Fracturing,caving propagation and influence of mining on groundwater above longwall panels–a review of predictive models

Historically there have been a number of different hypotheses and empirical models developed in an attempt to describe the nature of fracturing above longwall panels in underground coal mining.The motivation for suchmodels varies,ranging fromunderstanding the impact ofmining on surface subsidence,to back-analysis of caving behaviour in the immediate roof behind the longwall face.One of the most critical motivating factors that is taking on increased importance in many coalfields,is the need for better understanding,and hence prediction of the impact of mining on overlying strata,particularly strata units acting as aquifers for different groundwater horizons.This paper reviews some of the major prediction models in the context of observed behaviour of strata displacement and fracturing above longwall panels in the southern coalfields of New South Wales,south of Sydney.The paper discusses the parameter often referred to as"height of fracturing"in terms of the critical parameters that influence it,and the relevance and appropriateness of this terminology in the context of overlying sub-surface subsidence and groundwater impact.The paper proposes an alternative terminology for this parameter that better reflects what it is and how it is used.The paper also addresses the potential role of major bedding shear planes mobilised by mining and their potential influence on overlying subsidence and groundwater interference.

Theoretical description of drawing body shape in an inclined seam with longwall top coal caving mining

Understanding the characteristics of drawing body shape is essential for optimization of drawing parameters in longwall top coal caving mining.In this study,both physical experiments and theoretical analysis are employed to investigate these characteristics and derive a theoretical equation for the drawing body shape along the working face in an inclined seam.By analyzing the initial positions of drawn marked particles,the characteristics of the drawing body shape for different seam dip angles are obtained.It is shown that the drawing body of the top coal exhibits a shape-difference and volume-symmetry characteristic,on taking a vertical line through the center of support opening as the axis of symmetry,the shapes of the drawing body on the two sides of this axis are clearly different,but their volumes are equal.By establishing theoretical models of the drawing body in the initial drawing stage and the normal drawing stage,a theoretical equation for the drawing body in an inclined seam is proposed,which can accurately describe the characteristics of the drawing body shape.The shape characteristics and volume symmetry of the drawing body are further analyzed by comparing the results of theoretical calculations and numerical simulations.It is shown that one side of the drawing body is divided into two parts by an inflection point,with the lower part being a variation development area.This variation development area increases gradually with increasing seam dip angle,resulting in an asymmetry of the drawing body shape.However,the volume symmetry coefficient fluctuates around 1 for all values of the seam dip angle variation,and the volumes of the drawing body on the two sides are more or less equal as the variation development volume is more or less equal to the cut volume.Both theoretical calculations and numerical simulations confirm that the drawing body of the top coal exhibits the shape-difference and volume-symmetry characteristic.

Trial of small gateroad pillar in top coal caving longwall mining of large mining height

Coal seams in Tashan Mine of Datong Coal Group in China average 15 m thick and have been mined by the top coal caving longwall mining method of large mining height. Mining height was 3.8 m and the top coal caving height was 11.2 m. The gateroad pillar between panels was 38 m. During retreat mining,serious bumps occurred in the gateroads on both sides of the pillar affecting safety production. Therefore,pillarless mining was experimented. Using numerical modeling and comparative study of cases of similar mining condition,it was decided to employ a 6 m wide pillar,rather than the previous 38 m wide pillar.Support system for the gateroads was designed and implemented. During gateroad development,pillar failure conditions and entry deformation were monitored. Hydraulic fracturing method was employed to cut off the K3 sandstone along the entry rib so as to reduce the abutment pressure induced during retreat mining. Support reinforcement method combining grouting and advanced reinforcement methods was proposed to insure stable gateroad ahead of mining. Methane drainage and nitrogen injection were implemented to eliminate hazards associated with mine fire and spontaneous combustion. Since the development of gateroad has just completed,and retreat mining has not begun,the effectiveness of the proposed methods is unknown at this point. However,monitoring will continue until after mining.The results will be published in a separate paper.

Systematic principles of surrounding rock control in longwall mining within thick coal seams

Effective surrounding rock control is a prerequisite for realizing safe mining in underground coal mines.In the past three decades, longwall top-coal caving mining(LTCC) and single pass large height longwall mining(SPLL) found expanded usage in extracting thick coal seams in China. The two mining methods lead to large void space left behind the working face, which increases the difficulty in ground control.Longwall face failure is a common problem in both LTCC and SPLL mining. Such failure is conventionally attributed to low strength and high fracture intensity of the coal seam. However, the stiffness of main components included in the surrounding rock system also greatly influences longwall face stability.Correspondingly, surrounding rock system is developed for LTCC and SPLL faces in this paper. The conditions for simultaneous balance of roof structure and longwall face are put forward by taking the stiffness of coal seam, roof strata and hydraulic support into account. The safety factor of the longwall face is defined as the ratio between the ultimate bearing capacity and actual load imposed on the coal wall.The influences provided by coal strength, coal stiffness, roof stiffness, and hydraulic support stiffness,as well as the movement of roof structure are analyzed. Finally, the key elements dominating longwall face stability are identified for improving surrounding rock control effectiveness in LTCC and SPLL faces.

Rock mechanical investigation of strata loading characteristics to assess caving and requirement of support resistance in a mechanized powered support longwall face

Longwall mining is one of the most acclaimed and widely used in underground method for coal extraction. The interaction of powered supports with the roof is the key issue in strata mechanics of longwall mining. Controlled caving of rock mass is a prerequisite pro thriving exploitation of coal deposits by longwall retreat with caving technique and support resistance has evolved as the most promising and effective scientific tool to predict various aspects related to strata mechanics of such workings. Load density,height of caving block, distance of fractured zone ahead of the face, overhang of goaf and mechanical strength of the debris above and below the support base have been found to influence the magnitude of load on supports. Designing powered support has been attempted at the different countries in different methods. This paper reviews the mechanism of roof caving and the conventional approaches of caving behaviour and support resistance requirement in the context of major strata control experiences gained worldwide. The theoretical explanation of the mechanism of roof caving is still continuing with consistently improved understanding through growing field experiences in the larger domain of geo-mining conditions and state-of-art strata mechanics analysis and monitoring techniques.

A new fracture model for the prediction of longwall caving characteristics

A new numerical model is presented to simulate fracture initiation and propagation in geological structures. This model is based on the recent amalgamation of established failure and fracture mechanics theory, which has been implemented to the finite difference FLAC code as a constitutive FISH userdefined-model. Validation of the model has been studied on the basis of comparing the transitional failure modes in rock. It is shown that the model is capable of accurately simulating fracture distributions over entire brittle to ductile rock phases. The application of the model during longwall retreat simulation highlighted several caving characteristics relevant to varying geological condition. The distribution and behaviour of modelled fractures were both realistic and shown to provide an enhanced post failure analysis to geological structures in FLAC. Moreover, the model introduces new potential insight towards the failure analysis of more complicated problems. This is best suited towards improving safety and efficiency in mines through the prediction of various key fractures and caving characteristics of geological structures.

Numerical simulation of roof cavings in several Kuzbass mines using finite-difference continuum damage mechanics approach

An essential stage of mine design is an estimation of the steps of the first and periodic roof caving in longwall mines.Generally,this is carried out using the field experience and can be much enhanced by numerical simulation.In this work,the finite-difference method was applied coupled with the continuum damage mechanics(CDM)approach to simulate the stress-strain evolution of the rock mass with the underground opening during coal extraction.The steps and stages of roof caving were estimated relying on the numerical simulation data,and they were compared with the field data from several operating mines in the south of the Kuznetsk Basin,Russia.The dependence of the first roof caving step in simulation linearly correlates with field data.The results correspond to the actual roofs of longwall panels of the flat-dipping coal seams and the average rate of face advancement is approximately 5 m/day.

工作面综放开采工艺研究

为进一步提升综采工作面的生产能力和生产效率,以21051综采工作面为例在对其基本条件及地质条件综合分析的基础上,确定采用放顶煤采煤工艺对其进行开采;然后,基于PFC2D软件对走向长壁放顶煤和倾斜长壁放顶煤两种开采工艺参数对应的采出率和含矸率等指标进行对比,得出对应开采方式下的最佳放煤工艺参数,研究成果可指导该工作面的实际生产。

Numerical investigation on the caving mechanism with different standard deviations of top coal block size in LTCC

The size distribution of the broken top coal blocks is an important factor,affecting the recovery ratio and the efficiency of drawing top coal in longwall top coal caving(LTCC)mining panel.The standard deviation of top coal block size(dt)is one of the main parameters to reflect the size distribution of top coal.To find the effect of dt on the caving mechanism,this study simulates experiments with 9 different dt by using discrete element software PFC.The dt is divided into two stages:uniform distribution stage(UDS)whose dt is less than 0.1(Schemes 1–5),and nonuniform distribution stage(NDS)whose dt is more than 0.1(Schemes 6–9).This research mainly investigates the variation of recovery ratio,drawing body shape,boundary of top coal,and contact force between particles in the two stages,respectively.The results showed that with the increasing dt,the recovery ratio of the panel increases first and then decreases in UDS.It is the largest in Scheme 3,which mainly increases the drawing volume at the side of starting drawing end.However,the recovery ratio decreases first and then increases quickly in NDS,and it is the largest in Scheme 9,where the drawing volume at the side of finishing drawing end are relatively higher.In UDS,the major size of top coal is basically medium,while in NDS,the size varies from medium to small,and then to large,with a distinct difference in shape and volume of the drawing body.When the major size of top coal is medium and small,the cross-section width of the initial boundary of top coal at each height is relatively small.Conversely,when the top coal size is large,the initial boundary of top coal has a larger opening range,the rotating angle of lower boundary is relatively small in the normal drawing stage,which is conducive to the development of drawing body and reduces the residual top coal,and the maximum particle velocity and the particles movement angle are both larger.This study lays a foundation for the prediction of recovery ratio,and suggests that the uniform top coal is more manageable and has a larger recovery ratio.

我国综放开采40年及展望

我国应用和创新发展综放开采技术40 a来,取得了巨大成功和重大科技创新,实现了厚煤层一次采全高和高产高效低成本开采,综放开采技术已经成为我国开采厚煤层的主要方法,也是我国在世界煤炭开采行业的标志性技术。在综放开采技术发展过程中,先后试验了高位放煤、中位放煤、低位放煤,最终形成了我国的低位放煤综放开采技术。支架结构形式从早期的四柱支架发展到今天四柱与两柱并存的局面,对于智能开采工作面两柱支架更具优势。我国的综放开采技术可以分为2个阶段。在前20 a的探索和推广应用阶段,针对我国煤层条件,充分发挥了综放开采的技术优势,形成了高产高效综放开采模式、“三软”厚煤层综放开采模式、“两硬”厚煤层综放开采模式、急倾斜厚煤层综放开采模式等。在最近20 a的技术创新和输出阶段,我国创造性地开发了特厚煤层大采高综放开采技术、急倾斜厚煤层综放开采技术等,在智能化开采与智能放煤技术等方面取得了突破性进展,实现了综放开采技术从引进到输出的飞跃式发展。我国已经形成了一批专注综放开采技术研究的科技队伍,无论是综放开采的理论成果,还是技术成果都遥遥领先于世界其他国家。综放开采技术的进一步发展需要集中到综放工作面的全面智能化开采、智能放煤技术开发,提高工作面端头顶煤采出率技术,开发基于采动岩层运动的保水开采、绿色开采和减灾开采等技术,建立覆岩运动的统一场理论,研究综放开采低碳技术路径。

四面采空“孤岛”综放采场矿压控制的研究与实践

论文通过理论和实践研究,提出了四面采空“孤岛”采场的顶板结构和相关矿压控制技术。此类采场矿压控制的理论问题,是采场覆岩的多层空间结构运动及其与采动应力场的关系;其工程问题,是煤柱失稳造成的灾害形式及其判定方法、煤柱失稳的灾害的控制技术以及水、火和瓦斯的控制问题。论文通过河南义马煤业集团的实例,系统介绍了灾害监测技术和控制方法。开采结果表明,论文中提出的对岩层结构的认识和采取的控制技术是正确的,可以在条件相似的矿区推广应用。

长壁开采全过程采场矿压数值模拟方法

针对传统数值模拟方法对长壁开采矿压显现的模拟结果与实际偏差较大的问题,在对长壁开采全过程进行分析的基础上,基于塑性理论、损伤软化模型和双屈服模型,提出了一种长壁开采全过程采场矿压数值模拟方法。该方法的理论基础考虑了开采过程中煤岩体的损伤软化、顶板垮落、采空区充填及承压等物理过程。利用FLAC3D FISH语言开发了数值算法并得以数值实现。采用该方法对同忻煤矿某工作面的开采过程进行了数值模拟,所得结果与现场实际具有很好的一致性,监测的覆岩垂向位移曲线的形态特征能有效的判断三带范围及其分界线,监测的支架压力数据反映了周期来压和来压步距。研究成果为长壁开采矿压显现规律的数值模拟研究提供了一种新的思路和手段。

“三软”煤层综放工作面综合防尘技术试验研究

以典型工作面为例研究了煤体注水、自动喷雾、磁化水喷雾、化学防尘等综合防尘技术在“三软”煤层综采放顶煤工作面的实施方法。

急倾斜极软厚煤层走向长壁综放开采技术研究

以峰峰集团山西大远煤业1201急倾斜工作面为背景,系统研究了急倾斜厚煤层走向长壁综放开采的基本问题,指出支架的合理设计是该类煤层成功开采的首要条件,支架设计要充分考虑急倾斜厚煤层综放开采的顶板活动规律,以工作面上部顶板的冲击载荷确定支架的工作阻力。同时急倾斜支架要有足够的抗侧向挤压能力,结合工作面采放工艺确定侧护板的抗挤压能力。急倾斜厚煤层综放开采的顶煤放出体与煤岩分界面具有明显的不对称性。结合顶煤放出规律、支架稳定性等综合确定采放工艺,提出"下行动态分段、段内上行放煤"的采放工艺,适用于急倾斜厚煤层走向长壁综放开采,可最大限度地减少采放过程中对支架的不利影响,并可获得较高的顶煤采出率。工作面安装前需对软弱底板进行加固。

综放开采顶煤裂隙及其对渗透性研究的意义

在对综放工作面的煤样进行微观研究基础上 ,首次运用多重分形理论对顶煤裂隙的不均匀性和各向异性进行了定量表征 ,并将顶煤裂隙不均匀系数运用于渗透系数研究中 ,计算结果与实测基本吻合 ,表明运用多重分形方法 ,可为研究顶煤节理裂隙及其渗透性研究提供一个新的方法。

综放开采松软顶煤落放规律的理论研究

考虑放煤口位态影响,分别对高位及低、中位支架单口放出时松软煤矸的移动规律进行了理论探讨,提出拱-椭球放出体、放出体轴线平均偏转角等新观点,并推导建立了相应的放出体及漏斗母线方程。

综放面采空区温度场动态数学模化及应用

根据采空区浮煤氧化自燃过程的主要影响因素，建立了采空区温度分布的数学模型，计算分析了采空区浮煤温度分布随工作面推进速度的变化规律，确定了工作面的最小推进速度．并用ＦＯＲＴＲＡＮ语言编程。

综放开采煤层支承压力分布规律现场实测分析

依据谢桥矿1151(3)综放面开采地质及技术条件,采用钻孔应力计对工作面回采过程中煤柱和工作面煤层及巷帮侧向实体煤的应力进行观测。研究分析表明,在非对称开采条件,回采期间工作面及巷道周围煤层应力分布规律明显不同,煤柱和工作面煤层沿走向在工作面前方存在支承压力峰值,但巷帮侧向实体煤沿走向在工作面前方并不存在应力峰值,其峰值在采空区后方,而煤柱和巷帮侧向实体煤沿倾向均存在应力峰值,邻近工作面煤柱及工作面煤层应力均处于降低区。

采场上覆岩层结构理论及其新进展

回顾了采场上覆岩层结构理论的发展过程,介绍了平面结构模型各个发展阶段的主要学术观点及其新进展,重点阐述了采场上覆岩层结构的稳定性及综放采场顶板结构理论的研究现状,提出了大范围顶板控制问题如采动覆岩空间结构与应力场的动态关系、顶板的异常压力控制等问题,这些问题是今后采场矿压研究的重点,并介绍了这些方面的最新研究成果。

露天边帮煤井工长壁综放开采工作面顶板压力计算

受露天矿剥离和内排回填的影响,采用井工长壁采煤法回采露天矿边帮煤的采煤工作面的顶板结构、覆岩活动规律、矿山压力显现规律与常规井工矿井地层条件下的工作面相比存在显著差别,因此,顶板压力计算方法也需要根据露天矿边帮煤特殊的顶板覆岩结构特点和应力场环境进行理论和实验研究。以黑岱沟露天矿一采区西帮和南帮煤为研究对象,分析其边帮煤层顶板覆岩结构特点。对于经过内排回填,且剥离较多的低台阶处煤层仅保留较薄且强度较低的基岩顶板,其上覆盖厚层的松散排土。这种软弱薄基岩厚松散覆盖层的顶板本质上不存在关键层,露天矿台阶下保留的软弱薄基岩仅仅作为井工长壁工作面的直接顶提供有限的保护作用。由于厚层的松散排土载荷作用,软弱薄基岩的破碎也较为严重。按照传统的矿山压力理论,这种不存在关键层的软弱薄基岩厚覆盖层的顶板压力应当按照支架上方全部岩层重量进行计算。然而,根据相似材料实验结果显示,支架实际受压比其上方岩层重量小得多。实验证明在没有关键层的顶板岩层中仍然存在某种保护结构。根据实验观察和矿山压力理论分析,结合放矿理论,提出了软弱薄基岩厚覆盖层的椭球拱结构和直接顶自承能力相结合混合力学模型。在露天矿边帮下煤层还存在另一种未进行内排回填的薄基岩无覆盖层的顶板覆岩类型。对于这种顶板覆岩类型,提出了薄基岩无覆盖层的台阶式悬臂梁结构和直接顶自承能力的混合力学模型。本文总结了露天矿边帮煤层3种顶板覆岩类型,推导建立了露天边帮煤井工长壁开采工作面3种顶板结构条件下的顶板压力计算解析公式。按照软弱薄基岩厚覆盖层的椭球拱结构和直接顶自承能力相结合混合力学模型计算的顶板压力与相似材料实验结果相吻合。考虑了直接顶自承能力计算的顶板压力比传统不考虑直接顶自承能力的顶板压力小约14%。采用实验和理论分析方法,对露天边帮煤井工长壁工作面开采过程顶板垮落、覆岩活动,顶板压力计算等矿山压力问题进行了系统研究。