Open Stope Research Articles

Stability Evaluation and Structural Parameters Optimization of Stope Based on Area Bearing Theory

A reasonable and stable stope structure is the premise of realizing safe mining of underground metal ore. To safely mine the gently inclined medium-thick ore body, stope stability in Bainiuchang Mine was analyzed based on the pillar area bearing theory, and stope stability with regard to nine groups of structural parameters was numerically simulated. The results show that the existing stope structural parameters failed to maintain stability requirements and tended to be exposed to the risk of stope collapse. The middle section of the pillar as well as stope roofs and floors are vulnerable due to tensile stress when mining by open stoping, and the compressive stress concentration is prone to occur at the junction of the pillars, stope side walls, roofs and floors. Shear stress contributes little to pillar failure. The reasonable stope structural parameters of open stoping for the gently inclined medium-thick ore body in Bainiuchang mine are optimized using ANSYS numerical simulation: stope height 4.5 m, pillar diameter 4 m, pillar spacing 7 m and pillar row spacing 8 m. The onsite trial shows that the ore recovery rate reaches 82% under these parameters, which also realizes the equilibrium of safety and economy.

Stability analysis of a free-standing backfill wall and a predictive equation for estimating the required strength of a backfill material - a numerical modelling approach

Cemented backfilling has enabled underground hard rock mines to extract orebodies more safely with improved ore recovery. It is important to estimate the minimum required strength parameters for a backfill mix by optimizing the binder percentage to enable cost-effective and safe stoping operations. We conducted three-dimensional numerical modelling to study the effect of various stoping parameters on the stability of a free-standing backfill wall. A Mohr-Coulomb material model was used for the backfill material and the rock-fill interface. A strength reduction technique, excluding friction angle, was employed to arrive at a minimum stable strength value for the backfill. For a given combination of strength values, the stability state of the backfill wall could be demarcated using a displacement and yield zone tracking method. The numerical modelling results are compared to some earlier theoretical models. From the simulations, a predictive equation is developed to predict the required strength parameters for a backfill mix to ensure a stable free-standing wall. Examples are given of the successful use of the predictive equation at some underground hard rock mine sites in India.

The Consolidated Mathews Stability Graph for Open Stope Design

The stability graph method for open stope design is one of the most widely used approaches for predicting the stability of stopes in underground metalliferous mines. The primary purpose of this work is to propose a new stability chart, which includes all relevant case histories, and to exclude parameters with uncertainties for determining stability number. The modified stability number was used to achieve this goal, and the Extended Mathews database was recalculated and compared with the new stability graph. In this study, a new refined Consolidated stability graph was developed by excluding the entry mining methods data from the Extended graph data, and only the non-entry methods data was used. The applicability of the proposed Consolidated stability chart was demonstrated by an open stope example. The probabilities of stability for each stope surface were determined using the logistic regression model and the developed Consolidated stability chart. Comparing the stability analysis results with that of other published works of the same example shows that the determined Consolidated chart, in which the entry-method data is excluded, produces a more conservative and safer design. In conclusion, the size and quality of the dataset dictate the reliability of this approach.

Technology for mining of complex-structured bodies of stable and unstable ores

The object of the study is a rock massif represented by complex-structured ore bodies mined by underground systems with open stoping or systems with bulk caving. Deposits of Kryvyi Rih iron ore basin are represented by different types of ferruginous quartzite, which enables application of a great number of mining systems to mining blocks. There are also barren rock inclusions within the block which are extracted from the block along with ore, and this reduces the iron content of the mined ore mass. Findings of the comprehensive study enable concluding that application of the selective method when mining a deposit can enhance recovery rates. Analysis of methods for determining rock stability results in ascertaining that in case of a 4–7 m thick inclusion of barren rocks it is advisable to use a system with bulk caving at a single stage, and in case of a 7–12 m thick inclusion – in two stages. The first stage involves mining the hanging wall reserves, the footwall reserves are mined at the second stage. This method is distinguished by leaving a barren rock inclusion in the block. The developed options of the mining system allow increasing the iron content in the mined ore mass by 2–4% and obtaining the expected economic effect from 3.0 M to 30.2 M USD depending on mining and geological conditions.

Analyzing Stability Conditions and Ore Dilution in Open Stope Mining

Ore dilution is a fundamental problem for the production process in underground mining operations. Especially in open stoping methods of underground mining, the continuous estimation, monitoring and treatment of instability issues is considered necessary in order to maintain the consistency of the production process. This paper aims to combine empirical nomograms of stability estimation and numerical approaches and thus link the extensive experiences of the empirical design and the quantitative data derived by numerical analyses. To facilitate this, a large number of different geomechanical conditions were modeled and analyzed in the pursuit of obtaining valid and applicable relationships between the empirical stability graphs’ approaches and the numerical simulation models. The parametric analysis was made to express the stability conditions and the dilution with specific design characteristics, using prevalent stability-graph approaches while the numerical models were tested using the RS2 software package. The obtained results include direct and easy-to-use mathematical expressions that can be applied during the initial design of the stoping process, especially for the case of sidewalls (hanging walls and foot walls). Furthermore, through the research, an initial proposal is made for a dilution-based stability graph that could be utilized for the early identification of dilution.

Strength Model of Backfill-Rock Irregular Interface Based on Fractal Theory

In the two-step open stope subsequent filling mining method, the determination of the strength model for the backfill-rock interface is of great engineering significance to study the stress distribution and stability of the backfill in the stope. Considering the deformation mechanism of the interface and the interaction of the asperities, a strength model for backfill-rock with irregular interface has been proposed based on fractal theory, which can effectively describe the shear mechanical behavior of interfaces with random roughness. The model has been compared with the two-body mechanistic model and good agreements have been achieved. The results show that the shear strength of the interface changes non-linearly with increasing fractal dimension D, when the fractal dimension D is in the range of 1∼1.12. The complete relationship between the interface shear strength and the fractal dimension is given, as the fractal dimension increases from 1 to 2 based on the presented model. At the same time, the quantitative relationship between the interface and backfill friction angles during direct shear testing is analyzed.

Development of a knowledge-based system for assessing unplanned dilution in open stopes

A reliable estimation of unplanned dilution is one of the most vital parameters in open stopping mining methods as unplanned dilution and sloughing can lead to an increase of the cost production and most importantly to safety concerns. Owing to this importance, several predictive models to estimate unplanned dilution and stope overbreak have been proposed. However, some of these existing models do not provide any physical meaning of the underlying dilution problem. To overcome this limitation, a fuzzy inference system (FIS) is proposed. In this paper, a new design tool capable of assessing unplanned dilution in a reliable manner is developed. The methodology relies on a fuzzy inference system (FIS) as many epistemic uncertainties that are associated with unplanned dilution are not systematically accounted for in existing methods. To this end, a dilution database from Ridder-Sokolny mine in Kazakhstan has been compiled. It consists of unplanned dilution cases and includes stope geometry, rock mass characteristics and stope reconciliation data. Subsequently, knowledge-based FIS was implemented. Overall, the results of the FIS show good classification performance (84% of accuracy). In comparison to the dilution graph developed based on traditional method, the FIS results show better performance. Hence, it was concluded that the proposed FIS could be an alternative tool for empirical open stope design.

Parametric justification of underground ore mining systems with induced caving

The current situation in underground mining and the related problems and challenges dictate theoretical advancement in the field of design and process solutions when justifying parameters of underground mining systems and adapting them to specific ore body conditions. The mining systems with induced caving of ore and enclosing rocks, especially sublevel stoping, outperform the systems of open stoping and with manmade support of mined-out voids by the most technical and economic indicators. The mining systems with induced caving feature high rate productivity and low cost, which makes them the most promising approaches. The deficient experience of design and operation of underground mines using the systems with induced caving is the cause of high loss and low quality of produced minerals, extra expenditures connected with mineral processing and ecological degradation. Based on the analysis of influence jointly exerted by physical, mechanical, structural and technical factors on the behavior of caved ore outflow, the parameters of an underground mining system with induced caving are justified. The review of the implemented researches and studies in the field of parametric justification of mining systems with caving in Russia and abroad allows a conclusion to be drawn that the widthwise shape of the ore outflow is essentially influenced by the particle-size distribution of broken ore. The outflow width expands with growing size of ore particles, other things being equal. Therefore, the outflow design should be based on the average size of broken ore particles, which is readily determinable in the mine environment.

Stochastic stope design optimisation under grade uncertainty and dynamic development costs

ABSTRACT Stope design optimisation defines three-dimensional extraction volumes aiming to maximise cashflows, subject to geotechnical and operational constraints. Available stope layout methods are deterministic, failing to account for grade uncertainty and variability that affect stope locations and sizes, as well as value. A two-stage stochastic integer programming model for stope design optimisation is proposed, integrating grade uncertainty quantified through geostatistical simulations, level allocation, variable stope and pillar sizes for different geotechnical zones, and development costs. An application at an underground gold mine employing sublevel open stoping highlights the integration and management of grade uncertainty to define risk-resilient stope design.

The stress distribution around the mining excavations under different tectonic loads

The article studies the fatures of stress distribution around the mining excavations under different tectonic loads on the case study of the Zhdanovskoe copper-nickel ore deposit, Murmansk region, Russia. The deposit is located in the Arctic zone on the Russia-Norway state boundary and is excavated by the Severny underground mine using simultaneously the sublevel caving and the open stoping methods. The combination of these mining methods in the same area affects the geomechanical conditions; moreover, the highest level of stresses is identified between the opposing excavation fronts. The deepening of mining increases the geodynamic risks caused by the stress state in the rock mass. In this regard, the study of the Zhdanovskoe deposit rock mass stress state and its transformation due to the mining progress is urgent. To address the issues a numerical geomechanical 3D model was created. To specify the correct boundary conditions, the results of the in-situ measurements were compared to visual observation results. To achieve the most accurate result, the direction and ratio of the tectonic loads were varied and the results were analyzed. The authors have analysed the modelling results in the form of the stresses and deformations distribution in the area around the excavations under different directions of tectonic loads. Based on this analysis, the authors have substantiated a relevant variant of direction of tectonic forces relatively the ore body strike. The results have been applied to forecast the changes in perspective stress-strain state during the mining progress.

Investigation of Backfilling Step Effects on Stope Stability

Cemented rock fill (CRF) is commonly used in cut-and-fill stoping operations in underground mining. This allows for the maximum recovery of ore. Backfilling can improve stope stability in underground workings and then improve ground stability of the whole mine site. However, backfilling step scenarios vary from site to site. This paper presents the investigation of five different backfilling step scenarios and their impacts on the stability of stopes at four different mining levels. A comprehensive comparison of displacements, major principal stress, and Stress Concentration Factor (SCF) was conducted. The results show that different backfilling step scenarios have little influence on the final displacement for displacement in the stopes. Among the five backfilling scenarios, the major principal stress and stress concentration factor (SCF) have almost the same final results. The backfilling scenario SCN-1 is the optimum option among these five backfilling scenarios. It can immediately prevent the increase of the displacement and reduce the sidewall stress concentration, thereby preventing possible failures. Using the same strength of CRF can achieve the same effects among the four mining levels. Applying backfilling CRF of the same strength at different mining depths is acceptable and feasible to improve the stability of the stopes.

A Feasibility Study on The Implementation of Neural Network Classifiers for Open Stope Design

Assessing the stability of stopes is essential in open stope mine design as unstable hangingwalls and footwalls lead to sloughing, unplanned stope dilution, and safety concerns compromising the profitability of the mine. Over the past few decades, numerous empirical tools have been developed to dimension open stope in connection with its stability, using the stability graph method. However, one of the principal limitations of the stability graph method is to objectively determine the boundary of the stability zones, and gain a clear probabilistic interpretation of the graph. To overcome this issue, this paper aims to explore the feasibility of artificial neural network (ANN) based classifiers for the design of open stopes. A stope stability database was compiled and included the stope dimensions, rock mass properties, and the stope stability conditions. The main parameters included the modified stability number (N’), and the stope stability conditions (stable, unstable, and failed), and hydraulic radius (HR). A feed-forward neural network (FFNN) classifier containing two hidden layers (110 neurons each) was employed to identify the stope stability conditions. Overall, the outcome of the analysis showed good agreement with the field data; most stope surfaces were correctly predicted with an average accuracy of 91%. This shows an improvement over using the existing stability graph method. In addition, for a better interpretation of the results, the associated probability of occurrence of stable, unstable, or caved stope was determined and shown in iso-probability contour charts which were compared with the stability graph. The proposed FFNN-based classifier outperformed the conventional stability graph method in terms of accuracy and better prabablistic interpretation. It is suggested that the classifier could be a reliable tool that can complement the conventional stability graph for the design of open stopes.

Substantiation of the parameters of open stoping mining method with abandonment of unrecoverable pillars

Relevance. Developing inclined ore bodies of low and uneven thickness using mining systems with caving results in great losses and dilution, deteriorating the conditions for ore gravitation. As a consequence, it becomes necessary to slice the barren rock in the lying side to form slopes with the desired angle. There are also characteristic ore losses at the dead end of the drawing drift independent of the ore body thickness, therefore acquiring a significant role under a lesser thickness. The height of the sublevels, limited by the ore body’s inclination, leads to a considerable amount of preparatory and development operations. The absence of actual in-process monitoring of useful component content in the ore drawing doses often leads to increased dilution rates due to the drawing of barren caved rocks of the previously worked overlying sublevel. Thus, the technological solutions search and their design parameters substantiation for the conditions of inclined ore bodies of low thickness is an urgent scientific and technical problem. Research aim is to substantiate the parameters of the open stoping mining which provides for leaving unrecoverable pillars which ensure the drawing of the maximum volume of pure ore under the cantilever of the hanging side rocks. Research methods. The work used a comprehensive research method based on search and design of technically rational options for geotechnology, their technical and economic assessment and mathematical modeling, and determination of stable parameters of mining system structural elements – the chamber span and the pillar width. Results analysis. The optimal variant of the open stoping mining system has been determined. In comparison with the basic technology of sublevel caving, the specific consumption of preparatory and development operations per 1000 tons of mined ore has been reduced by 34%, the cost of mined ore – by 12%, and ore losses and dilution – by 2 and 2.9 times, respectively. The stable parameters of the chamber span and pillar width have been established. Conclusions. The developed technology of sublevel open stoping with double chambers with frontal ore drawing using remote-controlled loading and hauling machines and subsequent caving of unrecoverable pillars of minimum width allows to significantly increase the efficiency of mining.

Grid Search Optimised Artificial Neural Network for Open Stope Stability Prediction

ABSTRACT The assessment of the stope stability is of great importance in the underground mine design process. The instability of stopes leads to serious economic and safety problems in mining operations. This paper aims to assist in the prediction of stope stability by making use of machine learning algorithms. For this purpose, a hybrid grid search-based Artificial Neural Network method is proposed. In the experiments, a total of 215 stope cases which have been collected from six underground mining operations located across Australia are used to prove the validity of the proposed method. Then, the performance of the proposed method is compared with k-nearest neighbour (kNN), naive Bayes (NB), support vector machine (SVM), decision tree (DT) and the stability graph method. Furthermore, several performance measures, such as accuracy, precision, specificity, recall, f-measure, and g-mean are considered during the performance comparison of the methods. The accuracy values are obtained as 91.63%, 81.86%, 77.21%, 76.74%, 73.95%, and 69.77% with the proposed hybrid grid search-based ANN method, SVM, DT, NB, kNN, and the stability graph method, respectively. The empirical results from the experiments indicated that the proposed hybrid approach outperformed the other aforementioned methods, which confirms that the proposed method is a useful tool to predict stope stability.

Stability analyses of side-exposed backfill considering mine depth and extraction of adjacent stope

Pillar recovery requires stable backfill upon side exposure in open stoping mining methods. Until now, the solutions used to estimate the required strength of side-exposed backfill are mainly based on a wedge model where the effect of rock-wall closure was neglected. In this paper, the influence of mine depth and adjacent extraction on the stability of side-exposed backfill is analyzed through numerical modeling with FLAC3D. The failure mechanism and cohesion at failure (cf) of backfill are investigated. Results show that the governing failure mechanism can be sliding or crushing, depending on the magnitude of rock-wall closure. When the mine depth and rock-wall closure are small, sliding failure is dominant and rock-wall closure has effect to improve the stability of exposed backfill. The values of cf can become even smaller than those obtained with immobile rock walls solutions. When the sliding failure is dominant, the cf decreases with the increase of mine depth, fill stiffness, rock pressure coefficient, and fill-rock interface strength (friction angle and adhesion). Increasing rock mass stiffness, stope height and width leads to an increase in cf. When mine depth and rock-wall closure are large, the crushing failure is dominant. The cf increases with the increase of mine depth, stope height and length, fill stiffness and rock pressure coefficient. It decreases with the increase of stope width and rock mass stiffness, and is insensitive to the variation of fill-rock interface strength. In all cases, the stability of side-exposed backfill is improved by increasing its internal frictional angle.

Stope stability assessment by the Mathews–Potvin method: a case-study of open stoping in salt rock mass under conditions of secondary stress field

Predicting the stability of open stopes is a challenging task for underground mine engineers. The introductory part of this paper presents main issues related to stability and safety of soil and rock mass in course of large earthworks and underground works. On the basis of former studies, this paper reviews the research findings on stope stability by the methods of Mathews and Potvin in the stoping design for rock salt mines operating at depths greater than 1 km under conditions of secondary stress fields, as well as the rock mass quality assessment by Barton’s method. The conservative stope design has been performed for a few scenarios of design parameters. In conclusions, it is underlined by the stability graph at the given parameters, that in the first case-study scenario, HR is 4.5 m and the Mathews–Potvin stability number is 68 m. It is concluded that the stopes will be stable and preserve their shapes. On the basis of presented case study (stope stability assessment in the test halite–polyhalite–polymineral salt deposit was carried out for a few scenarios at maximal effective stresses and under conditions of jointing), the authors provide the general provisions on salt rock mass assessment at great depths.

Evaluation of the use of sublevel open stoping in the mining of moderately dipping medium-thick orebodies

The flow of blasted ore during mining of moderately dipping medium-thick orebodies is a challenge. Selecting a suitable mining system for such ore bodies is difficult. This paper proposes a diamond layout sublevel open stoping system using fan blastholes with backfilling to mine such orebodies. To evaluate the performance of system the relationships between ore recovery and stope footwall dip angle, footwall surface roughness, drawpoint spacing and production blast ring burden were investigated. An ore recovery data set from 81 laboratory physical model experiments was established from combinations of the listed factors. Various modules in a back propagation neural network structure were compared, and an optimal network structure identified. An ore recovery backpropagation neural network (BPNN) forecast model was developed. Using the model and sensitivity analysis of the factors affecting the proposed open stope mining system, the significance of each factor on ore recovery was studied. The study results were applied to a case study at the Shandong Gold Group Jiaojia Gold Mine. The results showed that the application of a BPNN and sensitivity analysis models for ore recovery prediction in the proposed mining system and field experimental results confirm that the suggested mining method is feasible.

Evaluation of Stope Stability in Underground Mine; Hermyingyi (Sn-W Deposit) Mine in Myanmar

The stability of underground excavations has become an important issue in order to extend underground mining operations and extract deeper deposits. The increasing demand for Tin-Tungsten (Sn-W) for industry and its market price has created a motivation for mining companies to extract deep-seated Sn-W ore deposits in Myanmar. Thus, this paper aims to investigate the stability of underground openings, especially, the stope with considering the mining methods. To meet the objective, FLAC3D 5.0 simulation was used for the assessment of stope under different stress ratios, 0.5, 1.0, and 1.5 for two types of underground mines; Open stoping and Cut and Fill stoping. The results show that the risk of instability of stope is high under the stress ratio of K = 0.5 than that of K = 1.0 and K = 1.5 in both mining methods. However, the stability of the stope in open stope method is lower than that of cut-and-fill method obviously. This result shows that the appropriate mining method has to be selected for extraction of Sn-W deposit carefully in terms of the balance of safety and cost.

Effects of Sublevel Open Stope Underground Mining on Surface and Open Pit Slopes

Underground mining is an economically viable option for exploiting ore reserves deemed uneconomic after open pit mining. However, underground development can have adverse effects on the above existing open pit slope walls. As a goal of this paper, identification and assessment of potential slope instabilities prior to underground development is crucial for safe and sustainable mining. Towards goal achieving, this paper gives a comprehensive parametric study to investigate the influence of sublevel open stope (SLOS) underground mining on the surface and open pit slope walls. By means of numerical simulation, the SLOS design is tried against the existing open pit followed by adjustments of important slope parameters which are overall slope height (OSH) and overall slope angle (OSA). We found that underground mining may induce slope failure, particularly on the hangingwall side of the pit. Subsidence is prominent on the hanging wall and the surface, whereas, the uplift dominates the footwall and pit bottom. Pit wall closure is observed during underground mining. Although the assigned dimensions in the parametric study show a negligible effect of OSH and OSA, the high OSH experience low subsidence in comparison with low OSH. Overall, the results demonstrate that the slope walls on the hanging wall side are mostly affected by the underground mining and high-stress concentration prevails near slope toes and pit bottom. Additionally, slope deformation decrease from pit bottom towards the slope crest and surface. The results of this study add knowledge to open pit and underground mining interaction.

Comprehensive Engineering Geological Analysis on Large-Scale Anti-dip Slopes: A Case Study of Changshanhao Opencast Gold Mine in China

Flexural toppling failure of large-scale anti-dip slopes constitutes a significantly rare phenomenon. With regard to the large-scale anti-dip slope instability at the southwest stope of Changshanhao Opencast Gold mine, in this work, a field engineering geological survey on the Changshanhao open stope was conducted, to investigate the structural plane features of the corresponding slope rock mass. Moreover, an equatorial horizon projection and strike rose diagram were utilized to determine the preferred structural plane of the joint fissures; the exposed rock mass integrity zoning of slope and the rock sample property testing on the main lithological compositions in the stope were also established. Based on the results of previous field investigations and indoor rock mechanics experiments, along with the occurrence conditions of stratigraphic structures unmasked by existing engineering geology borehole data, geographical explorations and mining engineering, the failure mode of southwest stope slopes were also determined. Finally, several suggestions were provided on the reinforcement and treatment of dangerous slopes within the ultimate mining boundary, consequently laying a foundation for sustainable safe mining.