Underground Stope Research Articles

Application of supervised random forest paradigms based on optimization and post-hoc explanation in underground stope stability prediction

This study applies a supervised random forest (SRF) paradigm to predict the underground stope stability. To improve the model prediction accuracy, 400 actual stopes with three main features (the optimized stability number (N), the hydraulic radius (HR), and a rock mass quality factor (Q)) are used to train the model, and nine meta-heuristic (MH) optimization algorithms are utilized to select the optimal hyperparameters of the SRF model. The prediction results show that the tiki taka algorithm-based SRF (TTA-SRF) model obtains the most satisfactory classification performance, with the smallest variance in the overfitting evaluation (Precision of 0.0201, Recall of 0.0257, Specificity of 0.0061, Accuracy of 0.0125, and F1-measure of 0.0246). Furthermore, four post-hoc explainable techniques (i.e., model-independent) including the feature importance (FI), the Shapley additive explanations (SHAP), the partial dependence plot (PDP), and the individual conditional expectation (ICE) are adopted to explain the best prediction model. The results of the model interpretation show that all features are essential for predicting the stope stability and they have an opposite tendency on the stability prediction of stable and caved stopes. In particular, the limits at which HR, Q, and N do not contribute to the stope stability prediction are 18 m, 40, and 100, respectively. The results of optimization designs for a specific case stope based on the visualization program showed that the suggestions given by the prediction model are desirable. In conclusion, a high-performance and strongly explanatory prediction model is proposed in this study to facilitate the refinement of stope stability assessment in underground space.

Characteristics of an underground stope channel supplied by atmospheric precipitation and its water disaster prevention in the karst mining areas of Guizhou

Atmospheric precipitation floods mining areas, which seriously affects the safe production of coal mines. However, research on the mechanism underlying precipitation supplying coal mines, particularly in karst landform areas, remains far from sufficient. Based on the collection of a large amount of geological and hydrogeological mining data and some data related to mine atmospheric precipitation and mine water inflow, the channels of atmospheric precipitation supplying mines in the main coal-producing areas of Guizhou, China, are systematically analysed and studied. They are divided into weathering zone fractures, mining fractures, water diversion faults, water diversion collapse columns and karst channels. Recharge channels have the characteristics of surface infiltration, pipeline flow and layered recharge, as well as self-healing after being filled by surface loess and other materials. The supply of atmospheric precipitation to the coal mine stope is seasonal. The mine water inflow in the rainy season is 1.2 ~ 12 times that in the dry season, with an average of 1.9 times. The supply has hysteresis. The lag time of surface infiltration, pipeline flow and layered flow is 2 ~ 4 days, within 24 h and more than 2 days, respectively. The recharge is affected by the burial depth of the coal seam and the characteristics of the combined upper roof slate. Among the mines affected by atmospheric precipitation and water disasters, some mines have carried out research on the comprehensive treatment of water disasters, implemented supplementary exploration projects such as surface hydrogeological drilling and geophysical exploration, or carried out hydrochemical research. Some mines have adopted water prevention and control projects, such as blocking ground water diversion cracks, constructing water diversion projects, adjusting the mining time of the working face, transforming the drainage system and improving the drainage capacity, to ensure the safe production of mines. This research achievement may provide a theoretical basis and practical experience for the prevention and control of atmospheric precipitation infiltration in coal mines in karst areas.

Analyzing the Energy and Damage Constitutive of Cemented Backfill with Different Water Content under Dynamic Load.

The dynamic characteristics of the filling body are the key parameters for designing the filling ratio and evaluating the stability of an underground stope. The different environment (water-bearing state) of the filling body in the underground stope exerts a complex impact on the mechanical behavior of the filling body. Therefore, six groups of cemented filling body specimens with different states were formed and subjected to dynamic uniaxial impact tests. The effects of water content on the mechanical properties, fractal dimension, and deformation damage characteristics of the cemented backfill under dynamic load were analyzed in depth, and a dynamic damage constitutive model that considers water damage and the compaction stage was established. The results indicate the following: (1) Due to the change of the specimen from the dry state to the water saturation state, the dynamic compressive strength of the cemented filling body decreases from 5.03 Mpa to 1.79 Mpa; however, the ductility of the specimen generally increases, and the filling body specimens with different water contents mainly exhibit tensile failure. (2) There is a significant nonlinear relationship between the water content and the fractal dimension Db of the cemented backfill specimen, and the growth rate of the fractal dimension Db tends to slow down with the increase in the water content. (3) From the energy evolution perspective, the water content of the specimen exerts a significant effect on the elastic deformation and failure stage of the stress-strain curve, and the slope of the dissipated energy-strain curve decreases with the increase in water content. (4) Based on the Weibull distribution and damage theory, a statistical damage constitutive model of cemented backfill was established, and it was compared with the experimental curve to verify the rationality of the model. Therefore, the relationship between stress and damage and the strain curves is discussed, and it is inferred that the damage evolution curve of cemented backfill is a typical S-shaped curve that exhibits a stable development-rapid increase-tending to be gentle. This study can provide a theoretical reference for further understanding the dynamic behavior and stability of backfill under different water conditions.

Mathematical modelling of waste rock management through incorporating open-pit waste rocks in underground stope filling: An environmental approach

Reducing the footprint of waste rock dumping would result in cost savings and mitigate the environmental impacts of mining. This study develops a mixed integer programming model for waste rock management, aimed at identifying the optimal dump location among surface waste dumps and underground stopes. The model generates an optimal waste rock dumping schedule and proposes the best destination, with the objectives of minimizing the total cost of waste rock hauling and maximizing the supply of filling materials for backfilling underground stopes from the waste rocks. By applying the model to a real metal mine, a total cost reduction of approximately 14% was accomplished by utilizing open-pit waste rocks as underground filling materials, replacing alternative sources. Sensitivity analysis approved that supplying the filling materials from waste rocks was a priority, which resulted in a reduction of 3,750,000 tonnes of dumping waste rocks on the surface and a significant reduction of the waste dump environmental footprint. The model not only improves the economics of mining operations but also mitigates the environmental impacts of open-pit waste rock dumping by using waste rocks for backfilling underground stopes instead of supplying materials from outside the mine.

NetherDrone: a tethered and ducted propulsion multirotor drone for complex underground mining stope inspections

Underground stope mapping is crucial to evaluate the quantity of blasted rock and the site integrity. In recent years, lidar-equipped drones have been used to map stopes with higher precision and without blind spots. However, they have limitations, such as large size, challenging lidar positioning on the drone, limited flight time for detailed visual inspections, and unreliable communication underground. This paper discusses the development of a compact tethered drone called the NetherDrone, specifically designed for stope inspections. The NetherDrone uses custom ducted propulsion to increase thrust efficiency by 50%. It reduces the propellers’ diameter and overall frame while maintaining an adequate lifting capability with low power consumption. The drone features an onboard 120 m tether spool for communication and power transmission, as well as a rotating arm to deploy the cable and reduce yaw moments from the tether tension. Flights in a real stope demonstrated that the drone could effectively move at least 50 m deep into a complex stope, complete a detailed lidar scan, visually scan one face of the stope in close proximity during 20 min, travel a total distance of 270 m, and maintain communications with an operator at all times through the tether.

A Rock Engineering System Based Abandoned Mine Instability Assessment Index with Case Studies for Waihi Gold Mine

Mining as one of the fundamental elements of the economy has left tens of thousands of mines abandoned. With increased urbanisation and population growth, industrial or residential sites become prone to post-mining geotechnical hazards such as subsidence from abandoned mining sites. Instabilities of abandoned workings may, also, interfere with active mining, imposing risks and considerable delays in the operations. Proper knowledge of these geotechnical hazards and the mechanisms of instabilities are critical for the classification and prioritisation of the sites for rehabilitation or repurposing of abandoned mining sites.The long-term stability analysis of abandoned mine workings is a complex task for engineering geologists and rock engineers. Numerous factors have been identified here as affecting the stability of underground mining excavations including depth, age, shape, and geometry of the mining excavations, underground water, in-situ stress state in the area, geotechnical characteristics, discontinuities, seismic loading and most importantly weathering and deterioration of the rock masses.To provide an efficient approach for assessing the complex phenomena of geotechnical hazards associated with shallow abandoned underground stopes in hard rock environments, with interconnected parameters, this study proposes a systematic approach by adopting and modifying the Rock Engineering System, RES, (Hudson 1992b). An Interaction Matrix is developed, the critical factors and dominant modes of instabilities are identified, and the hazard potentials are determined. An empirical index known as the Abandoned Mine Instability Index, AMII, is proposed to enable a quick and preliminary assessment of geotechnical instability and subsidence hazards in post-mining areas. This systematic approach was tested against one case history involving multiple crown pillar collapses and sinkhole subsidence incidents related to historic underground stope mining followed by a modern adjacent open pit at the Waihi mine in New Zealand. Information from relevant literature was used to assess the post-mining geotechnical hazards. The outcomes of the study indicate that the methodology can be successfully applied to differentiate between stable and unstable areas at the same mine. Future comparisons of mines in different regions are required to fully confirm the calibration of the AMII.

Evaluation of Stope Stability and Displacement in a Subsidence Area Using 3Dmine–Rhino3D–FLAC3D Coupling

Underground stope stability has important implications for the safety and efficiency of metal mining, especially when there are subsidence zones above. To analyze the stope stability around the subsidence area of the Hongling ore body, this paper innovatively adopts the 3Dmine–Rhino3D–FLAC3D coupled modeling numerical method to model and predict the excavation process of the stope around the Hongling subsidence area; it monitors and analyzes the displacement and stress-distribution changes in a selected stope roof, and its left-hand side and right-hand side walls. This paper realizes the separation and extraction of the special ore body and surface topographic features of the Hongling ore body; it discusses the stope stability characteristics around a subsidence area based on the real topography of the mining area and the occurrence form of the ore body. The study found that the influence of the upper collapse area on the undesired displacement of the downhole stope is mainly reflected in the roof displacement. As the stope position moves away from the collapse area, the roof displacement decreases significantly, the maximum displacement is at the center of the stope roof, and the maximum principal stress occurs at the corner of the stope.

Optimisation of Three-Dimensional Stope Layouts Using a Dual Interchange Algorithm for Improved Value Creation

Optimisation of three-dimensional (3D) underground stope layouts is a computationally complex process since it involves the modelling of many variables and constraints. As the number of variables and constraints increases to reflect the actual mining practice, the model complexity and solution time tend to increase exponentially, making the optimisation problem intractable. Metaheuristic approaches have therefore been used predominantly to solve the problem, but do not guarantee ‘true’ optimality. To minimise this limitation, a dual interchange algorithm (DIA) was developed by combining the strengths of two metaheuristic generic algorithms, namely the particle swarm optimisation (PSO) and genetic algorithm (GA). The DIA performance was compared to that of the Mineable Shape Optimizer (MSO) on four different design scenarios. The DIA generated stope layout economic values (SLEV) for three scenarios which were 0.3%, 3.4%, and 8.3% higher than for MSO under fixed and variable stope width conditions, while MSO produced a SLEV which was 9.7% higher than the DIA for a fixed stope width. This paper demonstrates that the DIA is a novel way of solving 3D optimisation of stope layouts under variable stope widths as encountered in actual mining practice.

A Coupled THM Model for Simulating Mechanical Response of Open Pit and Underground Stope to the Filling of the Pit

A Coupled THM Model for Simulating Mechanical Response of Open Pit and Underground Stope to the Filling of the Pit

Safety Analysis of Synergetic Operation of Backfilling the Open Pit Using Tailings and Excavating the Ore Deposit Underground

The transition from open pit mining to underground mining is essential for mineral resources to achieve deep excavation. Recently, cemented paste backfill (CPB) has been proposed as a novel technology to achieve open pit backfill (OPB). The proposed method not only eliminates the danger of the open-pit slope but also reduces the disposal of waste tailings. In order to ensure safe mining during the synergetic operation of OPB and underground mining, it is of great significance to improve this technology. In the present study, an open-pit metal mine in Anhui Province was taken as the research object. Then, the safety of underground stope roofs, underground backfill pillars, and open-pit slopes was evaluated during OPB. To this end, numerical simulations were performed and experiments were conducted on a similar physical model. Accordingly, the backfill mechanical parameters were optimized. The obtained results show that backfill height exerts the most significant effect on the safety of roofs and underground backfill pillars, accompanied by small displacements along the vertical direction during the backfill process. Moreover, concentration was observed at the foot of the slope, while the overall structure remained stable with no considerable displacement. The overall safety factors met the safety requirements. Based on the obtained results, the optimal foundation strength, foundation height, backfill strength and backfill height were 4 MPa, 10 m, 1.5 MPa, and 120 m, respectively. Moreover, it was concluded that displacements in the abovementioned three regions tend to be stable when the backfill height exceeds 150 m without damage. The present article provides a certain theoretical and application guideline for OPB practices in similar metal mines and suggests possibilities for cleaner production.

Evaluation of Underground Mineable Reserve in Presence of Grade and Commodity Price Uncertainties

The uncertainty-based mine evaluation and optimization have been regarded as a critical issue. However, it has received less attention in the underground mines than in the open-pit mines due to the diversity of the underground mining methods, and the underground mining parameters' complexity. The grade and commodity price uncertainties play essential roles in mining projects. Mine planning by not incorporating these uncertainties is accompanied by risks. The evaluation and risk assessment of the mine plans is possible through evaluating the mineable reserve in the presence of such uncertainties. In the present work, we evaluate the effects of grade and commodity price uncertainties on the underground mining stope optimization and the resultant mineable reserve. In this regard, the stope boundary is studied both deterministically and stochastically in the presence of the grade and price uncertainties. For this purpose, in this work, we implement the conditional simulation in order to generate equally probable ore reserve models. Furthermore, we optimize the stope boundary using the floating-stope algorithm in each realization. Several decision support criteria including the 'mineable reserve,' 'metal-content,' 'profit,' and 'value-at-risk' are defined to assist the decision-maker in uncertain conditions. Finally, a procedure is defined in order to consider two types of uncertainty sources simultaneously in underground mining. It will guide the decision-maker toward the most appropriate stope boundary that best fits the mining company's requirements. The procedure is implemented in a bauxite mine, and the optimal stope boundary is determined concerning the different criteria.

Research on the Evolution Characteristics of Rock Mass Response from Open‐Pit to Underground Mining

This study is based on the engineering background of pit no. 2 in Jinning Phosphate Mine, China. In order to systematically analyze the movement, deformation, and failure laws of surrounding rocks in underground stopes. The room and pillar method is used to excavate and stop the ore bodies in the mining area. Combined with the similar physical model experiments and discrete element MatDEM numerical simulations, it reveals the deformation and failure laws and evolution characteristics of the surrounding rock of the stope in the process of converting from open‐pit to underground mining. The results show the following: (1) Along the inclination of the ore body, the farther the horizontal and vertical displacements are from the underground stope, the less the impact of mining stress. On the other hand, along the inclined vertical direction of the ore body, the farther the measuring point is from the stope, the smaller the range of mining influence will be. (2) In the process of ore body recovery, the rupture of the overlying strata of the stope has an obvious layered structure, with collapse zones, fissure penetrating zones, and microfracture loosen zones appearing from the bottom to top. In addition, the movement and destruction of the overlying strata of the entire stope is an “elliptical arch.” Therefore, the results of similar simulation experiments and numerical simulation are basically consistent.

Research progress in the mining technology of the slowly inclined, thin to medium thick phosphate rock transition from open-pit to underground mine

Abstract This paper takes the deep ore body of Yunnan Phosphate Group Co. Ltd, the largest open-pit chemical mining enterprise in China, as the research background, and systematically introduces the technical problems recognised by many Chinese researchers in the past eight years on the open-pit to underground mining of gently inclined thin to medium-thick ore bodies with a soft interlayer. It shows that the mining of open-pit transferred to underground is a complex engineering system, and the underground stope surrounding rock and overlying strata present a nonlinear failure process. Through mining process innovation, mining method innovation and improvement, research was undertaken on new processes and technologies for phosphorus mining under complex conditions. The relevant research results not only have important economic value and academic significance for Yunnan Phosphate Group Co. Ltd. but also have important guidance and impetus to the exploitation of a large number of similar phosphate resources in China.

Experimental Tests on a Small-Scale Model of a Mine Stope to Study the Behavior of Waste Rock Barricades during Backfilling

This paper presents a reduced-scale physical model of a mine stope used to reproduce the underground stope backfilling practiced in some Canadian mines. The objective is to study the geomechanical behavior of the waste rock barricades in interaction with the mine backfill. The instrumentations, along with visual observations and preliminary results, are presented. The main results demonstrated that: (i) the stability of the barricade depends on its physical properties (e.g., size, location, particles gradation and compaction) and the frictional behavior at the barricade/drift walls interface, and (ii) for two backfill formulations, cemented and uncemented, prepared with 70% of solid mass concentration, the volumetric strain due to self-weight consolidation of the backfilled room was higher for uncemented backfill (16%) than for the cemented one (4.5%). In addition, the results highlighted the importance of using shotcrete around the downstream face of the barricade, mostly at the top, to close the gaps and bind the barricade particles, which improves its stability.

Application of Sequential Gaussian Conditional Simulation to Underground Mine Design Under Grade Uncertainty

In mining projects, all uncertainties associated with a project must be considered to determine the feasibility study. Grade uncertainty is one of the major components of technical uncertainty that affects the variability of the project. Geostatistical simulation, as a reliable approach, is the most widely used method to quantify risk analysis to overcome the drawbacks of the estimation methods used for an entire ore body. In this work, all the algorithms developed by numerous researchers for optimization of the underground stope layout are reviewed. After that, a computer program called stope layout optimizer 3D is developed based on a previously proposed heuristic algorithm in order to incorporate the influence of grade variability in the final stope layout. Utilizing the sequential gaussian conditional simulation, 50 simulations and a kriging model are constructed for an underground copper vein deposit situated in the southwest of Iran, and the final stope layout is carried out separately. It can be observed that geostatistical simulation can effectively cope with the weakness of the kriging model. The final results obtained show that the frequency of economic value for all realizations varies between 6.7 M$ and 30.7 M$. This range of variation helps designers to make a better and lower risk decision under different conditions.

Liquid spread mechanisms in high-temperature underground stope leaching

Liquid flow behavior is a key factor that affects the leaching rate and metal extraction of stope leaching (SL), but related experimental studies are very limited. The liquid spread mechanism of SL is complicated because of the high-temperature conditions, the structural characteristics of the stope, and the pore characteristics of the ore heap. In this study, a device was developed to investigate the liquid spread mechanisms of the upper wall and foot wall of the stope. The influence of temperature, dip angle, and flow rate on the liquid spread was investigated. It is shown that the most significant feature of liquid spread in a stope is the liquid flow towards the bottom of the stope vertically rather than horizontal liquid spread. The increase of temperature accelerates the liquid spread, leading to the emergence of preferential flow paths. When the dip angle decreased from 90° to 60°, the liquid spread along the foot wall increased, including the change of the preferential spread direction, though the initial wetting time of the upper wall decreased. Increasing the flow rate within a certain range does not result in a more obvious liquid preferential flow path. Although the liquid spread of stope leaching still requires more studies in detail, the results of this work could be helpful to understand industrial stope leaching mechanisms.

Study on the Instability Mechanism and Grouting Reinforcement Repair of Large‐Scale Underground Stopes

The environmental conditions due to unreasonable mining in underground stopes, the slurry diffusion mechanism in the grouting reinforcement of a stope within its influence, the causes of large‐scale instability collapse, and the catastrophic stope process are analyzed, and limit upper line analysis theory and numerical analysis methods are comprehensively adopted, revealing the continuous catastrophic collapse mode of large‐scale underground stopes. The method of determining the stope instability collapse boundary and the slip surface within the range based on the theory of the maximum shear strain increment is proposed, and the diffusion radius and range of the grouting slurry during the reinforcement process, which considers the multifield coupling factors, are obtained. The results show that the U‐shaped hidden danger area formed after the collapse of the large‐scale underground stope. The influence range reaches six adjacent stopes, which are symmetrically distributed around the collapse; the mining instability is manifested as a catastrophic chain process of stress change, energy accumulation, state change, and instability collapse. The damage mode of instability collapse is a combination method of wedge collapse, circular arc rotation, triangular translation, and strip slip. According to the multiphysics coupling numerical calculation, the diffusion radius of the grouting slurry is 12 m, exhibiting an elliptical distribution. The research results can be used to comprehensively control the underground mining environment, thus effectively solving the safety problems faced by tunnel or roadway excavations above the goaf.

Assessment of expansion and strength properties of sulfidic cemented paste backfill cored from deep underground stopes

The expansion and strength of cemented paste backfill (CPB), especially when prepared from sulfide mill tailings, are valuable properties for the estimation of design parameters for underground stope filling. These properties are particularly important for deep mining. In this paper, the free expansion ratio of CPB specimens containing sulfides was investigated using an invented apparatus called the free expansion measuring instrument. Moreover, the unconfined compressive strength (UCS) of backfill materials cored from the deep stope was studied. In the laboratory, the filling specimens continued to expand, and substantial cracks were observed at 28 days. Moisture and other reactants entered the samples through the cracks, and the specimens continuously expanded until collapse. In the deep mining stope, the strength of the backfill body in the same horizontal plane presents a wave-type change along the flow direction of the paste (or y-direction) and a hammock-shaped change along the x-direction (or crossing the y-direction). In the deep mining stope, due to the wall convergence effect of the deep surrounding rock and the expansion effect of the filling body, the filling material underwent a consolidation process. The backfill material after consolidation had a finer pore mechanism and a more compact microstructure based on the results of scanning electron microscopy. Therefore, the strength of the backfill near the surrounding rock was higher than that in the central area. The mechanical properties of the CPB mass differ at different locations along the same height in the stope, and the expansion of the sulfidic cemented paste and the convergence of the rock wall play reinforcing roles.

The integrated optimization of underground stope layout designing and production scheduling incorporating a non-dominated sorting genetic algorithm (NSGA-II)

Stope layout designing and production scheduling are main phases to determine the profitability of an underground mining project. These are mainly related to the output of one phase, which has a significant impact on generating of the next phase. Individual optimization of these two phases results only in a local optimal solution. To date, the integrated optimization of these phases has been carried out to maximize net present value (NPV). In this paper, a multi-objective integer programming model (MOIP) was developed to optimize this integrated problem in a sublevel stoping operation. The non-dominated sorting genetic algorithm (NSGA-II) was incorporated to solve the objective functions. The Pareto front generated by NSGA-II showed good convergence and diversity with the solution time decreasing significantly (from 7-8 days to 6–7 h). The most preferred solution point on the Pareto front increased recovery by 18.55% in exchange for a 0.41% reduction in NPV with respect to the single objective optimization to maximize NPV.

Risk Evaluation, Design, Implementation, Instrumentation, and Verification for Crown Pillar Extraction at Pinos Altos Mine

The Santo Nino ore body at Pinos Altos Mine has been in production since 2009 using both open pit and underground mining methods. Between 2015 and 2017, an extensive study evaluated extraction strategy of the Santo Nino crown pillar. The Agnico Eagle project team evaluated geomechanical, operational, safety and economical risks before developing a comprehensive plan to extract the Santo Nino crown pillar by underground stoping. The geomechanical aspects of that study focused on interpreting in situ rock mass conditions and understanding the mechanical and hydrological/hydrogeological mechanisms influencing ground reaction during open pit and underground mine operations to make meaningful forward predictions for crown pillar extraction. Numerical modeling played a significant role in making forward predictions. To minimize uncertainty in those predictions, the model parameters were calibrated using measured pit slope deformations, back analysis of historical slope instabilities and underground observations of rock mass conditions. Ultimately, it was decided that long-hole stoping would be utilized for crown pillar extraction. The pit floor would be used as a drilling horizon, and the upper most level of the underground operations served as the mucking horizon. Various safety and operational risk mitigation strategies were developed and implemented; this included geomechanical monitoring using the in-pit radar system and numerous borehole extensometers installed underground to monitor ground deformations. The calibrated numerical model results provided valuable input for the design of the instrumentation program, and once the crown pillar came into production in late 2017, the instrumentation provided valuable data for model verification and stope design adjustments.