Selection of a site for a heap leach pad may be undertaken using procedures used for tailings impoundment site selection. One of the early, but still relevant and applicable papers available on site selection is by Robertson et al.1 A similar but expanded approach is described in Caldwell and Robertson.2 In essence, the basic approach described in these two papers involves identifying candidate sites, comparing alternatives, communicating the subjective and objective criteria affecting the choice of site, and validating the site selected. Modern decision making theory as applied to site selection is described by Robertson and Shaw.3

Because conditions often vary significantly between sites—even those in close proximity—careful selection of the site having the most favorable set of advantages is a major design option. The selection process may involve many technical disciplines and can include both non-technical issues such as property availability, as well as technical criteria that include topography, geology, hydrology, environmental and engineering conditions.

Site selection involves evaluations of a number of factors, including distance from the mining area and dumped ore, environmental suitability, and the amount of earthwork that must be done to provide grading of the slope for proper drainage of the heap. Additional factors to be considered in site selection are: flood potential, subsoil conditions, and future prospecting potential for economic mineralization. Thorough testing of soil and rock beneath the proposed heap leach site is necessary to ensure stability of heaps.

Site Investigation Methodology
A desk study is usually conducted which almost invariably involves the review of topography, geological and vegetation maps, as well as geological, climatological and site investigation reports. This is followed by site reconnaissance visits, which can help to validate information obtained during the desk study.

Fieldwork generally provides key technical data such as:
• The sequence of soil and rock strata with depth.
• Depth to bedrock.
• The distribution of soil and rock over the study area.
• Discontinuity intensities of rock at the site.
• Prediction of bedrock topography.
• Groundwater distribution and seasonal variation.
• General direction of groundwater flow and consequent contaminant threat of leachate to groundwater resources.
• Aquifer properties.

Site Selection
Mining companies face the challenge of effectively managing heap leaching facilities through a life cycle, from site selection to final closure. The first step in the selection of sites for heap leach pads, collection ponds, and the extraction plant is the delineation of an area of interest where sites can be defined and selected. Because sites located far from the mine may be uneconomic, a fixed radius from the mine will typically define the area of interest. The length of the radius is a practical distance from the mine dictated by hauling costs and other operational factors. For heap leaching facilities, the area of interest is within a one-half to 1-mile radius.

Within the area of interest, there may be areas where facilities should not be sited for reasons that include ownership, topography, potential flood areas, subsoil conditions, and mineralization. The remaining sites are then screened to eliminate areas that are less suitable; i.e, steep topography, unfavorable geologic conditions, unacceptable hydrologic conditions such as high runoff (which can not be diverted easily), risk of failure, environmental considerations, etc. The remaining candidates are then compared to determine the best (or sometimes the least unacceptable) site.

The site selection process is generally an extension of good judgement and common sense. For some projects, only one site may be available or one may be substantially better than all others, and the process is not required. For other projects, several good sites may be available but the optimum site among them is not readily apparent. In such a case, the site selection process is used to determine which site would be preferable. Costs involved in the site selection process may be offset by future engineering, construction or operational cost savings.

Schematic showing general aspects of topography evaluation for heap leach pad site selection.

The slope of the pad and associated liners is very often controlled by the existing topography. The slope of the pad must be steep enough to allow efficient drainage of the leachate but not so steep that the stability of the heap is jeopardized or that erosion of the liner occurs. In many cases, it is possible to construct a leach pad using the existing topography without significant cut and fill.

Meteorological Conditions
Characterization of the meteorological conditions of a heap leach site includes collecting historic data about site precipitation, evaporation, snow, humidity, temperatures, and wind. Interaction of climate and the design and performance of mine waste management facilities is described by Hutchinson and Ellison.4 Specifically with regard to heap leach pads, they consider the following climatic factors and their significance:
• Short-term–intense rainfall affects the size of ponds, ditches, and containment dikes that need to be designed to avoid erosion and/or overtopping.
• Long-term–weather patterns affect the overall water-balance performance of the pad as regards infiltration, evaporation, and hence the quantity of flow from the pad.

It is also necessary to evaluate wind direction and velocity in the potential sites. Wind direction and velocity are critical because of dust problems that can be caused by erosion, especially in sites up-gradient from towns, recreation areas, farm lands, etc. Persistent high-velocity winds can also complicate reclamation of abandoned heaps. An example of the use of climatic data in the design and performance evaluation of heap leach pads and similar mine waste disposal facilities is provided in Wels et al.5

Hydrology
Hydrologic studies consist primarily of the acquisition of data on precipitation and runoff. Runoff estimates indicate the rate at which non-process water can be expected to enter the site. Diversion systems (ditches, berms, etc.) should be designed to handle extraneous water. Precipitation can affect leaching operation in several ways. It reduces pH and density of the solutions. Collection ponds should be designed larger to store leaching solutions as well as rainwater collected from the heap and channels. The pregnant solution pond must be designed to receive runoff from a 100- year maximum flood occurring in the total heap area being drained. This pond must handle runoff from the annual spring snowmelt. Sometimes, it is necessary to cover the heap surface with impermeable materials to prevent rainwater from percolating through the heap.

Geology
The geology of a heap leach pad should be established comprehensively. This may be done during geological studies usually undertaken as part of the overall mine investigation, characterization, and development. Geological data about the origin, location, and engineering characteristic of site and local soils, rocks, and groundwater should be obtained.6

Hydrogeology
The interaction of site groundwater and a heap leach pad is discussed by Hutchinson and Ellison.4 In theory there is no interaction of the heap leach pad and groundwater because the liner placed at the base of the pad precludes passage of groundwater up into the heap and passage of seepage from the heap to groundwater. In practice, the following considerations give rise to a need to integrate heap leach pad design and operation with groundwater evaluations:
• Springs and/or artesian groundwater flow at the heap leach pad site may necessitate placement of drains beneath the liner,
• Holes in the liner may result in the passage of leachate to the groundwater,
• Water pressure beneath the pad may affect slope stability.
Therefore, the site selection process for the proposed facility should include consideration of subsurface hydrogeological conditions within and adjacent to the potential zone of influence of the proposed site. A risk assessment of potential impacts on groundwater chemistry, levels and appropriate groundwater pressure should be conducted as a minimum. It is essential to determine groundwater flow system in the vicinity of the site from which escaping solutions may be released to underground aquifers. The most appropriate rules for site selection according to hydrogeological characteristics are:
• Select the site underlain by the maximum possible thickness of unsaturated materials.
• Select the site underlain by materials with lowest possible permeability.
• Select the site not underlain by shallow aquifers without confining layers.
• Avoid sites that are adjacent to streams where groundwater and the contaminated seepage it collects will discharge immediately to the stream.
• Provide seepage collection facilities where seepage is anticipated.
• No heap leach facility shall be constructed on reclaimed or filled land.

Geotechnical Studies
The general geologic and geotechnical characteristics of potential areas should be among the first factors to be examined. Depth to and topography of buried bedrock should be determined for structural and drainage purposes. Unconsolidated materials should be mapped and their grain-size distribution, permeability, and engineering properties determined so that potential settlement, leakage, and other failure problems can be anticipated and incorporated into the design of facilities. In general, a geotechnical site investigation for a heap leaching project is conducted to gather the following information:
• Foundation conditions beneath the heap, ponds, and other areas where additional load is added to change the stress conditions in existing rock and subsoil.
• Excavation conditions, or how easily materials can be excavated in areas where cuts are required.
• Quality and quantity of potential construction materials available for liners, drainage material, and structural fill.
• Hazards associated with the site such as recent or active faulting, unacceptable groundwater conditions, or soft or collapsible soils.
• General identification of geologic and hydrogeologic conditions beneath structures such as the heap pad and collection ponds.

Techniques used to gather this information vary from surface reconnaissance to backhoe trenching and drilling.

One important consideration associated with such facilities is differential settlement. Settlement of the underlying foundation may lead to disruption of the leach pad system. Differential settlement typically results from one of three causes: the liner spans soils of differing compressibility, the slope and height of the heap are extreme or local settlement results from leaks in the liner. The later cause can be significant where the soils contain high concentrations of acid soluble salt.7

The geotechnical risk assessment may also include a seismic assessment of the site as well as an assessment of the liquefaction potential of foundation, construction and stored materials. The investigation(s) should provide a high level of confidence in the physicochemical characteristics, the engineering and hydrogeological properties, and the behavior of the foundation, construction and stored materials under normal operating and seismic loading conditions.

Leach pads—particular those involved in copper ore leaching—can be enormous, reaching areas in excess of 5 million m2. This often precludes selecting of the optimum site from a geotechnical standpoint. It also requires a liner system to span a wide range of geologic profiles.

General features of heap leach pad construction.

Exploratory procedures for construction materials involve pitting, trenching, and drilling. The exploration program should identify alternative sites as to quantities of the various classes of construction materials available. The materials at the alternative sites should be classed according to volume available, water content, grain-size distribution, compaction characteristics and shear strengths.

The characteristics of natural materials vary from site to site, and even within parts of a particular area. Because of the variability of these materials, and their potential use as an important component of the leach facility, their geotechnical characteristics (as well as their quantity) should be determined.

Environmental Considerations
Although heap leaching facilities have leak detection systems and often use double liners, most facilities employ leak detection or monitoring systems so that leaks can be discovered and remediated. However, there is always the possibility that detection will fail and leakage will escape to the underlying strata.

In general, heap leach facilities can cause the following major types of environmental impacts:
• Solution containing ponds and ditches can present an acute hazard to wildlife and birds.
• Spills of leaching solutions can enter surface water and contaminate drinking water sources; or they can enter groundwater systems and contaminate water supply wells.
• Chemical solutions in the active heaps and solution ponds may reach water sources through leaks from leach pads and liners.
• Catastrophic failure of heap leach pads can release chemicals into streams and groundwater.

Such problems associated with chemical solutions in heaps have led operators to explore preventive strategies. These include reviewing environmental parameters in the initial site selection. The proper location of heap leaching facilities can be the most simple and the most effective factor in enhancing the successful long-term protection of groundwater quality. Proper site selection also represents the most powerful tool available to the engineers in ensuring the long-term stability of a heap leach pad.

Preparing an environmental assessment requires a number of environmental studies in the region of the proposed site. These studies generally address geology, hydrology, soils, vegetation, wildlife, aquatic features, history and archaeological aspects of the area.

Heap leaching ponds may also attract animals and birds for water. Consequently, preventing bird poisoning on wet heaps and ponds, especially in desert climates where the ponds are attractive sources of water, is a concern.

However, the major environmental concern with heap leaching is contamination of surface and ground waters with chemicals in the leaching solutions. The closure of heaps which have ceased to be productive requires taking account of additional environmental considerations

Mineralization
Leached ore is typically left in place, which means that a site should be located where no economic mineralization lies under the permanent disposal of the leached ore residue. Some exploration drilling may be appropriate to ensure that the selected site does not overlay economic mineralization.

Distance from Mining Area
Leach pad foundations are designed for high stability to prevent movement or cracking of the pad liner under the weight of the ore heaps, which may reach heights of 60 to 70 m or more. However, heap movement may occur with high heaps or heaps located on steeply sloping sites. This problem should be considered in the site selection process in order to select sites safely removed from the mining area.

Conclusions
Experience across the mining industry has consistently demonstrated that the most significant catastrophic risk posed by mining operations on the environment arises from the construction and operation of tailings, heap leach pads and water storage facilities. Because of the long-term nature of the pollution and environmental impact of many such facilities, the selected site must serve as an effective area, not only in the short term, but also in the very long term. The proper location of heap leaching facilities can be the most simple and the most effective factor in enhancing successful long-term efficiency of the facility.

The authors wish to thank National Iranian Copper Industries Co. for supporting this research.

References
1. Robertson, A. et al. “Uranium Mill Tailings Impoundment Site Selection,” Symposium on Uranium Mill Tailings Management, Fort Collins, Colorado, (1980).
2. Caldwell, J.A., and Robertson, A., “Selection of Tailings Impoundment Sites,” The Civil Engineer in South Africa, (1983).
3. Caldwell, J. “Technology Review; Heap Leach Pads,” Infomine, (2006).
4. Hutchinson, I. P.G., and Ellison, R. D., “Mine Waste Management,” Lewis Publishers, pp. 453- 497, (1992).
5. Wels, C., et al. “Infiltration Test Plot study for Mine Rock Piles at Questa Mine, New Mexico.” 2001 National Meeting of the American Society for surface Mining and Reclamation, Albuquerque, NM, (2001).
6. Miguas, R.E. “Geology and Engineering Geology in the Siting of Complex Public Facilities,” Geotechnical Site Characterization, (1998).
7. http://www.geomembrane.com/PGI
Saeed Karimi Nasab is assistant professor, Mining Engineering Department, Shahid Bahonar University of Kerman, Email: kariminasab@mail.uk.ac.ir. Azadeh Hojat is a Ph.D. student of geophysics, Institute of Geophysics, University of Tehran, Tehran, Iran, E-mail: ahojat@ut.ac.ir. Mohammad Reza Mollaei Fard is a mining engineer, National Iranian Copper Industries Co. E-mail: mollaeifard@nicico.org.