Coal Pillar Design Research Articles

A proposed method for optimizing coal pillar design using coalfield-specific uniaxial compressive strength

The research described considers whether the variability in coal material strength, as derived through a series of uniaxial compressive strength (UCS) tests, could be used to indicate the variability in coal pillar strength. The aim is to be able to use a distribution of UCS tests as input into the coal pillar strength calculation. This will allow the pillar design to be expressed in terms of a probability of failure rather than as the commonly used safety factor. To achieve this, the bulk strength factor associated with commonly used pillar strength formulae was replaced with a distribution of UCS results divided by an adjustment factor. The factor was determined so as to ensure that the resulting bulk strength does not deviate from the statistically determined bulk strength published in the original formulae. This approach enabled pillar strength distributions to be obtained using industry-accepted strength formulae, subsequently allowing for a probability of failure to be calculated for a specific pillar design. Using a regional coal material strength curve as a baseline, coalfields in which the coal is stronger than the regional mean can be identified and the pillar designs optimized. This is based on the stronger coals achieving lower probabilities of failure at similar safety factors. The research has considered actual UCS data from multiple mines in the Mpumalanga coalfields of South Africa, and has proved that the variability in material strength between coalfields could allow for some optimization using the proposed approach. Based on the data used in the study, a 2.78% increase in extraction could be achieved. However, further research will be required to validate the results of the study in an underground environment.

Stability control of overburden and coal pillars in the gob-side entry under dynamic pressure

To improve the recovery ratio of coal resources and stress environment of the roadway, gob-side entries have been widely implemented to numerous coal mines in China. Traditional gob-side entry is generally excavated after the gob of the adjacent longwall face is stabilized. To achieve a balance between the seam mining and roadway excavation, numerous mine coals use the method of driving a roadway along the next gob, which heads adjacent to the advancing coal face, and maintaining a narrow coal pillar. Consequently, the dynamic pressure occurs at the gob-side entry; accordingly, the difficulty of roadway maintenance is increased. However, the traditional gob-side entry stability analysis is not applicable to a gob-side entry under dynamic pressure. In this study, the overburden structural model and coal pillar stress model of a gob-side entry under dynamic pressure were established considering the compression load of the immediate roof and gangue and influence law of the cantilever beam length of main roof. Subsequently, thickness, hardness, and deformation of the immediate roof on the stability of the gob-side entry were studied, and the theoretical criterion for the stability of “voussoir beam” structure and coal pillar of the gob-side entry under dynamic pressure were proposed. Based on the geological conditions in tailgate # 110505 of the Yushuling coal mine, reasonable measures were taken considering the aspects of roof cutting and pressure relief, coal pillar design, and surrounding rock reinforcement. After applying the proposed measures, the width of the coal pillar was reduced from 15 m to 4 m. Meanwhile, the grouting cable beam was used to strengthen the roof, and the narrow coal pillar was reinforced by a bidirectional grouting anchor cable was developed. As a result, after the roadway was stabilized, the roof-to-floor convergence and two-side convergence were 268 mm and 105 mm, respectively. Moreover, the stress concentration factor of the coal pillar was 1.20. The stability characteristics of the overburden and coal pillar of the gob-side entry under dynamic pressure were effectively controlled, and remarkable economic benefits were obtained in this coal mine. The techniques proposed for maintaining the stability of the 4 m wide coal pillar and ensuring the bidirectional grouting reinforcement have been successfully applied in a coal mine for the first time, which have high innovation and application values. This study provides a reference for the design and maintenance of the gob-side entry under dynamic pressure.

Failure analysis of coal pillars and overburden from underground water reservoir under the mining-water invasion coupling effect

The construction of coal mine underground reservoirs utilizing old goaf as storage space plays an important role in the ecological environment and economic development of arid mining areas. The destruction and design of coal pillars have always been the focus of research on the stability of coal mine underground reservoirs. Under the combined effect of mining and water immersion, the water-resisting coal pillar is easy to cause gradual damage and instability. Based on this, a numerical simulation method of fluid–solid coupling considering the zoned osmotic unsaturated seepage characteristics and the water immersion weakening effect of water-resisting coal pillars, and the collaborative water-blocking mechanism of coal pillars and overlying strata were proposed to quantify the stability of coal mine underground reservoirs. In the simulation process, the coupled distribution characteristics of the seepage field, plastic zone, and stress field of the permeable flow of coal pillars with different widths under mining and water immersion were realized, and the water-blocking mechanism of the overlying strata based on the distribution characteristics of the water-conducting fissure lateral boundary was studied. Meanwhile, it is proposed that the lateral boundary intersection of the water-conducting fissures on both sides of the overlying strata and the connection between the seepage zone on the waterlogged side and the plastic zone on the mining side inside the water-resisting coal pillar as the critical conditions for the failure of both. Therefore, a reasonable design method for the width of the water-resisting coal pillar was proposed to prevent failure, which includes “water seepage zone + elastic pressure-dense water-resistance zone + plastic zone” three-zone distribution and considers the distribution characteristics of the overlying strata, which are “water-conducting fissure zone + middle extrusion water-blocking zone”. The reasonable width was determined to be 150 m for the water-resisting coal pillar, and on this basis, the dynamic degradation mechanism of the load-bearing capacity of the 150 m coal pillar under multi-field coupling during the entire mining process was quantitatively analyzed. Therefore, an engineering method was proposed to support and reinforce the water-resisting coal pillar and reasonably drain the water in the upper segment. This study is expected to provide theoretical guidance for the stability research and reasonable width determination of the water-resisting coal pillar in the protection of water resources in the water-rich goaf and the safe mining of adjacent working faces.

The new way for realizing carbon neutrality of coal and analysis of bearing characteristics and stability of coal pillar in production

In this paper, the idea of Underground coal gasification coupled with supercritical carbon dioxide storage is proposed based on the problems of Underground coal gasification combustion area and carbon dioxide storage site, which provides a new technical approach for carbon peaking and carbon neutrality in the coal sector. At the same time, to solve the underground coal pillar design problem in the popularization and application of UCG-CCS, this paper adopts numerical simulation and theoretical analysis methods to reveal the deformation mechanism and bearing characteristics of “hyperbolic” type coal pillar in UCG-CCS. The results of this study are as follows: ① The injection of supercritical CO2 into the underground goaf can effectively reduce the load of the coal pillar bearing the overlying strata and improve the bearing capacity of the “hyperbolic” protective coal pillar. ② The size of the “hyperbolic” coal pillar in UCG-CCS slightly influences the coal pillar's stability under certain injection pressure. For safety in practical engineering, it is necessary to ensure that the CO2 injection pressure reaches its critical pressure of 7.38 MPa. ③ Based on numerical simulation results, when the width of the combustion cavity is 40 m, the width of the protective coal pillar is 56 m, the injection pressure of supercritical CO2 is 7.38 MPa, and the sealing depth ranges from 800 m to 1200 m, the protective coal pillars are in a stable state due to the pressure effect of supercritical CO2 on the coal pillar and the roof. The research results of this paper have important practical significance for promoting the site selection, product design, and application of UCG-CCS and provide a technical means for the coal industry to achieve carbon peak and carbon neutrality.

Influence of Moisture Content on Creep Mechanical Characteristic and Mic-Fracture Behavior of Water-Bearing Coal Specimen

The most challenging aspect of constructing an underground reservoir is ensuring that the safety coal pillar has long-term stability. It is of great significance to study the creep mechanical behavior of coal pillars under water-rock interaction for the construction of underground reservoirs. Five kinds of coal specimens with different moisture contents (dry, 2.42%, 5.53%, 7.55%, and saturated) were prepared. The effect of moisture content on the creep mechanical behavior of coal specimens was studied by graded loading, and the acoustic emission signals during the creep loading of coal specimens were monitored. The results showed that with the increase in moisture content, the instantaneous strain and creep strain of coal specimens gradually increase, and the total time of the creep test, creep failure stress, and long-term strength generally decreases. The creep failure stress to peak strength and the ratio of long-term strength to peak strength were 73%~85% and 57%~65%, respectively, which are basically independent of water content. As the water content of the coal increased, the cumulative acoustic emission energy decreased, and the failure form of the coal specimen gradually shifted from tension failure ( ω = 0 % and 2.42%), tension shear composite failure ( ω = 5.53 % ) to shear failure ( ω = 7.55 % and 10.08%). The research results can be used as a reference for the design and strength evaluation of coal pillars for underground reservoirs in coal mines.

Failure analysis of coal pillars and gateroads in longwall faces under the mining-water invasion coupling effect

The failure and design of coal pillars have always been the focus of research in the field of mining engineering, and there are often many water resources in goafs with rich aquifers. Under the influence of mining and long-term water invasion, it is easy to cause gradual failure and instability to coal pillars. Based on this, this paper proposes a numerical simulation method for the coupling of engineering scale mining-water invasion and a quantitative index of the damage ratio of coal pillars. The numerical parameters are calibrated with measurement data from coal-pillar stress. In the simulation process, the evolution of the strength, damage ratio, water pressure, stress, strain, and other parameters in coal pillars under the effect of mining-water invasion is realized, and the progressive failure mechanisms of coal pillars under mining-water immersion are revealed. On this basis, the effects of goaf water-level height and coal-pillar width on coal-pillar stability are quantitatively analyzed, and the critical goaf water-level height corresponding to different coal-pillar widths is obtained. Using the simulation results of this paper, the instability of a coal pillar in a coal mine is effectively solved, and the deformation of the coal-pillar rib and roof of the tailgate is reduced by 39.6% and 30.8%, respectively, which ensures the safe production of the longwall face.

An approach to estimate coal pillar strength

In this paper, a probabilistic design chart for coal pillar based on South African coal mine experience is proposed as a tool capable of estimating the pillar strength. A methodology based on the logistic regression is employed to identify failed and intact pillars with their corresponding probabilities of occurrence. The input parameters used for pillar design include the mine depth, pillar width, bord width, height of mining excavation and panel width. The results showed that 84.6% of the pillars, including stable and failed categories, were correctly classified. In addition, the probability associated with the pillar conditions was determined and plotted in form of a pillar strength graph. The results were in good agreement with the field data and existing studies. One of the merits of the current study is that the probability of having a coal pillar intact or failed can be easily determined. This is particularly useful for design purposes since a series of uncertainties are usually involved in collecting the rock mass parameters. It is concluded that the results of this study could improve the empirical design of coal pillars.

Simulation Research for the Influence of Mining Sequence on Coal Pillar Stability under Highwall Mining Method

Highwall mining, which is referred to the technique of extracting coal from the bottom of an exposed highwall, features safety, efficiency, and economy. According to existing highwall mining methods, the mining sequence has a great influence on highwall stability. Based on a highwall mining project in Australia, this study adopted the FLAC3D numerical simulation method to investigate the stability of coal pillars with different mining sequences. The results show that different mining sequences of boreholes exert a great effect on highwall stability. Compared with sequential mining, the skip mining method achieves higher speed of highwall stabilization and smaller plastic zone of coal pillar with its maximum strength decreasing by 12%. By adjusting the mining sequence scientifically, the coal pillar failure and roof collapse caused by the deviation of mining angle can be avoided. The results may provide a new angle for the studies on the coal pillar layout and stability design in highwall mining.

Protective coal pillar design for segmental lining-supported TBM mine tunnels using physical model tests

Protective coal pillar design for segmental lining-supported TBM mine tunnels using physical model tests

A physical model study of surrounding rock failure near a fault under the influence of footwall coal mining

A study of the deformation of the surrounding rock and coal pillars near a fault under the influence of mining is conducted on a physical model for the design of coal pillars to support and maintain the roofs of adjacent fault roadways. This research is based on the 15101 mining face in the Baiyangling Coal Mine, Shanxi, China, and uses simulation tests similar to digital speckle test technology to analyse the displacement, strain and vertical stress fields of surrounding rocks near faults to determine the influence of the coal pillar width. The results are as follows. The surrounding rock of the roadway roof fails to form a balance hinge for the massive rock mass. The vertical displacement, vertical strain and other deformation of the surrounding rock near the fault increase steeply as the coal pillar width decreases. The steep increase in deformation corresponds to a coal pillar width of 10 m. When the coal pillar width is 7.5 m, the pressure on the surrounding rock near the footwall of the fault suddenly increases, while the pressure on the hanging wall near the fault increases by only 0.35 MPa. The stress of the rock mass of the hanging wall is not completely shielded by the fault, and part of the load disturbance is still transmitted to the hanging wall via friction. The width of the fault coal pillars at the 15101 working face is determined to be 7.5 m, and the monitoring data verify the rationality of the fault coal pillars.

Coal pillar size design and surrounding rock control techniques in deep longwall entry

We combine experimental data, FLAC3D numerical simulation results, and on-site industrial tests to study the problem of coal pillar size design in longwall entry. The Zhaozhuang coal mine located in Changzhi City, Shanxi Province, China, is used as a case study. We use an inversion calculation of the double-yield constitutive model to improve the reliability of the simulation results. Calculated results are fitted using a theoretical value of the Salamon strength formula to obtain mechanical parameters of the rock mass in the goaf, which are used to numerically simulate its compaction-bearing characteristics. The stress distribution state and plastic zone failure characteristics of entry surrounding rock under different coal pillar widths during mining are obtained by numerical calculation. The results show that increased coal pillar size leads to a gradual shift of the peak stress from the solid coal side to the coal pillar side. Use of an 8-m coal pillar generates a lower stress environment and has sufficient bearing capacity to remain stable. Based on the results obtained here, we propose differentiated support techniques that address the differential deformation and stress distribution of surrounding rock both sides of the goaf-side entry. On-site industrial tests show that the proposed support technology effectively reduces the deformation variation and ensures safe entry use during servicing.

The coal pillar design method for a deep mining roadway based on the shape of the plastic zone in surrounding rocks

The coal pillar design method for a deep mining roadway based on the shape of the plastic zone in surrounding rocks

Field and simulation study of the rational coal pillar width in extra‐thick coal seams

AbstractThe principles of mining pressure in extra‐thick coal seams are different from those of medium‐thick and thick coal seams. Field investigation, laboratory test, and simulation analysis methods were used to study the correlation between the gob‐side entry (GSE) stability and coal pillar width during longwall top coal caving mining in extra‐thick coal seams. The test site is in Datong City, Shanxi Province, China. The mining pressure in N5209 tailgate with coal pillar of 30 m is severe based on the field investigations during the N5209 panel retreat. Global constitutive double‐yield and strain‐softening models were built to study the abutment pressures and plastic failure ranges of coal seams with different thicknesses and coal pillar widths. The simulation results show that the GSE with an 8‐m coal pillar is destressed and minimal GSE deformation occurs. In addition, the abutment stress in the coal pillar decreases with increasing coal seam thickness. The field tests indicate that the GSE with an 8‐m‐wide coal pillar is stable in extra‐thick coal seams. This study provides a reference for the coal pillar design in coal mines with similar geological and mining conditions.

Design of coal pillars of gob-side entry between mining faces with large differences in mining height in deep mine

ABSTRACT In order to solve the problems of large deformations of gob-side entry and the fracture of coal pillars at large-height mining faces in deep mines, a case study of the width design of coal pillars between a slice mining face and a large height mining face is studied by theoretical calculation, numerical analysis, and site engineering measurement. In this paper, the occurrence of the Ji16-17 coal seam and its mechanical properties are analyzed. In addition, driving disturbances on roadway deformations, and the effect of plastic area distributions under conditions of different widths of coal pillars and different falls in heights are studied when mining at a slice face turned to a large height face in a deep mine. The results show that the reasonable width of a coal pillar is 8 m with a fall in mining height of 2.5 m between the Ji16-17–24011 slice face and the Ji16-17 −24030 large mining height face (at a 5.5 m mining height). The monitored results show that at a mining height of 5.5 m, an 8 m panel pillar can effectively maintain the stability of a pillar and gob-side entry, and can meet the requirements for the safe production of a stable transition from the slice mining face to the large mining height face.

Study on Protective Coal Pillar Size Design for Ultra High Voltage Line Tower Mining in Mountain Areas

High voltage line towers in mining areas are sensitive to surface deformation caused by mining. Protective coal pillar design for high voltage towers is one of the commonly-used technical measures. Aiming to solve the coal mining safety problem under the Ultra High Voltage transmission line in Sihe Coal Mine of Shanxi Province, the angle and size of protective coal pillars with the vertical line method were analyzed in this paper. The effect of additional displacement caused by landslide or slippage mining in mountain areas and repeated mining was considered. Based on the principle of the vertical line method, the protective coal pillar range and size were calculated. The amount of coal deposited in coal pillars for high voltage line towers was compared and analyzed between the vertical line method and the linear structure method. The results showed that the angle of critical deformation decreased by 2~10° caused by slippage due to mining in a mountainous area, and the angle in the uphill direction of building decreased more than that in the downhill direction; when multi-seams were mined repeatedly, the angle of critical deformation in the lower seam coal mining was reduced by 5~10° compared with that of the upper seam. The protective coal pillar design with the vertical line method can protect the high voltage line towers more effectively, and the amount of protective coal pillars with the vertical line method was 5.8 million tons less, which avoided the waste of coal resources.

Reasonable coal pillar design and remote control mining technology for highwall residual coal resources.

Highwall mining (HWM) technology is an efficient method for exploiting residual coal resources in Chinese open-pit coal mines. However, on-site personnel and equipment can be damaged by the instability of the highwall mining residual coal pillars and subsidence of final end-walls. This paper considers the geological conditions of an open-pit mine in Shendong Coal Field (China) in order to prevent overlying rock fall accidents; the Mark-Bieniawski formula and the FLAC3D numerical simulation are used to analyse reasonable coal pillar widths outside and under the road, which were determined to be 1.7 m and 1.3 m, respectively. Using the EBH132 cantilever excavator for remote control mining, the field experiment shows that the recovery ratio of highwall residual coal resources was over 67%; hence, safety, efficiency and high recovery ratio of highwall mining were achieved for the residual coal resources of an open-pit mine.

Study on Efficient Utilization Technology of Coal Pillar Based on Gob-Side Entry Driving in a Coal Mine with Great Depth and High Production

Improving the utilization of non-renewable resources takes a crucial position in circular economy. Gob-side entry driving technology has been widely applied in coal mines in China, such as in the Shilawusu mine (Ordos City, Inner Mongolia), here considered as a case study due to its high safety and resource-recovery rate. However, at present the complexity of coal pillar utilization makes it hard to fully master the key technology for coal pillar size design, which leads to huge waste of coal resources. Based on theoretical calculation and numerical simulation, this study analyzed the basic mechanical structure of coal pillar and the characteristics of its weakening failure, providing theoretical reference for efficient recovery of coal resources. In general, results of this study can be helpful in pursuing the efficient, hence sustainable, development of mines with Gob-side entry driving technology.

Coal Strength Development with the Increase of Lateral Confinement

The high stress environment brings many challenges in underground coal mining. In order to address the strength behavior of coal under various confining stresses and hence shed light on coal pillar design optimization, compressive tests were conducted under the lateral confinement of 0–8.0 MPa, and the strength enhancement mechanism was studied from the grain scale using PFC modeling. The results show that the coal strength and cumulative axial strain at failure increased with the confinement, while the Young’s modulus of coal is independent of confinement. However, this confinement-dependent strength property can be significantly weakened by existing cracks. Compared to the significant increase in peak compressive stress, the crack initiation stress slightly increased with the confinement. The strength component mobilized with the confinement enhancement. In the early stage of loading, the high confinement restrained the development of microcracks, while in the later stage, it enhanced the frictional resistance strength component. The two mechanism shifted the compressive strength of coal together and the latter one contributed to the strength component mobilization. The coal showed three failure modes sequentially with the increase of confinement, namely axial splitting, mixed failure and shear failure mode. With regard to failure envelope, the Mohr-Coulomb, Hoek-Brown and S-shaped failure criteria can generally represent the confinement-dependent coal strength with R-square larger than 0.9. The confinement of rapid strength promotion section of S-shape failure envelope falls in a range of 1.5–3.0 MPa. This leads to the difficulty of S-shaped failure envelope justification due to the soft nature and heterogeneity of coal.

Determination of pillar width to improve mining safety in a deep burst-prone coal mine

Coal burst is the main threat to coal mining safety in China. Coal pillars of appropriate width play a significant role in controlling stress distribution and coal burst hazard. In the latest National Coal Safety Standard (China), a qualitative principle is given for coal pillar design in burst-prone coal mines. However, specific and quantitative regulations on coal pillar design are more desirable in practice. In this study, the influence of a width-decreasing coal pillar on roadway convergence in a deep coal mine was investigated by numerical simulation and field measurement, which provides both qualitative and quantitative guidance on safe pillar design. Results and analysis indicate that: (1) The mining-induced width-deceasing coal pillar successively experienced the Fully Stable Coal Pillar (FSCP) stage, the Partially Stable Coal Pillar (PSCP) stage, the Critical Coal Pillar (CCP) stage and the Yielding Coal Pillar (YCP) stage; (2) The demarcation points of these four stages were approximately 100 m, 40 m and 10 m in a deep coal mine; (3) The priority in barrier pillar design should be given to YCP, FSCP and PSCP in sequence; (4) The priority in stop-line pillar design should be given to FSCP, followed by PSCP on condition that the roof strata in the stop-line area are fractured and de-stressed in advance; (5) CCP should be avoided in both the barrier pillar design and the stop-line pillar design in all circumstances. The above principles of coal pillar design can be used to improve mining safety in deep burst-prone coal mines of similar conditions.

Limitations and potential design risks when applying empirically derived coal pillar strength equations to real-life mine stability problems

The method of determining coal pillar strength equations from databases of stable and failed case histories is more than 50 years old and has been applied in different countries by different researchers in a range of mining situations. While common wisdom sensibly limits the use of the resultant pillar strength equations and methods to design scenarios that are consistent with the founding database, there are a number of examples where failures have occurred as a direct result of applying empirical design methods to coal pillar design problems that are inconsistent with the founding database. This paper explores the reasons why empirically derived coal pillar strength equations tend to be problem-specific and should be considered as providing no more than a pillar strength “index.” These include the non-consideration of overburden horizontal stress within the mine stability problem, an inadequate definition of super-critical overburden behavior as it applies to standing coal pillars, and the non-consideration of overburden displacement and coal pillar strain limits. All of which combine to potentially complicate and confuse the back-analysis of coal pillar strength from failed cases. A modified coal pillar design representation and model are presented based on coal pillars acting to reinforce a horizontally stressed overburden, rather than suspend an otherwise unstable self-loaded overburden or section, the latter having been at the core of historical empirical studies into coal pillar strength and stability.