Experimental research of overburden movement and subsurface water seeping in shallow seam mining

Shallow seam coal field has the largest coal reserve in China. Mining in shallow depth causes serious problems, and subsurface dewatering is a major issue. In this paper, the physical simulating models were prepared to study overburden movement and aquiclude stability in the shallow seam mining of Yushuwan Coalfield, China. According to the characteristic of clay aquiclude and bedrock in the overburden, the proper simulation materials for simulating the plastic clay aquiclude layers and brittle bedrock layers were determined by the stress-strain tests and hydrophilic tests. The physical simulating models of solid medium and two phases of solid-liquid medium were carried out to simulate the failure and caving process of the roof and overburden, as well as the subsurface water seeping. Based on the simulation, it was found that the movement of clay aquiclude follows the movement of the underlying bedrock layers. The stability of aquiclude is mainly affected by cracks in fracture zone. The tests also showed that the best way to control the stability of aquiclude is to reduce the subsiding gradient, and there is a possibility of ground water conservation under longwall mining in Yushuwan Mine. This research provides a foundation for further study on mining dewatering and water conservation.

Effects of gully terrain on stress field distribution and ground pressure behavior in shallow seam mining

This study proposes a novel approach to study stress field distribution and overlying ground pressure behavior in shallow seam mining in gully terrain.This approach combines numerical simulations and field tests based on the conditions of gully terrain in the Chuancao Gedan Mine.The effects of gully terrain on the in situ stress field of coal beds can be identified by the ratio of self-weight stress to vertical stress(η) at the location corresponding to the maximum vertical stress.Based on the function η =j(h),the effect of gully terrain on the stress field of overlying strata of the entire field can be characterized as a significantly affected area,moderately affected area,or non-affected area.Working face 6106 in the Chuancao Gedan Mine had a coal bed Jepth <80 m and was located in what was identified as a significantly affected area.Hence,mining may cause sliding of the gully slope and increased loading(including significant dynamic loading) on the roof strata.Field tests suggest that significant dynamic pressures were observed at the body and foot of the gully slope,and that dynamic loadings were observed upslope of the working face expansion,provided that the expanding direction of the working face is parallel to the gully.

Roof structure theory and support resistance determination of longwall face in shallow seam

This paper presents the structure models founded in shallow seam, the roof asymmetry arch with three articulations in roof first weighting and the step voussoir beam in roof periodic weighting. These structure models are differ from classic theory, it establishes the new roof control theory of instability structure roof, especially in shallow seam. Based on the new roof structure theory, the support working state of "given sliding load" is put forward, and the factor of load transmitting is introduced to determine the load on roof structure. Therefore, the proper and accurate calculating methods of support resistance are established. Based on this, the dynamic structure theory in shallow seam could be predicted.

Practice and adaptable conditions classification of aquifer-protective mining in longwall coalface for shallow seam with thin bedrock

A set of adaptable conditions classification of aquifer-protective mining in the Iongwall coalface for shallow coal seams with thin bedrock was put forward to deal with the conflict between water protection and high efficiency for the mining field in west China. This classification was suitable for shallow coal seams with different thickness and was beneficial to the local environmental protection. Using the 3-Universal Distinct Element Code （3DEC） numerical software, the height of the fractured zones for shallow coal seams with thin bedrock was calculated and analyzed, and its predicting formula was achieved. Meanwhile, according to the lithology and the weathering degree of the shallow coal seam the thickness of the protective layer was determined as 10 m and the overlying water body of loose water-bearing sand for shallow coal seams with thin bedrock was divided into three types, namely, weak, medium and strong. Based on these, the necessary bedrock thickness of the Iongwall coalface for shallow coal seams with thin bedrock was confined according to the different mining height and water yield nature of the overlying loose water-bearing sand. Combined with the present mining status, a set of new methods of adaptable conditions classification of aquifer-protective mining technology in the Iongwall coalface for shallow coal seams with thin bedrock was put forward.

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.

STUDY ON FRACTURE AND MOVEMENT OF OVERLYING BEDROCK IN SHALLOW SEAM UNDER THICK SAND

Based on the field observation results of ground pressure in shallow seam, the movement characteristics of overlying strata in front and behind and the subsidence of the earth’s surface are studied. Also, the paper analyzes the stability of overlying bedrock according to the theory of voussoir beam’s "S-R" stability and the hypothesis of step subsidence. It points out that slide instability is the fundamental form of overlying bedrock movement.

Study on water resisting property of subsurface aquiclude in shallow coal seam mining

According to the characteristic of clay aquiclude of overburden,the proper simulation materials and proportions of mixture for simulating the plastic clay aquiclude layers were developed,and the plastic similarity conditions were setup.Thus,the simili- tude and simulation method in whole stress-strain was progressed.Furthermore,the simulation condition and material proportions in water reaction property and crack closing property were also put forward.Based on systematical tests,the development and distri- bution of mined cracks in roof and subsurface aquiclude was found and the stability of aquiclude was analyzed at all.At last,the key section and key index of aquclude stability was advanced.It is found that the movement of clay aquiclude follows the movement of the underlying bedrock layers.The basic caving mechanism of the overburden roof strata was also presented.

Mechanism of Mining-induced Slope Movement for Gullies Overlaying Shallow Coal Seams

This paper provides an improved understanding of the movement mechanisms of both bed-rock gully and sandy soil gully when underground mining occurs underneath,followed by systematic analysis of the contributing factors such as mining advance direction,gully slope angle,gully erosion coefficient and mining height.This paper presents the results from monitoring,theoretical analyses and up to date modeling based on the geological features in the gully affected area,and the implications of these results to the success of roof support trial.It was observed that when mining occurred towards the gully,sliding of slope block along the fracture surface occurred,which resulted in unstable roof condition;when mining progressed away from the gully,polygon blocks developed in the gully slope and rotated in reversed direction forming hinged structure;within the bed-rock slope,the hinged structure was unstable due to shear failure of the polygon block;however,within the sandy soil slope,the structure was relatively stable due to the gradual rotating and subsiding of the polygon block.The increase of the value of slope angle and mining height lead to a faster and more intensive fracture development within the gully slope,which had a pronounced effect on gully slope stability and underground pressure.Various remediation approaches are hence proposed in this paper including introducing more powerful support and reasonable mining height,setting up working face along or away from gullies,using room and pillar,strip mining and backfill instead of longwall mining.

Simulation research on the influence of eroded primary key strata on dynamic strata pressure of shallow coal seams in gully terrain

In Huojitu Coal Mine of Shendong mining area, the dynamic strata pressure (DSP) accidents occurred when the working faces passed the gully terrain. Focusing on this problem, we used physical simulation experimental method to thoroughly study the influence of eroded overlying primary key strata (PKS) in the gully terrain on DSP of shallow coal seams in this paper. The result show that when mining activities took place in the uphill section of shallow coal seams in gully terrain and the PKS were eroded, the blocks could not form stable bond-beam structures since the horizontal force of PKS blocks in adjacent sloping surfaces were relatively small. The sliding instability of blocks caused rapid increase of the load on the sub-key strata (SKS) blocks, which resulted into coal slide and roof fall as well as sharp drop of active columns. This led to DSP phenomenon. When the PKS blocks were intact, there was no DSP phenomenon to enable blocks provide certain horizontal force to maintain stable bond-beam structure. The simulation results were verified by the mining practices of working face 21306 crossing the gully terrain in the Huojitu Coal Mine.

Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application

Based on break characteristics of roofs in fully mechanized top-coal mining of thick shallow coal seams, a fracture mechanics model was built, and the criterion of crack propagation in the main roof was derived using the fracture mechanics theory. The relationships between the fracture length of the roof and the working resistance of the supports were discovered, and the correlations between the load on the overlying strata and the ratio of the crack's length to the thickness of the roof were obtained. Using a working face of Jindi Coal Mine, Xing county Shanxi province as an example, the relationships between the fracture length of the roof and the working resistance of the supports were analysed in detail. The results give a design basis in hydraulic top coal caving supports, which could provide useful references in the practical application. On-site experiment proves that the periodic weighting step interval of the caving face is 15–16 m, which is basically consistent with the theoretical analysis results, and indicates that the mechanized caving hydraulic support is capable of meeting the support requirements in the mining of a super-thick but shallowly buried coal seam.

Characteristics and stability of slope movement response to underground mining of shallow coal seams away from gullies

Underground pressure is abnormal during mining of shallow coal seams under gullies. We studied gully slope movements, subject to underground mining, with physical simulation and theoretical analysis. The rules disclose that the slope rock slides horizontally in response to mining in the direction of gullies and rotates reversely with the appearance of a polygon block in mining away from gullies. We focused our attention on the case of mining away from a gully. We built a mechanical model in terms of a polygon block hinged structure and investigated the variation of horizontal thrust and shear force at the hinged point in relation to the rotation angle under different fragmentations. The Sliding-Rotation instability conditions of the polygon block hinged structure are presented based on the analyses of sliding instability and rotation instability. These results can serve as a theoretical guide for roof control during mining away from gullies in a coalfield defined by gullies.

Underground pressure characteristics analysis in back-gully mining of shallow coal seam under a bedrock gully slope

We studied underground pressure and its mechanism during back-gully mining in a shallow coal seam under a bedrock gully slope,by means of physical simulation,numerical modeling and field monitoring.The results show that the intensity of underground pressure is related to its relative position at the coalface.The underground pressure is intensive and the support resistance reaches a maximum when the coalface is at the bottom of the gully,whereas the underground pressure is moderate and decreases gradually when the coalface passes the gully.The mechanism of these changes is analyzed when the slope rotated in a reversed direction to the slope dip during back-gully mining and form an unstable,multilateral block hinged structure,due to slipping.The subsidence of multilateral blocks is considerable when the block fragmentation is small,resulting in enormous changes in the underground pressure.With an increase in the mass of the block body,the block displacement will be reduced in conjunction with an increased clamp effect by both the unbroken rocks and broken rocks in the goaf,resulting in a decrease of the underground pressure.

Ground fissure development regularity and formation mechanism of shallow buried coal seam mining with Karst landform in Jiaozi coal mine: a case study

A comprehensive study was undertaken at Jiaozi coal mine to investigate the development regularity of ground fissures in shallow buried coal seam mining with Karst landform,shedding light on the development type,geographical distribution,dynamic development process,and failure mechanism of these ground fissures by employing field monitoring,numerical simulation,and theoretical analysis.The findings demonstrate that ground fissure development has an obvious feature of subregion,and its geographical distribution is significantly affected by topography.Tensile type,open type,and stepped type are three different categories of ground fissure.Ground fissures emerge dynamically as the panel advances,and they typically develop with a distance of less than periodic weighting step distance in advance of panel advancing position.Ground fissures present the dynamic development feature,temporary fissure has the ability of self-healing.The dynamic development process of ground fissure with closed-distance coal seam repeated mining is expounded,and the development scale is a dynamic development stage of“closure→expansion→stabilized”on the basis of the original development scale.From the perspective of topsoil deformation,the computation model considering two points movement vectors towards two directions of the gob and the ground surface is established,the development criterion considering the critical deformation value of topsoil is obtained.The mechanical model of hinged structure of inclined body is proposed to clarify the ground fissure development,and the interaction between slope activity and ground fissure development is expounded.These research results fulfill the gap of ground fissures about development regularity and formation mechanism,and can contribute to ground fissure prevention and treatment with Karst landform.

Research on downward crack closing of clay aquiclude in shallow coal seam safety mining

The water resisting property of aquiclude is the key factor of water conservation and safety mining, and the mining induced cracks in aquiclude is major factor of water resisting property. The aquiclude is composed by loess layer and red clay layer in Yushuwan Coal Mine, and the water reaction property of clay and loess of aquiclude was tested by soil mechanics method. The permeability coefficient of the loess is 0.856 m/d and the clay is 0.434 m/d. The dilatability coefficient of the loess is 16.1% and the clay is 14.6%. Through physical solid-liquid simulation with whole stress-stain similarity, the distribution of ＂downward crack zone＂ and ＂upward crack zone＂ was found to be the major factor of aquiclude stability. The downward crack closing length is about 30% of the downward crack length. The expanding of clay and loess with water are principal factors of downward crack closing. At last, the mechanical model of downward crack closing was constructed, and the criterion of crack closing was put forward at all. This work will provides the theoretical base for aquiclude stability research and safety mining in shallow seam.

浅埋厚煤层开采地表采动裂缝发育演化规律研究

基于串草圪旦煤矿典型生产工作面覆岩赋存情况及地表采动裂缝发育特征,采用现场实测、理论分析和数值计算相结合,研究了覆岩关键层失稳运动形式及其对地表采动裂缝发育形态的影响,以及基采比对地表采动裂缝演化规律的影响。结果表明:浅埋厚煤层开采过程中,关键层失稳运动对地表采动裂缝发育类型具有控制作用,关键层滑落失稳引起地表产生台阶型裂缝,关键层回转失稳引起地表产生张开型裂缝;覆岩关键层失稳运动过程导致地表采动裂缝尺度参数发生动态变化,随着基采比的增大,地表采动裂缝的垂直错动量呈减小趋势,水平张开量呈增大趋势,裂缝水平张开量与关键层回转角线性相关。

基于减压区开采理论的浅埋煤层群同采工作面合理错距研究

为提高煤层开采效率,采用理论分析和数值计算相结合的方法确定了赵家梁煤矿浅埋煤层群同采时上下工作面的合理错距。采用减压区开采理论分析得出了赵家梁煤矿4^(-2上)和5^(-2)煤层同采时工作面合理错距范围在22.6~32.6 m;通过数值计算得出,同采煤层错距较小时,4^(-2上)煤层与5^(-2)煤层超前支压应力场叠加效应会对同采煤层产生较大影响,下部煤层工作面前方出现“应力增高区”“应力降低区”和“应力增高区”;同采错距在30~35 m范围时采场的应力集中程度较小。

厚煤层大断面回采巷道支护参数优化研究

长壁工作面回采巷道保持“掘-采”全生命周期内的围岩稳定,对于保障工作面高效生产、井下作业人员安全至关重要。杭来湾煤矿302盘区回采巷道为典型的浅埋深、厚煤层、大断面煤巷,为掌握其矿压显现规律,在302盘区选取“三条、两类”回采巷道为研究对象,通过矿压实测、煤岩力学测试、数值模拟及工程实践等方法开展了浅埋厚煤层大断面回采巷道矿压规律研究,对杭来湾煤矿工作面回采巷道支护参数进行了优化设计,在有效保障巷道围岩稳定的同时,实现了高强度、低密度支护,同时提高了掘巷速度,改善了采掘接续关系,促进了矿井实现安全高效生产。

Plastic zone analysis and support optimization of shallow roadway with weakly cemented soft strata

Based on a shallow roadway with weakly cemented soft strata in western China, this paper studies the range and degree of plastic zones in soft strata roadways with weak cementation. Geological radars were used to monitor the loose range and level of surrounding rocks. A mechanical model of weakly cemented roadway was established, including granular material based on the measured results. The model was then used to determine the plastic zone radium. The predicted results agree well with measured results which provide valuable theoretical references for the analysis of surrounding rock stability and support reinforcing design of weakly cemented roadways. Finally, a combined supporting scheme of whole section bolting and grouting was proposed based on the original supporting scheme. It is proved that this support plan can effectively control the deformation and plastic zone expansion of the roadway surrounding rock and thus ensure the long-term stable and safe mining.

富水沟谷区浅埋煤层导水裂隙演化特征

富水沟谷区域下浅埋煤层赋存条件特殊,采场上覆岩层导水裂隙发育演化特征复杂。为了揭示沟谷区浅埋煤层在回采扰动作用下覆岩裂隙演化规律,以朱家峁煤矿1305-2工作面过沟谷区回采阶段为工程背景,采用理论分析、物理相似模拟、数值模拟与现场效果验证的方法,建立了覆岩裂隙−渗流场概念模型,开展了覆岩结构发育与微震能量演化研究,分析了覆岩变形与塑性破坏分布特征,提出了针对沟谷区下浅埋煤层导水裂隙防治措施,并应用于现场工程实践。结果表明:开采扰动下裂隙−渗流场模型呈“梯台”结构,并依次划分为初渗区域、稳渗区域、紊渗区域3个区域;将所研究矿井的工况数据代入模型结构,计算出各个渗透区域范围,并根据计算结果对矿井的稳渗区域采取注浆措施。工作面回采至沟谷区段,覆岩裂隙域形态呈现“拱形-梯形-复合梯形”的扩展演化特征,裂隙纵向发育高度达到163 m,并与沟谷区地表贯通。随工作面推进,地表裂隙依次经历“滑移-挤压-撕裂”过程;沟谷区域位移云图呈现出滞后开采“高位梯形”破断形态,在沟底处下沉位移最大,达3.47 m。针对开采导致的裂隙大范围扩展贯通,提出在地面进行采动裂缝注浆处理,在工作面上覆岩层进行注浆封堵,实现过沟谷区开采“井上−井下”联合防治,保证安全开采。该研究结果可为浅埋煤层的过沟谷区开采、采动裂隙防治及富水区“保水采煤”提供新的科学依据。

浅埋近距离煤层过平行煤柱开采强矿压机理研究

为了揭示浅埋近距离煤层工作面过平行煤柱开采动载传递特征及强矿压机理,以榆家梁煤矿43207工作面为工程背景,采用现场实测、物理模拟、理论分析相结合的方法,研究了煤柱覆岩“倒梯形”结构随下部间隔岩层的运动及其载荷传递特征,分析了超前支承压力和支架载荷的转化规律,建立了“煤柱-间隔岩层台阶岩梁”耦合结构力学模型,得出了强矿压瞬间和顶板切落稳定后的支架载荷计算公式。研究结果表明:煤柱覆岩“倒梯形”结构的集中应力和载荷主要集中于煤柱后段,后段集中载荷是导致工作面过煤柱强矿压的根源;工作面进煤柱初期,煤柱应力表现为双峰;临近出煤柱时,应力集中为单峰,位于煤柱后段。工作面出煤柱4 m后,煤柱后区的超前支承应力系数达到峰值6.00,间隔层开始产生超前裂隙;出煤柱5 m时发生强矿压,煤柱应力下降43%。强矿压期间,煤柱应力传递至煤柱下7 m层位(间隔岩层关键层中部)下降了39%,表明顶板结构承担了相当大的载荷。基于间隔岩层台阶岩梁结构的理论计算,得出强矿压瞬间的支架载荷计算公式,按照煤柱平均应力计算得出支架载荷为22.9 MN/架,按照强矿压时刻煤柱后段应力计算得出支架载荷为24.1 MN/架;煤柱传递至支架的载荷仅为煤柱总载荷的12%~18%,仍超过现场支架额定工作阻力9.2 MN/架。因此,为防止压架,支架额定工作阻力需达到15 MN/架,或者对煤柱及间隔岩层结构进行处理,以保证安全开采。研究结果可为类似条件工作面过平行煤柱开采提供参考。