In general, shallow deposits are mined by open-pit methods. When deposits reach a substantial depth, it is possible to combine open-pit and underground mining methods. In this case, mine planning and optimisation play an important role in the decision-making process. The transition from open pit to underground mining is one of the major challenges in mining engineering. Mines that are likely to transition from open-pit to underground mining will reach a transition point where a decision must be made whether to expand the pit or go underground. Determining the optimum transition point is the transition problem. In the current traditional approach to combined mining, the ultimate pit boundary is determined and the pit design is carried out as a first step. At this stage, the ultimate pit limit can be optimally determined using algorithms such as the Lerchs-Grossman algorithm. Optimisation and design of the underground mine will begin after the standard ultimate pit limit. However, if the economic potential associated with underground resources is taken into account, the final pit can be significantly reduced. It is therefore essential to consider both the open pit and underground potential simultaneously, in order to maximise the profitability of the project. 🔄The schematic section illustrates three critical elements to be considered in mining transitions: 1️⃣ Ultimate Pit Boundary: The limit of economically viable open pit mining. Determines the optimum time to switch to underground mining. 2️⃣ Optimal Transition Point: The point at which the marginal costs of deepening the mine exceed those of going underground. Must take into account: geology, metal prices and operating costs. 3️⃣ Crown Pillar: Buffer zone between the two operations. Thickness calculated for : ✓ Preserve shaft stability ✓ Maximise ore recovery.

Ore dilution and quality control in open-pit mines 1. Definition Dilution in the mining sector refers to the inclusion of sterile material that is not separated from the ore during mining operations and is extracted with the ore. This waste material mixes with the ore and is then sent to the processing plant, resulting in an increase in total tonnage and a decrease in average grade compared with the initial plan. 2. Internal and external dilution Dilution in mining operations can be internal or external. - Internal dilution: occurs inside a mining block in which pockets of waste rock cannot be separated and are extracted with the block; also when there is low-grade material surrounded by high-grade material. - External dilution: refers to waste rock outside the ore body that is extracted within the mining block. It varies according to the geology, the shape of the ore body, drilling and blasting techniques, the scale of operations and the size of the equipment. 3. Impact of dilution on the value of a mine One of the main consequences of dilution is the reduction in mill feed grade. A lower feed grade means lower revenues. For an ore of marginal grade, dilution can reduce the grades to such an extent that it becomes uneconomic to process, in other words, dilution can turn a block of ore into barren. So, as a result of dilution : - There will be a loss of material and the mine's overall reserves will decrease in a given pit. - The energy and materials used in the processing plant to process the waste rock portion of the feed are wasted. By Consequently, the unit operating cost of the crusher increases directly as a function of the dilution factor. 4. Some methods for controlling of ore dilution Dilution control is essential to maximise ore recovery and minimise unnecessary waste rock, which can have a significant impact on the overall profitability of the mine: - Accurate geological modelling and Knowledge of the ore deposit: improvements in the accuracy of geological models and knowledge of the ore deposit are required to accurately delineate the boundaries of ore and waste rock zones ; - Effective grade control: effective grade control procedures must be grade control procedures to accurately determine the grade of ore mined, thereby reducing the risk of dilution due to the mixing of high and low grade materials; - equipment selection and operation: use appropriate mining equipment mining equipment capable of selectively extracting ore without unnecessary dilution; - monitoring and reporting: implement comprehensive monitoring and reporting systems to track dilution levels and identify potential areas for improvement.

The main underground mining operations can be summed up under seven headings corresponding to the life cycle of the operation: 1️⃣ Preparatory work to access the ore by shaft or chute : Depending on the configuration of the deposit (topography, depth, spatial envelope, etc.) and the geomechanical characteristics of the host rock and ore, access routes can be created by digging a shaft or a drift. Shafts and galleries are dug using drilling and blasting equipment (for hard and/or abrasive ground)💥, or using tunnel boring machines or point-attack machines (for softer and/or less abrasive ground). 2️⃣ Blasting 💣 Blasting is one of the basic operations involved in moving an infrastructure work forward (digging a downspout), but it is also part of the ore mining cycle (tracing) or digging the host rock. The ore blasting operation is carried out : 📍 Either using explosives🧨, in which case blast holes must first be drilled. 📍 Or directly by mechanical blasting using specific machines (spot blasters, cutters, etc.), blasting the materials according to their geomechanical characteristics and the opening of the blasting site. 3️⃣ Purging and underpinning ⚠️: 📍 The basic purging operation, which is quite delicate, consists of removing from the vault and the face any unstable blocks and flakes that may have been shaken by the blast. This safety measure must therefore be carried out systematically during the advance cycle before the underpinning operation. It can also be carried out before a manual drilling operation (depending on the terrain) and is a prerequisite for safe access to the site. 📍 The underpinning operation is carried out after the site has been made safe by purging. It consists of reinforcing the stability of the walls by various means, such as timbering or metal frames, anchoring, bolting, gunning and walking underpinning (which is rarely or never used at present). 4️⃣ Loading/Transport (Marining) 🚛: The marining operation consists of loading the cuttings and evacuating them quickly so as not to penalise the production cycle. Marining is generally carried out using wheeled loading equipment (more rarely on rails) adapted for the bottom and capable of driving to a stockpile at the bottom or to a lorry loading point for haulage. More rarely, on small, low-productivity sites, clearing is carried out using a winch for scraping towards a jet stack, or using a vibrating feeder (racking). 5️⃣ Dewatering 💧 Dewatering is the technical name given to operations to remove water from mining sites by pumping or gravity flow. 6️⃣ Ventilation 🌬️ Ventilation is essential to the safety and well-being of miners working underground. 7️⃣ Backfilling 🔄 This operation makes it possible to: ➕ Increase the recovery rate in certain cases (e.g. chamber and pillar mining). ➕ Resolve problems of ground instability. ➕ Minimise driving distances (rising/dependent sections, tamping).

This is one of the most important aspects of underground mining to ensure safe mining. 📍 Basic objectives of mine ventilation systems: (1) To provide airflows of sufficient quantity and quality to provide oxygen in the working area. (2) To dilute contaminants(hazardous gases and dust) to safe concentrations and remove them from the mine. (3) To ensure the appropriate quality of the composition of the atmosphere within the mine, in terms of oxygen content and dilution of noxious gases. (4) To ensure sufficient climatic conditions, in terms of temperature and humidity. 📍 Origins of gas mixtures present in mining operations : 1- Endogenous origin : gases contained in the reservoir and/or in the surrounding rock before exploitation. The composition of these gas mixtures is linked to the nature and geological environment of the materials extracted.These gases are continuously released during mining, through an expansion mechanism, and migrate into the atmosphere of the underground workings. 2- Exogenous origin: these are gases produced by specific chemical reactions during mining activity. Most often these are carbon monoxide and dioxide produced by the oxidation of certain ores and hydrogen sulphide produced by the decomposition of sulphurous materials such as wood; various gases linked to the use of explosives and combustion engine machinery. 📍Ventilation principle for underground mines. The general principle is to supply fresh air in sufficient quantities to all parts of the mine that are still active, and to evacuate used air to the surface. Mine ventilation is classically achieved through : (1) A main ventilation system that ensures the continuous introduction of atmospheric air and the discharge of used air to the surface via the return structures (shafts). The air circulation is ensured by main fans of variable power and flow installed on the inlet or return structures. The main ventilation system is most often suction, more rarely blowing. (2) Secondary ventilation, which recovers fresh air from the main ventilation circuits and provides local ventilation in specific areas. This is achieved by means of secondary fans and circulation ducts. 📍Elements of mine ventilation systems: + Interconnected airways and working zones within the mine. + Connections to the surface. + Fans to produce airflow. + Control devices to ensure that the air courses around the mine as required. 📍Main factors in sizing ventilation - Geometry of the mining network; - Mining method and method of blasting (by explosives or mechanised); - Nature and volumes of gases present (mineralogical and geochemical characterisation of the deposit and surrounding rocks); - Number of people employed.