Failure laws of narrow pillar and asymmetric control technique of gob-side entry driving in island coal face

In allusion to the problems of complex stress distribution in the surrounding rock and deformation failure laws, as well as the difficulty in roadway supporting of the gob-side entry driving in the island coal face, 2107 face in Chengjiao Colliery is researched as an engineering case. Through physical mechanical test of rock, theoretical and numerical simulation analyses of rock, the analysis model of the roadway overlying strata structure was established, and its parameters quantified. To reveal the deformation law of the surrounding rock, the stability of the overlying strata structure was studied before, during and after the roadway driving. According to the field conditions, the stress distribution in coal pillar was quantified, and the surrounding rock deformation feature studied with different widths of the pillars in gob-side entry driving. Finally, the pillar width of 4 m was considered as the most reasonable. The research results show that there is great difference in support conditions among roadway roof, entity coal side and narrow pillar side. Besides, the asymmetric control technique for support of the surrounding rock was proposed. The asymmetric control technique was proved to be reasonable by field monitoring, support by bolt-net, steel ladder and steel wire truss used in narrow pillar side.     

Principle and practice of coupling support of double yielding shell of soft rock roadway under high stress

In order to ensure the safety and stability of the soft rock roadway under high stress, based on the characteristics of the surrounding rock deformation and failure, this paper presented the support technology “coupling support of double yielding shell”, then gave the design method of inner and outer shells and analyzed the principle and requirements of the support technology by taking the −850 m east belt roadway of Qujiang coal mine as the background. The field application results show that the support technology can control the soft rock roadway deformation better under high stress. The displacement between roadway sides was 851 mm, the displacement of the roof was 430 mm, and the displacement of the floor was 510 mm.     

Zonal extraction technology and numerical simulation analysis in open pit coal mine

In order to enhance coal recovery ratio of open pit coal mines, a new extraction method called zonal mining system for residual coal around the end-walls is presented. The mining system can improve economic benefits by exploiting haulage and ventilation roadways from the exposed position of coal seams by utilizing the existing transportation systems. Moreover, the main mining parameters have also been discussed. The outcome shows that the load on coal seam roof is about 0.307 MPa and the drop step of the coal seam roof about 20.3 m when the thickness of cover and average volume weight are about 120 m and 0.023 MN/m³ respectively. With the increase of mining height and width, the coal recovery ratio can be improved. However, when recovery ratio is more than 0.85, the average stress on the coal pillar will increase tempestuously, so the recovery ratio should also be controlled to make the coal seam roof safe. Based on the numerical simulation results, it is concluded that the ratio of coal pillar width to height should be more than 1.0 to make sure the coal pillars are steady, and there are only minor displacements on the end-walls.     

Roadway deformation during riding mining in soft rock

“Riding mining” is a form of mining where the working face is located above the roadway and advances parallel to it. Riding mining in deep soft rock creates a particular set of problems in the roadway that include high stresses, large deformations, and support difficulties. Herein we describe a study of the rock deformation mechanism of a roadway as observed during riding mining in deep soft rock. Theoretical analysis, numerical simulations, and on site monitoring were used to examine this problem. The stress in the rock and the visco-elastic behavior of the rock are considered. Real time data, recorded over a period of 240 days, were taken from a 750 transportation roadway. Stress distributions in the rock surrounding the roadway were studied by comparing simulations to observations from the mine. The rock stress shows dynamic behavior as the working face advances. The pressure increases and then drops after peaking as the face advances. Both elastic and plastic deformation of the surrounding rock occurs. Plastic deformation provides a mechanism by which stress in the rock relaxes due to material flow. A way to rehabilitate the roadway is suggested that will help ensure mine safety.     

Determining the rational layout parameters of the lateral high drainage roadway serving for two adjacent working faces

To determine the rational layout parameters of the lateral high drainage roadway(LHDR) serving for two adjacent working faces, a mechanical model of the LHDR under mining influence was established, and the overburden fissure, mining-induced stress distribution rules were analyzed. First, the development characteristics of mining-induced overburden fissure and the stress distribution law of the upper section of the working face were analyzed. Second, by analyzing the distribution law of vertical stress at different layers, the lateral distance of the LHDR was determined as 25 m. Third, by analyzing the surrounding rock deformation effect, stress distribution law, and overburden fissure distribution law of the LHDR at the heights of 20, 25, and 30 m away from the roof, the rational horizon of the LHDR was determined to be 25 m. Finally, an example of a LHDR located 25 m above the roof of the No. 2 coal seam and 25 m away from the No. 2-603 working face was presented. Results show that when the No. 2-603 coalface is being mined, the surrounding rocks lag 80 m or even further and the working face tends to be stable. The relative deformations of the roof and floor of the roadway and both of its walls were 583 and 450 mm,respectively. The reduction rate of the roadway section was 21.52%–25.32%. The section of the roadway was sufficient to extract the pressure relief gas in the overburden of the No. 2-605 working face. The average gas concentration and the pure volume at the branch pipeline were 24.8% and 22.3 m~3/min,respectively, showing that the position of high-level boreholes was reasonable.     

Deformation mechanism of roadways in deep soft rock at Hegang Xing’an Coal Mine

Engineering geomechanics characteristics of roadways in deep soft rock at Hegang Xing’an Coal Mine were studied and the nature of clay minerals of roadway surrounding rock was analyzed. This paper is to solve the technical problems of high stress and the difficulty in supporting the coal mine, and provide a rule for the support design. Results show that mechanical deformation mechanisms of deep soft rock roadway at Xing’an Coal Mine is of I_ABII_ABCIII_ABCD type, consisting of molecular water absorption (the I_AB-type), the tectonic stress type + gravity deformation type + hydraulic type (the II_ABC-type), and the III_ABCD-type with fault, weak intercalation and bedding formation. According to the compound mechanical deformation mechanisms, the corresponding mechanical control measures and conversion technologies were proposed, and these technologies have been successfully applied in roadway supporting practice in deep soft rock at Xing’an Coal Mine with good effect. Xing’an Coal Mine has the deepest burial depth in China, with its overburden ranging from Mesozoic Jurassic coal-forming to now. The results of the research can be used as guidance in the design of roadway support in soft rock.     

Stress and deformation analysis on deep surrounding rock at different time stages and its application

Based on the characteristics of the deep circular tunnel, the surrounding rock was divided into three regions: the cracked region, the plastic region and the viscoelastic region. The process of rock stress deformation and change was divided into three stages after the roadway excavation. By using the elastic–plastic mechanics theory, the analytical solutions of the surrounding stress and displacement at different stages and the radii of cracked and plastic regions were formulated. We additionally explained the surrounding rock stress and displacement which appeared in practical project. Simultaneously, based on the problem which emerged from a mine in Xuzhou during the excavating process of rock roadway’s transport, we got the theoretical solutions for the stress and displacement in the process of rock roadway’s excavation and considered that the broken area of rock roadway was largely loosing circle. The results indicate that according to the rheological characteristics of surrounding rock, in the primeval excavation of rock roadway, we should increase the length of anchor bolt and cooperate it with anchor nets cable-U steel supporting frame. In addition, when the deformation rate of the surrounding rock is descending after the 15 days’ excavation, we should use the “three anchor” supporting method (anchor bolt spray, anchor note and anchor rope) and set aside about 20 cm as the reserved deformation layer.     

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

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.     

超长"孤岛"综放面大煤柱护巷的数值模拟

基于兖矿集团兴隆庄4324超长“孤岛”综放工作面开采条件，采用离散元数值计算程序UDEC3．0，模拟研究了不同宽度煤柱护巷条件下巷道围岩的应力分布和变形特征，分析了不同宽度煤柱的承载状况和应力峰值区的位置．研究表明，采用20m大煤柱维护巷道，可以改善巷道围岩应力环境；煤柱的承载能力适应超长“孤岛”综放工作面顶板的活动规律；围岩变形量小；能够保障巷道的整体稳定性和有效断面．通过现场实测，讨论了大煤柱维护巷道的矿压显现规律和对巷道的良好维护效果．     

3-D modeling of rock burst in pillar No. 19 of Fetr6 chromite mine

Fetr6 is an underground mine in which chromite is extracted using stope and pillar mining method. Despite of all improving works such as roof supporting and replacing of ore pillars with concrete pillars, pillar No. 19 failed and other pillars failed progressively as a domino effect and 4000 m² of mine collapsed within a few minutes, consequently. For detail investigation, two 3-D numerical models were developed by 3Dec. The first, a base model, was used for estimation of stress on pillars just before failure and the other for investigation of rock burst in pillar No. 19. The results show that discontinuity parameters such as friction angle and shear stiffness is critical parameters in this pillar failure. In addition, it indicates that W/H ratio equal 0.3, the lack of ore extraction strategy and inadequate roof support are the major reasons for this failure. In this paper, the procedure of study was described.     

Coal and rock fissure evolution and distribution characteristics of multi-seam mining

Henan Pingdingshan No.10 mine is prone to both coal and gas outbursts. The E_9–10 coal seam is the main coal-producing seam but has poor quality ventilation, thus making it relatively difficult for gas extraction. The F₁₅ coal seam, at its lower section, is not prone to coal and gas outbursts. The average seam separation distance of 150 m is greater than the upper limit for underside protective seam mining. Based on borehole imaging technology for field exploration of coal and rock fracture characteristics and discrete element numerical simulation, we have studied the evolution laws and distribution characteristics of the coal and rock fissure field between these two coal seams. By analysis of the influential effect of group F coal mining on the E_9–10 coal seam, we have shown that a number of small fissures also develop in the area some 150 m above the overlying strata. The width and number of the fissures also increase with the extent of mining activity. Most of the fissures develop at a low angle or even parallel to the strata. The results show that the mining of the F₁₅ coal seam has the effect of improving the permeability of the E_9–10 coal seam.     

护巷煤柱宽度与巷道围岩变形的关系

本文在分析研究大量受采动影响巷道的矿压显现规律和围岩变形的基础上,提出了巷道在采动期间的围岩变形量,以及采动稳定期间的围岩变形速度同护巷煤柱宽度之间的关系,并得出巷道服务期间的围岩变形总量与护巷煤柱宽度之间的关系式,为选择护巷煤柱宽度提供主要依据。     

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.     

Unanticipated multiple seam stresses from pillar systems behaving as pseudo gob–case histories

Underground coal mining in the U.S. is conducted in numerous regions where previous workings exist above and/or below an actively mined seam. Miners know that overlying or underlying fully extracted coal areas, also known as gob regions, can result in abutment stresses that affect the active mining. If there was no full extraction, and the past mining consists entirely of intact pillars, the stresses on the active seam are usually minimal. However, experience has shown that in some situations there has been sufficient yielding in overlying or underlying pillar systems to cause stress transfer to the adjoining larger pillars or barriers, which in turn, transfer significant stresses onto the workings of the active seam. In other words, the overlying or underlying pillar system behaves as a ‘‘pseudo gob." The presence of a pseudo gob is often unexpected, and the consequences can be severe. This paper presents several case histories, summarized briefly below, that illustrate pseudo gob phenomenon:(1) pillar rib degradation at a West Virginia mine at 335 m depth that contributed to a rib roll fatality,(2) pillar rib deterioration at a Western Kentucky mine at 175 m depth that required pillar size adjustment and installation of supplemental bolting,(3) roof deterioration at an eastern Kentucky mine at 400 m depth that stopped mine advance and required redirecting the section development,(4) coal burst on development at an eastern Kentucky mine at 520 m depth that had no nearby pillar recovery, and(5) coal burst on development at a West Virginia mine at the relatively shallow depth of 335 m that also had no nearby pillar recovery. The paper provides guidance so that when an operation encounters a potential pseudo gob stress interaction the hazard can be mitigated based on an understanding of the mechanism encountered.     

Investigation of overburden behaviour for grout injection to control mine subsidence

This paper describes a field and numerical investigation of the overburden strata response to underground longwall mining, focusing on overburden strata movements and stress concentrations. Subsidence related high stress concentrations are believed to have caused damage to river beds in the Illawarra region, Australia. In the field study, extensometers, stressmeters and piezometers were installed in the overburden strata of a longwall panel at West Cliff Colliery. During longwall mining, a total of 1000 mm tensile deformation was recorded in the overburden strata and as a result bed separation and gaps were formed. Bed separation was observed to start in the roof of the mining seam and gradually propagate toward the surface as the longwall face advanced. A substantial increase in the near-surface horizontal stresses was recorded before the longwall face reached the monitored locations. The stresses continued to increase as mining advanced and they reached a peak at about 200 m behind the longwall face. A numerical modelling study identified that the angle of breakage (i.e., the angle of the boundary of caved zone) behind the longwall face and over the goaf was 22–25° from vertical direction. This is consistent with the monitoring results showing the high gradient of stresses and strains on the surface 150–320 m behind the mining face.     

High-resistance controlled yielding supporting technique in deep-well oil shale roadways简

In order to avoid the deep-well oil shale roadway being deformed, damaged, or difficult to maintain after excavating and supporting in Haishiwan coal mine, this paper has analyzed the characteristics of the deformed roadway and revealed its failure mechanism by taking comprehensively the methods of field geological investigation, displacement monitoring of surrounding rock, rock properties and hydration properties experiments and field application tests. Based on this work, the high-resistance controlled yielding supporting principle is proposed, which is: to “resist” by high pre-tightening force and high stiffness in the early stage, to “yield” by making use of the controlled deformation of a yielding tube in the middle stage, and to “fix” by applying total-section Gunite in the later stage. A high-resistance controlled yielding supporting technique of “high pre-tightening force yielding anchor bolt + small-bore pre-tightening force anchor cable + rebar ladder beam + rhombic metal mesh + lagging gunite” has been established, and industrial on site testing implemented. The practical results show that the high-resistance controlled yielding supporting technique can effectively control the large deformation and long-time rheology of deep-well oil shale roadways and can provide beneficial references for the maintenance of other con-generic roadways.     

Control technology and coordination deformation mechanism of rise entry group with high ground stress

Based on engineering practices of Wuyang Coal Mine, we carried out X-ray diffract researches on No. 3 coal; and the rocks of its roof and floor by XRD meter, and simulated the interactive effect of the surrounding rock deformation by FLAC^2D5.0 numerical simulation software under the condition of different tunneling method of multimine roadway in parallel. The internal structures of the surrounding rocks of 76 belt roadway were monitored by borehole observation instruments; and then, we analyzed the reason of failure and deformation of surrounding rocks of several rise entry, and proposed the technical measures for controlling interactive effect of several rise entry surrounding rock deformation at last. For the thickness seam rise roadway, two conclusions were drawn: one is that the co-deformation among roadway groups mainly reflect on that both shear failure and deformation in coal pillar among roadways have decreased the width of pillar core region and clamping action of coal pillar to roof strata, increased the actual span of roof strata, intensified the flexural failure of roof strata and prized the bed separation of roof deep rock strata. The other conclusion is that the factors controlling the interactive deformation among roadways is obvious when appropriate re-adjustment in construction sequence of the tunneling of multimine parallel roadways because the construction sequence among roadways also has great effects on deformation of the surrounding rock in roadway.     

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.     

Pore structure characteristics of the relative water-resisting layer on the top of the Ordovician in Longgu Coal Mine

In order to study the permeability and water-resisting ability of the strata on the top of the Ordovician in Longgu Coal Mine, this paper tested the permeability and porosity of the strata, investigated the fracture and pore structure features of the strata, and identified the main channels which govern the permeability and water-resisting ability of the strata. The permeability of the upper, central and lower strata shows as 2.0504 × 10⁻³–2.782762 × 10⁻³, 4.1092 × 10⁻³–7.3387 × 10⁻³ and 2.0891 × 10⁻³–3.2705 × 10⁻³ μm², respectively, and porosity of that is 0.6786–0.9197%, 0.3109–0.3951% and 0.9829–1.8655%, respectively. The results indicate that: (1) the main channels of the relative water-resisting layer are the pore throats with a diameter more than 6 μm; (2) the major proportion of pore throats in the vertical flow channel and the permeability first increases and then sharply decreases; (3) the fractures occurring from the top to 20 m in depth of the strata were filled and there occurred almost no fracture under the depth of 40 m; and (4) the ratio of turning point of the main flow channel in the strata on top of Ordovician can be used to confirm the thickness of filled water-resisting layers.     

地应力对锚杆支护的沿空巷道的影响

在数值模拟的基础上,研究了地应力大小及方向对沿空巷道表面位移、深部位移、锚杆布置及护巷煤柱尺寸等的影响,同时对研究结果在现场的应用情况进行了分析．从地应力影响的角度研究了窄煤柱护巷巷道采用锚杆支护的可行性．研究结果表明：地应力（尤其是最大水平主应力）的大小及方向是影响锚杆支护巷道围岩稳定性的关键因素之一,最大水平主应力与巷道轴线的夹角愈小愈有利于巷道围岩的稳定．研究结果不仅为确定合理的护巷煤柱尺寸,而且为科学、合理设计回采巷道锚杆的支护参数提供了依据