Pillar Extraction Research Articles

Breaking law of overburden rock and key mining technology for narrow coal pillar working face in isolated island

When conducting retreat mining in segmented coal pillars, the dynamic evolution of stress and overlying strata structure is more complex than conventional working faces due to the influence of adjacent working faces. Understanding and mastering the dynamic evolution patterns of overlying strata structure after retreat mining in segmented coal pillar working faces is essential for guiding the safe recovery of coal pillar resources under similar conditions. Through statistical analysis of the types of residual coal and the mining techniques, the current research status of residual coal mining system in China has been summarized. Based on the safety recovery technology system for multi-type residual coal pillar resources at Zhaogu No.2 Mine, this paper focuses on narrow coal pillar working faces in sections with fully mined-out areas on both sides. By using research methods such as on-site measurement, theoretical analysis, numerical simulation, and engineering experiments, starting from the stress state analysis and evolution law of coal seam mining, the dynamic evolution law of the overlying rock structure of sectional coal pillars has been mastered. On this basis, a stress arch mechanical model of the mining area is constructed, and the working resistance of the support is calculated and determined, ensuring the safe recovery of the working face. The research results show that before the backfilling of the sectional coal pillar working face, the working face is affected by the overlapping mining of the goaf on both sides, presenting a “bimodal” stress distribution pattern, with a stress concentration coefficient between 1.78 and 3.2. After the extraction of segmented coal pillars, stress arches consisting of high-stress zones form along both the strike and dip of the working face. The structural support provided by stress arches undergoes a dynamic evolution process of “formation-development-elevation-stabilization” as the working face advances. Following the instability and rupture of the lower basic roof hinge structure, the stress-bearing structure shifts to the higher basic roof, continuing to provide support for the surrounding rock stress in the mining space of the working face. A stress arch mechanical model for the dip and strike of the mining area is constructed , and the shape characteristics of the overlying rock stress arch in the coal pillar working face is mastered. Based on the stress distribution law and stress arch evolution characteristics of the surrounding rock of the coal pillar working face, the maximum working resistance of the support in the working face is theoretically calculated to be 9153.48kN. Compared with the measured mine pressure data, the selected support effectively ensures the safety production of the working face.

Evaluation of rock stability challenges affecting the formation of sustainable pit lake towards mine closure following crown pillar extraction in open pit-underground mines

The extraction of high-grade ore from the crown pillar (CP) in open pit-underground mines poses stability challenges and potential environmental risks. While an open pit has the potential to transition into a pit lake, the extraction of CP can induce failure in the surrounding walls, preventing the formation of the lake. There is also a concern that the backfilling material may not effectively confine toxic water within the pit, thereby risking contamination of the underground environment. To address these issues, a case study was conducted using FLAC3D and 3DEC models to evaluate the extent of failure caused by CP extraction. On-site observation, along with modelling, has revealed rock damage, including deformation stretching about 4 m from the pit wall and extending vertically from the pit floor to the ramp. The study identified three primary factors leading to pit wall failure or damage: steep pit slopes reaching approximately 70° near the pit floor, an underestimated CP thickness by about 4 m, and the concurrent extraction of ore from the pit wall alongside CP. Based on these findings, rehabilitation measures are suggested, including excavation of the deformed wall and cantilever, as well as partial pit backfilling. There is a substantial increase in the volume of backfill material as the extent of failure increases, which raise concerns about the decision-making process regarding CP extraction. Therefore, this article aims to raise environmental awareness and evaluate whether the benefits of ore extraction outweigh the considerations for pit wall support and the rehabilitation efforts during mine closure.

Coal pillar destabilization prediction and mining method in residual mining areas under remining disturbances

AbstractTo address the problem of pillar instability in residual mining areas under repeated mining disturbances, the prediction of pillar instability time and mining technology is studied by using numerical simulations, theoretical examinations, and practical experimentation techniques utilizing the remaining facade of the Shenghua Coal Industry 3103. On the basis of the findings, employing a fully mechanized mining technique exerts greater pressure on the coal pillar, increasing the likelihood of plastic failure. However, adopting the segmental filling mining approach can effectively disperse this pressure, consequently mitigating potential damage to the coal pillar. Initially, the coal pillar undergoes a progressive yielding or collapsing process at its periphery, leading to instability once the area of the projectile core of the coal pillar decreases beyond a specific threshold. The dimensions of both the affected roadway and the coal pillar play a pivotal role in determining the coal pillar's load‐bearing capacity. A prediction model for pillar instability is established. If the area of the projectile core is less than 34% during remining, the mining method should be changed or pillar reinforcement should be implemented. The accurate prediction and adaptation of the model's mining approach ensure the secure extraction of coal pillars within the remaining mining region, with successful validation of the model's performance.

Machine learning approach for the prediction of mining-induced stress in underground mines to mitigate ground control disasters and accidents

The bord and pillar method is commonly employed in Indian underground coal mines, and the extraction rate varies between 30 and 65%. During pillar extraction, pillars are subjected to severe stress conditions. Due to this, the natural state of stress equilibrium is disturbed, resulting in severe strata control problems leading to sudden, unpredictable failure such as a premature collapse of pillars, severe roof or side fall, and sometimes leading to serious/fatal injury or burial of machinery. This paper deals with the prediction of mining-induced stress during pillar extraction using Machine Learning (ML) techniques like Random Forest and Multilayer Perceptron. The various factors used for the formulation of the models for predicting the mining-induced stresses are Depth of the workings (H), Panel width/length (W/L), Pillar width/working height (w/h), Goaf length, and Area of extraction. This paper highlights the importance of operational parameters rather than geological parameters. The Correlation coefficient (R2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${R}^{2}$$\\end{document}) of mining-induced stresses for the case studies discussed in the paper is 0.85 for Random Forest and 0.76 for Multilayer Perceptron, which shows Random Forest results have a comparative edge over Multilayer perceptron. With this developed prediction models, the stress conditions of pillars can be predicted.Graphic abstract

The role of progressive brittle fracture in the 1931 landslide at Dogface Rock, Katoomba

The 1931 Dogface Rock landslide in Katoomba NSW was a complex, progressive cliff collapse with a failure volume in the order of 100,000 m3 that was triggered by the extraction of remnant coal pillars from the Katoomba Colliery, about 200 m below the top of the escarpment. Although underground coal mining is generally accepted as a cause of the rockslide, previous studies have not explicitly investigated the role of progressive brittle fracture in the collapse. This paper presents an integrated study which incorporates remotely piloted aircraft photogrammetry with a discrete element method numerical investigation of the landslide, and thereby explores the role of progressive brittle fracture, and re-examines the failure mechanism and runout motion of this multi-stage landslide. Remotely piloted aircraft photography is used to build a georeferenced 3D model of the site with Structure-from-Motion photogrammetry software. A digital geotechnical mapping workflow is demonstrated to investigate the morphology of the landslide scar, extract statistics on discontinuity orientation, persistence, and spacing, and undertake trace mapping of newer brittle fractures that interacted with pre-existing high persistence joints as the landslide rupture surface developed. A series of discrete element method numerical laboratory tests are used to calibrate bonded block contact properties that reproduce laboratory scale intact rock index parameters including UCS and tensile strength. Upscaled rock block contact parameters are then applied to a cliff-scale model that investigates the progressive development of rock mass damage induced by mining. Following extraction of the remnant pillars, rock mass damage develops mostly by extensile strains that produce tension cracks. Brittle fractures propagate upwards from the mine level and eventually initiate toppling of massive sandstone slabs defined by high persistence pre-existing subvertical joints. The investigation illustrates how the integration of photogrammetry with discrete element numerical methods can be used to characterise progressive brittle failure and runout of large rock slope failures.

A study of UG2 pillar strength using a new pillar database

A recent experimental pillar extraction project at a UG2 bord-and-pillar mine presented a unique opportunity to compile a new pillar database. Currently, the South African hard rock bord-and-pillar mines are designed using the Hedley and Grant formula with a modified K-value. This empirically derived formula was developed for uranium mines in the Elliot Lake district of Canada. The use of this formula for the design of pillars in South Africa is questionable. Very few pillar failures have nevertheless been observed and its current calibrations for the various reef types are possibly too conservative. A new UG2 pillar database of 66 pillars, of which seven are classified as failed, was compiled by the authors. This enabled a revised 'first-order' calibration of the K-value for the Hedley and Grant formula. The new estimated value for the UG2 is K = 75 MPa. This gives a pillar strength that is more conservative than the PlatMine formula. This work should nevertheless be considered as only a preliminary calibration as the database was small. Further work is also required to determine whether the exponents in the formula for the width and height parameters are appropriate for UG2 pillars.

A bulking model to simulate stope convergence in deep tabular excavations

This paper investigates the use of a novel time-dependent bulking model in a displacement discontinuity code to simulate the convergence between the roof and the floor of deep tabular stopes. This study is important as the current layout design criteria that are based on elastic theory have become outdated. This approach ignores rock failure and fails to predict the actual magnitude and the time-dependent nature of the stope convergence. This has implications for support design and for estimating the stress acting on remnants in mature mines. A rock mass modulus reduction strategy, often used to circumvent this problem, reflects the expected lower effective rock modulus for near-stope fractured material, but must be applied for the entire rock mass region in the displacement discontinuity solution procedure for tabular layout problems. As an alternative, it is proposed in this paper that the rock bulking can be expressed as a function of the elastic closure by using a constitutive rule for the reef-normal compressive reaction stress that is expressed as a function of the compaction strain. The proposed model was tested using convergence data collected from a shaft pillar extraction in a deep gold mine. The bulking model, in conjunction with a limit equilibrium model to simulate the face crushing, seems capable of replicating the underground behaviour. As the stresses on remnants and pillars can be more accurately simulated, this approach holds promise of improving current design criteria. Calibration of the model nevertheless remains a challenge and a programme of routine convergence measurements will have to be implemented on the mines.

Artificial crown pillar construction and in-pit tailings disposal at the Pinos Altos Mining Complex, Mexico

ABSTRACT The Oberon de Weber open pit is part of the Pinos Altos Mining Complex owned by Agnico Eagle Mines Ltd. in Mexico. A project was initiated in 2013 to enable the deposition of tailings in this soon-to-be exhausted pit while mining the underground portion of the deposit extending below the pit. A conceptual plan was proposed for constructing the tailings facility and mining the crown pillar below it. A series of investigations, preliminary and detailed studies, and a risk assessment were conducted to prepare a plan that was technically sound, safe, and environmentally secure during operations and after closure. Several alternatives were considered and analyzed to optimize the recovery of the high-grade ore. Economic and technical analyses supported the construction of an artificial crown pillar within a narrow trench excavated at the pit bottom, combined with a 10-m rock pillar above the underground stopes. This paper describes construction of the facility, water management, and instrumentation of the crown pillar. The Oberon de Weber Tailings Storage Facility has been in operation since the end of 2015 and is performing well. The crown pillar extraction has been successful, exceeding the planned resource recovery.

Robust Extraction of Multiple-Type Support Positioning Devices in the Catenary System of Railway Dataset Based on MLS Point Clouds

Due to the external environment, the support positioning device (SPD) of the railway catenary is prone to break down. Therefore, the automatic detection of SPDs is of great research significance to ensure the safe operation of the railway system. However, the small-scale SPD expresses weak features due to the complexity of railway scenes. Therefore, we propose a new algorithm for automatically extracting multiple-type SPDs quickly. Firstly, a preprocessing approach of equidistant scene segmentation is proposed based on trajectory points to divide the whole scene into several independent region units for batch processing. Then, a new point spacing-based spatial clustering of applications with noise (PSBSCAN) algorithm is proposed for the extraction of pillars in the pillar corridor of each region unit. Adopting point spacing as the core standard and the parallel iteration mode with only accessing edge points can reduce the influence of the uneven point cloud density and the large-scale point clouds. Next, to locate SPDs, the spatial relationship between the trajectory point, the pillar, and the SPD in the scene is introduced into the corresponding objects to build the spatial index with semantic information. Finally, contact wires installed on the SPD are filtered through multiple-level voxel segmentation based on octree to extract SPD accurately. Through the test of 10km railway datasets, the average <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> by the proposed method is 98.61%, and the average recognition rate of SPD is 99.65%, which shows that this method can achieve efficient extraction of multiple-type SPDs in railway scenes with different point densities.

Development of pillar extraction strategy for a coal panel and rib pillar stability index under the influence of the goaved-out panels

Development of pillar extraction strategy for a coal panel and rib pillar stability index under the influence of the goaved-out panels

Panel Destressing Strategies for Remnant Pillar Extraction

Large-scale panel destressing is a rockburst mitigation technique employed in deep hard rock mines during remnant pillar extraction. Panels are choke blasted in the pillar footwall to cutoff the far-field major stress in the mining area and deviate them around the pillar. In this study, the effects of panel geometry and far-field stress magnitude are investigated. Destress blast performance is assessed by measuring change to the energy release rate (ERR) of all mining steps during the extraction of a simplified remnant pillar due to destressing. It is demonstrated that the energy release rate (ERR) of critical stopes is reduced by 30% with the base panel geometry. The panel thickness is shown to have the most influence on the efficiency of destressing, followed by the stand-off distance between the panel and the pillar and the overhang length of the panel. The effect of far-field stress magnitude on the ERR is also investigated, and the destress blast performance is expressed as an equivalent major principal stress reduction. It is shown that with the base panel geometry, the destressing program offers the same ERR reduction as a 9.6 MPa reduction in the far-field stress for the most critical stopes. Finally, the Copper Cliff Mine (CCM) panel destressing program is presented as a case study. The ore at risk and ERR are calculated over the extraction and destressing sequence in the pillar with a pillar-wide numerical model.

Stability Analysis of Underground Developed Pillars of Opencast Mine Using Three-Dimensional Finite Element Method Authored

The major coal producing companies of the India are running most of the opencast mines to excavate the underground developed pillars as the underground mines possess various strata control issues, subsidence, ensures low production and makes losses. During extraction of such developed pillars, the men and machineries may trap in the developed galleries. Hence, the conditions of the pillars and galleries need to be assessed before approaching the developed pillars. In this study, a case study of Jharia Coal field is chosen for detailed stability analysis of the workings. For this, 3D numerical modelling technique is applied to assess the condition of the underground pillars and galleries. From the study, it is observed that the pillars and galleries of the workings for a maximum of 15 m from the slope surface fails severely. It is therefore, these area requires special precautions while approaching the developed coal seam/workings.

Safe and Efficient Recovery Technique of Horizontal Isolated Pillar under Loose Tailings Backfill; A Case Study in a Zinc-Lead Mine

The presence of loose medium backfill above the horizontal pillar will technically hinder the efficient recovery of the pillar since the improper design and preserved roof protection layer height will potentially lead to casualties and equipment damage caused by large area collapse of filled tailings as well as roof fall accidents. In this study, a safe and efficient technique for the recovery of isolated pillars under loose tailings backfill was carried out via field investigation, theoretical analysis, numerical simulation, and analytic hierarchy process using the isolated pillars in the 855 middle sublevel of Hongling Zinc-Lead Mine, Chifeng, Inner Mongolia, as a practical engineering background. Current studies have revealed that the optimal scheme for an isolated horizontal pillar recovered via the cut-and-fill stoping of a drift vertical to ore body strike involves preserving a 1.0-m roof protection layer above the crown pillar combined with a spaced mining extraction sequence. This design minimizes ore dilution and losses during the pillar extraction process under safe operation. Our research results provide theoretical and technical support for the safe and efficient recovery of isolated pillars under loose tailings backfill in similar mines.

Risks and challenges affecting opencast pillar mining in previously mined underground bord and pillar workings

South Africa is one of the leading producers and exporters of coal globally. A significant amount of the country's production is obtained from previously mined underground bord and pillar workings. This coal is in the form of pillars and remnants on the roof and floor of the old workings. The good quality coal pillars were left behind as primary support during underground bord and pillar mining operations. Due to the depletion of virgin coal reserves, the pillars and remnant coal are now removed using opencast mining rather than underground methods. However, the secondary extraction of pillars and remnant coal from the old workings using opencast methods entails some serious challenges that have a negative impact on the safety and productivity of the operations, affecting both personnel and machinery. If these risk factors and challenges are managed properly, then the opencast mining operations could remove the pillars safely at recoveries competitive with those of virgin coal operations. In this study we review the recurring challenges affecting opencast pillar mining by means of field investigations and consultations with experts at five opencast pillar mining operations, and evaluate the best practices used to combat these challenges. It was found that each mine has its own unique conditions and challenges.

Stress-state monitoring of coal pillars during room and pillar extraction

Stress-state monitoring of coal pillars during room and pillar extraction

Field and Simulation Studies for Mechanised Depillaring below Incompetent Geological Formations of an Indian Coal Mine

Abstract A fully mechanised mining method using continuous miner (CM) for pillar extraction in single-pass is introduced in the year 2010 at Pinoura Mine of South Eastern Coalfields Limited, Bilaspur in India. Overall mining cycle time is reduced by single-pass pillar extraction as it does not require drivage of split galleries in pillars and support installation for such galleries. Further, inherent technical abilities of the deployed CM for faster cutting and quad-bolter for supporting, helped in faster recovery of pillars without creating any strata control issues. Rate of face advance varied between 5-15m/day with an average of 8m/day. Field observations found that CM based single-pass extraction is capable in dealing with the strata control issues during depillaring under weak and laminated (incompetent in nature) roof layers having rock mass rating value around 42. A total of eleven panels, have been depillared under the weak and laminated geology, ranging depth of cover from 59 to 149m in different panels of the mine. During field investigations in these panels, a maximum of 2.89MPa induced vertical stress is recorded and influence of caved roof over underground mining structures is observed up to 15m in the out-bye from the goaf edge. This paper deals with estimation of snook size and limiting roof movement value based on numerical simulation and field studies during the single-pass extraction below the incompetent geological formations.

Design of remnant pillar in mechanized depillaring using continuous miner

Ribs and snooks are the critical natural support at the goaf edge in the mechanized depillaring operation of the bord and pillar mining system. The pillar extraction has been carried out by taking the slices and leaving ribs and snooks during the depillaring operation. Remnants are the remaining portion of the extracted pillar. The depillaring operation leads to an unsupported roof, and the immediate unsupported roof imposes its weight on the pillar (remnant) under extraction. The remnant’s purpose is to provide a necessary reaction to the overhang to restrict roof failure until the pillar’s final slice. The remnant’s stability during depillaring operation has been accessed in the study using three-dimensional numerical simulations. A scheme has also been proposed in the study to evaluate the factor of safety (FOS*) of the remnant pillar in the residual phase at different stages of slicing operation. A case of an Indian coal mine using the fish-bone method has been chosen for the study. A typical depillaring stage has been selected for the extraction of the pillar using the fish-bone method. The numerical simulation of the considered panel provides the vertical stress and yielding profile on the pillars at different stages of depillaring. The simulation results show the influence zone up to one pillar from the goaf edge. The immediate intact pillar shows considerable yielding of about 60% of the pillar area. The remnants have completely yielded during the slicing operation but provide a reaction to the immediate strata. The remnant should provide the reaction to the immediate roof till taking the final slice from the pillar. The remnant’s FOS* is calculated by taking the ratio of reaction offered by the remnant (numerical simulations) and the weight of the overhang (estimation). The area’s borehole section shows two layers of medium to coarse-grained sandstone as an immediate stratum. The weight of the immediate strata has been estimated in the study considering the immediate strata’s thickness. Two different scenarios of immediate strata thickness (i.e., 4.75 m and 9 m) have been considered in the study to evaluate the remnant’s FOS at different depillaring stages.

Underground Coal Pillar Extraction Under Shale Roof and Caved Goaf of Upper Lying Seam in Raniganj Coalfields

Underground Coal Pillar Extraction Under Shale Roof and Caved Goaf of Upper Lying Seam in Raniganj Coalfields

Experimental Study on the Temporal and Spatial Variation Law of Gas Flow in Different Rock Pillar Drainage Boreholes

In order to study the temporal and spatial variation of gas flow in different rock pillar extraction boreholes, gas pressure, gas content, gas emission from 100-meter coal holes and coal seam permeability coefficient were measured on site. The site inspected the gas flow in the boreholes at 15m, 7.5m, directly above, 7.5m, and 15m at the lower slab of different rock pillar floor roadways. It analyzed the change law of gas flow in boreholes of 710 floor lanes and 505 floor lanes, which provided a basis for the layout of gas drainage boreholes.

A Limit Equilibrium Model of Tabular Mine Pillar Failure

An improved understanding of pillar strength and pillar failure mechanisms is required to optimize tabular mine layout designs. The paper describes the application of a limit equilibrium model for pillar failure analysis. It is shown that the model is capable of reproducing a hardening or softening response for uniformly compressed strip or square pillars. The stress–strain behavior depends on three non-dimensional parameters Q, M and β. Q represents the ratio of the limit failed uniaxial strength to the intact material strength, M is the ratio of the failed limit strength envelope slope to the intact strength envelope slope and β is proportional to the pillar width to height ratio. The model is implemented in a displacement discontinuity solution scheme using unstructured triangular elements to allow irregular plan-view pillar shapes and mining step increments to be represented. The seam-parallel confining stress distribution in the fracture zone is determined using a fast marching solution algorithm. A case study of an experimental pillar extraction site in a platinum mine is presented to illustrate the capability of the model to simulate the evolution of pillar failure as pillar extraction proceeds. Good qualitative agreement to observed failure trends are obtained but the detailed calibration of the model parameters remains a challenge. Further work is required to enhance the representation of pillar edge spalling processes.