Well-designed, haul roads serve as vital arteries for ultra-heavy mine trucks delivering payloads of ore to heap leach pads for leaching. However, subsurface hydrologicy challenges can arise when haul roads, built on leach pads, impede the flow of leaching solutions.
The Effects of Ultra-Heavy Mine Truck Haul Roads on Heap Hydrology
Article by: Dr. Dale Rucker | Chief Technical Officer | hydroGEOPHYSICS
Mining haul roads are one of the unrealized champions of heap leach mining operations. The placement, performance, and maintenance of these superstructures has a direct impact on operational efficiency, costs, and safety. Well-designed, haul roads serve as vital arteries for ultra-heavy mine trucks delivering payloads of ore to heap leach pads for leaching. However, subsurface hydrology challenges can arise when haul roads, built on leach pads, impede the flow of leaching solutions.
Subsurface hydrology challenges can arise when haul roads, built on leach pads, impede the flow of leaching solutions.
Ore stacked on leach pads can be transported by an automated conveyance system or manually with haulage equipment. For ultra-heavy haulage equipment, it is necessary to design and maintain wide roads for two-way traffic and safe maneuverability. Road widths can exceed 200 feet for a mile or more across leach pads: haul trucks, graders, dozers, water trucks, and light-duty vehicles will use these roads during the stacking process.
Cyclical and vibrational compaction of the ore from the high traffic loads can significantly impact how the ore will respond to wetting
The cyclical and vibrational compaction of the ore from the high traffic loads can significantly impact how the ore will respond to wetting, leaching, and drainage. Permanent damage to the leach pad can result from high axle-load traffic when ore compaction is not resolved before leaching operations begin.
Electrical resistivity is a geophysical method used to characterize and map subsurface hydrological conditions. This technology is often applied to leach pads to understand how leaching solutions move through the ore and to assist with mitigation efforts when uneven wetting exists. The method is used strictly on the surface of the pad with no drilling or invasive digging. Despite operating from the surface, the method can map the hydraulic conditions deep within the leach pad, all the way to the liner or native bedrock. This includes leach pads that are over 500 ft deep!
Example of a mining dump truck running on top of a leach pad and compacting the ore. The high traffic loads can significantly impact how the ore will respond to wetting, leaching, and drainage.
Electrical resistivity technology takes advantage of how and why rock, soil, and water interact with electrical current. Wet and dry areas in the ground create contrasts in the electrical properties revealing hydrologic anomalies in the subsurface. On leach pads, electrical resistivity highlights moisture conditions as it relates to electricity flowing through the ore. The effects of haul road damage will show as very resistive (dry) material, indicating where the leaching solution does not migrate. In contrast, areas of a leach pad on either side of a haul road should be highly conductive, showing wet or saturated ore. (See example below)
For more than 20 years, hydroGEOPHYSICS has been mapping leach pads with electrical resistivity, including gold, silver, and copper facilities across North and South America. In each case, mining operators have realized considerable value through a holistic understanding of subsurface hydrological properties.
Geophysical characterization of heap leach pads has the following benefits:
Reveals the effects of haul road compaction
Reveals hidden hydrological features in heaps
Shows contrasting wet and dry areas
Detects and tracks seepage
Shows internal structure that controls solution movement through heaps
Provides actionable information to optimize performance
Example:
A copper mine in Arizona was having issues with ponding along a 250ft section of a leach pad. The ponding was caused by reduced percolation, likely from compacted material below it. To investigate, the mine contracted HGI to look inside the heap to gain insight into the problem. The electrical resistivity profile revealed resistivity variations that were interpreted as either moist or dry, where low resistivity material (blue and purple contours) in the section below is indicative of wetter ore. A layer of wetted ore was disrupted by a large resistive feature (red blob from 1400 to 1650 ft along the line). The resistive feature was later discovered in historical aerial photos to be the location of a historical haul road. Despite ripping and conditioning the road for leaching, the extensive damage to the pad was obvious and permanent down to the native bedrock (in black dashed line). It was estimated that over one million tons were inaccessible for surface leaching.
A cross-sectional view of a resistivity survey showing how a haul road has caused significant compaction in a heap leach pad impeding the flow of leaching solution.
When it comes to the understanding of large scale, long term hydraulic conditions, and PLS movement in leach pads, we will leverage our ingenuity with your needs. HGI will support every aspect of your geophysical project, from design and acquisition to interpretation and actionable solutions.
Follow the link below for more information on geophysical characterization and mapping. If you have a project where this technology can benefit you or your client, call us! We are passionate about what we do and honest with applications and limitations of geophysical technologies.
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About the Author: Dr. Dale Rucker | hydroGEOPHYSICS
Dr. Dale Rucker currently acts as the Chief Technical Officer (CTO) for HGI. He is a geophysicist and hydrogeologist with a strong background in engineering and publishing. As CTO, Dr. Rucker has been instrumental in bringing to HGI new geophysical-based technologies to solve complex problems involving water resource, mining, engineering, and geotechnical issues.
Dr. Rucker is also the editor-in-chief of a well-regarded geophysical journal, the Journal of Environmental and Engineering Geophysics (JEEG). Over the past 15 years, he has published over 40 peer reviewed papers and book chapters in subjects of mining, karst, hydrogeology, and geophysics. Dale holds a BS in Mechanical Engineering, MS in Civil Engineering, and a Doctorate in Hydrology and Water Resources from the University of Arizona.
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