Interface shear behaviour of different geomembranes with rough surfaces in Heap Leaching applications

Abstract

The interface shear strength properties of the different geosynthetic layers of the liner system are examined for heap leach pads applications. A series of large scale direct shear tests is used to examine the effectiveness of four geomembranes (GMBs) with different surface roughness with different soil subgrades, geotextiles (GTXs) with different mass per unit areas, and a geocomposite drain (GCD) at moderate to high normal stresses. Four different subgrades, including sand, two different coarse-grained underliners, and clay representing the layers directly underlying the GMB liner in heap leach pads, are used to examine the shear strength of the GMB-Subgrade interfaces at normal stresses between 50 and 1000 kPa. It was found that increasing the normal stresses can change the mechanisms contributing to the shear resistance at the interface. This resulted in a statistically insignificant increase in the interface friction of the GMB-Granular soils interfaces when using GMBs with surface roughness relative to the smooth GMB. Furthermore, depending on the type of subgrade, establishing the shear envelopes over a wide range of normal stresses may over/underestimate the shear strength at the field stresses even when linear regressions present the best fit for the data. The effect of using GTX cushion to protect the GMB liner from excessive punctures at high stressed zones of the pad on the interface shear behaviour of the liner system is investigated by examining different GMB-GTX interfaces. Using GMBs with high surface roughness and/or decreasing the mass per unit area of the GTX increased the peak friction angles of the GMB-GTX interface. However, the shear strength of these GMB-GTX interfaces was still lower than the shear strength of most of the GMB-Subgrade interfaces even when the smooth surface of the GMB was in contact with the subgrade. Additionally, while preageing the GTXs to low mechanical properties prior to the direct shear experiments was found to increase the peak interface friction angles of different single layer GTXs due to the softening of geotextiles, it resulted in aniii internal failure of a heat bonded two-layer GTX at low shear displacements. This was due to the reduction in the bond strength between the two layers of the GTXs due to chemical degradation of the GTX.