Pillar Recovery Research Articles

Optimization of ore pillar recovery based on weighting combined with uncertainty measurement theory

Abstract Given the multi-objective nature and inherent uncertainty in evaluating ore pillar mining schemes, the analytic hierarchy process (AHP) and entropy method were applied to determine factor weights. A new combined weighting method was proposed to obtain more objective and accurate results. Seven quantitative and four qualitative indicators were selected based on various mining performance factors and grading standards. A combined weighting–uncertainty measurement theory evaluation model was then constructed, integrating weight values and confidence levels from the recognition criteria. This model was applied to evaluate three proposed schemes, determining their relative merits and ranking. The evaluation demonstrated that the combined weighting–uncertainty measurement model outperformed both the fuzzy theory and the TOPSIS method in delivering comprehensive, objectivity and reliable results for assessing ore pillar mining schemes.

Stability Analysis of a Mine Wall Based on Different Roof-Contact Filling Rates

This study takes the mine wall of the isolated mine pillar in the Dongguashan Copper Mine as the research object. Based on the mechanical model of the mine wall under the trapezoidal loading of the backfill, the expressions for calculating the safety factor of the mine wall were derived by considering the load-bearing conditions of the backfill–mine-wall system under different roof-contacted filling rates. On this basis, the variation law of the safety factor of the mine wall with the roof-contacted filling rate was obtained, and the calculation result was verified by a numerical simulation and a field test. The research shows that for the same mine wall width, when the roof-contacted filling rate exceeds 9.53%, the safety factor of the mine wall exhibits a "trapezoidal" variation pattern with the increase in the roof-contacted filling rate. Moreover, the comprehensive benefits of isolated pillar recovery are made more credible by maintaining a wall width of 3 m and a filler jointing rate between 30% and 74.49%. This study analyzes the effect of the roof-contacted filling rate on the stability of the mine wall, which can provide a theoretical basis for mining isolated pillars by the filler method in deep mines.

Study on roadway layout and surrounding rock control of isolated island panel

During the re-mining of historical residual coal resources, the stress environment is complex, the surrounding rock conditions are bad, the mining roadway is significantly affected by ground pressure, the layout is difficult, and the safety is poor. Taking the recovery of isolated island coal pillar in 4# coal seam as the research background, based on the difference in the distribution morphology of the goaf on both sides of the isolated island coal pillar, the stress and failure law of the isolated island panel boundary are studied by numerical simulation method. (1) The peak stress difference of multiple goaf boundaries on both sides of the isolated island coal pillar is between 0.18 and 4.51 MPa. The peak stress is affected by the change of the length of the roof “cantilever beam” at the stopping line of the goaf, so that the peak stress of the goaf boundary is periodic. (2) The high stress is mainly concentrated in the center of the pillar. The peak stress at the end of each pillar is 35–40 MPa. The coal pillar bears high stress, and the stress zone of the original rock moves to the end of the coal pillar. (3) There is a plastic zone of 8–20 m at the corner of the end of each coal pillar. On the basis of the stress zone and failure zone distribution of the goaf boundary on both sides of the isolated island panel, the roadway layout of the isolated island panel is determined, that is, the air-return roadway of the isolated island panel is arranged at random, and the width of the isolated island coal pillar d1 is selected as 10 m. The transport roadway is arranged straight, and the transport roadway of the isolated island panel is in the width section area of the goaf X4103. The width d1 of the isolated island coal pillar is selected to be 8 m, and the length d5–d7 of the mining roadway layout in the width of the coal pillar is 24 m. The roadway of isolated island panel is divided into 4 areas for support control, and the drilling pressure relief technology is proposed for high stress roadway. Through the field monitoring data, it can be seen that the mining roadway can meet the requirements of isolated island coal pillar recovery, which provides reference for the layout and control of abandoned coal roadway in this mine and other mines.

Numeric Investigation on the Stability of a Preformed Roadway under Backfill Body Subjected to Blasting Load

Tungsten, essential in the industrial, military, and civilian domains and deemed a strategic resource by various nations, necessitates careful consideration in room and pillar mines due to the potential instability and safety hazards posed by untouched mine pillars, making tungsten recovery crucial for worker safety and economic gain. This research aims to provide guidance for recovering tungsten from mine pillars and making mining operations safer for workers in the Xianglushan mine. Numerical simulations are conducted to study the mechanical response of a preformed roadway in a backfill body subjected to static and dynamic loads with various explosive distances and positions. Blasting vibration velocity and blasting-induced damage in the backfill body are extracted to evaluate the mechanical response of the backfill body. The numerical results indicate that the steel frame and preformed roadway remain stable under the influence of both gravity and the impact from blasting, using a charge of 3.00 kg per blasthole. By analyzing these indicators, potential safety hazards in the backfill body and preformed roadway are identified, and the numerical results provide guidance for mine pillar recovery practices.

Study on the Mechanics and Micro/Macroeconomics of Multiple Strip-Shaped Pillar Recovery

Study on the Mechanics and Micro/Macroeconomics of Multiple Strip-Shaped Pillar Recovery

Cooperative mining technology and strata control of close coal seams and overlying coal pillars

Mining of close coal seams envisages reservation of a large number of section coal pillars of a total width ranging from 30 to 40 m. This results in an enormous waste of coal resources and risks of spontaneous combustion, instability, and damage to the remaining coal pillars. This study attempts to mitigate the above problems via the proposed cooperative mining technical method for close seams and overlying coal pillars. It expounds on the technical principles, the required mining equipment, technology, and the system layout, and analyzes the key influencing factors of strata control. Using the numerical simulation via FLAC3D and PFC2D commercial software packages, the effect of horizontal distance between the lower coal seam roadway and the overlying coal pillar (i.e., spacing) on the surrounding rock control and rational layout of the roadway is studied. The mechanism of the overlying coal pillar width effect on the ground pressure of the stope and coal pillar recovery rate is clarified. The engineering case study shows that when the horizontal spacing was increased from 0 to 15 m, the surrounding rock deformation, surrounding rock stress peak value, and plastic zone area of the roadway decreased significantly. When the coal pillar width in the overlying section was increased from 25 to 40 m, the peak values of lateral and advance bearing stresses in the stope dropped by 21 and 12 MPa, respectively. Besides, the ground pressure driving strength of the stope was significantly reduced, and the recovery rates of the overlying coal pillar and rock between the close coal seams were increased by 7.45 and 4.29%, respectively. Combined with the top coal pillar drawing pattern, optimization concepts and their realization methods were proposed.

Evaluation of different methods of pillar recovery in coal mining by numerical simulation: a case study

Evaluation of different methods of pillar recovery in coal mining by numerical simulation: a case study

Study on Occurrence Mechanism and Prevention Technology of Rock Burst in Narrow Coal Pillar Working Face under Large Mining Depth

This paper presents a collaborative control scheme involving “unloading-solidifying” to prevent rock bursts during narrow pillar recovery at large mining depths. In this study, the stress distribution rule of coal rock mass during the excavation and mining process is studied, and the energy accumulation characteristics of the overlying hard and thick roof structure are investigated. In this way, the rock burst inducing mechanism of the narrow coal pillar working face under complex conditions is investigated. The results show that the peak lateral bearing pressure of the goaf and the maximum horizontal principal stress provide the static load condition for the occurrence of rock burst during roadway excavation. Affected by the superposition of “near-field high static load + far-field dynamic load”, it is extremely easy to reach the critical destabilization value during the mining period at the narrow coal pillar working face. According to the monitoring results, the developed coordinated control scheme, which focuses on the strong pressure relief and strong support in near-field high-bearing pressure coal mass and the pressure relief in far-field high-level hard roof with an advanced pre-cracking roof, can effectively avoid the occurrence of rock burst accidents on narrow coal pillar working face.

Investigation and Stability Assessment of Three Sill Pillar Recovery Schemes in a Hard Rock Mine.

In Canada, many mines have adopted the sublevel stoping method, such a blasthole stoping (BHS), to extract steeply deposited minerals. Sill pillars are usually kept in place in this mining method to support the weight of the overburden in underground mining. To prolong the mine’s life, sill pillars will be recovered, and sill pillar recovery could cause failures, fatality, and equipment loss in the stopes. In this paper, three sill pillar recovery schemes—SBS, SS1, and SS2—were proposed and conducted to assess the feasibility of recovering two sill pillars in a hard rock mine by developing a full-sized three-dimensional (3D) analysis model employing the finite element method (FEM). The numerical model was calibrated by comparing the model computed ground settlement with the in situ monitored ground settlement data. The rockburst tendency of the stope accesses caused by the sill pillar recovery was assessed by employing the tangential stress (Ts) criterion and burst potential index (BPI) criterion. All three proposed sill pillar recovery schemes were feasible and safe to recover the sill pillars in this hard rock mine, and the scheme SBS was the optimum one among the three schemes.

Study on Energy Dissipation Characteristics and Damage Law of Backfill under Cyclic Impact

This research aims to study the damaging effect of underground blasting mining pillars on adjacent cemented filling bodies. The filling bodies were made by mixing ore rock with ash sand by the ratio of 1 : 4 and 1 : 6, respectively, which were subjected to cyclic impact tests with the split-Hopkinson pressure bar under the pressure of 0.2 MPa and 0.24 MPa to analyze the energy dissipation characteristics and damage law. The results showed that as the stress wave induced by the cyclic impact was transmitted to the composite specimen, the energy was absorbed for crack growth and development. Then, the energy reflection ratio increased while the energy dissipation ratio and transmission ratio decreased. For the combined specimen with the cement-sand ratio of 1 : 4, after five cycles of impact under the condition of Ip = 0.2 MPa, the damage variables were 0.07, 0.11, 0.51, and 0.56, respectively, since the second time; under the condition of Ip = 0.24 MPa, the damage variables were 0.17, 0.29, 0.55, and 0.66, respectively. After reaching the damage threshold of 0.63, the damage variable showed nonlinearity. Moreover, it was found that the mechanical properties of the filling body affect the whole combined specimen, and the dynamic strength of the combined specimen with the cement-sand ratio of 1 : 4 was higher than that of the cement-sand ratio of 1 : 6. Therefore, it can be concluded that during the two-step pillar recovery, the amount of blasting explosive can be appropriately reduced, and the number of blasting can be increased to reduce the damaging effect of blasting impact on the cemented pillar and reserved pillar to maintain stability.

Study on the Optimization of the Sequence of Continuous Mining and Backfilling of 3 Coal in the Protected Coal Column

In the process of coal mining, unreasonable recovery sequence is likely to lead to rock subsidence and surface deformation. In order to ensure the safety of shaft works as well as surface buildings and structures during coal pillar recovery and extend the service life of the mine. In this paper, we adopt the continuous mining and charging mining method to recover the protected coal pillar with the engineering background of 3 coal recovery from the coal pillar under the dense building (structure) of Z coal mine industrial square. Based on the comprehensive consideration of the stability of coal pillar and resource recovery rate, the research methods such as theoretical analysis and numerical simulation are used to explore the overburden and surface subsidence reduction methods from the microscopic perspective of optimizing the sequence of supporting lane of coal mining with continuous mining and continuous charging for coal pillar 3 coal in coal mine industrial wide. It provides new ideas for reasonable recovery of coal pillar for mine protection.

Pillar recovery using new wireless blasting technology: a case study in Vazante mine, Brazil

The application of explosives has never been considered as a major influencing factor during either the designing of a mine or the selection of the mining method. However, this has changed with the launch of WebGenTM wireless blasting system, which allowed underground mines to exploit their orebody applying methods previously not possible due to limitations imposed by the use of wired detonators. The wireless blasting system is based on magnetic induction communication, and its signal is capable of overcoming hundreds of meters through rock, water, and air, to reach individual primers in the blastholes without any physical connection.A noble application of wireless detonators is being used in Vazante mine, Brazil, an underground zinc mine where ore pillars are left in the mined stope to secure stability and minimize dilution by limiting the hydraulic radius. The recovery of these pillars is financially desirable but involves extra time and costs associated with scaling, backfilling, reinstalling infrastructure, accessing areas previously blasted (less stable), drilling, charging with explosives, and subsequently, firing and mucking out the blasted material with expected high dilution. Applying 100% wireless detonators made it possible to safely preload the pillar together with production blasts before losing access to the area, a method named Temporary Rib Pillar (TRP). After all the stope is mined and the pillar accomplished its objectives, the primers are initiated without neither the need for the extra cycles described previously, nor the need to re-enter the area. Thus, it was possible to reduce the exposure of people and equipment, reduce operational cycles, and increase ore recovery, directly contributing to anticipate the ore production while guaranteeing the safety of the teams involved.

STUDY OF THE INFLUENCE OF BACKFILLING ON STABILITY AND LOCATION OF REMAINING STOPES IN SILL PILLAR RECOVERY

To extract steeply dipping orebodies, many mines in Canada have adopted the sublevel stoping method, such as blasthole stoping (BHS) and sublevel longhole retreat (SLR). In such methods, sill pillars are initially kept in place to support the weight of the overburden in underground mining. After the stope mining is complete, the stope voids will be backfilled with cemented rockfill (CRF). The strength of the CRF affects the stability of the adjacent stopes in the sill pillar recovery excavation scheme. Sill pillar recovery may cause prolonged failure, fatality, and equipment loss. Choosing a rational location of the last mined stope in sill pillar recovery can effectively eliminate the possible instability caused by the sill pillar recovery process. This paper uses the Finite Element Method (FEM) to present a comparison of sidewall swellings, floor heaves, and the roof subsidence of the crossing cuts in each stope among different last mined stope location scenarios. At all the 21 location scenarios, most of the roof corner displacements in undercuts are less than 10 cm, while in the overcuts, all the displacements in roof corners are over 10 cm. The values of the floor heave in the overcuts are more than 7 times that in the undercuts. Roofs in both undercuts and overcuts are prone to failures. Floors in the undercuts are more stable than the floors in the overcuts. For the optimum location of the last mined stope in sill pillar recovery, the last mined stope should be at least four-stope-width away from the two sill pillar edges

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.

A Method for Multihole Blasting Seismic Wave Prediction and Its Application in Pillar Recovery

Long-hole blasting in mines is likely to cause strong vibration of surficial infrastructure, greatly damage the rock mass surrounding goaf near explosion center, and possibly induce blast vibration disasters. In this article, an improved method for multihole blasting seismic wave prediction is proposed to estimate far-field blast vibration. In this method, the fundamental vibration waveforms are firstly measured through the field blast with a single deck at an underground pilot area. The fundamental vibration waveforms are then used to simulate the vibration waveforms for a single-deck case in the production blast by considering the difference of the equivalent distances from the production blast site and the pilot area to the surface measuring point. The vibration waveforms for the single-deck case are linearly superposed to predict the possible vibration waveforms in production blast with multiple long holes and decks according to the designed delay time between decks. Based on these predicted waveforms, the blast vibration can be estimated and the blast design can be optimized to determine a rational delay time in accordance with the vibration limit. The proposed method was applied in pillar recovery of Hongling Polymetallic Mine to optimize the long-hole blast design to manage blast vibration. The rational delay time for the 716 production blast design was recommended as 26 ms. The practice showed that the blast vibration induced by the 716 production blast has been managed, and the predicted and the measured waveforms agree well. It provides an effective method for multihole blast design to control blast vibration.

The current perspective of the PA 1957 gas well pillar study and its implications for longwall gas well pillars

Many states rely upon the Pennsylvania 1957 Gas Well Pillar Study to evaluate the coal barrier surrounding gas wells. The study included 77 gas well failure cases that occurred in the Pittsburgh and Freeport coal seams over a 25-year span. At the time, coal was mined using the room-and-pillar mining method with full or partial pillar recovery, and square or rectangle pillars surrounding the gas wells were left to protect the wells. The study provided guidelines for pillar sizes under different overburden depths up to 213 m (700 ft). The 1957 study has also been used to determine gas well pillar sizes in longwall mines since longwall mining began in the 1970 s. The original study was developed for room-and-pillar mining and could be applied to gas wells in longwall chain pillars under shallow cover. However, under deep cover, severe deformations in gas wells have occurred in longwall chain pillars. Presently, with a better understanding of coal pillar mechanics, new insight into subsidence movements induced by retreat mining, and advances in numerical modeling, it has become both critically important and feasible to evaluate the adequacy of the 1957 study for longwall gas well pillars. In this paper, the data from the 1957 study is analyzed from a new perspective by considering various factors, including overburden depth, failure location, failure time, pillar safety factor (SF), and floor pressure. The pillar SF and floor pressure are calculated by considering abutment pressure induced by full pillar recovery. A statistical analysis is performed to find correlations between various factors and helps identify the most significant factors for the stability of gas wells influenced by retreat mining. Through analyzing the data from the 1957 study, the guidelines for gas well pillars in the 1957 study are evaluated for their adequacy for room-and-pillar mining and their applicability to longwall mining. Numerical modeling is used to model the stability of gas wells by quantifying the mining-induced stresses in gas well casings. Results of this study indicate that the guidelines in the 1957 study may be appropriate for pillars protecting conventional gas wells in both room-and-pillar mining and longwall mining under overburden depths up to 213 m (700 ft), but may not be sufficient for protective pillars under deep cover. The current evaluation of the 1957 study provides not only insights about potential gas well failures caused by retreat mining but also implications for what critical considerations should be taken into account to protect gas wells in longwall mining.

Instability and pillar burst mechanism in roof-pillar system with rock beam embedded in elastic foundation

Pillar burst depends not only on the pillar itself but also on other components of the roof-pillar system. Instability theory is applied to optimize the critical condition of pillar instability, thus minimizing the likelihood of a pillar burst event. The instability mechanism and evolution process of a pillar burst for a rock beam embedded in an elastic foundation (EF) are investigated. The unmined orebody (UMO) is considered to be an EF; the pillar’s strain-softening behavior is described by a Weibull distribution. Pillar burst events occur in the post-peak strain-softening stage of the pillar and are mainly dependent on the pillar strength and stiffness ratio rK of the UMO-roof-pillar system. The EF coefficient kf reflecting the UMO property is an important factor affecting rK. The maximum deviation of rK reaches up to 198% when the UMO is regarded as a rigid foundation (RF), which emphasizes the importance of kf in terms of overall system instability. A typical case of partial pillar recovery is simulated to demonstrate that EF hypothesis accurately reflects the UMO response in such a system. The instability conditions obtained by simulation analysis are consistent with the theoretical results. The instability of the UMO-roof-pillar system is the result of many factors, among which the thickness hb and span l of the rock beam are extremely sensitive. The lateral pressure coefficient does not significantly affect pillar instability and can be neglected to some extent. The pillar burst evolution process can be divided into a stability stage, subcritical instability stage, instability stage, and post-instability stage.

Instantaneous and long-term deformation characteristics of deep room-pillar system induced by pillar recovery

Abstract The goaf may face a series of deformation and settlement problems when the room-pillar mining method is used to excavate ore and pillars in the deep strata. To this end, a deep room-pillar model with two levels was made, and the pillar recovery was carried out. The instantaneous deformation responses during the pillars recovery and the long-term settlements after the pillar recovery were analyzed. During the pillar recovery, different regions of surrounding rocks suffer from different dynamic disturbances which can be divided into three types, including (I) the combined action of blasting disturbance and unloading disturbance, (II) the sequential action of blasting disturbance and unloading disturbance, and (III) the action of unloading disturbance. After the pillar recovery, the settlement above the first recovering pillar is the largest, which has a traction effect on the settlement in other areas. The settlement process can be divided into two stages, stable displacement stage and unstable displacement stage. When the pillar-room system undergoes the unstable displacement stage, rock spalling and further cascading collapse will occur.

Study on the Characteristics of Top-Coal Caving and Optimization of Recovery Ratio in Steeply Inclined Residual High Sectional Coal Pillar

This paper is aimed at solving the technical problems such as low recovery ratio and frequent disasters in steeply inclined and extrathick coal seams at residual high sectional coal pillar. It takes the Wudong Coal Mine as an engineering background, a typical mine of steeply inclined and extrathick coal seams; the structural features of the top-coal caving at the steeply inclined residual high sectional coal pillar were analyzed using methods such as field monitoring and numerical simulation; a mechanical model of the top-coal arch structure was constructed, and the calculation method of top-coal caving height and related influencing factors was obtained. The results showed that the top-coal caving in the steeply inclined residual high sectional coal pillar was characterized as arch. Due to the existence of arch structure, the smooth caving of the top coal was hindered, resulting in a low top-coal recovery ratio, low support pressure at the working face, and differences detected by borehole television on the distribution of the top-coal cracks. With the advancement of the working face, the top-coal arch structure was in the process of dynamic evolution, as the old arch balance system was continuously replaced by the new arch balance system, and it continuously moved towards the upper top coal. The top-coal caving height was affected by factors such as length of the working face, bulk density of overlying coal rock, and cohesion of the top coal. The top-coal caving height increased with the length of the working face and the bulk density of the overlying coal rock mass but was inversely proportional to the cohesion of the top coal. Under the current mining conditions, the top-coal caving height was 39.8 m, which was much lower than the residual high sectional coal pillar height (71 m); the top coal cannot collapse completely. Based on the characteristics of the top-coal caving structure, the technology of sublevel advanced presplitting blasting was adopted to weaken the top coal in engineering practice, so that the top-coal caving structure moved up naturally. The daily coal production in the working face has increased by an average of 2419.6 tons, which has significantly improved the top-coal recovery ratio and production efficiency. The result provided a theoretical basis and application reference for similar residual high sectional coal pillar recovery.

Utilizing concrete pillars as an environmental mining practice in underground mines

Ground control is an integral element of mine design and worker safety. The use of concrete pillars for underground mines is of paramount importance to maintaining the economic and operational security of structures. This paper deals with the use of fiber-reinforced concrete (FRC) as pillars via laboratory and field tests. The strength performance of prepared concrete reinforced with glass, polypropylene and polyacrylonitrile fibers was researched by a mechanical press and a computed tomography (CT) tool. Samples were tested for fiber volume fractions of 0, 0.4, 0.8 and 1.2 wt%, respectively. Results have indicated that, with the addition of fibers, the strength was improved first due to a bridging effect and then decreased due to a pull-out effect. Compared to the reference sample, the absorbed energy prevents FRC from deterioration by mechanisms of matrix cracking, fiber-matrix interface debonding and fiber rupture. The peak strains of FRC linearly rise with increasing fiber. The gray value distribution curves have also good correspondence with 2D CT pore and crack distributions, which reveal that gray value processing could depict the structural behavior of concretes reinforced with or without fiber. Theoretical analyses show that the pillar remains stable for sustainable mining. Besides, the location and size of FRC pillars are suitable for numerical calculations of the trial stope. The findings of this study can offer a key reference for the orebody pillar recovery in underground mines.