Fractal characterization for the mining crack evolution process of overlying strata based on microseismic monitoring technology

In order to study the evolution laws during the development process of the coal face overburden rock mining-induced fissure,we studied the process of evolution of overburden rock mining-induced fissures and dynamically quantitatively described its fractal laws,based on the high-precision microseismic monitoring method and the nonlinear Fractal Geometry Theory.The results show that:the overburden rock mining-induced fissure fractal dimension experiences two periodic change processes with the coal face advance,namely a Small→ Big→ Small process,which tends to be stable;the functional relationship between the extraction step distance and the overburden rock mining-induced fissure fractal dimension is a cubic curve.The results suggest that the fractal dimension reflects the evolution characteristics of the overburden rock mining-induced fissure,which can be used as an evaluation index of the stability of the overburden rock strata,and it provides theoretical guidance for stability analysis of the overburden rock strata,goaf roof control and the support movements in the mining face.     

Overburden fracture evolution laws and water-controlling technologies in mining very thick coal seam under water-rich roof

Considering the danger of water inrush in mining very thick coal seam under water-rich roof in Majialiang Coal Mine, the universal discrete element (UDEC) software was used to simulate the overburden fracture evolution laws when mining 4# coal seam. Besides, this study researched on the influence of face advancing length, speed and mining height on the height of the water flowing fractured zones (HWFFZ), and analyzed the correlation of face advancing length and change rules of aquifer water levels and goaf water inflow. Based on those mentioned above, this research proposed the following water-controlling technologies: draining the roof water before mining, draining goaf water, reasonable advancing speed and mining thickness. These water-controlling technologies were successfully used in the field, thus ensured safely mining the very thick coal seam under water-rich roof.     

厚煤层大采高采场煤壁的破坏规律与失稳机理

基于大采高采场煤壁稳定性控制需要,在现场实测基础上,采用数值模拟分析了煤层采动裂隙的发展演化规律,并用滑移线理论分析了煤壁失稳的力学过程.研究表明:仅含层理煤层的采动剪切破坏面由倾向相反的共轭面组成;含节理煤层中,硬煤的采动破坏面为剪切破坏面与节理张裂面组成的倾向相反的共轭面,软煤采动破坏面为倾向采空区的单向平面;超前塑性区内硬煤的后继剪切破坏面仍为倾向相反的共轭面,软煤内则为倾向煤壁的单向平面.采用塑性滑移线确定了煤壁片帮的危险范围,影响煤壁失稳的主要因素为端面距与砌体梁结构的回转变形压力.     

不等长工作面覆岩破坏规律的数值模拟分析

针对不等长工作面煤层开采日渐增多的现状,采用数值模拟与理论分析相结合的方法,对采场覆岩的破坏特征及支承压力的分布状态进行系统研究,并相应模拟出工作面前方应力场与位移场的演化规律。结果表明：工作面自开切眼开始向前推移,推进到工作面“见方”期或斜长的整数倍位置时,顶板活动剧烈,覆岩空间结构发生新旧更替,形成了“0”型破断区;不等长工作面推进过程中岩层运移极不规则,推进距离在衔接面前后20～30m的范围内,应力波动较大,数值变化明显;回采期间支承压力对覆岩活动产生了重要影响,其大小约为水平应力的1．5～2倍;就采动过程中竖向位移的变化而言,巷帮移近量远大于顶底板变形量,故工程实践中应特别注意对巷帮及顶板的加固和维护。     

岩层采动裂隙分布在绿色开采中的应用

岩层采动裂隙分布的研究与水体下和承压水上采煤、卸压瓦斯抽放、离层区充填与开采沉陷控制等工程问题紧密相关，通过试验与理论分析，对岩层移动过程中的覆岩采动裂隙动态发育特征及其影响因素进行了深入研究，结果证明：覆岩关键层对离层及裂隙的产生、发展与时空分布起控制作用．基于关键层破断前后采动裂隙动态发育特性与差异，提出了“覆岩离层分区隔离充填减沉法”和卸压瓦斯抽放的“O”形圈理论，并分别应用于我国不迁村采煤试验和卸压煤层气开采实践。     

Mechanics criterion and factors affecting overburden stability in solid dense filling mining

The effect of controlling strata movement in solid filling mining depends on the filling rate of the goaf.However, the mechanical property of the overburden in the backfill stope and the designed size of the backfill mining workface should also be considered. In this study, we established a main roof strata model with loads in accordance with the theory of key strata to investigate the stability of the overburden in solid dense filling mining. We analyzed the stress distribution law of the main roof strata based on elastic thin plate theory. The results show that the position of the long side midpoint of the main roof strata failed more easily because of tensile yield, indicating that this position is the area where failure is likely to occur more easily. We also deduced the stability mechanics criterion of the main roof strata based on tensile yield criterion. The factors affecting the stability of the overburden in solid dense filling mining were also analyzed, including the thickness and elasticity modulus of the main roof strata, overlying strata loads, advanced distance and length of workface, and elastic foundation coefficient of backfill body.The research achievements can provide an important theoretical basis for determining the designed size of the solid dense filling mining workface.     

Numerical simulation of gas migration into mining-induced fracture network in the goaf

Gas extraction practice has been proven for the clear majority of coal mines in China to be unfavorable using drill holes in the coal seam. Rather, mining-induced fractures in the goaf should be utilized for gas extraction. To study gas migration in mining-induced fractures, one mining face of 10 th Mine in Pingdingshan Coalmine Group in Henan, China, has been selected as the case study for this work. By establishing the mathematical model of gas migration under the influence of coal seam mining, discrete element software UDEC and Multiphysics software COMSOL are employed to model gas migration in mining-induced fractures above the goaf. The results show that as the working face advances, the goaf overburden gradually forms a mining-induced fracture network in the shape of a trapezoid, the size of which increases with the distance of coal face advance. Compared with gas migration in the overburden matrix, the gas flow in the fracture network due to mining is far greater. The largest mining-induced fracture is located at the upper end of the trapezoidal zone, which results in the largest gas flux in the network. When drilling for gas extraction in a mining-induced fracture field, the gas concentration is reduced in the whole region during the process of gas drainage, and the rate of gas concentration drops faster in the fractured zone. It is shown that with gas drainage, the gas flow velocity in the mininginduced fracture network is faster.     

Measurement of overburden failure zones in close-multiple coal seams mining

In the Kaiping Coal field, mining of five coal seams, located within 80 m in the Kailuan Group, #5, #7, #8,#9 and #12 coal seam, is difficult due to small interburden thickness, concentrated stress distributions,high coal seam metamorphism, and complex geological conditions. By using the ZTR12 geological penetration radar(GPR) survey combined with borehole observations, the overburden caving due to mining of the five coals seams was measured. The development characteristics of full-cover rock fractures after mining were obtained from the GPR scan, which provides a measurement basis for the control of rock strata in close multiple coal seam mining. For the first time, it was found that the overburden caving pattern shows a periodic triangular caved characteristic. Furthermore, it is proposed that an upright triangular collapsed pile masonry and an inverted triangular with larger fragments piled up alternately appear in the lower gob. The research results show that the roof structure formed in the gob area can support the key overlying strata, which is beneficial to ensure the integrity and stability of the upper coal seams in multiple-seam mining of close coal seams.     

Mining pressure monitoring and analysis in fully mechanized backfilling coal mining face-A case study in Zhai Zhen Coal Mine

Fully mechanized solid backfill mining(FMSBM) technology adopts dense backfill body to support the roof. Based on the distinguishing characteristics and mine pressure control principle in this technology, the basic principles and methods for mining pressure monitoring were analyzed and established. And the characteristics of overburden strata movement were analyzed by monitoring the support resistance of hydraulic support, the dynamic subsidence of immediate roof, the stress of backfill body, the front abutment pressure, and the mass ratio of cut coal to backfilled materials. On-site strata behavior measurements of 7403 W solid backfilling working face in Zhai Zhen Coal Mine show that the backfill body can effectively support the overburden load, obviously control the overburden strata movement, and weaken the strata behaviors distinctly. Specific performances are as follows. The support resistance decreases obviously; the dynamic subsidence of immediate roof keeps consistent to the variation of backfill body stress, and tends to be stable after the face retreating to 120-150 m away from the cut. The peak value of front abutment pressure arises at 5-12 m before the operating face, and mass ratio is greater than the designed value of 1.15, which effectively ensures the control of strata movement. The research results are bases for intensively studying basic theories of solid backfill mining strata behaviors and its control, and provide theoretical guidance for engineering design in FMSBM.     

顶板隔水层关键层耦合作用规律研究

在煤矿开采过程中，如果覆岩裂隙扩展至贯穿隔水层，则会诱发地下水或地表水大量涌向采场，导致煤矿淹井事故．利用RFPA^2D-Flow软件建立了隔水层关键层耦合的采场推进模型，计算并分析了裂隙场的发育和分布，绘制了顶板水渗流量曲线．讨论了与裂隙发育密切相关的覆岩支承压力与中间岩层厚度、关键层厚度及破断闻的关系．结果表明：关键层未破断时，中问岩层厚度对隔水层裂隙发育作用不明显；厚关键层对隔水层能起较好的保护作用．     

Simulation of recovery of upper remnant coal pillar while mining the ultra-close lower panel using longwall top coal caving

With the depletion of easily minable coal seams, less favorable reserves under adverse conditions have to be mined out to meet the market demand. Due to some historical reasons, large amount of remnant coal was left unrecovered. One such case history occurred with the remnant rectangular stripe coal pillars using partial extraction method at Guandi Mine, Shanxi Province, China. The challenge that the coal mine was facing was that there is an ultra-close coal seam right under it with an only 0.8–1.5 m sandstone dirt band in between. The simulation study was carried out to investigate the simultaneous recovery of upper remnant coal pillars while mining the ultra-close lower panel using longwall top coal caving(LTCC). The remnant coal pillar was induced to cave in as top coal in LTCC system. Physical modelling shows that the coal pillars are the abutments of the stress arch structure formed within the overburden strata. The stability of overhanging roof strata highly depends on the stability of the remnant coal pillars. And the gob development(roof strata cave-in) is intermittent with the cave-in of these coal pillars and the sandstone dirt band. FLAC3 D numerical modelling shows that the multi-seam interaction has a significant influence on mining-induced stress environment for mining of lower panels. The pattern of the stress evolution on the coal pillars with the advance of the lower working face was found. It is demonstrated that the stress relief of a remnant coal pillar enhances the caveability of the pillars and sandstone dirt band below.     

Formation and evolution of gas flow channels in the abutment pressure area

The permeability of coal ahead of the working face obviously changes dues to changes in abutment pressure. The formation and evolution of gas flow channels within the abutment pressure area was studied by analyzing the fracture extension mechanism and fracture development in different zones of the abutment pressure area. Fracture and damage mechanics theory is used to understand the observations. The following two techniques were used to understand the evolution of gas flow channels: field observation of the characteristic fractures at different positions relative to the working face and fluorescence micrographs of prepared coal samples. Bending tensile fractures develop along an approximately vertical direction that forms a microscopic network of channels in areas of stress concentration. The abutment pressure affects the local stress and, hence, the local gas conduction. The fractures induced by large deformation and plastic flow form macroscopically networked channels in the reduced stress area. Closer to the working face the gas flow channels evolve from microscopic to macroscopic and from isolated to network. Gas permeability continuously increases during this time. This is corroborated by field observations of the displacement of top coal and the gas flow from gas extraction drillings.     

Underground ground control monitoring and interpretation,and numerical modeling,and shield capacity design

Mine or longwall panel layout is a 3D structure with highly non-uniform stress distribution. Recognition of such fact will facilitate underground problem identification/investigation and solving by numerical modeling through proper model construction. Due to its versatility, numerical modeling is the most popular method for ground control design and problem solving. However numerical modeling results require highly experienced professionals to interpret its validity/applicability to actual mining operations due to complicated mining and geological conditions. Underground ground control monitoring is routinely performed to predict roof behavior such as weighting and weighting interval without matching observation of face mining condition while the mining pressures are being monitored, resulting in unrealistic interpretation of the obtained data on mining pressure. The importance of ground control pressure monitoring and simultaneous observation of mining and geological conditions is illustrated by an example of shield leg pressure monitoring and interpretation in an U.S. longwall coal mine: it was found that the roof strata act like a plate, not an individual block of the size of a shield dimension, as commonly assumed by all researchers and shield capacity is not a fixed property for a longwall panel or a mine or a coal seam. A new mechanism on the interaction between shield's hydraulic leg pressure and roof strata for shield loading is proposed.     

Hydraulic support crushed mechanism for the shallow seam mining face under the roadway pillars of room mining goaf

While the fully-mechanized longwall mining technology was employed in a shallow seam under a room mining goaf and overlained by thin bedrock and thick loose sands, the roadway pillars in the abandoned room mining goaf were in a stress-concentrated state, which may cause abnormal roof weighting, violent ground pressure behaviours, even roof fall and hydraulic support crushed(HSC) accidents. In this case,longwall mining safety and efficiency were seriously challenged. Based on the HSC accidents occurred during the longwall mining of 3-1-2 seam, which locates under the intersection zone of roadway pillars in the room mining goaf of 3-1-1 seam, this paper employed ground rock mechanics to analyse the overlying strata structure movement rules and presented the main influence factors and determination methods for the hydraulic support working resistance. The FLAC3 D software was used to simulate the overlying strata stress and plastic zone distribution characteristics. Field observation was implemented to contrastively analyse the hydraulic support working resistance distribution rules under the roadway pillars in strike direction, normal room mining goaf, roadway pillars in dip direction and intersection zone of roadway pillars. The results indicate that the key strata break along with rotations and reactions of the coal pillars deliver a larger concentrated load to the hydraulic support under intersection zone of roadway pillars than other conditions. The ‘‘overburden strata-key strata-roadway pillars-immediate roof" integrated load has exceeded the yield load that leads to HSC accidents. Findings in HSC mechanism provide a reasonable basis for shallow seam mining, and have important significance for the implementation of safe and efficient mining.     

基于断层影响因子与断层分维特征的断层突水危险性定量化分析

断层是诱发矿井突水事故的重要影响因素,新汶煤田受奥灰岩溶突水威胁严重,针对断层诱发突水问题,在系统总结新汶煤田矿井断层构造分布特征基础上,建立了断层对奥灰岩溶突水影响定量化分析体系。引入断层影响因子概念,对底板突水危险性进行了定量分析;采用非线性理论对煤矿地质构造发育特征进行定量化研究,揭示了矿井断层分维特征,提出了断层分维特征及其对突水影响危险性分区的原则及方法。采用断层影响因子等值线和断层分维等值线相互融合技术,得到新汶煤田协庄煤矿奥灰岩溶裂隙分布规律。根据不同单元岩溶的富水性,圈定出煤层突水危险区,对煤炭开采防治突水灾害具有指导意义。     

Coupling effects of coal pillars of thick coal seams in large-space stopes and hard stratum on mine pressure

Concerning the issue of mine pressure behaviors occurred in fully mechanized caving mining of thick coal seams beneath hard stratum in Datong Mining Area, combined with thin and thick plate theory, the paper utilizes theoretical analysis, similar experiments, numerical simulations and field tests to study the influence of remaining coal pillars in Jurassic system goaf on hard stratum fractures, as well as mine pressure behaviors under their coupling effects. The paper concludes the solution formula of initial fault displacement in hard stratum caused by remaining coal pillars. Experiments prove that coupling effects can enhance mine pressure behaviors on working faces. When inter-layer inferior key strata fractures, mine pressure phenomenon such as significant roof weighting steps and increasing resistance in support.When inter-layer superior key strata fractures, the scope of overlying strata extends to Jurassic system goaf, dual-system stopes cut through, and remaining coal pillars lose stability. As a result, the bottom inferior key strata also lose stability. It causes huge impacts on working face, and the second mine pressure behaviors. These phenomena provide evidence for research on other similar mine strata pressure behaviors occurred in dual-system mines with remaining coal pillars.     

Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining

To investigate the abnormal ground pressures and roof control problem in fully mechanized repeated mining of residual coal after room and pillar mining,the roof fracture structural model and mechanical model were developed using numerical simulation and theoretical analysis.The roof fracture characteristics of a repeated mining face were revealed and the ground pressure law and roof supporting conditions of the repeated mining face were obtained.The results indicate that when the repeated mining face passes the residual pillars,the sudden instability causes fracturing in the main roof above the old goaf and forms an extra-large rock block above the mining face.A relatively stable ‘‘Voussoir beam" structure is formed after the advance fracturing of the main roof.When the repeated mining face passes the old goaf,as the large rock block revolves and touches gangue,the rock block will break secondarily under overburden rock loads.An example calculation was performed involving an integrated mine in Shanxi province,results showed that minimum working resistance values of support determined to be reasonable were respectively 11,412 kN and 10,743 kN when repeated mining face passed through residual pillar and goaf.On-site ground pressure monitoring results indicated that the mechanical model and support resistance calculation were reasonable.     

Spatiotemporal evolution rule of rocks fracture surrounding gob-side roadway with model experiments

A series of plane-strain physical model experiments are carried out to study the spatiotemporal evolution rule of rocks fracture surrounding gob-side roadway, which is subjected to the pressure induced by the mining process. The digital photogrammetry technology and large deformation analysis method are applied to measure the deformation and fracture of surrounding rocks. The experimental results indicate that the deformation and fracture of coal pillars are the cause to the instability and failure of the surrounding rocks. The spatiotemporal evolution rule of the rock deformation and fracture surrounding gob-side roadway is obtained. The coal pillar and the roof near coal pillar should be strengthened in support design. The engineering application results also can provide a useful guide that the combined support with wire meshes, beam, anchor bolt and cable is an effective method.     

急斜煤层分段放顶煤开采合理段高选择研究

为确定急斜煤层水平分段综放开采条件下工作面段高的合理取值,将开采后裸露顶板岩层简化为三边固支的矩形薄板,采用薄板破断理论对受横向和纵向载荷作用下的斜置矩形薄岩板进行力学分析.研究表明:岩板走向长度、倾斜长度、岩层倾角、覆岩载荷及泊松比是影响段高取值的5个主要因素.应以在工作面上方采空区煤矸体沿着顶煤采出后形成的槽形采空区域下移前岩板不发生沿根部折断为极限段高的选择标准.工作面合理段高取值分为3类:第Ⅰ类煤层倾角45°～55°,取值在29 m范围内;第Ⅱ类55°～75°,取值在51 m范围内;第Ⅲ类75°～90°,该范围内开采工艺及设备选择对合理段高取值的影响更为重要.     

Relevance between abutment pressure and fractal dimension of crack network induced by mining

Based on the geological conditions of coal mining face No. 15-14120 at No. 8 mine of Pingdingshan coal mining group, the real-time evolution of coal-roof crack network with working face advancing was collected with the help of intrinsically safe borehole video instrument. And according to the geology of this working face, a discrete element model was calculated by UDEC. Combining in situ experimental data with numerical results, the relationship between the fractal dimension of boreholes’ wall and the distribution of advanced abutment pressure was studied under the condition of mining advance. The results show that the variation tendency of fractal dimension and the abutment pressure has the same characteristic value. The distance between working face and the peak value of the abutment pressure has a slight increasing trend with the advancing of mining-face. When the working face is set as the original point, the trend of fractal dimension from the far place to the origin can be divided into three phases: constant, steady increasing and constant. And the turning points of these phases are the max-influencing distance (50 m) and peak value (15 m) of abutment pressure.