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By Ruixiang Gu

The de-confinement mechanisms of unstable failures as they occur in deep coal mining conditions are investigated using a distinct element numerical modeling code. The Mohr-Coulomb and continuously yielding joint constitutive models were used to study the compressive failures in coal and slip behavior of coal-rock interfaces, respectively. The effect of strength variations within interfaces on the failure mechanism of mining faces is also studied. The stability of interface slip failures was found to have a significant role in the failure mode of coal sidewalls. When the interfaces experience stable slip failures, the sidewalls are more likely to fail in a stable manner. With occurrence of unstable slips at interfaces, unstable compressive failures of sidewalls are triggered. The numerical modeling results support the proposed de-confinement mechanisms and emphasize the importance of including the coal-rock interface behavior in the analyses of unstable compressive failures. The significance of the proposed mechanisms in underground mining conditions is that rock discontinuities with large cohesion drop can become potential factors, inducing unstable compressive failures at ribs and mining faces. Therefore, the de-confinement mechanisms should be taken into consideration in mine design.

Jan 1, 2014

By I. L. Follington

A breaker line support (BLS) monitoring system, BLSmon, has been designed and constructed by the CSIRO and installed and commissioned at Laleham No.1 Colliery, Queensland. This system was designed to record hydraulic leg pressure and canopy position measurements and relay these, in real time, to the surface. The system has shown itself to be capable of operating on the BLSs without any adverse affect on production, Analysis of the results of the monitoring exercise has shown that rate of change of leg pressures on the BLSs could be used as an aid in the prediction of adverse mining conditions. Under favourable mining conditions the supports were observed to initially unload from the set pressure, this unloading reduced with time and by the end of a typical lift cycle had either ceased or reversed slightly. This behaviour was considered to be due to one of four possible causes, these being; the compaction of floor debris, and/or compaction of the floor, and/or crushing of the roof and /or support creep. The actual cause can only be determined through detailed monitoring of both roof and support convergence. However observational data would suggest that compaction of debris and softened floor material beneath the supports was the most likely cause. The few positive loading rates recorded under these conditions were of short duration and occurred towards the end of individual lifts. Where mining conditions were unfavourable the negative rates of change of pressure, unloading of the support, were reduced throughout a lift, in both duration and magnitude, when compared with favourable mining conditions. Over the same period the positive rates of change were observed to be greater. The rate of change of loading and convergence can be utilised to identify the onset of instability in the lift area. However, before this facility can be utilised by the mine for the reliable prediction of instability more monitored data and sophisticated processing techniques are required. A mechanism has been postulated for the deformation behaviour of the roof and pillars in a mechanised pillar extraction operation from the continuously monitored data, observational data and results of previous studies. The 7 m wide fenders were observed to yield under abutment stresses before lifting commenced. The stability of these fenders throughout the lifting sequence was considered to be dependant upon the magnitude of their post peak load carrying capability. This is influenced by the post peak stiffness and the extent of roof lowering in the vicinity of the fender. Difficulties with fender extraction occurred where remnant coal left standing in the goaf caused uncaved goat to bridge to the mined fender. Difficulties were also experienced where sub-vertical jointing occurred in the roof of the split reducing it's ability to bridge effectively. Despite the relatively short monitoring period, the potential role and application of continuous monitoring in advancing understanding of the rock mass response to mechanised pillar extraction operations has been demonstrated. This understanding now requires enhancement and shows great potential for aiding the optimisation of mechanised pillar extraction operations.

Jan 1, 1992

By Xinzhi (Thomas) Du

"In order to monitor the overburden strata deformation over a longwall mining area in the Pittsburgh Coal Seam, three boreholes were drilled across the longwall panel. Each borehole employed multiple-point borehole extensometers (MBE) to monitor overburden strata movement during and after the longwall face passed under the study area. The longwall face in the study area was 1,428 ft wide and 4,000 ft long. The average mining height was about 7 ft. The overburden depth ranged from 550 ft to 650 ft with an average of 600 ft. The final extensometer readings indicated that overburden strata initially showed sign of movement when the longwall face was 700 ft inby the MBE. The rate of strata movement accelerated when the face was 300 ft inby MBE. When the face was directly under MBE, a sudden anchor displacement occurred, indicating rapid strata movement once under-mined gob. The readings showed insignificant strata movement after the longwall face was 400–700 ft outby MBE.INTRODUCTIONThe longwall mining method is an efficient mining method of underground mining. Surface and subsurface strata are inevitably disturbed above the longwall mined out gob. Figure 1 shows the four zones of disturbance in the overburden strata in response to longwall mining.A longwall shear cuts the coal seam and allows the immediate roof strata to cave into irregular blocky material to fill the void. This is called the caved zone. The caved zone extends from the mining horizon to six or eight times the mining height, depending on the bulk factor of each caving strata layer. The fractured zone is located above the caved zone, in which the strata breaks into regular blocks by vertical or sub-vertical fractures caused by bending forces due to the weight of the underlying strata. The surface soil and rock crack zone extends to approximately 50 ft below the surface. The surface cracking is caused by downward movement of the surface. Between the fractured zone and surface crack zone is the continuous deformation zone. The strata in this zone deforms gently without causing any major cracks that extend long enough to cut through the thickness of the strata, as in the fractured zone (Peng, 2006).The objective of this project is to provide fundamental rock mechanics research about overburden strata movement response to longwall-related activity. The findings aid in better understanding pillar stability, gas well subsidence management, hydraulic impacts, and numerical modeling."

Jan 1, 2018

By Jialin Xu

Bed separations position, size, development characteristics and their influence factors have been analyzed in-depth by experimental and theoretical methods. The results show that the key strata determine the creation, development and space-time distribution of the overburden bed separations and fractures. Based on the dynamic growth principle during the two-stage development of bed separations, the theory of section-grouting for the overburden bed separation space and the O-ring theory for the pressure-relieved methane drainage are proposed. The technique has been applied in some coal mines in China to reduce surface subsidence and for methane drainage. Keywords: rock strata movements; key stratum; bed separation; bed separation grouting; pressure-relieved methane drainage

Jan 1, 2003

By Gennaro G. Marino

As part of the development of a shopping center in West Virginia, a significant bench had to be cut into a mountainside that was over 700 ft. high. As a result, a steeply stepped, over 400-ft.- high wall was created. The high wall exposed old abandoned room-and-pillar coal workings at the toe of the cut. Because of exposure to the elements, the mine openings began to severely deteriorate with significant potential of undermining of the rock cut face and causing a large rock fall(s). Exacerbating this wall problem was a set of fairly planar sub-parallel and sub-vertical joints creating, in essence, thin rock slabs. Also present were stress relief fractures that were observed in the exposed pillars. Given the proximity of the project building to this high rock face, mitigation of the rock fall potential was deemed necessary. In order to prevent continued damage and deterioration of these exposed mine entries, the rubblized openings were filled with bulkhead grout and pressure injected with grout. The total amount of grout installed in these openings was about 3,945 CY. Also, a rock drape was attached to the entire face to mitigate small rock falls from natural causes. There also existed another level of mining below the ground surface and under an existing building at a depth of about 160 ft. Under a portion of the building, the pillars in these old works were second mined, and the building seemed to have been already affected by limited subsidence above this area. Also, calculated pillar safety factors in this high extraction section of the mine were too low. Consequently, these old works were stabilized by saturation grouting. An indication of overall roof distress was the significant fracturing encountered in the bedrock overburden during the drilling of the injection holes and the volume of grout placed in these holes. The total grout installed in the high extraction section of the lower mine was about 6,660 CY. This paper describes the investigation and the grouting techniques employed at the rock cut face and under the existing building to mitigate continued damage to avoid future subsidence problems at the face and the building premises.

Jan 1, 2013

By John C. Stankus

The 3 Right pre-developed longwall recovery entry at Signal Peak Energy?s Bull Mountain No. 1 mine was designed to prevent the abnormal roof conditions encountered in its firsttwo recovery entries. Unique geological conditions, such as the low overburden of approximately 200 feet and prevalence of roof joints in the sandstone overburden, contribute significantly to abnormal conditions during longwall recovery. The 3 Right recovery entry was 42 ft wide and developed in two stages, 21 ft wide in each stage. The entry roof was reinforced with steel wire mesh/non-metallic recovery mesh, high-capacity primary bolts and cables. After all the support was installed, the entry was completely backfilled with specially designed, cuttable, low-density cement. The roof bolting and backfilldesign were based on current practices in this mine and successful practices in other mines in the U.S. and Australia. Extensive instrumentation was installed in the recovery entry for continuous monitoring of load transfer as the face approached and cut into the recovery entries. The support design was completely successful with a smooth cut-through on January 29 and 30, 2014, significantly reducing the shield recovery time and meanwhile protecting the safety of miners. Based on actual observance and practices of the 3 Right recovery entry, recommendations on improvement and modificationsof roof support for 4 Right recovery room are presented.

Jan 1, 2014

By Xuexin Ding, Weijie Wei, Jinwang Zhang, Yi Chen, Shengli Yang

"Many ultra-thick coal seams exist in Xinjiang and the northern part of the Inner Mongolia. These coal seams are more than 30-40m thick. This kind of the thick coal seam is too difficult to extract in one slice, so the top-coal caving longwall mining method was designed for this purpose. In order to study the movement characteristics of the overburden strata and the development of the ""three zones"" in ultra-thick coal seams, the physical and numerical simulations were carried out based on the in-site condition of Baiyinhua coal mine. The result shows that, when extracting the upper part of the l 5m thick coal seams, the first weighting interval was 50-58m and the periodic weighting interval was I 8-20m. The caving zone, fractured zone and bending and sinking zone were 38m, 92m, and 50m respectively in height. The fractured zone height increased with the advancing distance of the face, but in some localities, a ladder-like distribution due to the periodic weighting existed. When extracting the lower part of the 25m thick coal seams, the roof was extremely broken and the collapse range of overlying strata continued to increase in irregular form.INTRODUCTIONThe thick coal seams occupy 44 percent of coal reserves in China and account for 45 percent of the gross domestic coal production. There are three main methods for extracting the thick coal seams: sublevel mining, large height mining, and longwall top-coal caving (LTCC) (Wang, 2009). Due to the rapid development of LTCC in the past two decades, it has become the most extensively used mining method. It possesses the advantages of high-production, high-efficiency, low development ratio, and low-cost (Yang, Zhang, Chen, and Zhengyang, 2016; Wang and Zhang, 2015). Baiyinhua coal mine's coal reserves are full of uniformly distributed thick coal seams, with the minable thickness of the main coal seam between 25 to 40m, i.e. the main coal seam is an ultra-thick coal seam. Sublevel longwall top-coal caving (SLTCC) which is an efficient method for extracting the ultrathick coal seams has led to multiple disturbances Xie, Chen, and Wang, 1999; Singh and Singh, 1999; Xie, Chang, and Yang, 2009. Therefore, using the case of Baiyinhua coal mine, the movement features of overburden strata and development characteristics of the three zones in ultra-thick coal seams were studied by physical and numerical simulations (Qian, et al, 2003; Qian, 1994)."

Jan 1, 2016

By D. W. H. Su

This paper presents the results of ill situ horizontal stress measurements using a Minifrac system in several northern Appalachian coal mines. The effect of stress magnitude and orientation on longwall gate entry stability was analyzed using a series of three-dimensional finite element analyses. The best headgate stability is achieved if the maximum horizontal stress is aligned with the direction of longwall retreat or if the panel is oriented such that the maximum stress relaxes over the headgate due to the presence of the newly formed gob. The degree of horizontal stress damage to longwall gate entries depends largely on the stress magnitude and roof geology. Accurate assessments of support or design techniques which may mitigate longwall headgate or tailgate instability in areas of thinly laminated roof and high horizontal stresses must account for the in situ horizontal stress state and site specific roof geology. The ability to measure stresses and map geology is thus essential to prevent ground control failures at the longwall headgate.

Jan 1, 1995

By Mark G. Colwell, Russell Frith

"An Analytical Model for Coal Mine Roof Reinforcement (AMCMRR) has been developed. AMCMRR utilizes a Factor of Safety (FOS) approach, which is commonly used in all forms of engineering. The starting point in the development of AMCMRR was an existing analytical roof behavior and roof support design methodology/model, originally developed by Colwell. This technique was used successfully in the Australian underground coal industry for roof support evaluation and design prior to and during the course of the research project, and when used, it was essentially calibrated on a site by site basis.It has long been recognized that bolts and longer tendons can modify the behavior and load bearing capacity of the reinforced roof via the concept of beam building. The break-through in developing AMCMRR was combining the original analytical model with a comprehensive database (associated with Australian collieries) to effectively quantify this reinforcing effect, and this in turn provided the ""platform"" by which this analytical model can be calibrated for the entire Australian underground coal industry.This paper focuses on the application and use of AMCMRR and the analyses undertaken to quantify the reinforcement beam building offers to the immediate roof. Furthermore, this paper demonstrates that a combined empirical and analytical approach is currently the most practical way of developing credible geotechnical design tools for the Australian or any other underground coal industry.BACKGROUNDBy the start of 2006 the ALTS (Analysis of Longwall Tailgate Serviceability-Colwell et al., 2003) and ADRS (Analysis and Design of Rib Support-Colwell, 2006) design methodologies were being routinely and widely used within the Australian underground coal industry. As a result of their success, Colwell Geotechnical Services commenced a research project entitled, ""The Future Development and Integration of ALTS & ADRS for Improved Underground Roadway Design. "" The project (which became known as the ALTS 2006 Project) was funded directly by several of the major coal producers as well as individual Australian collieries."

Jan 1, 2010

By Jimin Su, Enguang Zhu, Kangning Zhang, Yang Li

"The effect of interburden on overlying strata movement during the mining of a lower group coal seam is still a challenge in China. Based on the multiple coal seams in western China, numerical simulation and field tests are applied to study the interburden and overburden movement during the undermining process. This study focuses on the impact of the thickness and structure of the interburden, the proportion of hard rock, and the ratio of the overburden thickness to interburden thickness (OB/IB). In this paper, four typical underground coal mines are simulated for lower seam longwall mining with different interburden parameters. The results indicate that the ratio of the overburden thickness to the interburden thickness is not the dominant actor for multiple seam longwall mining, especially in the mines in western China,which feature shallow depth. The thick and hard interburden could strongly affect the lower coal seam mining process. INTRODUCTION The project background is based on four typical underground coal mines in western China.The information on the overburden and interburden is shown in Table 1. Different overburden to interburden thickness (OB/IB) (Ellenberger, 2009) values of the four coal mines are also studied: 6.2 in Bulianta coal mine, 7.2 in Kaida coal mine, 8.2 in Shigetai coal mine, and 9.4 in Halagou coal mine. The proportions of hard rock in the interburden of the four coal mines are also different: 0% in Kaida coal mine, 70% in Shigetai coal mine, 80% in Bulianta coal mine, and 100% in Halagou coal mine. To be specific: the thickness of interburden are listed as follows: 7m in Halagou coal mine, 6m in Kaida coal mine, 10m in Shigetai coal mine, 28m in Bulianta coal mine. The depth of the upper coal seam are: 66m in Halagou coal mine, 54m in Kaida coal mine, 79m in Shigetai coal mine, 182m in Bulianta coal mine. NUMERICAL SIMULATION UDEC 4.0 is applied in this paper to analyse the overlying strata movement and the interburden failure.(Liu, 2007) This is based on mining in the lower coal seam under the mined-out upper coal seam (Majdi, 2012, Morsy, 2006)."

Jan 1, 2017

By Edward McHugh

While less than 1% of reported accidents are associated with slope stability problems. slope failures were responsible for about 15% of all U.S. surface mine fatalities between 1995 and 2001. Small rockfalls, which may involve hand-sized rocks weighing only a few kilograms, can cause fatal injuries to workers away from the protection of large machinery. Massive highwall failures containing a million cubic meters of material or more can be fatal even for heavy equipment operators. As part of an ongoing study at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health, several remote-sensing technol¬ogies are being evaluated as tools to monitor slopes for hazards and to assess slope stability. Field tests of a hyperspectral imager were conducted to assess its value for improving geologic maps of poten¬tially unstable alteration zones on mine slopes. An interferometric radar device capable of detecting very small displacements on slopes awaits final assembly and field testing. Computerized moni¬toring methods using images from digital and video cameras are being assessed for application to mine slope surveillance. These and other techniques will eventually provide new tools to augment current methods for monitoring ground control hazards in mines.

Jan 1, 2002

Results of direct shear tests on rock discontinuities and triaxial compression tests on intact rocks are presented and - discussed. The main parameters considered area stress functions, angle of internal friction and cohesion variations. Primary objective of this work is to determine relationships that adequately describe the rock behaviour for design of mining excavations, while still maintaining a certain degree of simplicity.

Jan 1, 1987

By Brian Daniels, James Farroni

"Draw rock (small rock that forms/falls when bolting the roof and/or rib) is the cause of numerous injuries annually. These injuries result in lost work hours and low morale. The report below explains how ""Mine Safe Draw Rock Shields"" (Pans) are used in an underground operation. This report will chronicle the application of these pans in a Southern West Virginia coal mine along with testing data that measures their effectiveness. Since the introduction of these pans in 2002 the amount of reportable accidents relating to draw rock has decreased by 80%. The report below reinforces that perimeter contact is critical to the performance of these pans. In addition the strength of these products at the perimeter will determine their effectiveness. The results of the testing data outlined in this paper, along with the physical records at the study mine, confirm that the use of pans has significantly reduced draw rock falls at this facility.BACKGROUNDSupplemental support is defined as ground control that is used in addition to required support in an underground mine. Some examples of supplemental support are:• Pans• Mesh/Geogrid• Mats• Half Headers• Boards• ChannelA typical roof condition that warrants the use of pans is when loose small rock is dislodged while bolting. If the top is very scaly (loose, thin rock) additional supplemental support can be used in conjunction with pans such as mesh or geogrid (See Figure 1). Pans will provide protection where used and approximately 12 in (250 mm) out from perimeter. This is achieved by contact with a surface which will put scale in tension up to 12 in (250 mm) from perimeter of pan.INTRODUCTIONThe ease of applying a roof support system is critical to effectively operating a deep mine. Figure 2 shows how a pan is installed in conjunction with a roof bolt and bearing plate. The Mine Safety and Health Administration (MSHA) requires that supplemental support cannot decrease the performance of required support. The goal of using all supplemental support is to provide additional safety for the miner which equates to increased productivity.The ability of skin control products to contact a surface is the single most critical characteristic (Figure 3 and Figure 4). If the material does not contact the surface it cannot be effective. Therefore, the flexibility, design and proper installation of the products are critical in determining their effectiveness. Figure 5 shows the proper installation of the roof pan.To reiterate, all skin control must be applied in conjunction with a roof bolt and ASTM approved bearing plate as shown in Figure 4."

Jan 1, 2010

By J. R. Koehler

The failure of yield pillar-based gate mad designs to provide adequate ground control performance is primarily related to the use of "critically" sized chain pillars. A "critical" pillar is one that falls into a range of pillar sizes that are too large to either yield nonviolently or yield before the roof and floor sustain permanent damage, but are too small to support full longwall abutment loads. To directly compare the in-mine performance of critical and yielding pillar designs, the U.S. Bureau of Mines recently completed a field study in a tapering gate road at the Sunnyside No. 1 Mine, Sunnyside, UT. Extreme pillar stresses and associated coal bumps characterize the response to first panel mining of a 16.8-m-wide critical design. Significantly lower pillar stresses, early yielding of the pillar and adjacent panel rib, and an absence of coal bumps suggest that a narrower 12.2-m-wide design more closely approaches proper yield pillar dimensions. Probehole drilling of several 10.6-m-wide pillars revealed low stress levels and substantial pillar and panel rib yielding prior to abutment onset. suggesting a properly functioning yield pillar design.

Jan 1, 1995

By David Hill

"End of panel longwall face recovery is almost certainly the most involved recurring geotechnical problem faced by operators, with major loss potential should problems occur. The paper presents Australian experiences and updates the outcomes of an industry sponsored research project on the geotechnical issues associated with conventional longwall recoveries.A number of critical features of the geotechnical environment, support design and mining geometry have a pronounced impact on ground control during face recovery. A model of roof behaviour at the take-off point has been developed and validated. Key geotechnical issues include low roof competency, an adverse weighting environment, geological structure, horizontal stress concentrations at the gate ends and the capacity and. integrity of the shields. All of these are identifiable either at the support design stage or, at worst, prior to the start of powered support removal, which is the critical stage of the recovery process.Key aspects of the geometry and process are the ability to maintain powered support resistance during bolt-up and recovery, the direction of shield removal, the impact of chutes and the speed of the shield removal process.The author presents case studies that illustrate these issues and the associated ground behavior.INTRODUCTIONEnd of panel face recovery is almost certainly the most involved recurring geotechnical problem faced by operators, with major loss potential should problems occur. However, unlike most geotechnical aspects of coal mining, very little research has historically been undertaken on the issue of conventional longwall recoveries. This is in marked contrast to the design of pre-driven recovery rooms, an issue which has attracted considerable research interest (Thomas, 2008). In 2006, Strata Engineering completed an Australian industry (ACARP) sponsored research project aimed at defining and minimising the geotechnical threats during longwall face recovery (Strata Engineering, 2006). The research project had the overall objective of developing guidelines for the specification of ground control strategies, so as to minimise the likely geotechnical threats relating to the safety, operational costs and production delays associated with the recovery of a longwall face."

Jan 1, 2010

By Anthony T. Iannacchione

This study, funded by the Appalachian Research Initiative for Environmental Science (ARIES), examines how mine layout, mining method, and geology interacted to allow water from the Grove No.1 mine pool to discharge into local surface waters. Federal and State regulations prohibit the discharge of acid or iron-producing waters from underground coal mines. Mining companies design barriers around their operations to prevent mine waters from discharging to the surface. The Grove No.1 represents one mine where these designs proved inadequate. Unique geologic conditions at the Grove No.1 mine pool, combined with retreat room-and-pillar mining, contributed to a discharge that required passive water treatment.

Jan 1, 2013

By Anton Sroka

The German hard coal production predominantly derives from underground multiseam mining activities. Beside common surface damages, ?inner mine damages? on underground infrastructures (i.e. main roads, cross cuts and shafts) due to mining excavations occur to an increasing extent. As these "inner mine damages" dramatically increased because of the development towards high performance longwall mining technologies, this matter raised to be a severe problem concerning mining economics. To protect durable and long lasting underground infrastructures, precautionary measures such as suitable mine layouts, adapted planning of mining operations and moderate face advances have to be considered. Some examples of "inner mine damages" recorded by means of mine surveying methods are given in this paper. An empirical-mathematical model based on the principles of mining subsidence engineering methods will be introduced. This model allows to minimize ?inner mine damages? and helps to support and optimize mining processes.

Jan 1, 1999

By Robert Trueman

This paper details the results of an assessment aimed at understanding the shield loading mechanisms associated with strata-related issues on a longwall face. Shield load cycle analysis theories developed by the authors were used to quantify the interaction between the shields and strata. Shield pressure data was back-analyzed using a version of the Longwall Visual Analysis (LVA) software that has been extended to enable efficient load cycle analysis to be undertaken. The software outputs enabled the shield strata interactions to be characterized. The results of the analyses indicated that the major cause of the roof falls was high-level periodic weighting, resulting in periodic shield overload. Exploration borehole data was analysed to identify and characterize the overburden unit that was the likely cause of the shield overloading.

Jan 1, 2011

By Liu Xiao, Lin Xiaoying, Ma Geng, Tao Yunqi

"The soft coal in China has several issues, low permeability, high gas content and prone to gas outburst, making it difficult to drill and improve its permeability. Using the rock mass loading test, the relationship of stress-strain-permeability-time of sandstone was investigated. The fracture network that communicates with coal seams to provide the pathways for gas migration can be established by drilling, hydraulic fracturing or loose blasting in the roof and floor about 5m from coal seams top and bottom. The gas migration can be summed up as ""desorption-diffusion-two levels of seepage"" for gas drainage of the quasi-protection-layer in hard coal seams, and ""desorption-two levels of diffusion-seepage"" in soft coal seams. In the quasi-protection-layer, different gas drainage techniques were used during different stages of mining. Gas drainage of the quasi-protection-layers was demonstrated at Hebi Zhongtai Mining Co., Ltd. The results showed that the technique promotes the gas seepage flow, and improves the efficiency of drainage. The influence zone of the quasi-protection-layer was 1601 m2 and the average amount of gas drainage was 328m3/d, with the maximum being 661m3/d. INTRODUCTIONGas is one of the main factors that cause disasters and influence safety production in underground coal mines (SACMS, 2009). The coal-bearing formations in China have been subjected to multiphase tectonic stress field during the period of geological evolution. Coal body was often crushed into fragmented coals and mylonitic coals under the tectonic stresses (Su and Lin, 2009). The gas permeability of coal is generally poor. So in underground, drilling and hydro-fracturing technology have been used to improve the permeability of coal, such as intensive drilling, hydraulic punching, hydraulic cutting, hydraulic extrusion and deep hole blasting. But the gas drainage rate was low, only 23% (Liu, Li, and Li, 2006; Li and Wei, 2006; Henan Polytechnic University, 2008; Ma, et al., 2014; Lin and Wu, 2009; Zhou, et al., 2011) that was much lower than the rates achieved in the main coal-producing countries such as America and Australia whose average rate of gas drainage was 50%. The protection layer mining technology has been used widely as an effective technique to improve the permeability of coal in the mining of multiple coal seams (Cheng and Yu, 2003; Diaz and Gonzalez, 2007; Karacan, et al., 2007; Palchik, 2003; Latham, et al., 2013; Yuan, et al., 2013; Shi and Liu, 2007; Wang, et al., 2008, Guo, et al., 2015; Lu, et al., 2015; Slazak, Obracaj, and Swolkien, 2014 ). In the single low permeability coal seam, there is no protection layer. This kind of coal seam has low permeability, high gas content, soft coal body, and is prone to serious gas outbursts. During drilling, the holes very often spray, collapse, stuck and wedged the drilling tool. The hole length drilled is difficult to reach the required minimum, the service life of holes for gas drainage is short, the construction cost is high, and the holes are difficult to maintain. The gas drainage efficiency of single low permeability coal seam, especially soft coal seams, cannot be improved even though hydro-fracture technology has been implemented, because its permeability is not improved."

Jan 1, 2016

By S. S. Peng

In recent years many roof falls have been conveniently attributed to the adverse existence of a high horizontal stress. The normal practice of not conducting a follow-up study in a roof fall investigation usually leads to a wrong conclusion. This paper describes two cases of massive oriented roof falls in two separate room and pillar mines. The investigation in each case involved the analysis of past history and current conditions from which remedial measures were recommended and implemented. A follow-up period of up to three years was performed to evaluate the effectiveness of the recommended remedial measures. In both cases, high horizontal stress was proven positively not the cause, contrary to the original conviction of mine management Rather, a weak immediate roof was the basic cause Under a weak immediate roof, a proper roof bolting plan and adequate entry width and intersection span must be strictly observed.

Jan 1, 1999