was observed when the final GCL thickness beneath the gravel contact was less than 3.5 mm. The GCL was much more susceptible to failure from internal erosion when a geotextile protection layer was installed above the GCL as a result of the increased transmissivity between the gravel and GCL allowing the large head to act on the thinnest part of the GCL. The head required to result in failure of the GCL by internal erosion decreased as the minimum thickness of the GCL decreased and it was very sensitive to thickness being less than 1 m for a GCL thickness less than 2 mm and at least 100 m for a GCL thickness greater than 3.5 mm. The results suggest that if a protection layer is selected to limit the tensile strains in an overlying geomembrane to less than 3%, the reductions in GCL thickness from bentonite extrusion would be sufficiently small such that the particular GCL tested would not be at risk of hydraulic failure from internal erosion for the conditions tested for most practical cases (i.e. heads much less than 100 m).