Pillar Working 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.

Research on Distribution Law of Advanced Support Pressure in Narrow Coal Pillar Working Face

Research on Distribution Law of Advanced Support Pressure in Narrow Coal Pillar Working Face

Analysis and Prevention of Rock Burst Risk of Working Face under the Influence of Continuous Irregular Triangular Coal Pillar Stress Concentration Area.

Irregular coal pillars inevitably appear in the layout of the current long-wall mining method, which easily forms stress concentrations and becomes a heavy disaster area of rock burst. In order to solve the impact risk of irregular coal pillar working face, it is necessary to study the instability mechanism of the coal pillar and put forward effective prevention and control measures. Based on the research background of 14320 working face of the Dongtan Coal Mine in the Yanzhou mining area of China, this paper studies the prediction and prevention of rock bursts in this kind of coal pillar by means of theoretical calculation, numerical simulation, engineering analogy, and field monitoring. The results show that (1) the absolute stability of coal pillar is that the width of coal pillar B reaches twice the support pressure of 2L, and the possibility of instability from large to small is coal pillars 2, 5, 3, 1, and 4. (2) The ratio of coal pillar strength to its average load determines the stability coefficient of the coal pillar, and it is judged that coal pillars 1 and 4 are in a stable state, coal pillars 3 and 5 are in a limit equilibrium state, and coal pillar 2 is in an unstable state. The numerical simulation shows that the maximum stress value inside the coal pillar during the mining process is basically consistent with the theoretical calculation of the bearing strength of the coal pillar. (3) The new evaluation method is used to evaluate the rock burst risk degree of the working face roadway: 156.75 m is a strong rock burst risk zone, 728.18 m is a medium rock burst risk zone, and 176.88 m is a weak rock burst risk zone. (4) Regional prevention and local prevention measures are proposed for the risk of rock burst in the roadway, which reduces the stress concentration of the coal pillar. It is verified that the pressure relief effect is remarkable, and the safe mining of such an irregular coal pillar working face is completed, which provides a solution for studying and solving such rock burst risk.

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.

Surrounding Rock Control Technology When the Longwall Face Crosses Abandoned Roadways: A Case Study

When a working face is crossing the abandoned roadways, problems such as roof subsidence, rock fracture, and instability will occur, resulting in widespread roof fall and rib spalling, which seriously affect safe and efficient mining on the working face. In this paper, the no. 23 coal pillar working face of Juji coal mine is taken as the engineering background, a mechanical model of crossing the abandoned roadways is constructed aimed at the problem of the working face crossing the abandoned roadway group, the collapse of the abandoned roadway roof is analyzed, a scheme of crossing the abandoned roadways is designed, and the development law of the stress and plastic zone after the reinforcement scheme is stimulated and analyzed. The results show that when the working face advances to the abandoned roadway, key block B crosses the abandoned roadway and the solid coal to form a “cross-roadway long key block.” It is calculated that the minimum support resistance required for the abandoned roadway is 6700 kN. Based on the results of numerical comparison, it is concluded that filling wood pile when the working face passes through the roof abandoned roadway and adding anchor cables for reinforcement support when the working face crosses the coal seam abandoned roadway effectively reduce the stress concentration of surrounding rocks, decrease the development of the plastic zone, and achieve safe and efficient mining when the working face crosses the abandoned roadways.

Research of inorganic fire‐extinguishing materials on preventing and controlling gas and coal spontaneous combustion in the narrow coal pillar and adjacent goaf

SummaryTo reduce the pressure on underlying coal seam and maximize the mining efficiency of coal resource, narrow coal pillar mining technology is often used. However, for coal mines with high gas and spontaneous combustion, this technology can cause gas and coal spontaneous combustion composite disaster easily. In this paper, we adopted a new type of inorganic fire‐extinguishing material (IFEM) to prevent and control gas and coal spontaneous combustion composite disaster. The result shows that the destruction of narrow pillar caused by stress extrusion and mining disturbance is the internal cause of gas and coal spontaneous combustion composite disaster. IFEM can go deep into the cracks of coal mass to seal up the cracks and reduce gas exchange between roadway and adjacent goaf. Meanwhile, it is full of moisture in the interior, which can reduce the temperature of coal mass through evaporation and heat absorption. The IFEM has the characteristics of high water retention, strong fluidity, and strong flame retardant, which can inhibit the generation of free radicals in coal mass at both physical and chemical levels, prevent the spontaneous combustion of coal mass, and reduce the occurrence of composite disasters of gas and coal spontaneous combustion.Highlights The crack inside the narrow coal pillar develops, and the gas permeability is good. The combined disaster in the narrow coal pillar working face is serious. IFEM can seal cracks and increase the strength of narrow coal pillars. IFEM can inhibit the generation of free radicals in coal and prevent coal from contact with oxygen.

Coal spontaneous combustion and N2 suppression in triple goafs: A numerical simulation and experimental study

Coal pillar fragmentation caused by mining stress will cause multiple goafs in series on the isolated island's small coal pillar working face. Due to the unclear rule of air leakage in the mined-out area and coal pillar, once the spontaneous combustion occurs, it will threaten the safety of the mine, causing a large number of Waste of resources. To examine this problem, taking a coal mine in Shaanxi Province as an example, this study analyses the air leakage laws and areas prone to coal spontaneous combustion (CSC) in the series goafs through experimental tests, field observations, and numerical simulations and use the inert goaf method to prevent and control CSC. The results indicate that there is a large air leakage in the goaf of the isolated island small coal pillar working face, particularly on the air inlet side of the goaf. The three series goafs have the hidden danger of spontaneous combustion. The relationship between the air leakage intensity and the buried depth in coal pillar is determined. The ideal inert gas injection position for controlling the spontaneous combustion of the coal seam lies in the transition area between the heat dissipation zone and the oxidation zone. After the injection of inert gas, the area of the oxidation zone in the goaf decreased by 25%. The research results are of scientific guiding significance for the control of air leakage in the triple goafs and the prevention of coal seam fire.

Determination of some mining parameters of the long wall for the effective use of plough systems in the Western Donbas mines

Theoretical studies of stress strain state of rock under the extraction pillar working by mechanized complex with plough systems were conducted. As a result of studies the mining parameters of the wall face working are presented. The mining parameters include the width of coal shavings continuously removed before reaching the critical rock pressure in the face and duration of the technological pause between removal of coal shavings, that leads to “relaxation” of rock pressure in the bearing zone to a safe state for continued mining. The use of the values of these parameters during the extraction pillar working ensures efficient and no-failure operation of high-performance mechanized complexes equipped with plough systems as shown by the example of mining and geological conditions of one of the mines of the Western Donbas.

Study on Optimization of Mining Pillar in Isolated Island Working Face of Thick Coal Seam under Complex Conditions

In this paper, a thick coal seam in Shandong Province is used as a prototype, which is located below the upper layer of 6 m residual pillar, and is a typical isolated island coal face based on the mining theory of upper liberation layer. The optimization study on the coal pillar of the island coal pillar working face formed by the ower stratification is carried out. When mining the coal face with stratified island pillar, it is easy to be affected by the upper residual coal pillar and the lateral support stress of the two side goaf. The isolated island surface is easy to form high concentration stress, the roof movement is violent, the impact ground pressure is easy to be induced, and the support press frame is easy to be formed. Mining earthquake and other dynamic disasters seriously threaten the safety of working face. Therefore, the reasonable determination of the width of coal pillar is guaranteed. By using UDEC numerical simulation method, this paper studies the mining optimization of the coal face under the influence of the upper stratified residual pillar, which is the important factor of the safe mining of the separated island working face under the thick coal seam, and the optimization of the mining of the coal face under the influence of the upper stratified residual pillar. This paper analyzes the stress distribution and evolution law of coal pillar in lower layer protecting roadway, and determines that it is appropriate to keep 12 m coal pillar in isolated island working face of lower stratified coal pillar, and the internal dislocation is 6 m in the residual pillar of upper stratification, which can reduce its impact risk at this time. It can ensure the safe mining and high recovery rate of coal.

Vertical stress and stability of interburden over an abandoned pillar working before upward mining: a case study.

Upward mining of the residual coal seam over an abandoned pillar working is one of the effective measures to alleviate the contradiction between limited resources and increased consumption. Interburden stability over an abandoned pillar working plays a significant role in guaranteeing the safety of upward mining; however, it has not yet been extensively studied and understood. In this study, the vertical stress of the interburden over an abandoned pillar working was first investigated. The mechanical model of the interburden was established and the damage conditions were analysed. Then, the stability of the interburden over 38502 abandoned workings in Baijiazhuang coal mine was determined by mechanical analysis and field monitoring. The results show that: (i) Vertical stress of the interburden over abandoned mining zones is clearly lower than the initial stress, indicating the existence of a de-stressed effect. Moreover, vertical stress of the interburden over residual coal pillars is greater than the initial stress, which is the evidence of a stress concentration effect. (ii) The interburden over an abandoned pillar working should be regarded as an elastic rectangular plate supported by generalized Kelvin bodies in mechanical modelling. (iii) The interburden over abandoned mining zones may experience two damage stages. In the first stage, initial plastic damage appears at the central region of interburden. In the second stage, the plastic damage evolves from the central point to the surrounding areas. (iv) The mechanical analysis and field monitoring both indicate the initial damage occurred at the central region over 38502 abandoned workings in Baijiazhuang coal mine before upward mining. Related rock control measures should be implemented in that region to guarantee the safe mining of the residual coal seam.

Research on reasonable coal pillar width of roadway driven along goaf in deep mine

Driving roadway along a goaf is commonly adopted for mining face of thick seam in a deep mine. Determining a reasonable width of coal pillar is a key scientific problem for driving roadway along a goaf in a deep mine. The paper took a roadway driven along a goaf at Zhaolou coal mine which is a typical kilometer-deep mine in China as engineering background. Field monitoring, model test, and numerical experiment are conducted. Stress and displacement evolution mechanism are analyzed with different pillar widths. The test results show that with the increase of coal pillar width, the peak stress value at the coal pillar working slope and integrated coal beside the roadway increases firstly and then tends to be stable, its position is transferred to the side of the roadway, and the deformation of coal pillar decreases gradually during roadway excavation. The coal pillar deformation and roadway vertical displacement increased as the coal pillar width increases under high abutment pressure. In order to reduce the waste of non-renewable resources and meet the requirements of bearing capacity and stability of coal pillars, a method is proposed for setting a reasonable width of coal pillars and the specific width of coal pillars is designed and applied in engineering practices based on the above research. All the tests are significant in the study of driving roadway along a goaf in a deep mine.

A New Blastability Index for Hard Roof Management in Blasting Gallery Method

The presence of hard and massive sandstone above the coal seam in underground coal mines often leads to delay in caving of overlying rock beds thereby causing excessive load on supports and posing danger to underground workings. The problem is more prominent in blasting gallery (BG) as well as longwall mining methods in Indian coal mines. Induced caving by blasting is a promising means for hard roof management in underground coal mines. Based on extensive studies and data collected from different mines in India, a Blastability Index (BI) has been developed which can be used for the classification of roof according to the degree of ease in caving by induced blasting. Different charge factors have also been suggested based on the Blastability Index. Due to wide change in the method of extractions, ‘Cavability Index’ for longwall panel was found ineffective in case of BG method of working as well as bord and pillar working. For this reason, this proposed Blastability Index would be of immense help for caving of hard roof by induced blasting.

Research on Walling-up with Concrete Block Technology of Gob-Side Entry Retaining Based on Fully Mechanized Face in Medium-Thickness Coal Seam

Gob-side entry retaining is an advanced and green mining technology, which can avoid using pillar and island working face, reduce driving workload, alleviate the tension between extraction and driving, and achieve a Y-type ventilation. The coal mine in this research successfully reserved a 485m roadway in No.10302 fully mechanized coal face by employing wide lane driving, pre-driven roadway, pre-controlling immediate roof in the roadway, and gob-side entry retaining by walling up with multi-standard concrete block using break-joint vertical stacking technology. From doing so, the supporting effect is significant, the roadway is integrated and the stable wall is free of distortion or cracking. This research paves a new way for gob-side entry retaining of fully mechanized mining in China.

Modelling study for assessment of strata behaviour in bord and pillar working under soft floor condition

Safe and successful mining operation in bord and pillar workings under soft floor conditions requires a proper understanding of strata behaviour for better planning of mining layouts and selection of suitable mining horizon for better strata control and management of conditions arising due to excessive roof–floor convergence in the workings. This paper illustrates a modelling approach to assess the caving behaviour of strata, stress concentration, and roof–floor reactions in a depillaring bord and pillar working of Singareni coalfields in India. Assessment of floor heaving and roof–floor convergence in junctions of advance galleries is conducted with progressive goaf exposure. The calibrated model is used to predict the strata behaviour and roof–floor reactions in another depillaring panel. The modelling study has been helpful in better understanding of strata control issues for a confident decision making to ensure better safety and productivity of mine workings.

Stability of parting rock between level contiguous coal pillar workings

The factors affecting stress distribution in thin parting between contiguous pillar working have been identified as depth of cover, parting thickness, lateral shift between pillars, percentage extraction and modulus ratio. A large number of two-dimensional finite element models covering these variables ultimately give an expression for the maximum tensile stress. Barton's rock classification [1] is next modified considering this stress and three-dimensional finite element analysis of four failed cases. In all, six collapsed cases and four stable cases have been used to estimate the safe roadway junction spans as against the existing spans, using the modified classification equation. Safety factors of parting rock in all case studies have also been calculated using a recently developed failure criterion for jointed rock masses [2] as well as the Hoek and Brown criterion [3], comparing their performance. Design guidelines are presented for contiguous pillar workings.

Main heading development and pillar working techniques at oakdale colliery 1978–1982 : Hamment, J D Aust J Coal Min Technol Res, N3, 1983, P25–36

Main heading development and pillar working techniques at oakdale colliery 1978–1982 : Hamment, J D Aust J Coal Min Technol Res, N3, 1983, P25–36

Pressure exerted by cut coal and stowage in pillar working to the rise

Pressure exerted by cut coal and stowage in pillar working to the rise

光弾性実験による規則正しく配列した空洞の周りの応力の研究

This investigation was carried out for the purpose of obtaining materials for the layout of pillar working or of driving close parallel headings. The stress state in an elastic ground with six parallel openings, square in section, was analysed by means of photo-elastic experiments under the initial stress in one direction as well as in two directions. The results are as shown in Fig. 3 and 4. The conclusion to this research will be given in the 2nd report.