One of the most important aspects in the geotechnical heap leach pad design is related to the interface strength of the underlying geomembrane liner system which isolates the leach solutions and mineral from the natural ground (pad foundation). The liner system characteristics will depend of the environmental regulations, site conditions, risk associates to the solution leaks, operation characteristic, equipment of operation above the liner, normal stress for ore heaping, adequate liner strengths for slope stability, etc. In the world, the tradition liner system design considers a double contention system formed by a geomembrane (GM) liner over a compacted layer of low permeability soil (LPS), the over liner (OL) and pipes collection system are collocated over the GM. The OL generally is a granular soil that reduces the damage in the GM and allows to drainage the leach solutions.

The interface shear strength between GM and a soil is estimated through large-scale direct shear (LSDS) test according to the ASTM D 5321, the LSDS test provides the peak and post peak shear strengths. In the practice, the slope stability analysis for heap leach pads is running with the lowest post peak shear strength in the liner system. Usually, the executed LSDS tests show the GM-LPS interface has the lowest post peak shear strength, likewise these LSDS tests reported GM-OL interface shear strength higher than GM-LPS interface. For this reason, GM-OL interface shear strength is not used in the slope stability analysis, however this focus is changing because the tendency of the lowest shear strength changed to the interface GM-OL due to a minimum request for asperity height of the geomembrane of 0,4 mm (according to GRI – GM 17 Standard Specification).

This paper proposes a focus and methodology for the estimation of the shear strength of a liner system composed of a LPS layer, GM and OL based on the analysis of 400 LSDS tests. In addition, this research proposes a correlation to estimate the GM-LPS interface post peak shear strength based on asperity height of GM and LPS classification (according USSC), also a correlation for GM-OL interface is proposed based on gravel content of OL. For slope stability analyses, the use of peak shear strength for flat zones and post peak shear strength for stepped zones of heap leach pads is evaluated, the findings are compared with the results of numerical analysis.