Protective Seam Research Articles

Representation of mining permeability and borehole layout optimization for efficient methane drainage

The development of a cost-effective and accurate permeability characterization method for large-scale mining overburden conditions is topical for engineering assessment of mine water and gas migration characteristics. Based on laboratory test results, a new permeability characterization method considering loading–unloading times and elastic–plastic stress evolution is proposed, which is realized by fish language embedded in FLAC 3D software. The proposed method is first compared with other available models via the inversed experimental results and then applied to evaluate the protected seam permeability-enhanced range. The COMSOL Multiphysics and FLAC data interaction is used to optimize the protected seam extraction borehole layout. It is shown that the proposed model can characterize the permeability evolution more accurately than other (elastic, damage strain) models. The protected seam stress paths have non-equal loading and unloading amplitudes in vertical and horizontal directions, while its permeability is divided into six areas and the enhanced area exceeds that of the gob. Based on the permeability zoning characteristics, the non-uniform layout parameters of extraction boreholes are quantified to achieve the identical-size layout of working faces of the protected and protective seams and reduced workload of extraction boreholes.

Influence of hard-roof on gas accumulation in overlying strata: A case study

Protective seam mining is one of the most commonly used technique to realize stress relief and permeability enhancement of reservoirs, so as to improve gas drainage efficiency. However, for coal reservoir with a hard roof, such as the tight sandstone, magmatic rocks, the distribution and evolution of destressed zone and gas field have not been fully understood, which leads to an inefficient gas control in the field. In this work, taking the geological background of Pingmei 12th Coal Mine as an example, we numerically investigated the evolution and distribution of stress, permeability and gas field in the hard-roof stope during protective seam mining. The results show that in the zone with a distance greater than 3.7 m, stress in the hard-roof stope is significantly higher than that in the conventional stope and the permeability enhancement ratio in the hard-roof stope is significantly lower than that in the conventional stope. In the overlying strata, the existence of the hard roof inhibits the upward extension of the fractured zone, and the range of the fractured zone in the hard-roof stope is obviously smaller than that in the conventional stope. Therefore, in the hard-roof stope, the relieved gas accumulates in the gob, instead of entering the overlying fractured zone, which may put a threat to the mining safety. To solve this problem, the presplitting blasting technique was applied to cut the hard roof. After the collapse of the hard roof, the overlying fractured zone is fully developed, and the gas accumulated in the gob enters the fractured zone driven by the concentration difference and buoyancy. This can not only prevent gas overrun, but improve gas drainage efficiency of the upward borehole. This practice could be used as a valuable example for other coal mines with similar geological conditions.

Permeability Enhancement and Gas Drainage Effect in Deep High Gassy Coal Seams via Long-Distance Pressure Relief Mining: A Case Study

Coal 3 in group A is employed as a protective layer to release long-distance coal 4 in group B in Paner colliery (approximately 80 m vertical interval) as the mining depth extends downward, which is the first engineering test in the Huainan coal mining area. To evaluate the validity of the scheme, the permeability distribution, and evolution law, gas pressure distribution characteristics, swelling deformation, pressure relief range, and gas drainage volume of the protected coal seam are analyzed using a FLAC3D numerical simulation and field measurements. Therefore, different stress-permeability models are adopted for caved, fractured, and continuous deformation zones, and a double-yield model is applied in the goaf based on compaction theory to improve the accuracy of the numerical simulation. The results indicate that the extraction of coal 3 has a positive effect on permeability enhancement and pressure relief gas drainage. However, the dip angle of coal measurements causes asymmetric strata movement, which leads to the pressure relief and permeability enhancement area shifting to the downhill side, where the permeability enhancement effect of the downhill side is better than that of the uphill side. The permeability enhancement zone is an inverted trapezoid, but the effective pressure relief range is a positive trapezoid. The permeability of the protected coal seam in the pressure relief zone is significantly higher than that in the compressive failure zone. The permeability in the pressure relief zone will decrease again due to the recompaction of the coal seam with an advancement of the longwall face. Thus, pressure relief gas drainage is suggested during long-distance protective coal seam mining to eliminate gas hazards.

Layout of the First-Mining Working Face in the Protected Seam for Pressure Relief by Peak Stress Shift

An appropriate layout of the first-mining working face of protected seams is key to rockburst prevention in coal seams. By taking the first-mining working face in the 2-2# middle coal seam in Hulusu Coal Mine as the background, a mechanical model for the stress distribution after mining the upper protective seams was established through theoretical analysis and in situ measurement. The problem concerning the layout of the first-mining working face in the protected seam was transformed into one concerning the optimal horizontal distance L. Furthermore, based on the theory of stress transfer within a semi-infinite plane, the distribution of the vertical stress in the protected seam was deduced, for which numerical simulation was performed. Through calculation, it is best (in terms of roadway excavation) to set L to between 50 and 75 m; the mining-induced disturbance shows the least influence on the two roadways at L = 80 to 140 m and at L = 40 to 100 m, and the influence of the remaining coal pillar on mining of the working face is the least significant. Based on the analytic hierarchy process (AHP) evaluation method, the weights of three groups of contradictory criteria and the weighted values of the peak stress under different values of L were calculated. The results show that the global stress on the first-mining working face in the protected seams is lowest at L = 60 m. The results can provide a reference for the layout of the working faces in protected seams in adjacent coal mines.

Stress Distribution and Gas Concentration Evolution during Protective Coal Seam Mining

Coal and gas burst is one of the significant and catastrophic hazards in underground longwall operations. To date, the protective coal seam mining has been recognized as the most effective mining method for minimizing or even avoiding the effect of the coal and gas burst. In this paper, numerical modelling and field test were carried out for the longwall operation in Qidong Coal Mine in order to investigate the induced stress and coal seam gas drainage performance in the protected coal seam after the complete extraction of the protective coal seam. It was found that four stress zones can be classified in the protected coal seam being the original stress zone, stress concentration zone, stress relief zone, and recompaction zone. In addition, the monitoring data of gas concentration and volume change in the field agree well with the numerical modelling results.

The pressure relief protection effect of different strip widths, dip angles and pillar widths of an underside protective seam.

To design underside protective seam strip layout. Similarity model experiments, numerical simulations and theoretical calculations are used to quantitatively study the pressure relief protection effect of different strip widths, dip angles and coal pillar widths of a thin underside protective seam under deeply buried conditions. The optimal strip width range is obtained according to the change law of strain during the mining process of the underside protective seam in a similar model experiment. The change law of the expansion of the protected coal seam is obtained and the fitting surfaces among the dip angle and strip width of the coal seam with the protection distance and pressure relief angle along the strike and dip of the protected coal seam are established according to the numerical simulation results of underside protective seam mining. It is concluded that the best pressure relief effect can be achieved when the dip angle is 16.7° and the strip width is 70 m. According to the stability threshold of coal pillars considered in strip mining theory, the coal pillar width is calculated to be 50 m. Similarity model experiments and numerical simulations of protected coal seam mining verify the pressure relief effect of the designed protective seam strip width and pillar width. A calculation method of the protective seam strip width, position and pillar width required by the specific width of the protected seam is proposed.

Geological evolution process, mechanism, and application of protective layer in semi-coal and rock face

Under the influence of geological structure, the surrounding rock of semi-coal seam is easy to be loose and fractured, and the repairing rate is high. Therefore, it is of great theoretical and practical significance to study semi-coal seam mining's mechanism and control technology. In this paper, the geological evolution principle of the semi-coal seam is analyzed theoretically. The mechanic's relationship between the mining height of the semi-coal face and the lower coal seam's pressure relief is interpreted. The mechanism of eliminating bad blind areas in non-pillar mining of upper protective seam is analyzed. Through numerical simulation analysis, it is concluded that the expansion deformation rate of a protective layer increases linearly with the increase of mining thickness of the protective layer, and the width of undesirable blind zone of coal seam increases linearly with the rise of protective layer thickness. This paper puts forward the technology of non-pillar mining in the protective seam's semi-coal and rock face. Field industrial test results show that the technology achieves the overall pressure relief and reflection reduction of low permeability and high gas coal seams. The underlying 3 + 4 protective layer achieves the general pressure relief and reflection reduction of low permeability and high gas coal seams. Coalbed methane can be pre-drained up to 18 m3/min with a concentration of 90%.

Analysis of the Influence of Upper Protective Layer Mining on the Effect of Pressure Relief and Protection of Coal and Rock Masses between the Lower Overburden Layers

Protective seam mining is an effective gas pressure relief method in deep mining. Effective theoretical calculation methods in the current studies on the prediction of pressure relief protection effect of interbed coal and rock masses and their distribution laws are lacking. Thus, the evaluation and research with respect to pressure relief effect in protective seam mining relatively lag behind. This situation restricts the engineering feasibility evaluation and decision making in the protective seam mining. Therefore, the influence of upper protective seam mining on the pressure relief protection effect of coal and rock mass between underlying beds was investigated in this study. On the basis of an analysis of concrete engineering projects, a mechanical model was constructed for the pressure relief protection effect of upper protective seam mining on the coal and rock mass between underlying beds. The distribution equation of pressure relief expansion ratio in the underlying protected seam was also derived. The influence laws of main influencing factors on the pressure relief protection effect of the protected seam were revealed as well. In the end, the pressure relief effect was analyzed and verified for the protected seam before and after mining through numerical simulation and similarity simulation test. The pressure relief effect of upper protective seam mining on the coal and rock mass between underlying beds and the distribution characteristics were deeply explored in this study, which could provide a theoretical reference for the decision making in the gas extraction engineering design and pre‐evaluation of extraction effect. Results show that the effective pressure relief zone (expansion rate>0.3%) of the protected seam beneath the goaf is located within the range of approximately 40 m from the coal wall to the rear part. It also presents an approximate “Λ‐shaped distribution characteristic,” that is, it experiences migration and evolution with the advancement in the working face. Moreover, the peak pressure relief lags behind the coal wall on the working face by nearly 10–20 m. In the numerical simulation, the expansion ratio in the goaf also presents an approximate “Λ‐shaped distribution.” Its effective pressure relief zone is the 50 m range from the coal wall to the rear part of the goaf, and the peak value lags behind the coal wall by around 15 m. The theoretical results and numerical simulation results are basically consistent with the physical experiment results. The expansion rates are 1.25%, 1.268%, and 1.32%, respectively. The elastic modulus E of coal seam and interbed spacing H are the main influencing factors of the swelling deformation and are negatively correlated with the expansion ratio. In the actual mining process, E and H of the protected layer can be measured to infer the expansion deformation of the protected layer.

Elimination of Coal and Gas Outburst Dynamic Disasters in Dengfeng Coalfield through Gas Extraction Based on Extremely Thin Protective Coal Seam Mining

No. 21 coal seam is a full‐thickness structured soft coal in Dengfeng coalfield. The coal seam gas‐bearing capacity is high, and the permeability is poor, thus resulting in serious coal and gas outburst dynamic disasters. According to the gas geological conditions of Baoyushan Mine, No. 17 coal seam without outburst danger, which is 0.5 m thick and 23.4 m under No. 21 coal seam, was mined in advance as the lower protective seam. At the same time, a gas extraction roadway was constructed in No. 21 coal seam floor. Cross‐layer boreholes were constructed to extract the pressure relief gas of No. 21 coal seam for comprehensive treatment of mine gas. The mobile deformation of the overburden coal and rock mass after mining No. 17 coal seam, the fracture development characteristics of No. 21 coal seam, the pressure relief gas migration of the coal seam, the gas extraction, and the outburst danger elimination were studied. The research findings showed the following: (1) after mining No. 17 coal seam, the overburden hard and extremely thick limestone roof sagged slowly, albeit leading to no craving zone. (2) The permeability of No. 21 coal seam was increased by about 394 times, from 0.0012 mD to 0.4732 mD. (3) After the extraction of pressure relief gas through the gas extraction roadway on the floor through the cross‐layer borehole, the gas pressure of No. 21 coal seam decreased from 1.17 MPa to 0.12 MPa, while the gas content decreased from 9.74 m3/t to 3.1 m3/t, which suggested that the coal and gas outburst dynamic danger of No. 21 coal seam was totally eliminated and the goal of safe and efficient mining in the mine was realized.

Study on Gas Control Methods Optimization for Mining Safety

Effective gas control is of significance for safe efficient coal mining in Haizi Coal Mine and other mines with similar geological conditions. This study concentrates on gas control theories and techniques in multiple coal seams of Haizi Coal Mine (No. 7, No. 8, No. 9, and No. 10 coal seam from top to bottom). To minimize risk of high gas emission and outburst hazard, No. 10 seam was mined first as a protective seam prior to the mining of its overlying outburst‐prone No. 7, No. 8, and No. 9 seam. Four gas drainage measures were determined for gas control, including cross‐measure boreholes into overlying coal seams, surface goaf wells, roof boreholes, and roof gas drainage roadway. These gas control measures, if implemented through entire coal seam extraction, would be possibly uneconomic. An investigation was undertaken to analyze effects of those four measures on gas emission, methane concentration, and gas drainage quantity in No. 2 1024 mining panel of No. 10 seam. Results indicate that the highly expensive gas drainage measure of a roof roadway has poor drainage performance and could be effectively replaced by roof boreholes. When adopting the optimized combination of gas drainage measures, drainage efficiency of No. 7 seam, No. 8 seam, and No. 9 seam could reach 58.64% and decrease gas pressure to be below 0.74 MPa. Outcomes of this study could provide beneficial guidance not only for gas drainage design optimization in Haizi Coal Mine but also for other multiple‐seam mines with similar mining and geological conditions, for increasing gas drainage efficiency and guaranteeing mining safety.

Gas Flow Characteristics and Optimization of Gas Drainage Borehole Layout in Protective Coal Seam Mining: A Case Study from the Shaqu Coal Mine, Shanxi Province, China

With increasing demands for coal resources, coal has been gradually mined in deep coal seams. Due to high gas content, pressure and in situ stress, deep coal seams show great risks of coal and gas outburst. Protective coal seam mining, as a safe and effective method for gas control, has been widely used in major coal-producing countries in the world. However, at present, the relevant problems, such as gas seepage characteristics and optimization of gas drainage borehole layout in protective coal seam mining have been rarely studied. Firstly, by combining with formulas for measuring and testing permeability of coal and rock mass in different stress regimes and failure modes in the laboratory, this study investigated stress–seepage coupling laws by using built-in language Fish of numerical simulation software FLAC3D. In addition, this research analyzed distribution characteristics of permeability in a protected coal seam in the process of protective coal seam mining. Secondly, the protected coal seam was divided into a zone with initial permeability, a zone with decreasing permeability, and permeability increasing zones 1 and 2 according to the changes of permeability. In these zones, permeability rises the most in the permeability increasing zone 2. Moreover, by taking Shaqu Coal Mine, Shanxi Province, China as an example, layout of gas drainage boreholes in the protected coal seam was optimized based on the above permeability-based zoning. Finally, numerical simulation and field application showed that gas drainage volume and concentration rise significantly after optimizing borehole layout. Therefore, when gas is drained through boreholes crossing coal seams during the protective coal seam mining in other coal mines, optimization of borehole layout in Shaqu Coal Mine has certain reference values.

Mechanical Properties and Control Rockburst Mechanism of Coal and Rock Mass with Bursting Liability in Deep Mining

In order to study the mechanism of the dynamic disaster of rockburst in a deep coal mine and the prevention and control measures of weakening shock, the MTS815.03 servocontrolled rock mechanics test system is used to test the coal, rock, and combined specimens with the buried depth of nearly 1200 m in Xinwen Mining Area. And their mechanical properties, energy evolution, and bursting properties are studied and analyzed. The rationality of the test results is also verified by the in-situ engineering practice. The key conclusions are as follows: (1) There is a relation between the ratio of elastic modulus Ee before peak strength to descending modulus Ed after peak strength and the bursting properties. For the fractured coal, the descending modulus Ed is relatively small, and the Ee/Ed is relatively large and presents progressive ductile failure with low probability and risk of rockburst. For the less fractured rock, the descending modulus Ed is relatively large, and the Ee/Ed is relatively small and presents brittle failure, which is very similar to the characteristics of rockburst. (2) For the same type of rock, with the increase of confining pressure, the Ee/Ed gradually increases, indicating the reduction of rockburst strength. Therefore, the greater the support strength provided to the surrounding rock surface of the roadway, the smaller the failure degree of rockburst. (3) With the increase of confining pressure, after peak strength, the elastic energy of coal specimens decreases slowly, and the dissipated energy increases slowly, indicating that the increase of confining pressure can effectively limit the energy dissipation and release after coal specimen failure. So, in the in-situ engineering practice, it is an important measure to improve the surface restraint and support strength of the coal roadway for reducing the occurrence intensity and probability of rockburst. (4) The combined measures of “the mining of double liberating seam + the implementation of large-diameter pressure relief borehole in advance of working face” is the very effective way to eliminate the rockburst accidents of working face in a protected coal seam and has an important guiding significance for the safe mining of rockburst mine.

Rib Spalling 3D Model for Soft Coal Seam Faces with Large Mining Height in Protective Seam Mining: Theoretical and Numerical Analyses

Fully-mechanized mining of coal face with a large cutting height is generally jeopardized by rib spalling disaster in the working face. Preventive measures based on undisturbed coal seam conditions fail to provide safe predictions, as large-scale fractures in soft coal face frequently appear before excavation due to mining-induced stresses. This paper investigates a case study of the Paner Mine 11224 working face in the Huainan mine area, China, which features an overlying protected layer in the protective seam mining. To simulate the failure process in such a mine, we elaborated a simplified physical-mechanical model of a coal wall that underwent shear failure and sliding instability, in compliance with the triangular prism unit criterion. Similar simulation experiments, theoretical calculations, and borehole monitoring were used to comprehensively analyze the overburden fracture and movement after mining the lower protective seam. The development height of three overburden zones was determined, and the characteristics of the protected layer affected by mining were obtained. The results show that the failure is mainly related to the roof load, coal cohesion, internal friction angle, coal seam inclination, and sidewall protecting force. The key to limiting the frictional sliding of a slip body is to reduce the roof load and increase the sliding coefficient and cohesion of the main control weak surface (MCWS). Besides, a self-developed three-dimensional numerical calculation software RFPA3D (Realistic Fracture Process 3D Analysis), which considered the rock heterogeneity, was used to reproduce a weak triangular prism’s progressive failure process. The numerical simulation results agreed with the fracture pattern predicted by the theoretical model, which accurately described the rib spalling mechanisms in a soft coal face with a large cutting height and a protective layer.

Numerical Study on Stress Relief and Fracture Distribution Law of Floor in Short-Distance Coal Seams Mining: A Case Study

During the mining process of short distance coal seams group with high gas content, excavation of the protective coal seam will result in stress relief and fracture development which provides pathways for gas migration. Gas stored in the lower coal seam moves along the mining-induced fracture to the floor of the upper working face, which may cause the gas over-limit at the upper coal seam. This paper takes the short distance coal seams group in Xinghua coal mine as an example, theoretical analysis, numerical simulation and field monitoring are comprehensively applied to study the floor failure mechanism, pressure relief law and dynamic evolution of cracks induced by the mining of the second-right working face. The results showed that as the upper coal seam gradually advances, the pressure relief angle of roof and floor gradually decreases, and the pressure relief range and floor failure depth gradually increase. After the working face of the upper coal seam is advanced 100 m, the range of the pressure relief in the surrounding coal and rock masses approaches stable; the cracks in the shallow depth are mainly with small angles or type II shear cracks, and the cracks in the large depth are mainly shear-expansion with lager angles, and the amount of fractures in the floor reduces with the increase of the depth of the floor. It was found that the mining-induced fractures in the floor is distributed in the shape of an “O” ring, and as the working face advances, the range of this “O”-shaped crack circle keeps moving forward but the width of the crack circle is basically unchanged. Finally, the low-level drilling field was used to control the gas from lower coal seams according to the stress relief and fracture distribution law, which achieved satisfied application effect and provided guidance for mine gas extraction in similar mining conditions.

Using stress path-dependent permeability law to evaluate permeability enhancement and coalbed methane flow in protected coal seam: a case study

Pressure-relief mining is the primary method to remove gas and coal outbursts and realize coal and gas simultaneous extraction in deep, high-gas, and low-permeability coal seams. Numerical modeling has been widely used to evaluate the effectiveness of protective coal seam (PCS) longwall mining, which usually employs experiment-derived permeability models to describe permeability changes and coalbed methane (CBM) flow. Due to the difficulty of tests and data available, published models and parameters were commonly utilized in simulations, which, however, may limit the accuracy of the results because of their case-dependent nature. In this paper, a series of laboratory tests were performed on different damage degree rock mass, i.e., broken rock samples (caving zone), fracture coal samples (fracture zone and swelling deformation zone), and raw coal samples (original zone), to obtain permeability models for different damage zones surrounding the longwall face. Besides the stress states, the experiments also highlight that the damage degree and stress path play vital roles in the permeability changes. The stress sensitivity of permeability in unloading stages is found to be significantly lower than that in loading stages, indicating the importance of the stress path when simulating longwall mining-induced permeability changes. A stress path-dependent permeability law was proposed, and the associated algorithm was designed and embedded into the FLAC3D code. For the first time, the stress path-dependent permeability law was employed to simulate the pressure relief, permeability enhancement, and CBM flow in the protected coal seam (PDCS) when longwall mining is executed in the upper PCS. Field measurements proved the feasibility and accuracy of the new numerical method. Numerical results also suggest the common loading permeability model overestimates the effectiveness of the pressure relief mining. This study highlights the significant role of stress path-dependent permeability laws to accurately predict the permeability changes and CBM flows in pressure relief mining.

Permeability Enhancement of Coal Seam by Lower Protective Layer Mining for Gas Outburst Prevention

Risk prediction of dynamic disasters such as rock burst, gas outburst, and water inrush is closely related to the permeability evolution of coal seam. According to the characteristics of the lower protective layer mining, the basic assumption of gas-solid coupling model of the coal was proposed in this paper. The permeability enhancement coefficient of equivalent layer spacing was first put forward. Based on the three-zone-shaped dynamic evolution of the permeability of the overlying protective layer during the lower protective layer mining, the theories of seepage mechanics and damage mechanics were applied to introduce the permeability enhancement coefficient of equivalent layer spacing. A mathematical model of permeability evolution of the protected coal seam in the lower protective layer mining was established. Based on the engineering background of the lower protective layer mining in Changping Coal Mine, the numerical simulation using the proposed mathematical model was performed. The results showed that the stress and permeability of the protected layer in #3 coal seam evolved dynamically with the advancement of the working face of the protective layer in #8 coal seam. When the working face of the protective layer in #8 coal seam advanced to 80 m, the stress reduction rate in the relief area tended to be stable, and the stress in the stress reduction area was about 50% of the original rock stress. When advanced to 80 m, the permeability of the protected layer of the #3 coal seam increased sharply, and the permeability increased by 873 times. With the continuous advancement, the permeability of the protected layer in #3 coal seam tended to increase steadily, and the permeability increased by 1100–1200 times. The calculated magnitude of permeability increment is consistent with that in the engineering practice, indicating that the permeability evolution model is basically reasonable. The research provides a theoretical guidance for the gas drainage field application in the lower protective layer mining for prevention of coal and gas outburst.

Key technologies and engineering practices for soft-rock protective seam mining

Severe gas disasters in deep mining areas are increasing, and traditional protective coal seam mining is facing significant challenges. This paper proposes an innovative technology using soft rock as the protective seam in the absence of an appropriate coal seam. Based on the geological engineering conditions of the new horizontal first mining area of Luling Coal Mine in Huaibei, China, the impacts of different mining parameters of the soft-rock protective seam on the pressure-relief effect of the protected coal seam were analyzed through numerical simulation. The unit stress of the protected coal seam, which was less than half of the primary rock stress, was used as the mining stress pressure-relief index. The optimized interlayer space was found to be 59 m for the first soft-rock working face, with a 2 m mining thickness and 105 m face length. The physicochemical characteristics of the orebody were analyzed, and a device selection framework for the soft-rock protective seam was developed. Optimal equipment for the working face was selected, including the fully-mechanized hydraulic support and coal cutter. A production technology that combined fully-mechanized and blasting-assisted soft-rock mining was developed. Engineering practices demonstrated that normal circulation operation can be achieved on the working face of the soft-rock protective seam, with an average advancement rate of 1.64 m/d. The maximum residual gas pressure and content, which were measured at the cut hole position of the protected coal seams (Nos. 8 and 9), decreased to 0.35 MPa and 4.87 m3/t, respectively. The results suggested that soft-rock protective seam mining can produce a significant gas-control effect.

Development Range Determination of Gas Conducting Fissure Zones for Lower Protective Seam Mining in JINJIA Coal Mine

Under the circumstance that lower protective seam mining may give rise to mining-induced fracture development in the upper coal and rock mass, the key to gas extraction during lower protective seam mining is investigated based on mining-induced fracture development law of overlying strata. Fracture evolution processes are seldom described by the traditional “Three Zone” theory of overlying strata. Consequently, its field application in gas extraction during the lower protective seam mining is limited. While theoretical analysis and analog simulation are adopted to analyze mining-induced fracture development rules for the lower protective seam mining, a plastic hinge is selected to theoretically derive a process from internal fracture development to occurrence of rock beam fracturing. In combination with the theory of key strata that the movement of overburden is controlled by key bond strata, a computing method is obtained to figure out development height of a gas conducting fissure zone during the lower protective seam mining. Taking geological conditions in Jinjia Coal Mine as the engineering background, development height of the gas conducting fissure zone in No. 22 seam there is calculated to be 42.2 m. On this basis, engineering design is performed for pressure relief gas extraction of the lower protective seam mining. During working face mining, gas emission quantity and sources are also monitored according to gas conducting features of strata within a range of the gas conducting fissure zone. Hence, accuracy in development range computations for this zone is verified. As demonstrated by research findings, combining the plastic hinge theory with a control action of key strata has the potential to effectively work out development height of the gas conducting fissure zone during the lower protective seam mining. Without a doubt, an accurate theoretical basis can be provided for pressure relief gas extraction in the course of lower protective seam mining.

Decrease in coal losses during mining of contiguous seams in the near-bottom part at Vorkuta deposit

The problem of formation of extended zones with high rock pressure (HRP) from safety pillars at the boundaries of extraction pillars formed due to the mine layout of complex geometry is considered at the example of JSC Vorkutaugol mines. A detailed analysis of the remaining reserves of the near-bottom part of the deposit was carried out to estimate losses and the impact of HRP zones from the Chetvertyi protective seam to mining operations on the Troinoi upper seam along with the possibilities for the reduction of sizes of HRP zones at the account of expanding the underworked space. Due to research on the near-bottom part of the Vorkuta deposit, within the framework of the accepted layout, a zone at the Komsomolskaya mine and two zones at the Zapolyarnaya-2 mine were singled out, at which losses at the boundaries of the extraction pillars amount up to 13-22 % of the total resources of the mine field. The high volume of losses in these pillars indicates the relevance of research on the priority extraction impact of protective seams on the efficiency and safety of mining operations in the working area of underworked and HRP zones. Based on the analysis of foreign and Russian experience in the pillar cleaning-up at the boundaries of working areas and the methodical guidelines and instructions, a technological scheme was developed that allows increasing the coal mining recovery factor in the near-bottom part of the Vorkuta deposit from 0.75 to 0.9 without fundamental changing of the ventilation and transport networks and also without purchasing any additional mining equipment.
 The conducted economic calculations confirmed the effectiveness of implementing the new technological scheme for cleaning-up reserves at the boundaries of extraction districts. The economic effect is from 0.079 to1.381 billion rubles of additional profit from coaxial extraction pillars, depending on the mining and geological conditions and the size of the pillars.

A protective seam mining method by cutting roof with chainsaw arm machine

Abstract Multiple-layered coal seams widely exist in main coal mining areas of China. When these coal seams are exploited, the pillar mining method is always employed. This leads to many coal pillars left in the upper coal seams as a protective barrier. As a result, these residual pillars will not only cause the loss of coal resources but also could trigger environmental issues and a serious of mine disasters. A theoretical model was built to analyse the effect of the residual pillars. From the theoretical model, it was found that four stress concentration areas were formed by the upper residual coal pillars. To address the issues of the residual coal pillars, Datong Coal Mine Group has developed an innovative technology of the roof cutting with a chainsaw. A new protective coal seam mining method using chainsaw roof-cutting technology is introduced. A numerical model is constructed to analyse the mining pressure distribution law in working face within the lower layer coal seam. From the numerical simulation, the new protective layer mining method could reduce about 15.2% of the advancing stress, which contributes a lot to controlling the mining pressure within the lower layer. The field measurement showed that the hydraulic support utilised at the site was at lower pressure levels, which proves the new protective seam mining method can significantly reduce the working face pressure.