Stability and control of room mining coal pillars-taking room mining coal pillars of solid backfill recovery as an example

The stability of room mining coal pillars during their secondary mining for recovering coal was analyzed. An analysis was performed for the damage and instability mechanism of coal pillars recovered by the caving mining method. During the damage progression of a single room coal pillar, the shape of the stress distribution in the pillar transformed from the initial stable saddle shape to the final arch-shaped distribution of critical instability. By combining the shapes of stress distribution in the coal pillars with the ultimate strength theory, the safe-stress value of coal pillar was obtained as 11.8 MPa. The mechanism of instability of coal pillar groups recovered by the caving mining method was explained by the domino effect. Since the room coal pillars mined and recovered by the traditional caving mining method were significantly influenced by the secondary mining during recovery, the coal pillars would go through a chain-type instability failure. Because of this limitation, the method of solid backfilling was proposed for mining and recovering room coal pillars, thus changing the transfer mechanism of stress caused by the secondary mining(recovery) of coal pillars. The mechanical model of the stope in the case of backfilling and recovering room coal pillars was built. The peak stress values inside coal pillars varied with the variance of backfilling ratio when the working face was advanced by 150 m. Furthermore, when the critical backfilling ratio was 80.6%, the instability failure of coal pillars would not occur during the solid backfill mining process. By taking Bandingliang Coal Mine as an example, the coal pillars' stability of stope under this backfilling ratio was studied, and a project scheme was designed.

Analysis of stability of coal pillars with multi-coal seam strip mining

Strip mining is one of the efficient measures to control surface subsidence and mining damage. However, the researches on the laws of the geological mining factors to upper and lower pillar's stability are still deficient in multi-coal seam strip mining at present. Based on the three dimension fast Lagrangian analysis of continua (short for FLAC3D) numerical simulation software, the laws of the stress increasing coefficient on the coal pillar and its stability were systematically studied for different depths, different mining widths, different interlayer spacings, different mining thicknesses, different properties of interstratified rock and the spacial relations of the upper and lower pillars in vertical alignment in multi-coal seam strip mining. The function relation between the stress increasing coefficient of upper and lower pillars with the mining depth, mining widths, interlayer spacing, mining thickness, property of interstratified rock and the spatial relationship were obtained.

Stability of coal pillar in gob-side entry driving under unstable overlying strata and its coupling support control technique

Considering the situation that it is difficult to control the stability of narrow coal pillar in gob-side entry driving under unstable overlying strata, the finite difference numerical simulation method was adopted to analyze the inner stress distribution and its evolution regularity, as well as the deformation characteristics of narrow coal pillar in gob-side entry driving, in the whole process from entry driving of last working face to the present working face mining. A new method of narrow coal pillar control based on the triune coupling support technique (TCST), which includes that high-strength prestressed thread steel bolt is used to strain the coal on the goaf side, and that short bolt to control the integrity of global displacement zone in coal pillar on the entry side, and that long grouting cable to fix anchor point to constrain the bed separation between global displacement zone and fixed zone, is thereby generated and applied to the field production. The result indicates that after entry excavating along the gob under unstable overlying strata, the supporting structure left on the gob side of narrow coal pillar is basically invalid to maintain the coal-pillar stability, and the large deformation of the pillar on the gob side is evident. Except for the significant dynamic pressure appearing in the coal mining of last working face and overlying strata stabilizing process, the stress variation inside the coal pillar in other stages are rather steady, however, the stress expansion is obvious and the coal pillar continues to deform. Once the gob-side entry driving is completed, a global displacement zone on the entry side appears in the shallow part of the pillar, whereas, a relatively steady fixed zone staying almost still in gob-side entry driving and present working face mining is found in the deep part of the pillar. The application of TCST can not only avoid the failure of pillar supporting structure, but exert the supporting capacity of the bolting structure left in the pillar of last sublevel entry, thus to jointly maintain the stability of coal pillar.

Stability of inner dump slope under coal pillar support:case study in an open‑pit coal mine

The stability of an inner dump slope was investigated under the efect of coal pillar support considering the development position of dumping.Based on the instability mechanism and load distribution characteristics of the supporting coal pillar,the three-dimensional mechanical efects of the supporting coal pillar are characterized.Using the two-dimensional equivalent principle and the residual thrust method,the stability of the inner dump slope was analyzed under the efect of pillar support at diferent dump development positions.The quantitative efects of various factors on the inner dump slope stability were revealed,and the coal pillar shape parameters were optimized through numerical simulations.The results indicate that the slope stability coefcient is linearly related to the top width and height of the coal pillar,slope angle,and base inclination angle,and has an exponential relation with the coal pillar strike length and slope height increment.There are quadratic and absolute value relations with the coal pillar outer and the inner bottom angle,respectively.The top width of the coal pillar in the inner dump of Shengli East No.2 open-pit coal mine should be at a level of+824 m,and the optimal top width and height are 15 and 36.7 m,respectively.The instability mechanism of the supporting and retaining coal pillar obtained by numerical simulations and the stability of the inner dump are in good agreement with the theoretical analysis.Our results provide a theoretical basis for the design,treatment,and safe implementation of similar open-pit mine slope engineering.

Rib stability: A way forward for safe coal extraction in India

Ensuring rib stability during pillar extraction is of prime importance in bord and pillar(B&P) method of underground coal mining with caving. Rib stability has been assessed here by way of assessing factor of safety(FOS), a ratio of the strength of rib to stress on it. Earlier formulations for rib stability when applied to case studies gave very low FOS value suggesting significant ground control problems, which were contrary to the field observations. This has necessitated the need to revisit the concept of rib stability. The stress coming on the rib is estimated with the use of numerical modeling technique using the FLAC^(3D) software. The methodology of assessing rib-stability with the help of suggested rib-strength formulation has been validated at eight Indian coal mines. The outcome of this study finds relevance and importance in ensuring underground coal liquidation with improved safety and conservation.

Coal pillar design when considered a reinforcement problem rather than a suspension problem

Current coal pillar design is the epitome of suspension design.A defined weight of unstable overburden material is estimated, and the dimensions of the pillars left behind are based on holding up that material to a prescribed factor of safety.In principle, this is no different to early roadway roof support design.However, for the most part, roadway roof stabilisation has progressed to reinforcement, whereby the roof strata is assisted in supporting itself.This is now the mainstay of efficient and effective underground coal production.Suspension and reinforcement are fundamentally different in roadway roof stabilisation and lead to substantially different requirements in terms of support hardware characteristics and their application.In suspension, the primary focus is the total load-bearing capacity of the installed support and ensuring that it is securely anchored outside of the unstable roof mass.In contrast, reinforcement recognises that roof de-stabilisation is a gradational process with ever-increasing roof displacement magnitude leading to ever-reducing stability.Key roof support characteristics relate to such issues as system stiffness, the location and pattern of support elements and mobilising a defined thickness of the immediate roof to create(or build) a stabilising strata beam.The objective is to ensure that horizontal stress is maintained at a level that prevents mass roof collapse.This paper presents a prototype coal pillar and overburden system representation where reinforcement, rather than suspension, of the overburden is the stabilising mechanism via the action of in situ horizontal stresses.Established roadway roof reinforcement principles can potentially be applied to coal pillar design under this representation.The merit of this is evaluated according to failed pillar cases as found in a series of published databases.Based on the findings, a series of coal pillar system design considerations for bord and pillar type mine workings are provided.This potentially allows a more flexible approach to coal pillar sizing within workable mining layouts, as compared to common industry practice of a single design factor of safety(Fo S) under defined overburden dead-loading to the exclusion of other relevant overburden stabilising influences.

An analytic solution describing the visco-elastic deformation of coal pillars in room and pillar mine

Coal pillar deformation is typically nonlinear and time-dependent. The accurate prediction of this defor- mation has a vital importance for the successful implementation of mining techniques. These methods have proven very important as a way to excavate coal resources from under buildings, railways, or water bodies. Elastic and visco-elastic theory are employed with a Maxwell model to formulate an analytic solution for displacement of coal pillars in room and pillar mine. These results show that the visco-elastic solution adequately predicts the coal pillar deformation over time. We conclude that the visco-elastic solution can predict the coal pillar and roadway displacement from the measured geological parameters of the conditions in situ. Furthermore, this method would be useful for mine design, coal pillar support optimization, ground subsidence prediction, and coal pillar stability analysis.

Preliminary rib support requirements for solid coal ribs using a coal pillar rib rating(CPRR)

Researchers from the National Institute for Occupational Safety and Health(NIOSH)are developing a coal pillar rib rating(CPRR)technique to measure the integrity of coal ribs.The CPRR characterizes the rib composition and evaluates its impact on the inherent stability of the coal ribs.The CPRR utilizes four parameters:rib homogeneity,bedding condition,face cleat orientation with respect to entry direction,and rib height.All these parameters are measurable in the field.A rib data collecting procedure and a simple sheet to calculate the CPRR were developed.The developed CPRR can be used as a rib quality mapping tool in underground coal mines and to determine the potential of local rib instabilities and support requirements associated with overburden depth.CPRR calculations were conducted for 22 surveyed solid coal ribs,mainly composed of coal units.Based on this study,the rib performance was classified into four categories.A preliminary minimum primary rib support density(PRSD)line was obtained from these surveyed cases.Two sample cases are presented that illustrate the data collection form and CPRR calculations.

遗留煤柱边界下方撤面巷道合理位置确定研究

为了确定遗留煤柱边界下方撤面巷道合理布置位置,以内蒙古某矿为研究对象,基于遗留煤柱覆岩空间结构,探究应力在底板的传递规律,通过构建“上煤层遗留煤柱传递应力-下煤层巷道掘进应力”应力叠加计算模型,分析外错距离与撤面巷道围岩安全稳定之间的关系。研究结果表明:在遗留煤柱的“支撑”作用下,煤柱煤体及其上覆岩层近似成“T”型空间结构;10 m外错距离满足现场安全生产和巷道围岩控制的要求。研究结果可为相似开采条件下的巷道稳定评估和合理位置确定提供参考。

松软厚煤层区段煤柱剪切滑块运动机理及协同控制技术

松软厚煤层区段煤柱高、煤壁暴露面积大,加之煤质松软、裂隙发育,强采动作用下极易造成煤柱失稳,巷道维护难度极大。以山西伏岩煤业3号煤层开采为工程背景,基于剪切滑块理论,探究采掘扰动下煤柱变形破坏机理,求解煤柱剪切滑块运动范围及应力分布规律,揭示煤柱侧帮剪切滑块运动机理,提出煤柱稳定性协同控制对策并在现场进行工程实践验证。结果表明:①采用极限平衡理论与叠加理论,确定了煤柱剪切滑块运动范围及煤柱垂直应力分布规律,阐明煤柱剪切滑块安全系数分布规律:0~1.26 m深度,煤柱上部安全系数较小;在1.26~3.95 m处,煤柱中线部分大面积安全系数较小,易受顶板来压破坏。②提出了1种以“注浆加固—锚索强化—切顶卸压”为主体的区段煤柱协同控制技术,煤柱侧裂隙较无支护条件及原支护条件分别减少62.89%和46.26%,巷道围岩完整性大幅提高,形成了强承载结构,有效控制了煤柱变形及底臌。③根据松软厚煤层区段煤柱条件,合理确定了协同控制设计参数,并对煤柱防控效果进行试验监测评估。现场试验结果表明,煤柱裂隙得到充分填充,注浆后煤体强度提高63%以上;巷道位移、锚杆索受力、离层等均在可控范围,表明协同控制技术明显提高了煤柱承载力,回采巷道围岩变形得到有效控制,为工作面安全高效开采提供了空间保障。

基于BOTDR的区段煤柱水平变形监测

针对煤柱内部微裂纹萌生、变形破坏等潜在安全隐患,利用布里渊光时域反射(BOTDR)分布式光纤传感技术进行煤柱水平变形监测。首先,根据煤柱破坏碎胀特性理论分析,建立了光纤轴向应变与水平变形的转化方法;其次开展室内试验确定了金属基索状传感光纤轴向拉伸响应情况,并根据煤柱变形破坏规律修正了应变系数,最后以陕西大柳塔煤矿活鸡兔井为工程背景,对22206工作面区段煤柱水平变形实现现场监测。监测结果表明:区段煤柱采动侧和未采动侧存在明显的水平变形差异,采动侧变形量最大,约为未采动侧的5倍,但煤柱整体水平变形较小,仅出现弹性变形,煤柱较稳定;煤柱变形分区明显,呈现中部为弹性核区,两侧为塑性破坏区的分区特征;工作面从测点前20m推进至测点后40m时,煤柱水平应变呈指数增长,远离测点之后变形趋于稳定,反映煤柱变形与采动应力扰动之间的动态关联;通过分析煤柱水平应变与垂直应力,发现两者呈指数相关关系,定量建立了煤柱水平应变与垂直应力关系式,说明煤柱变形与破坏很大程度上取决于采动应力的分布与演化。利用BOTDR技术对区段煤柱内部变形进行监测可以精确获取区段煤柱内部的微观变形信息,对于煤柱留设尺寸确定、围岩控制及采空区压力评估等具有重要指导意义。

综放面“双硬”煤层临空煤柱宽度及承载强度校核

具有“双硬”特征煤层的工作面其护巷煤柱的极限尺寸确定与常规工作面有所不同。以榆树岭煤矿505工作面沿空掘进巷道为背景,首先测试了煤岩样的力学性质和不同高径比煤样的抗压强度,得到了不同高径比煤样峰值强度曲线;其次分析了沿空掘巷覆岩结构模型,建立了沿空掘巷护巷煤柱顶板力学模型,得到了不同宽度煤柱所受的载荷应力;建立了不同高宽比煤柱的数值模型,得到了不同高宽比煤柱的极限强度校核公式;最后通过对比不同宽度煤柱所受的载荷应力和极限强度,得出4m宽度的煤柱即可满足支护要求,并在此基础上提出了沿空掘进巷道支护工艺。现场监测结果表明,煤柱宽度为4m时,巷道整体变形量较小,巷道稳定性得到有效维护。研究结果可为小煤柱护巷宽度的确定提供参考。

浸水含瓦斯抽采钻孔煤柱稳定性研究及控制

窄煤柱护巷可有效提高煤炭资源采出率,改善采掘接替关系,但施工维护难度大,尤其是在采空区积水且矿井瓦斯含量较高的条件下,煤柱稳定性控制显得更为重要。基于石泉煤矿30108工作面沿空巷道掘进及支护的工程背景,结合钻孔可视化探测技术提出了一种基于MATLAB图像处理的钻孔裂隙定量表征方法。综合浸水煤柱力学模型和数值模拟,确定了合理煤柱宽度,提出了采空区疏水及“锚索网梁+窄煤柱侧索梁补强+窄煤柱侧注浆加固”的巷道高强非对称支护方案,并进行了工业性实践。研究结果表明:钻孔截面中裂隙面积占比随钻孔深度增加呈负对数减小趋势,煤柱宽度为10.00 m时正常区段及含瓦斯抽采钻孔区段的煤柱内部均存在较大范围的弹性区,具备较强的承载能力;相应支护方案控制效果较好,具有显著的经济效益。

近距离煤层沿空掘巷围岩稳定性分析

合理的煤柱留设宽度及巷道布置错距不仅能保证煤炭的安全生产,还能提高煤炭资源的产出率,降低巷道支护的难度。以色连二矿近距离煤层群12409工作面为研究背景,采用理论分析得出沿空掘巷煤柱留设宽度,通过数值模拟研究了留设8 m小煤柱时回采巷道的稳定性,并进行现场工业性试验,监测回采期间巷道矿压显现规律。研究结果表明:①基于极限平衡理论及工程经验设计留设8 m小煤柱;②留设8 m小煤柱,测点超前工作面17 m时,煤柱内3.5~5.9 m分布式光纤应变值为-1 470~-7 268,表明煤柱内形成承载区域,利于巷道围岩控制;③回采期间,巷道顶底板及两帮的最大移近量分别为250、123 mm,在可控范围内,保证了矿井安全高效生产。

房柱式采空区顶板破坏失稳机理研究

针对房柱式采空区失稳致灾问题,以榆林某矿房柱式采空区遗留煤柱为研究背景,采用理论分析、物理相似模拟、数值分析等研究方法,对房柱式采空区顶板失稳机理进行研究。研究结果表明:矿井采空区遗留煤柱稳定性安全系数为1.54,考虑煤柱在顶板长期载荷作用下发生蠕变作用,最终会发生大面积失稳;房柱式采空区煤柱破坏失稳诱灾影响因素包括煤体物理力学性质、风化作用、开采方式、积水浸蚀、蠕变作用等5种因素;房柱式采空区顶板灾变演化为“由点及面”的破坏规律,顶板由破坏到失稳分为初步破坏阶段、破坏扩展阶段、破坏加剧阶段及彻底失稳阶段4个阶段;采空区彻底失稳时煤柱应力峰值为15.08 MPa,应力集中系数为3.92,顶板下沉位移为392.44 mm。

基于突变理论的深部煤柱稳定性

针对神华新街矿区盾构工法建设煤矿长距离斜井,研究了深部条件下保护煤柱的自身稳定性。研究发现,根据煤柱不同区域受力特征的差异性,可将其分成弹性核区和边缘塑性区,在此基础上建立了煤柱尖点突变模型,并经推导得到了煤柱的失稳判别公式。尖点突变模型可有效描述煤柱失稳演化的宏观过程。通过数值计算分析了深部盾构斜井保护煤柱在开采过程中的稳定状态。试验结果表明,随着煤层的开采能量缓慢释放,120 m煤柱自边缘向内部逐渐屈服,并在单侧出现4.5 m的塑性区。经公式判别,该工程中保护煤柱可以在深部开采条件下保持稳定,研究结果对相似工程中合理煤柱的留设提供了借鉴。

窄煤柱沿空掘巷非对称支护力学特征与支护参数研究

为了研究窄煤柱沿空掘巷非对称支护下围岩变形与应力分布特征,采用理论分析方法,研究窄煤柱沿空巷道对称支护与非对称支护力学机理;基于马道头煤矿现场工程实践,采用数值模拟与工程效验方法,研究非对称支护下巷道围岩力学特征与稳定性。理论分析结果表明,非对称支护可有效降低巷道顶板弯矩,减小巷道顶底板与两帮变形量。模拟结果表明,煤柱内部压应力增高区与弹性核区保障了煤柱承载性能。现场实测结果表明,采用“锚索+槽型钢”非对称支护,巷道顶板、窄煤柱侧巷帮与实体煤侧巷帮最大变形量分别为100、79、62 mm,巷道松动圈与裂隙发育处于中等水平。

近距离煤层内错开采上下双层位煤柱稳定性研究

近距离煤层上下煤柱叠加,双煤柱稳定性关系到煤柱留设的成功,通过理论计算分别推导出内错时上窄下宽双层位煤柱的载荷与煤柱合理宽度计算公式。分析上窄下宽双层位煤柱稳定性影响因素为上下煤柱宽度比、上下煤层开采厚度比、上下煤层采空区宽度比,得出:随着下、上煤柱宽度比的增加,上窄下宽煤柱下煤柱荷载呈现递减,下、上煤柱宽度比一定时,下煤柱荷载随着采空区宽度增加而增大;随着采空区宽度的增加,上窄下宽煤柱下煤柱荷载呈现递增,采空区宽度一定时,下煤柱荷载随着下、上煤柱厚度比增加而增加;随着下、上煤柱厚度比增加,上窄下宽煤柱下煤柱荷载呈现递增,下、上煤柱厚度比一定时,下煤柱荷载随着下、上煤柱宽度比增加而降低。

基于关键块破断特征的区段煤柱设计研究

为了快速确定布尔台煤矿42204工作面区段煤柱的宽度,通过数值模拟手段,分析了不同关键块体对煤柱承载特征的影响,结合极限平衡理论,共同确定了区段煤柱宽度为25 m是最佳设计。实际工程应用结果显示,25 m区段煤柱有效维持了巷道的稳定性,煤柱横向位移量在合理范围,保证巷道稳定性的同时,间接提高了矿井的经济效益,符合矿井生产需求。