Focus to Assess the Shear Strength of a Liner System for Block Failure in High Heap Leach Pads

One of the most important aspects in the geotechnical heap leach pads design is related to the interface strength of the underlying geomembrane liner system which isolates the leach solutions and mineral from the natural ground. The liner system characteristics depends of the environmental regulations, site conditions, risk associates to the solution for 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 soil liner (SL), 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 SL is estimated through the large-scale direct shear (LSDS) test which 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 GM-SL interface has the lowest post-peak shear strength, likewise these LSDS tests reported that shear strength of OL-GM interface is higher than SL-GM interface. For this reason, OL-GM interface shear strength is not used in the slope stability analysis, however this focus could be changed because the tendency of the lowest shear strength changed to the OL-GM interface due to a minimum request for asperity height of the geomembrane of 0,4 mm according to GRI–GM 17 (2019).
This paper proposes a focus and methodology for the estimation of the shear strength of a liner system based on the analysis of 453 LSDS tests. In addition, this research proposes correlations to estimate the SL-GM interface post-peak shear strength based on asperity height of GM and SL classification (USSC), also correlations for OL-GM 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 are evaluated, the findings are compared with results of numerical analysis.