Numerical Study on an Applicable Underground Mining Method for Soft Extra-Thick Coal Seams in Thailand

The EGAT Mae Moh Mine is the largest open pit lignite mine in Thailand and it produces lignite about 16 million tons annually. In the near future, the pit limit of the mine will be reached and underground mine will then be developed through the open pit in the depth of 400 - 600 m from the surface. However, due to the challenges for underground mining such as poor geological conditions, extra thickness (20 - 30 m) of coal seams, and weak mechanical properties of coal seams and the surrounding rock, the success possibility of underground mining and an applicable underground mining method is being investigated at the present. The paper discusses the applicability of multi-slice bord-and-pillar method for the soft extra thick coal seams in the Mae Moh mine by means of numerical analyses using the 3D finite difference code “FLAC3D”.

Theoretical analysis and engineering application of controllable shock wave technology for enhancing coalbed methane in soft and low‑permeability coal seams

Coalbed methane(CBM)is a significant factor in triggering coal and gas outburst disaster,while also serving as a clean fuel.With the increasing depth of mining operations,coal seams that exhibit high levels of gas content and low permeability have become increasingly prevalent.While controllable shockwave(CSW)technology has proven effective in enhancing CBM in laboratory settings,there is a lack of reports on its field applications in soft and low-permeability coal seams.This study establishes the governing equations for stress waves induced by CSW.Laplace numerical inversion was employed to analyse the dynamic response of the coal seam during CSW antireflection.Additionally,quantitative calculations were performed for the crushed zone,fracture zone,and effective CSW influence range,which guided the selection of field test parameters.The results of the field test unveiled a substantial improvement in the gas permeability coefficient,the average rate of pure methane flowrate,and the mean gas flowrate within a 10 m radius of the antireflection borehole.These enhancements were notable,showing increases of 3 times,13.72 times,and 11.48 times,respectively.Furthermore,the field test performed on the CSW antireflection gas extraction hole cluster demonstrated a noticeable improvement in CBM extraction.After antireflection,the maximum peak gas concentration and maximum peak pure methane flow reached 71.2%and 2.59 m^(3)/min,respectively.These findings will offer valuable guidance for the application of CSW antireflection technology in soft and low-permeability coal seams.

Analysis of the Risk of Water Breakout in the Bottom Plate of High-Intensity Mining of Extra-Thick Coal Seams

In order to clarify the danger of water breakout in the bottom plate of extra-thick coal seam mining, 2202 working face of a mine in the west is taken as the research object, and it is proposed to use the on-site monitoring means combining borehole peeping and microseismic monitoring, combined with the theoretical analysis to analyze the danger of water breakout in the bottom plate. The results show that: 1) the theoretically calculated maximum damage depth of the bottom plate is 27.5 m, and its layer is located above the Austrian ash aquifer, which has the danger of water breakout;2) the drill hole peeping at the bottom plate of the working face shows that the depth of the bottom plate fissure development reaches 26 m, and the integrity of the water barrier layer has been damaged, so there is the risk of water breakout;3) for the microseismic monitoring of the anomalous area, the bottom plate of the return air downstream channel occurs in the field with a one-week lag, which shows that microseismic monitoring events may reflect the water breakout of the underground. This shows that the microseismic monitoring events can reflect the changes of the underground flow field, which can provide a reference basis for the early warning of water breakout. The research results can provide reference for the prediction of sudden water hazard.

Behaviors of overlying strata in extra-thick coal seams using top-coalcaving method

Accidents such as support failure and excessive deformation of roadways due to drastic changes in strata behaviors are frequently reported when mining the extra-thick coal seams Nos.3e5 in Datong coal mine with top-coal caving method,which significantly hampers the mine's normal production.To understand the mechanism of strata failure,this paper presented a structure evolution model with respect to strata behaviors.Then the behaviors of strata overlying the extra-thick coal seams were studied with the combined method of theoretical analysis,physical simulation,and field measurement.The results show that the key strata,which are usually thick-hard strata,play an important role in overlying movement and may influence the mining-induced strata behaviors in the working face using top-coal caving method.The structural model of far-field key strata presents a 'masonry beam' type structure when'horizontal O-X' breakage type happens.The rotational motion of the block imposed radial compressive stress on the surrounding rock mass of the roadway.This can induce excessive deformation of roadway near the goaf.Besides,this paper proposed a pre-control technology for the hard roof based on fracture holes and underground roof pre-splitting.It could effectively reduce stress concentration and release the accumulated energy of the strata,when mining underground coal resources with top-coal caving method.

Rockburst mechanism in soft coal seam within deep coal mines

A number of rockburst accidents occurring in soft coal seams have shown that the rockburst mechanism involved in soft coal seams is significantly different from that involved in hard coal seams. Therefore, the method used to evaluate rockburst in hard coal seams is not applicable to soft coal seams. This paper established an energy integral model for the rockburst-inducing area and a friction work calculation model for the plastic area. If the remaining energy after the coal seam is broken in the rockburstinducing area is greater than the friction work required for the coal to burst out, then a rockburst accident will occur. Mechanisms of ‘‘quaking without bursting" and ‘‘quaking and bursting" are clarified for soft coal seams and corresponding control measures are proposed as the optimization of roadway layouts and use of ‘‘three strong systems"(strong de-stressing, strong supporting, and strong monitoring).

Prediction of upper limit position of bedding separation overlying a coal roadway within an extra-thick coal seam

Failure of the surrounding rock around a roadway induced by roof separation is one major type of underground roof-fall accidents.This failure can especially be commonly-seen in a bottom-driven roadway within an extra-thick coal seam("bottom-driven roadway"is used throughout for ease of reference),containing weak partings in their roof coal seams.To determine the upper limit position of the roof interlayer separation is the primary premise for roof control.In this study,a mechanical model for predicting the interlayer separation overlying a bottom-driven roadway within an extra-thick coal seam was established and used to deduce the vertical stress,and length,of the elastic,and plastic zones in the rock strata above the wall of the roadway as well as the formulae for calculating the deflection in different regions of rock strata under bearing stress.Also,an approach was proposed,calculating the stratum load,deflection,and limiting span of the upper limit position of the interlayer separation in a thick coal seam.Based on the key strata control theory and its influence of bedding separation,a set of methods judging the upper limit position of the roof interlayer separation were constructed.In addition,the theoretical prediction and field monitoring for the upper limit position of interlayer separation were conducted in a typical roadway.The results obtained by these two methods are consistent,indicating that the methods proposed are conducive to improving roof control in a thick coal seam.

Macro and micro grouting process and the influence mechanism of cracks in soft coal seam

Grouting is an important method to reinforce soft coal roadway,and the presence of primary cracks in the coal body has an important influence on the grouting effect.With the discrete element simulation method,the grouting process of the soft coal seam was simulated.The mechanism of primary cracks on grouting was revealed,while the influence of fracture characteristics and grouting pressure on the grouting effect was analyzed.The results demonstrated that grouting in the soft coal seam involves the stages of seepage,rapid splitting,slow splitting,and stability.Due to the presence of primary cracks,the grouting diffusion radius increased significantly.Under the slurry pressure,the tensile stress concentration was formed at the crack tip,and the slurry split the coal once the splitting pressure was reached.In addition,the distribution characteristics of fractures are found to have a great influence on the grouting effect.It is observed that smaller fracture spacing is associated with a larger slurry diffusion radius and thus easier penetration of the primary crack tips.The fracture angle affects the direction of fracture propagation.The secondary fracture formed by splitting is a tensile fracture,which is more likely to extend along the direction parallel to the maximum principal stress.Overall,these simulation results have guiding significance for the setting of reasonable spacing of grouting holes in the practice of grouting engineering.

Strata behavior in extra-thick coal seam mining with upward slicing backfilling technology

Based on the character of upward slicing backfilling mining and the condition of Gonggeyingzi coal mine in Inner Mongolia,this paper describes the studies of the strata behavior and the stress distribution in the process of backfilling mining in extra-thick coal seams.This was achieved by setting up and analyzing the elastic foundation beam model using the ABAQUS software.The results show that:(1) With the gradual mining of different slices,the roof appears to bend continuously but does not break.The vertical stress in the roof decreases and the decreasing amplitude reduces,while the tensile stress in the roof grows with the mining slices and the maximum tensile stress will not exceed the allowable tensile stress.(2) The front vertical stress at the working face exceeds the rear vertical stress and both show a trend of decrease with decreasing amplitude of decrease.(3) The slices mined early have more influence on the surrounding rock than the later ones.Similarly,the strata behavior experiences the same trend.The field measured data show that the roof does not break during the mining process,which is consistent with the conclusion.

Outburst control in soft and outburst prone coal seam using the waterjet slotting technique from modeling to field work

The paper discussed a comprehensive numerical simulation and field work by the usage of waterjet slotting tech- nique to prevent the occurrence of outbursts in soft and outburst prone coal seams. This was based on the geological and ge- omechanical conditions of Jinjiachong Colliery, Guizhou Province, associated with varied waterjet slotting parameters such as slotting penetration, slotting thickness and slotting distance along the length of borehole. Also, to understand the variation of internal stress of coal seams after waterjet slotting application, the internal stress levels were compared with and without slot- ting application, and the results indicate that the internal effective stress levels can be reduced to 70% and 45% for the vertical and horizontal stresses, respectively, and the gas concentration can be increased up to 5 times when the waterjet slotting is ap- plied.

Disasters of gas-coal spontaneous combustion in goaf of steeply inclined extra-thick coal seams

In light of the escalating global energy imperatives,mining of challenging-to-access resources,such as steeply inclined extra-thick coal seams(SIEC),has emerged as one of the future trends within the domain of energy advancement.However,there is a risk of gas and coal spontaneous combustion coupling disasters(GCC)within the goaf of SIEC due to the complex goaf structure engendered by the unique mining methodologies of SIEC.To ensure that SIEC is mined safely and efficiently,this study conducts research on the GCC within the goaf of SIEC using field observation,theoretical analysis,and numerical modeling.The results demonstrate that the dip angle,the structural dimensions in terms of width-to-length ratio,and compressive strength of the overlying rock are the key factors contributing to the goaf instability of SIEC.The gangue was asymmetrically filled,primarily accumulating within the central and lower portions of the goaf,and the filling height increased proportionally with the advancing caving height,the expansion coefficient,and the thickness of the surrounding rock formation.The GCC occurs in the goaf of SIEC,with an air-return side range of 41 m and an air-intake side range of 14 m,at the intersection area of the“<”-shaped oxygen concentration distribution(coal spontaneous combustion)and the“>”-shaped gas concentration distribution(gas explosion).The optimal nitrogen flow rate is 1000 m3/h with an injection port situated 25 m away from the working face for the highest nitrogen diffusion efficacy and lowest risk of gas explosion,coal spontaneous combustion,and GCC.It has significant engineering applications for ensuring the safe mining of SIEC threatened by the GCC.

Discussion of the indexes of nonoutburst coal seam upgrade and its corresponding critical values

In view of the measurement difficulties of indexes recommended by the 50Items Experience of Coal Mine Gas Prevention in the process of the nonoutburst coalseam upgrade, this paper took the No.8 coal seam of Huainan Mining Group as research object. Discussed the suitability of indexes and corresponding critical values, putforward method in determining the indexes and its critical values by analysis and investigation of the gas geological condition and the-spot tracking near position where anoutburst occurred combined with laboratory experiment, and established the indexesand its critical values of nonoutburst coal seam upgrade in No.8 coal seam of HuainanMining Group. The results show that it is suitable to take gas content and tectonic softcoal thickness easily to gain in routine production as primary upgrade indexes that itscritical values are 7.5 m^3/t and 0.8 m, respectively. In addition, takefvalue and Ap valueas auxiliary indexes.

Influence of Water Injection Fracturing on Crack Damage in Soft Coal and Application in Gas Extraction

In order to study the hypotonic and rheological particularity of “three soft” coal seam in west Henan, China, this paper explored the stress and damage characteristics of crack in coal under condition of water injection fracturing based on ABAQUS platform;The cohesive element in T-P damage evolution criterion was used to describe the approximately linear relationship between crack width and extending distance in soft coal. The simulation results show that stress evolution and crack damage in soft coal is a gradually developing process under condition of water injection fracturing. When the static pressure is 4 - 10 MPa, and the injection time is about 1 - 2 hours, the damage range of crack in soft coal can basically reach an ideal data of 80 - 100 m, and then greatly improve the hypotonic performance of “three soft” coal seam.

煤层瓦斯含量测定技术及装备研究进展

瓦斯含量是煤与瓦斯突出危险预测、煤层瓦斯资源量估算、矿井瓦斯治理工程设计的重要参数。围绕如何在大区域准确快速测定煤层瓦斯含量,依托国家科技重大专项、国家自然科学基金和煤炭企业联合基金等项目科技攻关,在取样、测试方面取得了一定进展。主要表现在如下4个方面:①煤层瓦斯含量测定取样经历了孔口接样、岩心管定点取样、压力引射定点取样和密闭取样4个阶段,密闭取样装备保压能力达到11.5 MPa,煤心直径达到38 mm;②针对不同煤层地质条件,发展形成了顺煤层定向长钻孔密闭取样、底板穿层钻孔密闭取样和顶(底)板梳状定向长钻孔密闭取样3种取样技术;③在河南焦作和山西晋城矿区硬煤层中,顺层定向长钻孔取样深度达到516 m,密闭取样法测得煤层瓦斯含量较常规取样法分别平均提高了0.44倍和1.04倍。在安徽淮南矿区碎软煤层中,穿层钻孔密闭取样深度达到209 m,测得煤层瓦斯含量较常规取样法平均提高了0.26倍;在安徽淮北矿区碎软煤层中,顶(底)板梳状钻孔密闭取样深度达到484 m,测得煤层瓦斯含量较常规取样法平均提高了0.19倍,密闭取样法在煤层瓦斯含量测定精度、探测范围上优于常规取样法;④在瓦斯含量测试方面,除了传统解吸法测试,发展了系列煤矿井下瓦斯含量快速测试装备,可实现最快30 min内测得煤层瓦斯含量,一般用于百米孔内的瓦斯含量测试。提出了煤层瓦斯含量测定密闭取样装备需向小型化、轻量化的方向发展,并能实现随钻密闭取样。在测试上,应根据实际情况确定合理的解吸终止限,并将测试装备和密闭取样装备进一步结合,以实现深孔瓦斯含量快速准确测定。密闭取样技术已成为煤层瓦斯含量大区域精准勘查、预测的主要手段,是煤炭安全高效开采的重要技术保障。

松软薄煤层条带消突下斜定向钻孔成孔护孔技术研究与应用

针对松软薄煤层煤巷条带消突长距离下斜钻孔,存在成孔难、易钻遇煤层顶底板、积水堵塞瓦斯抽采通道的难题,提出了氮气排渣下斜定向钻进成孔与护孔技术,采用高压氮气排渣、大功率气动螺杆钻具定向、“波浪式”轨迹控制和三层管护孔排水技术,在河南省平煤股份一矿开展现场试验,完成8个孔深超300 m下斜定向长钻孔,煤层钻遇率90%以上,全孔段下三层护排水管,平均下入深度302 m,支管平均抽采纯量1.66 m^(3)/min,平均瓦斯浓度17.5%。试验结果表明:该技术可有效解决松软薄煤层下斜钻孔长距离定向成孔、全孔段筛管护孔和排水的技术难题,提高了钻孔成孔深度和煤层钻遇率,实现了下斜孔全孔段护孔和排水,保障了瓦斯抽采效果,为松软薄煤层煤巷条带长距离消突提供技术保障,在类似地层条件具有广泛的应用推广价值。

煤层顶板间接压裂裂缝扩展机制及影响因素

碎软低渗煤层在我国普遍发育,制约煤层气单井产气量提高和产业发展。间接压裂是通过在邻近层产生垂直裂缝沟通煤层进而实现煤层有效改造的一种压裂方式,可以有效应对钻井塌陷、煤粉产出、压裂液滤失和煤层厚度薄等不利因素。通过间接压裂物理模拟试验和扩展有限元数值模拟分析,揭示煤层顶板间接压裂裂缝扩展影响因素,明确裂缝扩展机制,以期为间接压裂技术提供指导。通过直接压裂煤层和不同起裂位置、垂向应力和施工排量影响下间接压裂试验表明,起裂压力高,更易产生长裂缝,且受原生裂缝影响程度减小;但起裂点距离煤层越远,起裂所需能量越大,高破裂压力会对煤层造成粉碎性破坏;大施工排量下,起裂压力对应升高,起裂时间变短,原生裂缝影响程度变小。考虑地应力、起裂位置、岩石力学和施工排量等参数的数值模拟结果显示,在模型参数设置条件下,最大水平主应力和垂向应力差在<4 MPa,煤层与顶板有效应力差>3 MPa、弹性模量差<15 GPa的地层和岩性组合适合间接压裂,起裂位置距离煤层最优距离为<6 m,施工排量需要根据力学性质、断裂能密度等参数确定最优范围。

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。

松软煤层钻进钻杆减重降阻机制及应用研究

施工瓦斯抽采钻孔是治理瓦斯危害的必要手段,针对松软煤层钻进因钻杆自重大引起的钻进阻力大、钻进效率低及劳动强度高等问题,提出了优化钻杆结构以减少自重并降低钻进阻力的研究思路,通过设置四翼内凹结构达到减小钻杆自重及降低钻进阻力的目的,并依此设计了四翼内凹刻槽钻杆,兼顾钻杆推进阻力和旋转扭矩损失,建立了钻杆钻进阻力模型,结合ANSYS数值模拟方法对四翼内凹刻槽钻杆进行强度校核分析与参数优化,研制了四翼内凹刻槽钻杆,并进行了现场工业性试验。研究表明:(1)对于100m钻孔深度,四翼内凹刻槽钻杆相对于圆状刻槽钻杆质量减轻了21.88%,在正常钻进时,四翼内凹刻槽钻杆与圆钻杆相比旋转扭矩损失降低了25.10%;与圆状刻槽钻杆相比降低了20.90%。在塌孔10m的情况下,四翼内凹刻槽钻杆钻进阻力相对于圆钻杆降低了51.44%,相对于圆状刻槽钻杆降低了43.65%。(2)对四翼内凹刻槽钻杆进行强度分析与参数优化,结果显示,在常规坑道钻机作用下,钻杆最大应力与内凹深度呈正相关,与壁厚呈负相关,壁厚对于应力的影响大于内凹深度的影响,且在内凹宽度为33mm、深度为4mm、壁厚为11.5mm时,安全系数为1.59。(3)现场工业性试验表明,四翼内凹刻槽钻杆排渣顺畅,强度可靠,钻进深度提高了18.18%,钻进效率提高了17.80%。在保证钻杆强度的前提下、通过钻杆结构创新有效降低钻杆质量,对于降低工人劳动强度、提高钻进效率和成孔率具有显著的效果,为推动复杂地层钻探技术发展提供了新的方法。

大埋深“三软”厚煤层智能综放开采技术研究

针对大埋深“三软”条件下厚煤层综放开采的过程中液压支架钻底以及巷道底鼓的问题,以淮北信湖煤矿818智能综放工作面为研究背景,对液压支架的适用性进行了分析,并介绍了818智能综放采煤工作面的液压支架自动化系统。通过理论计算得出液压支架的最大工作阻力为9940 kN,分析现场实测数据可知中部液压支架的工作阻力最高,其次是下部,上部最小。工作面支架前排立柱受力大于后排立柱,工作面来压时,液压支架前立柱工作阻力平均比后立柱工作阻力大44.5%,前排立柱工作阻力占整架工作阻力的比例大致在0.57~0.74之间,工作面液压支架平均工作阻力为28.11 MPa,因此动载荷计算法更适合818工作面支架工作阻力计算;分析了液压支架钻底的原因,并提出液压支架的管控措施;通过数值模拟得到工作面超前支承压力的影响范围为煤壁前方110 m,工作面中部峰值应力最大,其次是下部,最后是上部,峰值应力出现在煤壁前方12~19 m;由现场实测数据可知煤壁向外105 m范围内,巷道底鼓及两帮变形量相对较大,巷道顶板下沉及底鼓比较明显,在此基础上提出预防巷道底鼓的相关措施,以期为淮北矿区以及类似条件下开采的工作面提供借鉴与参考。

四翼内凹刻槽钻杆高效排渣钻进技术研究

为解决松软煤层钻进钻孔排渣通道堵塞这一制约钻孔施工效果的关键难题,基于优化钻杆截面形状以增大排渣空间、预防钻孔堵塞这一理念,提出四翼内凹刻槽钻杆几何设计构想,通过孔内排渣阻力力学方程计算验证、钻杆排渣效果数值模拟,分析钻杆结构参数对排渣性能的影响,研制四翼内凹刻槽钻杆并在某矿进行工业性试验。研究结果表明:相较于传统圆状刻槽钻杆,四翼内凹刻槽钻杆在施工过程中具有更大的排渣空间,通过孔内沿程阻力计算对比,四翼内凹刻槽钻杆孔内沿程阻力较圆状刻槽钻杆低13%;数值模拟计算结果表明,当弧形内凹切槽对钻杆的圆心角α=35°、切入深度|EF|=4 mm时,渣体运移距离最大,平均运移速度最大,排渣性能最优;工业性试验表明,使用四翼内凹刻槽钻杆施工钻孔,班进尺长度和终孔深度得到显著提升。研究结果可为煤矿瓦斯抽采钻孔钻具设计提供参考。

基于两堵两注通管直连技术的瓦斯抽采研究

为解决“两堵一注”封孔工艺及传统连管方式在孟津煤矿实施存在的弊端,提出了“两堵两注”通管直连技术,提高了瓦斯抽采浓度,延长了瓦斯抽采时间。为此,采用巷道松动圈理论分析了该矿合适的封孔长度;采用离散元数值模拟方法建立巷道模型,分析了巷道周围应力分布,确定了钻孔的最佳封孔深度;开展了“两堵两注”封孔地面实验和通管直连技术现场实验。结果表明:孟津煤矿二1煤层工作面顺层瓦斯抽采钻孔封孔长度至少为6.75 m;该矿12070工作面穿层钻孔最佳封孔深度为15~30 m;“两堵两注”技术能有效对抽采钻孔进行封堵;通管直连技术在现场应用中大幅度提高了瓦斯抽采浓度,延长了瓦斯抽采时间。