Tailings

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Tailings Dam Assessment

Tailings dams are often constructed as part of mining operations to contain mining waste. These types of earthen impoundments can be used to store earthen waste or to hold a slurried combination of solid waste and water, which can include heavy metals and minerals that cause the water to become highly acidic or basic. In addition, tailings dams are typically unlined and susceptible to unplanned release or transport of materials through the infiltration of rainwaters. A tailings dam failure has the potential to be a great environmental disaster.

 

Electrical resistivity measurements can provide insight into the tailings dam stability by identifying areas of differing electrical properties.

 

 

A small stream of water flows from a rocky, barren slope carrying traces of white and yellow mineral deposits down to a dark, still body of water below. The terrain, dry and rugged, shows patches of discolored earth indicative of various mineral compositions, perfect for tailings seepage mapping.
 

A Geophysical Investigation service provided by HGI is one way of reducing the probability of such a disaster from occurring, using non-invasive and cost-effective techniques. The structural integrity of a tailings dam can be evaluated using a variety of geophysical methods, such as electrical resistivity, seismics, and electromagnetics. Geophysical methods can determine whether the structure of a tailings dam is sound, as well as identify areas where materials (liquid or solid) may be escaping the dam. In theory, an engineered structure such as a tailings dam should have somewhat predictable and homogeneous properties. Any weaknesses or instabilities may stand out in contrast to the background, stable materials.

 

Electrical resistivity measurements can provide insight into the tailings dam stability by identifying areas of differing electrical properties, which in this case would be highly influenced by moisture content and material grain size. A survey would employ an array of stainless steel electrodes placed into the ground surface, along with the associated cabling required to relay measurements back to a field computer. The measurements are compiled and modeled to provide a cross-sectional profile along the dam, or potentially along the foot of the dam, to determine the presence of any weaknesses in the dam structure.

 

 

Image of electrical resistivity plots showing the directions of tailings dam seepage Flow – Hydrogeophysics Inc.
 

 

A seismic investigation would provide additional insight into the structural integrity of the tailings dam by detecting imperfections or anomalies within the dam. Seismic methods rely on the introduction of an active noise source (such as by striking a metal plate on the ground surface with a sledgehammer) and then analyzing the propagation of sound waves through the subsurface. A setup would appear similar to that of an electrical resistivity survey, only with seismic an array of geophone sensors would be used to measure the sound waves. Subtle variations in the material structure due to settling, faulting, the creation of void space, etc., can be detected because of the way these types of weaknesses affect the reflections and travel of sound waves through the dam. Survey results would be relayed in the form of a cross-sectional profile.

 

Electromagnetic methods are another effective way of evaluating a tailings dam. These types of surveying are often faster and easier to conduct, though there are some limitations. Measurements are typically made using a handheld device as the user walks the survey area. The sensors are able to detect changes in soil conductivity, which is again related to soil properties and moisture content. Typically, one or more sensor frequencies are used to evaluate different depths with lower frequencies penetrating to greater depths below the ground surface. This type of a survey provides increased horizontal resolution but decreased vertical resolution.