Effect of a lateral drainage layer on leakage through a defect in a geomembrane overlain by saturated tailings

Experiments are conducted to investigate the effect of a drainage layer in/below saturated tailings on leakage through a hole of various shapes and sizes in a 2-mm-thick HDPE geomembrane. Three cases: no drainage (Case 1), drainage layer between two layers of tailings (Case 2), and drainage layer below tailings and above the GMB (Case 3) are examined. Analytical solutions predicting leakage through a circular GMB hole overlain by tailings with an internal drainage in (Case 2) and below (Case 3) tailings are developed, and match the experimental and numerical results well. Results show that a drainage layer separated from the GMB by a thin layer of tailings (Case 2) gives leakage slightly greater than if no drainage layer is present, but 3-5 orders of magnitude lower than when the drainage is placed directly over the GMB (Case 3). In Case 3, leakage is dependent on the hydraulic conductivity k of both the drainage and subgrade, and is not affected by the tailings. Unlike Cases 1 and 2 where the subgrade has negligible effect on leakage, a low permeability subgrade with k less than 10% of k for the drainage layer is recommended in Case 3 to minimize leakage through geomembrane defects.