Barren solution injection wells are a recent technology used in the heap leach processing to extract additional minerals from the ore, particularly in older deeper lifts of ore in mature heap fills. Injection wells are typically installed along linear exterior slope access benches or along the top crest surface, which can contribute to shallow and deep open cracks in the loose ore heap fills from injection well hydraulic loading. These conditions in turn contribute to lower post-peak ore and liner interface strengths, if any slippage does occur along the planar base pad liner surface or at shallower depths within the heap fill itself.
Several large geomembrane lined gold and copper ore heaps in both North and South America have had crack movements or slope failures in recent times, which were directly related to barren solution injection well activity. Ore lifts are placed on the heap in controlled loose lifts and leached at low surface irrigation rates in the range of 5 to 10 liters/hour/square meter (L/hr/m2 ), which typically do not result in saturated ore material issues for most heap leach fills. However, shallow or deep injection of process solutions into the pile via injection wells several months to years after initial fully drained leaching of these ore lifts changes the short term and long term geotechnical and hydrological characteristics of the heap fill. As an example, the ore permeability may change at depth and laterally from rapid injection hydraulic loads. This causes both additional vertical settlement as well as lateral hydraulic compression movement in the loose fill by gravity or pump injection pressures. This is evidenced by the downhole buckling deformation that can be observed in the walls of PVC well pipes.
If the induced total weight from the injection well is a significant change in load stress above any deeper saturated loose heap fill zones, then there is a potential risk that static liquefaction may occur. In other words, any significant and rapid hydraulic loading on loose placed and saturated fill zones can cause excess pore pressures to temporarily develop and approach zero fill strengths. If the zones of saturation related to the injection well activities migrate toward the exterior slope of the heap in more pervious preferential pathways, then heap slope instability may occur either in a static saturation condition or dynamic earthquake related liquefaction condition.
This paper discusses the potential instability of lined copper and gold ore heaps from solution injection well activity. The results of slope stability analyses will be evaluated and presented for several selected case scenarios of solution injection, well-related saturation conditions within an idealized ore heap section. The focus and stability analyses for this paper can also be applicable to solid waste landfills
Click here for full paper.