Effects of blasting vibrations in heap leaching piles located in the vicinity of an open pit mine

Abstract

The current thesis project was done in collaboration with Terrafame, which is a large-open pit mining company located in Sotkamo, Finland. Terrafame extracts and process nickel and cobalt into battery chemicals, but also supplies zinc and copper to their customers for metal processing. Terrafame utilizes a bioleaching process to extract the minerals employing one primary heap leaching pile, and four secondary heap leaching piles, which annually receive approximately 18 M tons of crushed ore material.

Open pit is the most common mining method used for the extraction of minerals near the surface. If the blast is well designed most of the energy will be used to break the rock, nevertheless some of the leftover energy will be transformed into vibrations as ground motion or as air-blast.

Peak ground acceleration (PGA) is the most used strong motion parameter, although there is a lack of correlation with structural performance during past earthquakes. Thus, other parameters will be considered, including response spectrum, and Arias intensity (AI).

Slope stability is a must in the planning, operation, and post-closure stages of heap leaching piles. The most common causes for slope failure are the change in slope geometry, the variation of the pore water pressure, vibrations, and earthquakes.

Liquefaction occurs when the cyclic stresses bring the soil to a critical state where the material’s strength drops dramatically, and its structure is not strong enough anymore to resist the shear stresses.

In this case, four secondary heap leaching piles are under study. Following a logical behavior, the closest leaching pile to the open pit possesses the highest values for acceleration, velocity, and amplitude compared with the other three piles.

However, there is an uncertainty given that the lowest values for acceleration, velocity and amplitude do not belong to the farthest leaching pile to the open pit. A more accurate statement of this behavior cannot be given at this point because there are no more leaching piles in farther distances to study or more points of measure.

For the preliminary liquefaction assessment, a simplified analysis was performed for the leaching piles where the lowest values of factor of safety (FoS) against liquefaction due to blasting vibrations are in the first three meters of the piles, but none of those represent an imminent risk for the integrity of the structures.

Arias intensity has proved to be efficient as a liquefaction triggering index. In this case, the values obtained of Arias intensity in the leaching piles due to blasting vibrations are very low, which means that there is not a risk for liquefaction.

Finally, it is recommended to make a wider geotechnical campaign to characterize the piles material, and also perform a definitive geotechnical risk assessment which includes a numerical model of the leaching piles with their geometry, and an accurate analysis of the variation of the pore pressure during a ground motion.