Longwall Face Stability Index for Estimation of Chock-Shield Pressure and Face Convergence

The paper discusses the concept and methodologies for the development of longwall face stability index (LFSI). LFSI is used for estimation of chock-shield pressure and face convergence. The index comprises of engineering properties of main roof, depth of mining, different support capacities and mechanical properties of coal seam being mined and provides a numerical value in the range of −6.17 to 8.13. In this study, 324 finite element models of longwall panel are developed based on various combinations of geomining conditions of Indian coal measure strata. The LFSI is an outcome of the results from finite element models. This paper illustrates a real life example for the estimation of chock shield pressure and face convergence based on LFSI. Validation of the LFSI based calculation is carried out with the field monitored data and found that the LFSI based approach is sufficient to forecast face stability parameters at longwall face.     

A Geometric Computational Model for Calculation of Longwall Face Effect on Gate Roadways

In this paper a geometric computational model (GCM) has been developed for calculating the effect of longwall face on the extension of excavation-damaged zone (EDZ) above the gate roadways (main and tail gates), considering the advance longwall mining method. In this model, the stability of gate roadways are investigated based on loading effects due to EDZ and caving zone (CZ) above the longwall face, which can extend the EDZ size. The structure of GCM depends on four important factors: (1) geomechanical properties of hanging wall, (2) dip and thickness of coal seam, (3) CZ characteristics, and (4) pillar width. The investigations demonstrated that the extension of EDZ is a function of pillar width. Considering the effect of pillar width, new mathematical relationships were presented to calculate the face influence coefficient and characteristics of extended EDZ. Furthermore, taking GCM into account, a computational algorithm for stability analysis of gate roadways was suggested. Validation was carried out through instrumentation and monitoring results of a longwall face at Parvade-2 coal mine in Tabas, Iran, demonstrating good agreement between the new model and measured results. Finally, a sensitivity analysis was carried out on the effect of pillar width, bearing capacity of support system and coal seam dip.     

Strata deformation measurements on longwall coalfaces and their implications for design of powered support systems

Summary Recent developments in powered supports on British longwall faces include the advent of lemniscate designs, increased setting and yield loads, larger maximum extraction heights and a consequent rise in weight and cost. There was a requirement to examine the relevance of existing strata control theory to modern conditions. Measurements on five longwall coalfaces at four British collieries are described. Accurate measurements of support pressures, closures and inclinations, roof/floor convergence and lateral movement and hydraulic supply parameters were made with a digital data logging system. At one site magnetic extensometers were utilized to measure face and roof strains. Consideration of measured roof dilation and reclosure in conjunction with convergence provides a new perspective on the deformation mechanisms involved. The most important parameters governing strata conditions were identified as proximate lithology, foundation characteristics and adequate setting load densities. Recommendations are made regarding choice, design and operation of powered supports.     

Yield pillars for stress control in longwall mines — case study

Summary The demand for increased productivity and the problems associated with mining at greater depths have increased the interest in using the yield pillar concept in the United States. This paper summarizes chain pillar behaviour in a mine that historically experienced coal bumps in both room-and-pillar and longwall sections. Results indicate that, generally, the chain pillars yield as designed, but that yielding occurred either after development or with approach of the longwall face. The Bureau of Mines investigated several yield pillar design approaches to possibly explain observed differences in pillar behaviour. These approaches suggest that very localized conditions, such as coal and rock properties, cover depth, and extraction height, may influence the behaviour of any one pillar. At this mine, yielding chain pillars result in de-stressing of the longwall entries and the transfer of potentially dangerous stress concentrations to adjacent panels. Pre-longwall-mining behaviour indicates the existence of a pressure arch, the width of which increases with depth. Results indicate that use of yield pillars improves stress control, reduces bump potential, and increases resource recovery.     

Access tunnel convergence prediction in longwall coal mining

Summary The paper describes theoretical andin situ studies of tunnel deformation in longwall coal mining. It develops a method to predict tunnel convergence profiles from the faceline in longwall mining. The method accounts for the effect of panel width, extracted seam height, deformation moduli of the goaf material and coal pillar, depth of cover,in situ structural defects, tunnel shape and tunnel size in addition to the strength characteristics of surrounding strata. The analytical technique has been validated by reference toin-situ deformation measurements in 26 face-access tunnels in Cape Breton Coalfield mines. Based on this method a series of vertical convergence profiles for different depths and extracted panel widths have been presented.     

The future use of refrigeration in British coal mines

Summary This paper examines the methods presently available to apply refrigeration to a longwall district of a coal mine with particular reference to both the sources of heat and humidity around a district, and the specific locations which require cooling. Other methods to improve climatic conditions are also investigated and computer-predicted results from a district temperature prediction program are used for discussion.An approach to the solution of a climatic problem is explained with reference to other coal mine contaminants and an environmental design philosophy for deep high production districts in British coal mines is described.     

Calculation of periodic roof weighting interval in longwall mining using finite element method

The state of periodic loading and the interval of periodic roof weighting have an important role in geomechanical stability and, hence, in the continuity of longwall mining operations. In this paper, the mechanism of roof caving in longwall mining—together with the effect of engineering and geomechanical properties of surrounding rock masses on the magnitude and timing of periodic loading—is studied. For this purpose, a longwall mine is first modeled using Phase2 software, and then, by simulating the roof caving process, the periodic roof weighting intervals is calculated. Based on the numerical modeling, the first roof weighting interval and the periodic roof weighting interval are calculated as 27.2 and 12.1 m, respectively. Sensitivity analysis is then applied to determine the effect of changes in the mechanical properties of the rock mass, especially in the main roof and immediate roof. The results of the analysis show that as GSI and quality of the immediate roof increases, the periodic roof weighting interval also increases. Hence, the applied algorithm in this research study can effectively be utilized to calculate the periodic roof weighting interval in the longwall mining method.     

Positive and negative impacts of longwall mine subsidence on a sandstone aquifer

 Subsidence due to longwall underground coal mining changes the hydraulic properties, heads, yields, and in some cases the groundwater chemistry of overlying bedrock aquifers. A 7-year study of a sandstone aquifer overlying an active longwall mine in Illinois has supported a comprehensive model of these impacts. Subsidence caused increases in permeability and storativity over the longwall panel. These changes initially caused a major decline in water levels in the sandstone, but the aquifer recovered slightly within a few months and fully within several years after mining. The enhanced hydraulic properties combined with potentiometric recovery resulted in a zone of greater well yield. However, at sites with very poor transmissivity and inadequate recharge pathways, recovery may not occur. Also, at the study site, the physical enhancement was accompanied by a deterioration in groundwater quality from slightly brackish, sodium bicarbonate water to more brackish water with increased sulfate levels. Received: 17 March 1997 · Accepted: 9 September 1997     

Astrata control system and its application in West German coal mining

Summary Astrata control system has been developed to improve mine planning and design in deep coal mines in West Germany, where redistributed stresses from longwall workings in weak strata create difficult support problems. The system involves theoretical, laboratory and underground observations and studies of strata and support performance. Examples of the system covered in detail include planning the position and support of a cross-cut; improving a face layout; planning a gateroad; planning strata bolting and planning longwall face layouts.     

A study of the ground control effects of mining longwall faces into open or backfilled entries

Unusual circumstances may require that a longwall retreat into or through a previously driven room. The operation can be completed successfully, but there have been a number of spectacular failures. To help determine what factors contribute to such failures, a comprehensive international database of 131 case histories has been compiled. The cases include six failures where major rock falls occurred in front of the shields, and seven even more serious failures involving major overburden weighting. The case studies suggest two types of room failure mechanism. The first is a roof fall type failure caused by loading of the immediate roof at the face as the fender or remnant longwall panel narrows. The second is an overburden weighting type failure caused by the inability of the roof to bridge the recovery room and face area, and affecting rock well above the immediate roof. The data indicate that the roof fall type of failure is less likely when intensive roof reinforcement (bolts, cables and trusses) is employed together with higher-capacity shields. The overburden weighting failures, in contrast, occurred when the roof was weak and little standing support was used. Weighting failures were not greatly affected by the density of roof reinforcement. In one of the overburden weighting cases, in a Pittsburgh coalbed mine, stress cell, convergence, bolt load and extensometer data have been used to analyze the failure in detail.     

Coal Mine Roadway Stability in Soft Rock: A Case Study

Roadway instability has always been a major concern in deep underground coal mines where the surrounding rock strata and coal seams are weak and the in situ stresses are high. Under the high overburden and tectonic stresses, roadways could collapse or experience excessive deformation, which not only endangers mining personnel but could also reduce the functionality of the roadway and halt production. This paper describes a case study on the stability of roadways in an underground coal mine in Shanxi Province, China. The mine was using a longwall method to extract coal at a depth of approximately 350 m. Both the coal seam and surrounding rock strata were extremely weak and vulnerable to weathering. Large roadway deformation and severe roadway instabilities had been experienced in the past, hence, an investigation of the roadway failure mechanism and new support designs were needed. This study started with an in situ stress measurement programme to determine the stress orientation and magnitude in the mine. It was found that the major horizontal stress was more than twice the vertical stress in the East–West direction, perpendicular to the gateroads of the longwall panel. The high horizontal stresses and low strength of coal and surrounding rock strata were the main causes of roadway instabilities. Detailed numerical modeling was conducted to evaluate the roadway stability and deformation under different roof support scenarios. Based on the modeling results, a new roadway support design was proposed, which included an optimal cable/bolt arrangement, full length grouting, and high pre-tensioning of bolts and cables. It was expected the new design could reduce the roadway deformation by 50 %. A field experiment using the new support design was carried out by the mine in a 100 m long roadway section. Detailed extensometry and stress monitorings were conducted in the experimental roadway section as well as sections using the old support design. The experimental section produced a much better roadway profile than the previous roadway sections. The monitoring data indicated that the roadway deformation in the experimental section was at least 40–50 % less than the previous sections. This case study demonstrated that through careful investigation and optimal support design, roadway stability in soft rock conditions can be significantly improved.     

长壁孤岛工作面冲击失稳能量场演化规律

煤矿冲击地压一直是困扰中国煤矿安全的主要问题,而煤矿开采过程中跳采形成的孤岛工作面由于容易产生应力集中,来压强度提高,极容易发生冲击地压。基于唐山矿T2193下孤岛工作面的地质条件,从数值分析的角度研究了煤岩体材料的非均匀性,揭示了孤岛工作面顶板周期来压时煤岩体能量释放的动态特征,分析了工作面前方能量释放激增机制。数值模拟结果显示,长壁工作面回采过程中直接顶的不断垮落造成了老顶悬空距离的不断增大,工作面周期来压时,积聚于老顶岩层内的弹性应变能将瞬间释放,容易引发工作面及巷道的冲击失稳。孤岛工作面由于其特有的矿压显现特征,老顶周期破断时所释放的弹性应变能将更加剧烈,冲击地压势必愈加强烈。孤岛工作面顶底板和煤层的能量释放激增可以作为判断煤岩体冲击失稳的前兆信息。孤岛工作面前方发生冲击破坏的主要原因是由于工作面回采过程中围岩所积聚的大量弹性能在顶板断裂时所伴随的巨大能量释放而造成的。     

A New Blastability Index for Hard Roof Management in Blasting Gallery Method

The presence of hard and massive sandstone above the coal seam in underground coal mines often leads to delay in caving of overlying rock beds thereby causing excessive load on supports and posing danger to underground workings. The problem is more prominent in blasting gallery (BG) as well as longwall mining methods in Indian coal mines. Induced caving by blasting is a promising means for hard roof management in underground coal mines. Based on extensive studies and data collected from different mines in India, a Blastability Index (BI) has been developed which can be used for the classification of roof according to the degree of ease in caving by induced blasting. Different charge factors have also been suggested based on the Blastability Index. Due to wide change in the method of extractions, ??Cavability Index?? for longwall panel was found ineffective in case of BG method of working as well as bord and pillar working. For this reason, this proposed Blastability Index would be of immense help for caving of hard roof by induced blasting.     

A GIS-based Weights-of-Evidence model for mapping cliff instabilities associated with mine subsidence

The Weights-of-Evidence (W-of-E) technique was applied, within a geographic information system (GIS), to derive a model of rockfall potential associated with mining-induced subsidence. The purpose of the model was to describe the potential for rockfalls from up to 60 m high steep sandstone gorges and slopes associated with proposed underground longwall operations within the immediate vicinity of a previously mined area. Ten known rock falls associated with the previous mining operation were used as training points. Six evidential themes were considered-slope, cliff height, planform curvature, profile curvature, the distance of the cliff areas from the longwall panels, and the distance of the cliff areas from the river. Two models were created, one based on a mine layout in which longwall panels extend beneath the steep areas of a nearby river, and a second in which the mine layout is modified so that mining does not occur directly beneath or within 50 m of the steep slopes. This is to allow for the comparison of rockfall potential based on different mining configurations. The results demonstrate that the W-of-E method is a suitable tool for mine subsidence impact assessment, and suggest that not mining directly under the Nepean river may decrease rockfall potential, on average, by approximately ten times. Numerous limitations with the results, relating to the availability of appropriate evidential theme data and the accuracy of training points, are discussed.     

Discontinuous Modelling of Stratum Cave-in in a Longwall Coal Mine in the Arctic Area

This paper presents a discontinuous numerical approach for studying roof cave-in mechanisms and obtaining the required support capacity of longwall shields in a case study site, the Svea Nord coal mine in Svalbard. The block size in the roof strata and the mechanical parameters of the discontinuities for the numerical model were obtained through back-calculations. The back-calculations were conducted with a statistical method of design of experiment. Numerical simulations revealed that voussoir jointed beams are formed before the first cave-in occurs. The maximum deflection of a roof stratum in the study site prior to the first cave-in is about 70 % of the stratum thickness. The maximum span of the roof strata prior to the first cave-in depends upon the in situ horizontal stress state. The roof beams have a large stable span when they are subjected to high horizontal stress; but horizontal stress would increase the possibility of rock crushing in deflected roof beams. The simulations and field measurements show no periodic weighting on the longwall shields in the study site. Stiff and strong roof beams would result in large first and periodic cave-in distances. As a consequence of having large cave-in distances, the longwall shields must have high load capacity, which can be calculated by the presented numerical approach.     

Some observations on the influence of faults in mininginduced seismicity

The effect of faults on seismicity has been investigated for mining-induced seismicity associated with coal extraction in the North Staffordshire Coalfield, U.K. The studies show that seismic events occur if the mine workings cause the redistribution of strata pressure in such a way that sliding movements take place along major faults. In particular, movements occur when the active longwall face is driven in the footwall of the faults and parallel to the fault plane.     

Effect of the form of data on the quality of mine tremors hazard forecasting using neural networks

During hard coal mining operations conducted under conditions of rockburst hazard, one of the most important preventive measures can be the prediction of occurrence time and location of the strong seismic mine tremors of energy E _s ⩾ 10⁴ J. This is a very difficult task and the way it is being currently performed appears to be unsatisfactory. Therefore, attempts have been made to use neural networks, specifically trained for this application. The paper presents an approach for determining an influence of the type and shape of the input data on the efficiency of such a prediction. The considerations are based on a selected example of the seismic activity recorded during longwall mining operations conducted in one of the Polish mines.     

浅埋煤层长壁开采顶板岩层灾害控制研究

根据浅埋煤层顶板岩层的赋存特点和长壁开采时基岩老顶的断裂下沉特征,应用初始后屈曲理论和突变理论探讨了初次来压期间基岩老顶的灾害机制和灾害控制,补充了基岩老顶破断后控制岩块滑落失稳的充分条件和岩块触矸前后台阶下沉的判据,确定了台阶下沉量,给出了控制台阶下沉和维持顶板稳定的支护力计算公式,并以神东矿区大柳塔1203工作面为例给出了工程实例。研究发现,顶板破断后岩块处于不平衡状态,台阶下沉的机制是结构的滑落失稳;岩块触矸前后的台阶下沉分别与断裂下沉和开采高度之间存在着确定的关系;顶板强烈来压主要是由结构滑落失稳造成的,控制台阶下沉的实质是控制岩块沿煤壁的滑落失稳。研究结果表明,综合应用初始后屈曲理论和突变理论,可根据顶板结构的赋存状况和结构特征,确定基岩老顶触矸前是否会出现全厚度切落;根据工作面的开采高度和采空区有效充填厚度等,可确定基岩老顶触矸后岩块的运动形式;根据顶板的破断步距、断裂下沉量和岩块摩擦因数等,可确定岩块触矸前后控制台阶下沉的支护力     

Stability Control of the Equipment Recovery Passage in a Fully Mechanized Longwall Mining: Case Study

Equipment recovery passage (ERP) is being widely employed in longwall panel for the purpose of recovering heave mining equipment at the end of mining stage. The stability of the ERP, however, is always difficult to control although powerful supporting structures are employed, which restricts its further promotion. In the present paper, we focused on the ERP’s stability control through reducing the abutment stress imposed on the ERP’s surroundings rather than solely increasing roadway support intensity, based on a rigorous case study in China. First, the dynamic evolution of the abutment stresses, corresponding plastic zone and deformations in the surrounding rock of the ERP were analyzed through a meticulously validated FLAC3D numerical simulation, as the longwall face moved with different velocity. The simulated results indicate that the faster the longwall face moved, the lower the abutment stresses, the narrower the plastic zone and the smaller the deformations were. In terms of these analyses, two suggestions were proposed, including increasing longwall face moving velocity and roof structure optimization, and corresponding technologies were introduced, and potential effect were verified as well. Conclusions and suggestions of this paper might be helpful for increasing the flexibility of the ERP in similar geotechnical conditions.

     

Study on the Fracture of Hard and Thick Sandstone and the Distribution Characteristics of Microseismic Activity

The movement of hard and thick key stratum during underground coal extraction via longwall top coal caving differs from that of other types of overlying strata. Therefore, numerous problems such as roadway instability, rock burst and strong mining-induced mine seismicity will be encountered as a result of the fracture of a hard and thick key stratum. The key to controlling the behavior of strata is to understand the movement and fracture pattern of the hard and thick key stratum. Taking the 103_up02 working face of Baodian coal mine as a case study in this study, according to the in situ measured microseismic data, the rule of fracture of overlying hard and thick sandstone caused by working face mining is studied, and the mechanism of dynamic pressure impact induced by hard and thick strata fracture is explained. Finally, the dynamic pressure control technology is put forward. The research results are of great significance for coal mining under hard and thick strata, mastering the fracture rules of hard and thick strata and predicting dynamic disasters.