Leakage Phenomena of Tailings Overlying a 1 cm Diameter Circular Geomembrane defect

Abstract

Leakage phenomena for two types of tailings overlying a 1 cm diameter geomembrane defect were studied. The first type of tailings was a Silty Sand. Various underliners (DF15/DB85=5.6 to 13.5) not meeting typical retention criteria for filtration were placed beneath the tailings and geomembrane defect. The leakage rates and visual findings revealed that a critical stress condition exists where erosion continuously occurred for up to 24 hours. This condition resulted in a leakage rate that was 2 to 3 orders of magnitude higher than previous observed with filter compatible conditions and higher stresses. This condition could be encountered during early deposition or development of a reclaim pond at a tailings storage facility. Visual findings on the surface of the underliner suggested that contact likely had more influence on stability in this critical stress condition than gradation compatibility. Based on the areal coverage of tailings on the surface of the underliner, it was postulated that more intimate contact between the geomembrane and underliner leads to more stable leakage conditions. The second type of tailings studied was fine-grained (approximately 90% passing 75 um). The material was placed on top of a geomembrane with a 1 cm defect, overlying a filter compatible underliner. Leakage rates implied the hydraulic conductivity reduced as the tailing experienced more vertical effective stress (up to 200 kPa). Small samples (8 mm in diameter, 6 mm in height) were retrieved near the defect after the test and permeated at a variety of effective stresses (0 to 800 kPa). Although some disturbance likely occurred, the permeation results suggested the tailings hydraulic conductivity was in the range of 3 x 10-9 to 6 x 10-9 m/s, which confirmed values estimated using a saturated seepage model (SEEP W/). Permeation results and gradation analysis of the small samples suggested that significant migration of fines did not occur. Further seepage modeling assuming homogeneous tailings hydraulic conductivity suggested the leakage rate from a well-designed, well-constructed, lined facility containing the tailings is likely in the range of 1-2 L/ha/day. Without the liner, the leakage rate is likely 3-4 orders of magnitude higher.