长壁孤岛工作面冲击失稳能量场演化规律

煤矿冲击地压一直是困扰中国煤矿安全的主要问题,而煤矿开采过程中跳采形成的孤岛工作面由于容易产生应力集中,来压强度提高,极容易发生冲击地压。基于唐山矿T2193下孤岛工作面的地质条件,从数值分析的角度研究了煤岩体材料的非均匀性,揭示了孤岛工作面顶板周期来压时煤岩体能量释放的动态特征,分析了工作面前方能量释放激增机制。数值模拟结果显示,长壁工作面回采过程中直接顶的不断垮落造成了老顶悬空距离的不断增大,工作面周期来压时,积聚于老顶岩层内的弹性应变能将瞬间释放,容易引发工作面及巷道的冲击失稳。孤岛工作面由于其特有的矿压显现特征,老顶周期破断时所释放的弹性应变能将更加剧烈,冲击地压势必愈加强烈。孤岛工作面顶底板和煤层的能量释放激增可以作为判断煤岩体冲击失稳的前兆信息。孤岛工作面前方发生冲击破坏的主要原因是由于工作面回采过程中围岩所积聚的大量弹性能在顶板断裂时所伴随的巨大能量释放而造成的。     

Yield pillars for stress control in longwall mines — case study

Summary The demand for increased productivity and the problems associated with mining at greater depths have increased the interest in using the yield pillar concept in the United States. This paper summarizes chain pillar behaviour in a mine that historically experienced coal bumps in both room-and-pillar and longwall sections. Results indicate that, generally, the chain pillars yield as designed, but that yielding occurred either after development or with approach of the longwall face. The Bureau of Mines investigated several yield pillar design approaches to possibly explain observed differences in pillar behaviour. These approaches suggest that very localized conditions, such as coal and rock properties, cover depth, and extraction height, may influence the behaviour of any one pillar. At this mine, yielding chain pillars result in de-stressing of the longwall entries and the transfer of potentially dangerous stress concentrations to adjacent panels. Pre-longwall-mining behaviour indicates the existence of a pressure arch, the width of which increases with depth. Results indicate that use of yield pillars improves stress control, reduces bump potential, and increases resource recovery.     

Improving stress environment in development entries through an alternate longwall mining layout

Most coal mines in China use the longwall mining system. High stresses are frequently encountered around development entries at deep mines. This paper presents an alternate longwall mining layout for thick coal seams to minimize ground control problems. In a conventional longwall panel layout, development entries on both ends of the panel are located along the floor, and a coal pillar (chain pillar) is left between adjacent panels to ensure stability. Gateroads on either end of a longwall panel using the layout proposed in this paper are located at different vertical levels within a thick coal seam or in a geologically split coal seam for improved stability. The headgate entry/ies are driven along the floor while the tailgate entry/ies are driven along the roof. Therefore, a longwall face has a gradually elevated or curved section on one end of the panel. For the adjacent panel, the development entry may be located directly below the development entry of the previous panel or may be offset horizontally with respect to it. Based on physical and numerical modeling approaches, it is demonstrated that the stress environment for development entries employing the longwall layout is significantly improved; ground control problems are therefore minimized.     

A study of surface subsidence and coal pillar safety for strip mining in a deep mine

Some villages and bridges are located on the ground surface of the working district no. 7 in the Wanglou Coal Mine. If longwall mining is adopted, the maximum deformation of the ground surface will exceed the safety value. Strip mining is employed for the working district no. 7 which is widely used to reduce surface subsidence and the consequent damage of buildings on the ground surface. To ensure the safety of coal pillars and improve the recovery coefficient, theoretical analysis and numerical simulation (FLAC 3D) were adopted to determine the coal pillar and mining widths and to discuss the coal pillar stress distribution and surface subsidence for different mining scenarios. The results revealed that the width of coal pillars should be larger than 162 m, and the optimized mining width varies from 150 to 260 m. As the coal seam is exploited, vertical stress is mainly applied on the coal pillar, inducing stress changes on its ribs. The coefficient of mining-induced stress varies from 2.02 to 2.62 for different mining scenarios. The maximum surface subsidence and horizontal movement increase as the mining width increases. However, when the mining width increases to a certain value, increasing the pillar width cannot significantly decrease the maximum subsidence. To ensure the surface subsidence less than 500 mm, the mining width should not be larger than 200 m. Considering the recovery coefficient and safety of the coal pillar, a pillar width of 165 m is suggested.     

Determination of Suitable Pillar Size for Protecting Gas Well Drilled Through a Longwall Mining Abutment Pillar Using Numerical Modelling Approach: A Case Study

Gas well drilled through longwall mining abutment pillar could potentially face instability issue due to the strata deformation following longwall panel extraction. Therefore, it is imperative to adequately design the pillar size of a longwall mining in order to ensure the stability of the gas well penetrated longwall mining abutment pillar. In this paper, the determination of suitable pillar size for protecting gas well subjected to longwall mining operation was investigated. Two scenarios of longwall gateroad system including the three and four entry system with varying pillar sizes were assessed using numerical modelling approach. The results of this study indicate that the pillar geometry plays an important role on the vertical gas well stability. In addressing the suitable pillar size for the given case study considering three entry system, the suitable chain pillar and abutment pillar size were found to be 80 ft (24.4 m) wide by 120 ft (36.6 m) length and 104 ft (31.7 m) wide by 120 ft (36.6 m) length rib to rib, respectively. Whereas, if four entry system is used, the suitable chain pillar size is 48 ft (14.6) wide by 120 ft (36.6 m) length and the abutment pillar size is 104 ft (31.7 m) wide by 120 ft (36.6 m) length rib to rib. The proposed numerical modelling procedure presented in this paper can be a viable alternative and applied to other similar projects in order to determine an optimal pillar size for protecting gas well in longwall mining area.

     

In situ yield behaviour of a coal pillar

Summary An indication of the complete stress-strain behaviour of coal was obtained through instrumenting a headgate yield pillar and monitoring the changes in vertical and horizontal pillar stress, pillar strain, and roof-floor convergence as a function of time and distance from the moving longwall face. A semiautomated monitoring program was conducted over a 5 month period as the longwall face advanced toward and eventually passed the instrumented pillar. The development of a confined core and yield zone within the pillar was confirmed by the vertical and horizontal stress changes. Thein situ pre-failure (208 to 661 MPa) and post-failure moduli (55 to 208 MPa) of the coal were determined from the change in pillar stress versus pillar strain. Thein situ moduli are an order of magnitude lower than the 3450 MPa, commonly assumed for coal.     

Influence of Coal Extraction Operation on Shaft Lining Stability in Eastern Chinese Coal Mines

In order to find the relationship between the shaft lining stability and the coal extraction operation, a 3D numerical model of strata layers and shaft lining was established for simulating the influence of coal extraction operation on shaft lining. Certain factors including mining depth, safety pillar width, mining width and mining height were taken as the influence factors in the simulation. The results indicated that the coal extraction could lead to the initiation of the failure in the aquifer and rock layers. As the mining depth increases, the shear strain increment in aquifer becomes small. In this case, the distance between mining panel and aquifer should be larger than 220 m and the safety pillar width should not <70 m. The maximum principal stress in aquifer had a little relation to mining operations. The mining panel width should not exceed 50 m without any support.     

Access tunnel convergence prediction in longwall coal mining

Summary The paper describes theoretical andin situ studies of tunnel deformation in longwall coal mining. It develops a method to predict tunnel convergence profiles from the faceline in longwall mining. The method accounts for the effect of panel width, extracted seam height, deformation moduli of the goaf material and coal pillar, depth of cover,in situ structural defects, tunnel shape and tunnel size in addition to the strength characteristics of surrounding strata. The analytical technique has been validated by reference toin-situ deformation measurements in 26 face-access tunnels in Cape Breton Coalfield mines. Based on this method a series of vertical convergence profiles for different depths and extracted panel widths have been presented.     

Study of roof stability of the end of working face in upward longwall top coal

Low recovery of longwall top coal caving (LTCC) remains one of the most difficult engineering problems in this mining method and impedes its application. The top coal left in the gob at face end accounts for a large portion of the total coal loss, and the instability of the leftover triangle coal at face end has long been a threat to the safety of miners and the mining equipment. In this paper, based on the engineering background of Ruilong mine, we explore the stability of the roof at the end of the face by using theoretical analysis, numerical simulation, and field measurement. Results reveal that in the inclined longwall top coal caving face, the immediate roof forms an “arch” structure, and the basic roof forms a “masonry beam” structure after the roof collapses; working resistance of the support calculated by the method of ultimate bearing capacity was adequate to meet the requirement of roof load; roof load of coal pillar was related to the length of key block and fracture position; and increasing the size of coal pillar could ensure the stability of both coal pillar and roof.     

A Geometric Computational Model for Calculation of Longwall Face Effect on Gate Roadways

In this paper a geometric computational model (GCM) has been developed for calculating the effect of longwall face on the extension of excavation-damaged zone (EDZ) above the gate roadways (main and tail gates), considering the advance longwall mining method. In this model, the stability of gate roadways are investigated based on loading effects due to EDZ and caving zone (CZ) above the longwall face, which can extend the EDZ size. The structure of GCM depends on four important factors: (1) geomechanical properties of hanging wall, (2) dip and thickness of coal seam, (3) CZ characteristics, and (4) pillar width. The investigations demonstrated that the extension of EDZ is a function of pillar width. Considering the effect of pillar width, new mathematical relationships were presented to calculate the face influence coefficient and characteristics of extended EDZ. Furthermore, taking GCM into account, a computational algorithm for stability analysis of gate roadways was suggested. Validation was carried out through instrumentation and monitoring results of a longwall face at Parvade-2 coal mine in Tabas, Iran, demonstrating good agreement between the new model and measured results. Finally, a sensitivity analysis was carried out on the effect of pillar width, bearing capacity of support system and coal seam dip.     

大采高综放面区段煤柱合理留设研究

侧向支承压力分布、资源回收率以及煤柱和巷道的稳定性是大采高综放面区段煤柱宽度留设要兼顾的因素,为了确定大采高综放面区段煤柱宽度,以某矿8103面为工程背景,首先,采用理论计算和现场应力监测等方法确定大采高综放工作面倾向支承压力分布规律,得出应力降低区宽度约为8 m,原岩应力区为巷帮侧28 m外。其次,采用工程类比方法确定大采高综放工作面巷帮外侧煤体严重破裂区宽度约为4 m。最后,采用FLAC3D数值软件分析了下区段工作面回采时窄煤柱（6、8 m）和宽煤柱（28、30 m）的应力场、位移场及塑性区特征,获得不同煤柱宽度时巷道和煤柱力学特征。研究表明：当煤柱宽度6 m和8 m时,在采动支承压力下煤柱几乎无承载能力,且巷道变形量较大;当煤柱宽度28 m和30 m时,在采动支承压力下煤柱中央仍有一定的弹性核,煤柱保持稳定且巷道变形量较小。综合考虑资源回收、巷道稳定性、次生灾害控制等因素,确定大采高综放工作面区段煤柱宽度为28 m。     

Floor failure analysis at a longwall mining face based on the multiple sliding block model

This paper presents an analytical model of a floor failure at a longwall coal mining face based on the multiple sliding block model. During longwall mining, stresses and displacements of strata are constantly changing. High stress concentrations at the coal face can exceed the rock strength and initiate fractures in the strata that can, under unfavourable conditions, lead to large floor displacements and disruption of mining operations. Underground observations of the rock floor and the computational modelling of the longwall face indicate that two types of fracture dominate floor failure. Extensive lateral fracturing often develops along the numerous weak bedding planes that are typically present in the sedimentary strata while sub-vertical fractures form in response to changing stress abutments ahead of the longwall coal face. The fractures that initially develop ahead of the longwall face are subject to a secondary movement when exposed ahead of the longwall supports. In response to the stress relief as the coal is mined from above, strata move towards the opening and bending of the floor occurs. If fractures in the floor exist, the floor blocks will displace in response to the floor movement and interact at the fractured surfaces. This analysis attempts to explain how the stress distribution develops within the broken floor during an active movement of floor strata leading to high stress concentrations at the floor level. The analytical formulation in this paper is supplemented by numerical modelling, and results are presented to verify that the analytical solutions are in accordance with the numerical predictions.     

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.     

Stability of goaf-side entry driving in 800-m-deep island longwall coal face in underground coal mine

Goaf-side entry driving in underground coal mines could greatly improve coal recovery rates. However, it becomes more difficult to maintain stability, especially in deep coal mines. Pillar width plays a pivotal role in the stability of goaf-side entry driving. To obtain a reasonable and appropriate narrow pillar width, theoretical calculations of the widths of mining-damaged zone and limit equilibrium zone in the pillar are derived according to limit equilibrium theory. Based on the stability issues of goaf-side entry driving in the first island longwall coal face (LCF) at a depth of 800 m below the surface in Guqiao Coal Mine in China, a numerical model is established by FLAC software to analyze the stability of the surrounding rock of goaf-side entry driving during excavation, using various coal pillar widths and support schemes. The results obtained from theoretical calculations, numerical simulation, and engineering practice indicate that an 8-m-wide coal pillar is relatively reasonable, appropriate, and feasible. Field measurements show that deformations of the surrounding rock could be efficiently controlled 31 days after the support schemes were implemented in goaf-side entry driving with an 8-m-wide narrow pillar along the adjacent goaf side with a compaction duration of 10 months. The mining influence range of the overlying LCF on the stability of goaf-side entry driving is found to be the area from 50 m ahead of the LCF to 70 m behind the LCF as it passes over the measurement point.     

Borehole stress monitoring analysis on advanced abutment pressure induced by Longwall Mining

The paper presented the research on the dynamic advanced abutment stress induced by longwall mining with borehole stress meters on mining side coal mass. Twenty vibrating wire borehole stress meters were installed into the extracting coal mass wall of a first mining roadway of 910 m depth in Zhuji Coal Mine, China, and were used to monitor dynamic changes in vertical and horizontal stresses. Three months of continuous monitoring and further analysis showed that the impacting distance of advanced abutment stress induced by mining in the strike of the working face along its central axis was the farthest, greater than 250 m (the face length is 220 m); it gradually decreased in the radial direction of the face from its central axis outward; the pressure peak was located within 24 m in the front of the mining coal wall; non-synchronous caving of the layered mudstone roof at the stope occurred. Comparison between vertical and horizontal stress increments indicated that the horizontal stress was much smaller than the vertical stress in the coal mass of mining side, while the latter’s magnitude determined the drastic degree of mine pressure manifestation. The study has been applied to determine the advanced support length of the working face and further provide a reliable basis to forecast such dynamic disasters as rock burst, coal and gas outburst, etc., as well as to design the asymmetric supports on both sides of a gateway.     

Evaluation of coal longwall caving characteristics using an innovative UDEC Trigon approach

The paper presents an innovative numerical approach to simulate progressive caving of strata above a longwall coal mining panel. A proposed Trigon logic is incorporated within UDEC to successfully capture the progressive caving of strata which is characterized by fracture generation and subsequent propagation. A new damage index, D, is proposed that can quantify regions of both compressive shear and tensile failure within the modelled longwall. Many features of progressive caving are reproduced in the model and found to fit reasonably well with field observations taken from a case study in the Ruhr coalfield. The modelling study reveals that compressive shear failure, rather than tensile failure, is the dominant failure mechanism in the caved strata above the mined-out area. The immediate roof beds act like beams and can collapse in beam bending when vertical stress is dominant or in beam shear fracture when horizontal stress is dominant. The proposed numerical approach can be used to guide the design of longwall panel layout and rock support mechanisms.     

Longwall Face Stability Index for Estimation of Chock-Shield Pressure and Face Convergence

The paper discusses the concept and methodologies for the development of longwall face stability index (LFSI). LFSI is used for estimation of chock-shield pressure and face convergence. The index comprises of engineering properties of main roof, depth of mining, different support capacities and mechanical properties of coal seam being mined and provides a numerical value in the range of −6.17 to 8.13. In this study, 324 finite element models of longwall panel are developed based on various combinations of geomining conditions of Indian coal measure strata. The LFSI is an outcome of the results from finite element models. This paper illustrates a real life example for the estimation of chock shield pressure and face convergence based on LFSI. Validation of the LFSI based calculation is carried out with the field monitored data and found that the LFSI based approach is sufficient to forecast face stability parameters at longwall face.     

超长工作面过大断面空巷三维应力演化及综合治理技术

为解决超长工作面过大断面空巷极易发生片帮和大面积冒顶等难题,以晋城成庄矿某超长工作面为背景,建立大断面空巷的三维模型,将工作面顶板划分为煤柱顶板、空巷顶板和待采区顶板3部分。通过理论分析,推导了煤柱失稳的判据,并利用FLAC3D数值模拟分析了大断面空巷顶板应力演化过程。结果表明:煤柱宽度W≤40 m时,工作面超前支承应力与空巷超前支承应力在煤柱上叠加,煤柱开始发生塑性变形;W≤10 m时,煤柱顶板应力逐渐达到峰值16.6 MPa,煤柱发生破坏并失去承载能力,工作面超前支承应力向待采区转移,空巷顶板应力达到峰值12.7 MPa。根据空巷顶板应力演化规律,确定高水材料充填支柱支护的合理强度及空巷两帮煤壁注浆加固的时机,辅以空巷锚索梁补强,提出了大断面空巷综合治理措施,现场应用效果良好。      

辛安煤矿1402工作面临空窄煤柱采掘响应及动态加固

随着我国煤炭资源去产能整合煤矿的增多,复采工作面临空窄煤柱采动失稳问题日益凸显,已严重制约矿井安全高效生产。为此,针对辛安煤矿复采1402工作面辅运巷道5号钻场临空窄煤柱稳定性控制的工程难题,运用数值模拟与理论分析相结合的方法,探究5号钻场临空窄煤柱稳定性采掘扰动响应特征,提出5号钻场临空窄煤柱动态注浆加固技术方案并开展现场应用和效果检验。研究结果表明:1402工作面辅运巷道掘进对5号钻场临空窄煤柱稳定性影响较小;在1402工作面回采期间,距5号钻场18~6 m范围,临空窄煤柱集中垂直应力由非对称马鞍形分布逐渐过渡为拱形分布;距5号钻场6 m时,临空窄煤柱承载叠加垂直应力超过煤体强度,塑性区完全贯通,极易破坏失稳;现场采用MP364型注浆材料及专用注浆设备对5号钻场临空窄煤柱前后5 m区域进行加固,动态注浆始终超前工作面10 m,通过深孔窥视和气体监测手段验证临空窄煤柱良好的封堵固化效果,保障了工作面安全回采,为我国整合矿井类似条件下煤柱稳定性控制提供借鉴和参考。移动阅读      

泾河下综放开采隔离煤柱对覆岩破坏控制作用的物理模拟

根据地质资料,分析了下沟煤矿泾河下特厚煤层大面积综放开采的地质特点,为实现水体下安全回采,确定了在各工作面间留设一定宽度隔离煤柱的开采方案。通过相似材料模拟试验,分析了改本区地质条件下各综放工作面间留设一定宽度隔离煤柱对覆岩破坏的影响。研究证实,隔离煤柱对覆岩破坏起到有效的控制作用。根据试验得出的单工作面最大裂采比,通过最小防水安全煤岩柱垂高的计算,认为地质条件满足泾河下安全回采的要求。且研究成果成功指导了5个工作面安全回采,可为该区及类似条件其他矿井的开采提供参考