Open-pit mining is a low-cost mining method that allows for both a high degree of mechanization of its operations as well as handling large volumes of production. The method was initially designed to mine superficial mineral deposits with a wide range of ore grades, which could not be mined using underground methods. However, during the last decade, the depth of open pit mining has increased, and today it is possible to find pits operating at depths greater than 900 m, with plans to reach final pit depths of 1,000 to 1,200 m in the next 20 to 25 years. The fundamental economic factors in open pit mining, especially if it is deep, are related to slope stability and the efficiency and automation of waste and ore haulage (AHS, conveyor belts, transfer trolley systems among others). Depending on the ore grade and distribution, geometry and dimensions of the deposit, in some cases, underground mining by caving methods may be less costly than open pit mining operating at greater depths.
Today, there are many deposits that have a considerable vertical extension and, although their mining method is open pit, at a certain depth they will have to make very relevant decisions, such as continuing with deeper and deeper open pit mining, and the associated high costs for this type of operation, or changing to underground mining to reach the geological resources remaining below the final pit. Currently, there are several open pit mines that are planning or are in the process of transitioning to underground mining, such as Bingham Canyon, Resolution (USA), Chuquicamata (Chile), Grasberg (Indonesia), Palabora (South Africa), Oyu Tolgoi (Mongolia), among others.
In order to successfully develop a transition project from open pit to underground mining using caving methods, it is essential to establish at least three key aspects from the geotechnical point of view: First, determining if the rock mass will cave, to then define the most appropriate mining method for the characteristics of the deposit, i.e., sublevel, block or panel caving. Secondly, the minimum area and shape required to initiate caving must be defined. Finally, caving mechanics and propagation must be evaluated to ensure that the connection of the cavity with the surface or the open pit floor will occur. All this requires fundamental decisions to be made to define a transition project: height of the mineralized column to be caved and associated to this, dimensions of the basal area (footprint), feasibility of a simultaneous operation of the pit and the underground mine, and the undercutting and extraction strategies, which will define the mining plan of the future underground mine.
A transition seeks to maintain production levels that allow continuing to take advantage of the open pit mine infrastructure, so any underground mining must be massive and incorporate methods such as sublevel, block or panel caving to achieve high production rates and low development, preparation and operating costs. High production rates should be understood not only as reaching a certain tonnage, but also achieving an adequate ore fragmentation, which allows a continuous gravitational flow, and a minimum interruption to the unit operation of ore extraction from the extraction level. Meanwhile, low cost should be understood not only as a low cost per tonne produced, but also an optimized mine design that allows a maximum height of ore column, the minimum necessary initial development, and a mining strategy that allows a safe and reliable operation.
The essential concept in mining by block or panel caving is to take advantage of gravity by undercutting the base of the ore column to induce caving of that column. Therefore, the first geomechanical consideration must be to evaluate whether the rock mass to be mined will cave naturally, or whether pre-conditioning of the column of ore will be required. Hydraulic fracturing boreholes and destress blasting (DDE) are the common techniques applied for this activity. Current practice for caving propagation is usually based on empirical correlations between the geotechnical quality of the rock mass, expressed in terms of Laubscher’s MRMR index, and the hydraulic radius of the caved area, HR. However, this correlation must be used with caution and, preferably, as a basis for the development of a correlation adjusted to the local conditions at each mine. These analyses can be combined with complex 3D continuum and discontinuum numerical models to simulate macro-sequences, front caving performance, connection to surface, abutment stress concentration and potential risks of rockburts. Additionally, these models can provide a preliminary assessment for the collapse potential and verify the support defined.
Expert opinion article published in the GBR 'Chile Mining 2024' Report, Santiago, Chile, August 2024.
