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.

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.

Roadway layout for recycling residual coal pillar in room-and-pillar mining of thick coal seam

In the context of a room-and-pillar mining gob in Shanxi province in China,this paper numerically investigates the stress distribution and deformation rules of roadway surrounding rocks at various locations of residual coal pillars in room-and-pillar mining gobs using software FLAC3 D.It is found that the concentrated stress beneath coal pillars distributes in a shape of ellipse.A reasonable roadway layout is then proposed.In this design,it is indicated that roadways should be designed to avoid the supporting zones of pillars with increasing compression and take into account the roof falling and crushing in the upper gob.According to the surrounding rock deformation characteristics and mining roadway locations as well as the supporting principles of timely support,rock reinforcing,piecewise management and suiting local conditions,a new asymmetric shield supporting plan is proposed.The field surveying results show that this supporting plan can effectively control the roadway rock deformation,thus guarantee the safe and smooth construction of roadways.

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.

Mechanisms of support failure and prevention measures under double-layer room mining gobs——A case study: Shigetai coal mine

In the practice of mining shallow buried ultra-close seams,support failure tends to occur during the process of longwall undermining beneath two layers of room mining goaf(TLRMG).In this paper,the factors causing support failure are summarized into geology and mining technology.Combining column lithology and composite beam theory,the key stratum of the rock strata is determined.A finite element numerical simulation is used to analyze the overlying load distribution rule of the main roof for different plane positions of the upper and lower room mining pillars.The tributary area theory(TAT)is adopted to analyze the vertical load distribution of each pillar,and dynamic models of coal pillar instability and main roof fracture are established.Through key block instability analysis,two critical moments are established,of which critical moment A has the greater dynamic load strength.Great economic losses and safety hazards are created by the dynamic load of the fracturing of the main roof.To reduce these negative effects,a method of pulling out supports is developed and two alternative measures for support failure prevention are proposed:reinforcing stope supports in conjunction with reducing mining height,or drilling ground holes to pre-split the main roof.Based on a comprehensive consideration of economic factors and the two categories of support failure causes,the method of reinforcing stope supports while reducing mining height was selected for use on the mining site.

Optimization of room-and-pillar dimensions using automated numerical models

This paper presents an optimization methodology for the geometric configuration of a room–and–pillar mining project,considering safety and operational restrictions while maximizing ore recovery.An underground manganese mine was chosen as a case study to investigate the capabilities of the presented methodology.A software package(OPTIMINE)was implemented to address the computational demand in an automated manner.Three–dimensional finite difference analyses were performed in FLAC3D and used as implicit functions to consider safety in terms of the factor of safety and room convergence.The obtained results showed that recovery could be increased from 44%to more than 80%in a safe manner.

Design concerns of room and pillar retreat panels

Why do some room and pillar retreat panels encounter abnormal conditions? What factors deserve the most consideration during the planning and execution phases of mining and what can be done to mitigate those abnormal conditions when they are encountered? To help answer these questions, and to determine some of the relevant factors influencing the conditions of room and pillar(R & P) retreat mining entries, four consecutive R & P retreat panels were evaluated. This evaluation was intended to reinforce the influence of topographic changes, depth of cover, multiple-seam interactions, geological conditions, and mining geometry. This paper details observations were made in four consecutive R & P retreat panels and the data were collected from an instrumentation site during retreat mining. The primary focus was on the differences observed among the four panels and within the panels themselves. The instrumentation study was initially planned to evaluate the interactions between primary and secondary support, but produced rather interesting results relating to the loading encountered under the current mining conditions. In addition to the observation and instrumentation, numerical modeling was performed to evaluate the stress conditions. Both the La Model 3.0 and Rocscience Phase 2 programs were used to evaluate these four panels. The results of both models indicated a drastic reduction in the vertical stresses experienced in these panels due to the full extraction mining in overlying seams when compared to the full overburden load. Both models showed a higher level of stress associated with the outside entries of the panels. These results agree quite well with the observations and instrumentation studies performed at the mine. These efforts provided two overarching conclusions concerning R & P retreat mine planning and execution. The first was that there are four areas that should not be overlooked during R & P retreat mining: topographic relief, multiple-seam stress relief, stress concentrations near the gob edge,and geologic changes in the immediate roof. The second is that in order to successfully retreat an R &P panel, a three-phased approach to the design and analysis of the panel should be conducted: the planning phase, evaluation phase, and monitoring phase.

Subsidence over room and pillar retreat mining in a low coal seam

The objective of this paper is to study the behavior of a low thick and low depth coal seam and the overburden rock mass. The mining method is room and pillar in retreat and partial pillar recovery. The excavation method is conventional drill and blast because of the small production. The partial pillar recovery is about 30% of the previous pillar size, 7 m × 7 m. The roof displacement was monitored during retreat operation; the surface movement was also monitored. The effect of the blasting vibration on the final pillar strength had been considered. Due to blasting, the pillar reduced about 20%. The consequence is more pillar deformation and roof vertical displacement. The pillar retreat and ground movement were simulated in a three-dimensional numerical model. This model was created to predict the surface subsidence and compare to the subsidence measured. This study showed that the remaining pillar and low seam reduce the subsidence that was predicted with conventional methods.

Technology of back stoping from level floors in gateway and pillar mining areas of extra-thick seams

According to the special requirements of secondary mining of resources in gateway-and-pillar goaf in extra-thick seams of Shanxi, this paper presents a technical proposal of back stoping from level floors.Numerical simulation and theoretical analysis are used to investigate the compaction characteristics of cavities under stress as well as an appropriate mining height of the primary-mining layer based on different mining widths and pillar widths. For Yangjian coal mine, the mining thickness of the first seam during back stoping from level floor is determined to be 3 m, which meets the relevant requirements.Gateway-and-pillar goaf of a single layer has a range of influence of 9 m vertically. If gateway-and-pillar goaf occurs both in 9-1 and 9-5 layers, the range is extended to within 11.2 m. When the mining width of a gateway is less than 2 m or larger than 5 m, the gateway-and-pillar goaf in the upper layer of the primary-mining seam can be filled in and compacted after stoping. When the working face is 2 m away from the gateway and pillar before entering into it and after passing through it, the coal body under the gateway and pillar is subjected to relatively high stress. During mining of the upper layer, moreover, the working face should interlock the goaf in primary-mining layer for 20 m.

Geomechanical design of a room and rib pillar granite mine

The geomechanical and stability design of an underground granite mine located in Canal San Bovo （Trento district, Northeastern Italy） was described. The exploitation of the granite, which is used in the ceramic industry, was carried out by the rooms and rib pillars method. The rooms are 12 m wide while the pillars are 11 m wide and they cross the main discontinuity set of the rock mass in the perpendicular direction. To verify the stability condition of an underground mine, it is necessary to carry out the calcula- tions that are able to check both the local and global stability of the rock mass. In the studied example, this approach has been applied with the development of analytical and numerical parametric analyses and it has permitted to get the best orientation and to design the size of rooms and pillars.

3D reconstruction method and connectivity rules of fracture networks generated under different mining layouts

In current research,a series of triaxial tests,which were employed to simulate three typical mining layouts(i.e.,top-coal caving,non-pillar mining and protected coal seam mining),were conducted on coal by using MTS815 Flex Test GT rock mechanics test system,and the fracture networks in the broken coal samples were qualitatively and quantitatively investigated by employing CT scanning and 3D reconstruction techniques.This work aimed at providing a detail description on the micro-structure and fractureconnectivity characteristics of rupture coal samples under different mining layouts.The results show that:(i)for protected coal seam mining layout,the coal specimens failure is in a compression-shear manner and oppositely,(ii)the tension-shear failure phenomenon is observed for top-coal caving and non-pillar mining layouts.By investigating the connectivity features of the generated fractures in the direction ofσ_1,under different mining layouts,it is found that the connectivity level of the fractures of the samples corresponding to non-pillar mining layout was the highest.

Overlying strata movement of recovering standing pillars with solid backfilling by physical simulation

To analyze the overlying strata movement law of recovering room mining standing pillars with solid backfilling.Physical simulation experiments with sponge and wood as the backfilling simulation material were tested.The results show that:(i) The covering-rock mechanics of the overly strata comes from "two-arch structures + hinged girder + bend beam" to "backfilling material + hinged girder + bent beam" by increasing the fill ratio from 0%to 85%,the beginning of overlying strata movement appears later and the total duration of subsidence velocity increased from zero to the highest value increases.The trend of "single polarization" of the subsidence velocity curves becomes noticeable and the velocity variation trend becomes stable,(ii) The equiponderate aeolian sand was added to improve the anti-pressure ability of the loess,and the corresponding ground processing & transportation system was designed.

Evaluating performance of lignite pillars with 2D approximation techniques and 3D numerical analyses

This paper attempts to investigate the use of approximate 2D numerical simulation techniques for the evaluation of lignite pillar geomechanical response, formed via the room and pillar mining method.Performance and applicability of the developing methodology are assessed through benchmarking with a more direct and accurate 3D numerical model. This analysis utilizes an underground lignite mine which is being developed in soft rock environment. Through the decisions made for the optimum room and pillar layout, the design process highlights the strong points and the weaknesses of 2D finite element analysis, and provides useful recommendations for future reference. The interpretations of results demonstrate that 2D approximation techniques come near quite well to the actual 3D problem.However, external load approximation technique seems to fit even better with the respective outcomes from the 3D analyses.

Preventing roof fall fatalities during pillar recovery:A ground control success story

For decades, pillar recovery accounted for a quarter of all roof fall fatalities in underground coal mines.Studies showed that a miner on a pillar recovery section was at least three times more likely to be killed by a roof fall than other coal miners. Since 2007, however, there has been just one fatal roof fall on a pillar line. This paper describes the process that resulted in this historic achievement. It covers both the key research findings and the ways in which those insights, beginning in the early 2000 s, were implemented in mining practice. One key finding was that safe pillar recovery requires both global and local stability.Global stability is addressed primarily through proper pillar design, and became a major focus after the2007 Crandall Canyon mine disaster. But the most significant improvements resulted from detailed studies that showed that local stability, defined as roof control in the immediate work area, could be achieved with three interventions:(1) leaving an engineered final stump, rather than extracting the entire pillar,(2) enhancing roof bolt support, particularly in intersections, and(3) increasing the use of mobile roof supports(MRS). A final component was an emphasis on better management of pillar recovery operations.This included a focus on worker positioning, as well as on the pillar and lift sequences, MRS operations,and hazard identification. As retreat mines have incorporated these elements into their roof control plans,it has become clear that pillar recovery is not ‘‘inherently unsafe." The paper concludes with a discussion of the challenges that remain, including the problems of rib falls and coal bursts.

水仓防隔水煤柱安全评价及注浆加固治理研究

某煤矿水仓的砌碹巷道在开采的扰动、防隔水煤柱宽度留设不足、砌碹壁后充填不密实等情况下,使得周边采空区水渗漏至水仓,极大影响了水仓的安全稳定。为进一步加强水仓煤柱的稳定性,运用地面钻孔及井下钻孔相结合的方法进行注浆加固。采用地面钻孔注浆充填井下硐室,硐室充填沙子245~328 m^(3)、水泥487~650 t,充填材料体积远远大于硐室体积220 m^(3),有效增加了煤柱厚度;利用井下钻孔在砌碹处进行水泥注浆与水化学注浆,使得砌碹裂缝处涌水量由15 m^(3)/h降至为零,砌碹壁后空隙充填效果良好。通过对煤柱及砌碹壁后空隙的注浆充填,有效提高了水仓防隔水煤柱的稳定性。

深部特厚煤层综放沿空掘巷煤柱优化及巷道支护

为研究深部大采高综放工作面窄煤柱沿空掘巷巷道矿压控制问题,以国能宁煤集团枣泉煤矿2^(-2)特厚煤层130203大采高综放工作面回风巷道为背景,基于实用矿山压力理论,建立了巷道围岩内、外应力场动态结构力学模型,运用理论计算和数值计算针对不同尺寸煤柱煤体应力对比分析,将原留设15 m护巷煤柱缩小至5 m进行了煤柱优化。结果表明:在稳定的内应力场掘巷有利于巷道的稳定性,避免了顶板事故及冲击地压相关灾害的发生,现场5 m小煤柱护巷工程应用中,130203回风巷道小煤柱侧变形量为1050 mm,实体煤帮变形量为400 mm,两帮呈现不对称性变形,底板局部底鼓量为1400 mm;深部特厚煤层综放开采沿空掘巷采用5 m小煤柱护巷方案设计正确,极大改善了巷道围岩的应力环境,整体设计满足生产要求,现场应用良好。130203工作面小煤柱沿空掘巷技术成功应用,为矿井开采提供了可靠的科学依据。

浅埋房柱式采空区煤柱稳定性及控制研究

老采空区受矿区早期采煤方法的限制,具有较强的隐匿性,很难得到有效治理,为其地表构筑物建设带来巨大挑战。以昌盛煤矿房柱式老采空区治理为工程背景,采用理论分析、钻孔勘探、数值模拟以及现场瞬变电磁勘查等方法,分析了房柱式采空区覆岩裂隙特征;判别了房柱式采空区上地表的稳定性,数值模拟进一步分析得出,在新的单位载荷作用下,采场中央遗留煤柱将发生活化失稳,引起采场所有煤柱发生多米诺骨牌式破坏,进而导致房柱式采空区地表发生失稳。基于对房柱式采空区稳定特征及浆液在房柱式采空区流动的特性提出了散点式注浆充填方案,给出了浆液配比及注浆工艺。现场瞬变电磁勘查结果表明,散点式注浆充填法能有效实现老采空区的充分均匀充填,注浆效果整体良好,同时该方法可为对类似老采空区地表治理提供参考。

坚硬顶板切顶卸压技术对巷道围岩变形规律影响

针对特厚煤层坚硬顶板、宽煤柱条件下临空巷道面临的高围岩应力、大变形等问题,以老石旦煤矿16403综放工作面为工程研究背景,从宽煤柱顶板侧向破断结构角度对临空巷道大变形的影响因素进行了理论分析,采用数值模拟方法研究了对16402运输巷实施不同切顶卸压方案时,临近采空区的16403回风巷侧向顶板采动应力传递规律,并在现场施工水力压裂钻孔进行切顶卸压,实现临空巷道围岩变形控制。研究结果表明:“低位坚硬岩层悬臂梁+高位坚硬岩层砌体梁”破断结构是特厚煤层宽煤柱临空巷道大变形的主要原因,可采用切顶卸压技术破坏宽煤柱顶板侧向破断结构来控制临空巷道围岩大变形;切顶角变化可使关键块B长度发生改变,切顶角越大,则关键块B长度越小,临空侧顶板载荷向煤柱传递的程度越弱,临空巷道围岩承受的采动应力越小,切顶角为100°时临空巷道围岩垂直应力与变形量最小;在16402运输巷以切顶角100°施工水力压裂钻孔后,16403回风巷顶底板变形量较未实施切顶卸压的16402回风巷减小86.5%,两帮变形量减小87.1%,临空巷道围岩稳定性得到极大提高。