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By Yuanping Cheng, Jingyu Jiang, Qiang Zhang

"With the increase of mining depths, deep-seated coal seams face threats from high ground stresses, high gas pressures, high gas content, and low seam permeability, which significantly increase the likelihood of coal and gas outbursts. Challenges continue to grow in coalbed methane (CBM) extraction fields, especially for short-distance gassy multiple-seams. Research has proved that the most effective way to increase coal seam permeability is to decrease the ground stress. Protective seam mining technology is a surrounding rock control technology, which can effectively reduce the ground stress and increase the permeability of the protected coal seam. In this paper, we try to use upper protective seam mining technology to simultaneously extract coal and gas. Findings are based on a FLAC3D simulation of stress variation during the mining of the No. 12 coal seam in the Daxing coal mine, Tiefa Coalfield, China. Results indicate that, after mining the upper protective seam, the pressure-relief effects of the rock mass generated three belts and five zones below the protective seam, providing the space and time for gas pre-extraction engineering. Within the protected area, the numerical simulation indicates that the stress of the No. 13/14 coal seam reduces 95.84% and 88.25% respectively, while the expansion deformation of the protective seams reach 6.87 ‰ and 6.70 ‰ respectively, which demonstrate that the protective mining technology can provide a significant pressure-relief effect to the outburst-prone coal seam. Combining stress relief in this manner with pressure-relief from engineered gas drainage (performed by crossing boreholes from the floor rock roadway) applied before the mining of the protective seam, it would not only be beneficial to CBM drainage but also can reduce the gas outburst risk of the protected seams.INTRODUCTION Internationally, since France used protective seam mining technology to control coal and gas outburst in 1939, it has been promoted and applied in many countries (?????, 1966). Since 1958, China has taken the mining test and engineering practice of protective seam mining technology in the Beipiao, Tianfu, Nantong, Zhongliangshan, Songzao, Huainan, Huaibei, and other coalfields (Yu, 1992). Long-term theoretical research and mining practice of outbursts and dangerous coal seams show that protective seam mining technology is the most effective, safest, and most economical outburst prevention measure (Cheng, Fu, and Yu, 2009; Hu et al., 2008; Cheng and Yu, 2007). Under the condition of high gas content coal seams, the safety and efficiency of simultaneous extraction of coal and gas can be realized through a reasonable mining sequence and an effective gas extraction method (Cheng and Yu, 2003). The in situ stress distribution of the protective seam (No. 1 coal seam) and protected seam (No. 2 coal seam) during protective seam mining in the Haishiwan coalfield formation (Figure 1) (Li et al., 2014). A comparison of the stress and displacement (five zones and three belts) of the rock mass below the protective seam, and in a multiple-seam context in the Dalong Mine, is shown in Figure 2 (Jiang et al., 2015). After mining protection coal in a close-distance seam group, the small spacing between the coal seams causes the protective coal and the protected coal between part of the rock to crack through each other until the protective layer of gob is through the area. This provides a desorption of gas to the protective layer of the mining space channel, making the coal seam permeability increase to several hundred times and improving the gas discharge capacity (Zhai, 2008)."

Jan 1, 2017

By B. Macdonald

In 1991, roofbolts were introduced as a primary support for inseam retreat longwall entries at CBDC's Phalen Colliery. Increasing mine depth (+550 m) necessitated replacing conventional inseam steel sets in these entries because they were being severely damaged during wall retreat and subsequently causing gate end problems. Roofbolts had been used in the Sydney coalfield between 1949 and 1963 in room and pillar working, however, some concerns arose with using roofbolts as a primary support in long (2.2 - 3.4 km) single entry roadways. In order to alleviate these concerns, a phased approach was adopted whereby conventional steel densities remained along with roofbolts. The roofbolt design was such that the roofbolts would be the primary support and the steel sets would experience little or no loading. A detailed geotechnical monitoring program was developed by both CBDC and the Cape Breton Coal Research Laboratories technical staffs. The results of this monitoring were used to build-up confidence in roofbolts and subsequently reduce the steel set densities until eventually only roofbolts were used as a primary support. This paper will discuss briefly the phased introduction of rootbolts at Phalen Colliery and more specifically the development of geotechnical data collection and interpretation.

Jan 1, 1994

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 Dennis R. Barratt

Rockbolting is now used extensively as a method of primary support in UK coalmines. Over the past fifteen years an optimum bolting system has evolved, based on a high grade steel bolt and a very strong stiff resin. Where necessary, cablebolts are used in conjunction with rockbolts as a supplementary means of support. The rockbolting system, as currently used, is based on a maximum bolt length of 2.4 m whilst cablebolts are generally 6 or 8 m long. Based on the UK experience, there are situations where it is necessary to extend the bolted horizon above that normally achieved with a 2.4 m long rockbolt. Until recently the only options available were coupled rockbolts or cablebolts, both of which have inherent problems in terms of installation. Flexible Strand Rockbolts (FSR), in lengths of 4 to 6 m, provide the means of being able to extend the normal bolted horizon Other advantages include increased strength characteristics (tensile and shear), able to be installed in the UK standard hole size (26-28 mm), encapsulation using resin capsules, thus offering immediate support, high bond strength compared to conventional strand, and able to be installed using existing drilling and bolting equipment. The FSR is considered to be a new concept in terms of strata control and one that compliments the rockbolting system currently in use.

Jan 1, 1997

By Gregory Molinda

Extremely difficult ground conditions were encountered in the main belt entry in a room and pillar mine in West Virginia. Several generations of supplemental support, including cable bolts, cribbing, and Heintzmann jacks and beams failed to halt roof falls that threatened workers safety and the life of the mine. Polyurethane (PUR) was injected into 27 intersections in the roof and the results were monitored by borehole video camera. The mine had averaged 2-3 falls in the beltway per year and, since the roof injection, no roof falls have occurred. Pre- and post injection monitoring of test holes showed that stability can be achieved without filling all of the existing void spaces. Video logging showed that the project goal of building a beam from 2-6 ft into the roof was successful. Pre-injection monitoring of roof fractures uphole can delineate the zones to target for reinforcement, and avoid pumping PUR into large voids to little effect. Multiple-¬injection zones can be identified, and the entire intersection drill hole array can be optimized based on knowledge of the fractured zones. In addition, pre-monitoring can prevent excessive hydrofracturing in solid zones. Polyurethane injection is a proven method of rock stabilization in even the weakest, most broken ground. Optimization of the injection design by pre-injection video diagnostics can greatly contribute to the successful and efficient roof stabilization.

Jan 1, 2004

By Yi Luo

The efforts to mitigate longwall subsidence influences on various residential and farming structures by many coal mining companies have been successful in the recent years (Luo, et al, 2003). The successes were largely dependent on the accurate subsidence predictions, correct assessments of potential influences, appropriate design and implementation of the mitigation measures. However, such efforts for protecting large industrial structures are still very rare. In this paper, the successful assessment and mitigation of the subsidence influences associated with a very intensive dynamic longwall subsidence process on a large workshop are presented.

Jan 1, 2005

By William G. Pariseau, Michael K. McCarter

"The challenge of estimating mine-wide subsidence and linkages to seismicity over tabular deposits is addressed by a special finite element technique (dual node - dual mesh). Subsidence and mine-induced seismicity begins near the face when caving occurs and propagates to the surface as extraction reaches a critical extent. Thus, the challenge is to obtain details at the face at the meter scale and also at the surface over the whole mine at the kilometer scale. Interactions between old and new sections of a mine are automatically taken into account with this technique. The finite element method is well established technology based on fundamentals of physical laws, kinematics and material laws. With this technique, no empirical ""scaling"" or fitting computer output by input data ""adjustment"" to mine measurements is necessary. Capability is demonstrated for doing practical whole-mine subsidence analysis from first principles. Mine-induced seismicity is shown to correlate well with face advance and element failure.INTRODUCTIONSeismicity associated with coal mining has been under study for several years by a team at the University of Utah. Team members include mining faculty, research geophysicists and students from the Departments of Mining Engineering and Geology & Geophysics. Mines in central Utah that have been studied include the Trail Mountain Mine (Pariseau, 2015; Pariseau and McCarter, 2015), the Beehive Mine, also known as the Des-BeeDove (Pariseau, 2014), and the Crandall Canyon Mine (Pariseau, 2015). These mines were developed from outcrops in the Wasatch Plateau coal field west of Price, Utah, where the topography is characterized by . steeply incised canyon drainages and high plateaus with relief of a thousand meters or so.Seismicity associated with coal mining in central Utah has been of interest for many years and includes studies in the Book Cliffs to the east of Price, Utah, where mining is still active (Smith et al, 1974; Agapito, Goodrich and Moon, 1997; Arabasz et al, 2001; Arabasz et al, 2002; Fletcher and McGarr, 2005; McGarr and Fletcher 2005; Maleki, 2005) and to the northwest (Ellenberger et al, 2001). Results of many micro-seismic studies are summarized perhaps best by lannacchione, et al (2005) in stating, ""Deviations from normal strata response can provide useful stability information.""This study concerns a fourth underground coal mine in the southern portion of the Wasatch Plateau coalfield, hereafter referred to as the MINE. Focus is on mining from 2004-2008 with the objective of examining mining in relation to seismicity."

Jan 1, 2016

By Jessica M. Wempen, Michael K. McCarter

"Differential Interferometric Synthetic Aperture Radar (DlnSAR), a satellite-based remote sensing technique, has potential application for measuring mine subsidence on a regional scale with high spatial and temporal resolutions. However, the characteristics of Synthetic Aperture Radar (SAR) data and the effectiveness of DlnSAR for subsidence monitoring depend on the radar band (wavelength). This study evaluates the effectiveness of DinSAR for monitoring subsidence due to longwall mining in central Utah using L-band (24 cm wavelength) SAR data from the Advanced Land Observing Satellite (ALOS) and X-band (3 cm wavelength) SAR data from the TerraSAR-X mission.In the Wasatch Plateau region of central Utah, which is characterized by steep terrain and variable ground cover conditions, areas affected by longwall mine subsidence are identifiable using both L-band and X-band DlnSAR. Generally, using L-band data, subsidence magnitudes are measurable. Compared to X-band, L-band data are less affected by signal saturation due to large deformation gradients and by temporal decorrelation due to changes in the surface conditions over time. The L-band data tend to be stable over relatively long periods (months). Short wavelength X-band data are strongly affected by signal saturation and temporal decorrelation, but regions of subsidence are typically identifiable over short periods (days). Additionally, though subsidence magnitudes are difficult to precisely measure in the central Utah region using X-band data, they can often be reasonably estimated.INTRODUCTIONDifferential Interferometric Synthetic Aperture Radar (DlnSAR) is a satellite-based remote technique that can be used to measure surface displacement over large regions with high spatial resolution. Under good conditions, displacements can be measured with centimeter to subcentimeter accuracy (Buckley, 2000; Massonnet and Feig!, 1998). DlnSAR also has high temporal resolution, and imaging periods typically range from 10 to 50 days (Rosenqvist, Shimada, and Watanabe, 2004; Roth, 2004). In the last two decades, the application of DlnSAR for mine subsidence monitoring has been demonstrated in coal basins in Europe, Australia, China, and the United States (Camec and Delacourt, 2000; Ge, Chang, and Rizos, 2007; Ge et al., 2008; Ismaya and Donovan, 2012; Ng et al., 2010; Perski, 2000; Perski and Jura, 2003; Popiolek and Krawczyk, 2006; Wegmuller et al., 2005; Wright and Stow, 1999; Zhao et al., 2014; Xinglin et al., 2014). Overall, these studies have demonstrated good data resolution, strong relationships between mine development and subsidence, and reasonable agreement between displacements measured by DinSAR and displacements measured by conventional surveys."

Jan 1, 2016

By L. A. Sandbak, Yan Xing

"The purpose of this study is to evaluate the rock mass stability around the tunnels by three-dimensional (3-D) numerical modeling. Complex geological and tunnel features have been included in the model using the software package 3DEC™. Based on the available information on stratigraphy, geological structures, and mechanical properties of rock masses and discontinuities from the underground mine, 3-D stress analyses were performed on various cases with different K0 (lateral stress ratio) values, constitutive models, as well as support systems. Relations between these factors and the distributions of stress, displacement, and failure zones around the tunnels were obtained and discussed. Finally, comparisons between the field deformation measurements and numerical simulations are used as validations for the simulations, helping to determine the appropriate K0 value and material constitutive models for the underground mine.INTRODUCTIONThe case study site is an underground mine in the USA with a monthly ore production between 20,000 and 30,000 tons. The regional geology of the mine is structurally and stratigraphically complex. Rocks in the area include carbonaceous mudstone and limestone, tuffaceous mudstone and limestone, polylithic megaclastic debris flows, fine-grained debris flows and basalts, all part of the Cambrian-Ordovician Comus formation and related dikes. Both the mineralization and faults have general orientations striking N-S to NW-SE with high dip angles. The ore body in the mine has an incline of about 25---45 degrees located in relatively low quality rock; hence, cut-and-fill mining has been used.This underground mine has two vertical shafts used for ventilation, ore hoisting, and for personnel and material transportation. Development drifts were designed and driven to extract and transport the ore, as shown in Figure I. Bolts, mesh, and shotcrete have been applied in this mine for ground control. In order to reach various locations, the tunnel system is designed with different heading directions and inclines, which will be described in detail later. The area selected for modeling is a cubic region including all the tunnels inside the black square (Figure 1)."

Jan 1, 2016

By Meng Xiao, Peng Huang, Lizin Lan, Feng Ju, Shuai Guo

"Through changing the axial load on backfilling material compaction test to reflect different overlying strata pressure on backfilling material, the stress-strain relations in the compaction process of backfilling material under the geological condition can be obtained. Based on the characteristic of overlying strata movement in backfill mining, a model of roof thin plate is established. By introducing the stress-strain relation in compaction process into the model and using RIZT method to analyze the bending deformation of roof, the bending deflection and stress distribution can be obtained. The results show that the maximum roof subsidence and maximum tensile stress occurring at the center are 255mm and 5 MPa, respectively. Tensile fracture of roof under the geological condition of Dongping mine did not occur. The dynamic measurement results of roof in Dongping mine verify the theoretical result from the aforementioned model, thereby suggesting the roof mechanical model is reliable. The roof thin plate model based on the compaction characteristic of backfilling material in this study is of importance to research on backfill mining theories and application of backfilling material characteristics.INTRODUCTIONSolid backfilling mining (SBM) is a green mining technology in which the solid waste materials is placed into the gob to support the overlying strata and to control roof's subsidence and movement (Miao, 2012; Miao et al., 2010, 2015). SBM has been successfully used in several mines to solve many problems, including coal extraction under buildings, water bodies and railways, surface subsidence, and environmental problems. Good results have been obtained in many mines (Zhang et al., 2009). The key in the application of SBM technology is controlling strata movement. However, the critical factor influencing the controlling effect of strata movement is compaction characteristics of backfilling materials. Thus, a roof model of SBM is built based on the filling materials' compaction characteristics. A subsidence equation and the critical failure condition of roof was given and field verification was performed at Panel 15061 of Donping Mine."

Jan 1, 2016

By Dr. Daniel W. H. Su

t is both an honor and privilege for me to nominate Dr. Daniel W. H. Su for the inaugural Syd S. Peng Ground Control in Mining Award. I can think of no better candidate than Daniel that is deserving of this award ? an award to recognize technical excellence in advancing ground control technologies [and] approaches. Daniel?s education and career spans over three decades. Having graduated from National Cheng Kung University in Taiwan in 1972 with a B.S. degree in Mining and Petroleum Engineering, he spent the next four years in the Taiwanese military and mining industry, before moving to the United States, where he enrolled in West Virginia University?s Mining Engineering Department. He received his M.S degree in Mining Engineering from WVU in 1978; then in 1982 became the first person to ever receive a Ph.D. in Mining from WVU. His thesis was titled, ?Development of Ultrasonic Techniques for the Measurement of In Situ Stresses?.

Jan 1, 2006

By Jun Lu, Greg Hasenfus

"In this paper, the mining experience and challenges for the first right-handed longwall panel in the Pittsburgh seam are introduced. The longwall headgate T-junction experienced very high face convergence (up to 2 feet), accompanied by roof sag, floor heave, and rib loading. The headgate convergence was so large that, in a few places, it threatened longwall retreat and ultimately required the bottom to be re-graded.Different underground instruments, such as a roof scope, de-gas drill, tell-tale, laser meter, and Borehole Pressure Cell (BPC), were employed to explore the roof geology and to monitor the entry convergence and the stress changes in the pillar. In addition, the impact of other geologic factors, such as large overburden depth, laminated sandstone roof geology, soft floor, and large headgate equipment, were also analyzed. Subsequently, geotechnical solutions were provided to avoid or mitigate the impact of these challenging geologic factors.INTRODUCTIONIt is well known that high horizontal stress will affect the stability of roof, floor, and ribs within underground mines. In general, the headgate entries will experience stress concentration for the right-handed longwall face. On the contrary, the stress concentration will be in the tailgate entries for the left-handed longwall face (Hasenfus and Su, 2006). Without the stress shadow, the stress concentration in the headgate for the first longwall panel may be more significant (Figure 1).A mine in the CONSOL Pennsylvania Operation was extracting the first right-handed longwall panel in the Pittsburgh seam, which created high horizontal stress concentrations at the longwall headgate. Because of concern over soft wet floor within the syncline, floor coal had been left in place on development. Early on, upon retreat, the longwall headgate T-junction experienced very high face convergence, accompanied by roof sag, floor heave, and rib loading. The floor heave was exacerbated by the presence of floor coal, which buckled upward near the center of the beltline. Roof movement was occasionally aggravated by laminated roof conditions, but was kept in check by aggressive supplemental support including cable bolts, yielding props, cribbing, and roof mesh. Rib bolting and rib mesh helped to further maintain the entryway stability by keeping sloughage at a minimum. Nevertheless, despite the heavy support, headgate convergence was so large that, in a few places, it threatened longwall retreat and ultimately required the bottom to be re-graded."

Jan 1, 2018

By Behdeen Oraee

Constructing tunnels underground is generally described as a high-risk activity because conditions in the surrounding area tend to be unpredictable. This would and can create unique dangers in every situation. It is important to gain a deep understanding of the risks associated with tunneling in order to reduce the likelihood of their occurrence and severity of their consequences should they occur. For this to be achieved, different risks should first be evaluated and ranked according to their relative importance and criticality. The Amir Kabir tunnel is considered one of the most dangerous tunnels in Tehran, Iran, in terms of the risks involved in its excavation and their potential consequences. In particular, part T4 of the tunnel passes through a zone of different strata and also a compound of various soils. This paper studies part T4 of the Amir Kabir tunnel from a risk analysis and risk management point of view. In doing so, factors increasing total cost, causing delay in the project completion time, decreasing the operation rate and downgrading project quality were identified and ranked according to their importance using the aggregate primary index risk method. For this purpose, a comprehensive questionnaire was designed and completed by tunneling experts. By responding to the questionnaires, the experts determined the relative likelihood of occurrence of the studied factors and their potential consequences. As a result, the experts suggested several alternatives to reduce the risks involved in this particular study. Moreover, the experts completed new questionnaires whilst taking into account different alternatives. In the next step, the risks before and after applying the alternatives were compared. As a result of performing the said analyses, it was concluded first that the aggregate primary index risk is an effective tool in identifying the risks involved in such projects. Second, it was concluded that, by taking into account different alternatives when analyzing and ranking the risks, severity of the said risks can potentially be reduced. Also, by considering such matters when analyzing tunneling risks, the projects? efficiency can greatly increase as a result of reducing risks that threaten financial and human resources. The approach introduced in this paper, together with the methodology described, can be adopted by the mining design engineer in all similar situations. They will be of particular advantage in such methods as room and pillar mining, where the number of similar tunnels required is high.

Jan 1, 2014

By Sahendra Ram, Arnn Kr Singh, Dheeraj Kumar, Rajendra Singh

"Inherent existence of different rooms (openings) along the extraction line in a depillaring panel provides an easy path for the goaf to encroach the working area. Each of these openings in a mechanized depillaring (MD) operation along the extraction line is supported by the roof bolt-based breaker line support (RBBLS) to restrict the goaf encroachment. Different field studies by Council for Scientific and Industrial Research - Central Institute of Mining and Fuel Research (CSIR-CIMFR) found that the RBBLS works effectively during the depillaring under the shadow of stable surrounding ribs/fenders only. It is observed that, generally, these natural supports experience high value of induced stresses after a large amount of void creation inside the goaf. The induced stress creates spalling/loosening of the sides of the natural supports. Field studies also found that the positions of the RBBLS at the goaf edge need to be adjusted according to the depth of this spalling/ loosening. However, a systematic study in field for the correct positioning of the breaker line as per the spalling/loosening under different site conditions is difficult. Therefore, on the basis of the data and guidance provided by these field investigations, a detailed parametric investigation was conducted in the laboratory on simulated models for an effective design of RBBLS in a MD panel. FLAC3D is used for the simulation of the depillaring, and the available empirical formulations for Indian coalfields are used for subsequent calibration. An analysis of stress redistribution for different stages of the MD in simulated models showed the significance of the nature of overlying strata/induced stress for the RBBLS design. Discussing some results of field studies, this paper presents results of the laboratory investigations for an effective design of the RBBLS.INTRODUCTIONContinuous Miner (CM)-based mechanized depillaring (MD) is an important issue for Indian coalfields, where huge amounts of coal are locked up in standing pillars (Dixit and Mishra, 2010). Considerable variations in site conditions of these developed pillars bring a number of technical challenges into consideration during their final extraction. One of these challenges is goaf encroachment/overriding during caving of the competent roof strata. Goaf encroachment/overriding can effectively be controlled, mainly, by suitable design of natural supports (rib/snook/fender). However, there is a requirement of applied supports against the immediate roof as a considerable portion of this roof is exposed due to the presence of different openings along the line of extraction. These applied supports act as a fulcrum at the goaf edge between the working area and the goaf to facilitate the break out of the hanging roof inside the goaf. This is called breaker line support. An effective breaker line support is important for the efficiency of a depillaring operation. Generally, MD adopts roof bolt-based breaker line support (RBBLS) at the goaf edge (Singh, et al., 2014). For the first time in India, roof bolts were applied at the goaf edge as RBBLS in a CM-based MD (Leeming, 2003). In the beginning, three rows of roof bolts were installed as primary and secondary breaker lines support. On the basis of the observations of instrumented bolts, only two rows of roof bolts were used as secondary RBBLS (Ram, et al., 2014). Now, most of the MD operations in India are using only two rows of bolts as RBBLS (Table 1). However, the strata mechanics studies at different MD operations showed interesting characteristics resulting from this support (Ram, et al., 2015)."

Jan 1, 2016

By Adam Mackenzie, Dan Payne, Yass-Marie Rutty

"Historically, the Crinum mine has experienced significant falls of ground when longwall production slowed to prepare for recovery in weak roof areas. These conditions continue through the recovery process and result in both safety concerns and delays. When mine plans and exploration revealed that most of the mine's future longwall recoveries were located in weak roof areas, a decision was made to try pre-driven recovery roads as a solution to the problem. After completing eight pre-driven recovery roads with varying degrees of success and numerous lessons, the Crinum North mine now utilizes a modified Pre-driven Recovery Roadway (PDRR) to improve the longwall take-off process in weak roof areas. During mine development, a standard roadway is driven where the final recovery location of each longwall is planned. After the installation of secondary support, the PDRR is backfilled with a cement-flyash mix to provide support to the roof, and confinement to the ribs and floor of the roadway. The method has been refined over the last four years to provide greater strata stability and improved operational and safety performance compared with conventional take-offs at Crinum, and has resulted in a site record of longwall relocation in 11.5 days (pull mesh to picks in coal). This paper describes the evolution of the PDRRs from Crinum East to Crinum North, including lessons from initial attempts and changes to the secondary support regime, the operational approach during the final stages of retreat, the backfill strategy and plans for the future.INTRODUCTIONThe Crinum mine is the underground component of BHP Billiton Mitsubishi Alliance's (BMA) Gregory Crinum mine, located northeast of Emerald, Queensland (Figure 1).The Crinum mine has a history of roof control problems coming into and during longwall recoveries. Weak roof, combined with the slow mining, which is a consequence of preparing for equipment recovery, pulling mesh (which also creates tip-to-face issues), and the lengthy bolt-up process, often results in loss of immediate roof and subsequent roof falls. In fact, approximately 50% of the first 13 longwall recoveries (which were at depths of 180 to 200m) experienced roof fall delays, the longest of which took four months to mine the last pillar and recover the longwall. This recurring problem prompted the mine to investigate options to reduce or eliminate the problem."

Jan 1, 2016

By Peter Zhang, Daniel Su, Essie Esterhuizen, Mark Van Dyke, Berk Tulu, Dave Gearhart

"Roof falls in longwall headgate can occur when weak roof and high horizontal stress are present. To prevent roof falls in the headgate under high horizontal stress, it is important to understand the ground response to high horizontal stress in the longwall headgate and the requirements for supplemental roof support. In this study, a longwall headgate under high horizontal stress was instrumented to monitor stress change in the pillars, deformations in the roof, and load in the cable bolts. The conditions in the headgate were monitored for about six months as the longwall face passed by the instrumented site. The roof behavior in the headgate near the face was carefully observed during longwall retreat. Numerical modeling was performed to correlate the modeling results with underground observation and instrumentation data and to quantify the effect of high horizontal stress on roof stability in the longwall headgate. This paper discusses roof support requirements in the longwall headgate under high horizontal stress in regard to the pattern of supplemental cable bolts and the critical locations where additional supplemental support is necessary.INTRODUCTIONLongwall mining is the primary underground coal mining method in the United States and currently accounts for more than 60% of the underground coal production. The longwall headgate, as a passageway for the longwall crew, intake air, material supplies, and coal belt transportation, is critical for both safety and continuous production of the longwall panel. A roof fall in the longwall headgate would not only result in substantial interruption of production but could potentially cause injuries or fatalities. Rehabilitation of failed roof in the headgate would also expose miners to the risk of injuries. Roof falls in the headgate, though infrequent, mostly occur in the belt entry near the face. Considerable research has been conducted to determine the effects of various factors on headgate roof stability, as well as effective measures to support the roof in longwall mines in the Pittsburgh seam (Mark, Mucho, and Dolinar, 1998; Chen et al., 1998; Su and Hasenfus, 1995; Su, Thomas, and Hasenfus, 1999; Su, Morris, and McCaffrey, 2002; Su, Draskovich, and Thomas, 2003; Hasenfus and Su, 2006; Van Dyke et al., 2015). Previous studies have demonstrated that high horizontal stress and transitional roof geology are primary factors that cause headgate roof failure. These studies also showed that panel orientation, retreat direction, pillar sizes, and roof support played an important role in the stability of a headgate.In underground coal mines located in the eastern United States, the magnitude of the maximum horizontal stress is typically three times greater than the vertical stress, and about 40% greater than the minimum horizontal stress. Mark, Mucho, and Dolinar (1998) studied seven cases of headgate failures caused by high horizontal stress in different coal seams in the United States and stated that roof stability is affected, to a large extent, by rock type, entry orientation, and longwall orientation. The effects of horizontal stress can be summarized in these statements: (1) A laminated roof is very vulnerable to high horizontal stress. (2) Entries that are aligned with the maximum horizontal stress will suffer less damage on development than those perpendicular to it. (3) Horizontal stress concentration and relief depends on panel orientation, the direction of retreat, and the sequence of longwall panel extraction."

Jan 1, 2018

By Brent A. Slaker, Khaled M. Mohamed

"Understanding coal mine rib behavior is important for inferring pillar loading conditions as well as ensuring the safety of miners who are regularly exposed to ribs. Due to the variability in the geometry of underground openings and ground behavior, point measurements often fail to capture the true movement of mine workings. Photogrammetry is a potentially fast, cheap, and precise supplemental measurement tool in comparison to extensometers, tape measures, or laser range meters, but its application in underground coal has been limited. The practical use of photogrammetry was tested at the Safety Research Coal Mine, National Institute for Occupational Safety and Health (NIOSH). A commercially available, Digital Single-Lens Reflex (DSLR) camera was used to perform the photogrammetric surveys for the experiment. Several experiments were performed using different lighting conditions, distances to subject, camera settings, and photograph overlaps, with results summarized as follows: The lighting method was found to be insignificant if the scene was appropriately illuminated. It was found that the distance to the subject has a minimal impact on result accuracy, and that camera settings have a significant impact on the photogrammetric quality of images. An increasing photograph resolution was preferable when measuring plane orientations; otherwise a high point cloud density would likely be excessive. Focal Ratio (F-stop) changes affect the depth of field and image quality in situations where multiple angles are necessary to survey cleat orientations. Photograph overlap is very important to proper three-dimensional reconstruction, and at least 60% overlap between photograph pairs is ideal to avoid unnecessary post-processing. The suggestions and guidelines proposed are designed to increase the quality of photogrammetry inputs and outputs as well as minimize processing time, and serve as a starting point for an underground coal photogrammetry study.INTRODUCTIONThe stability of underground mine ribs has been a safety concern for underground coal mines for decades. Unfortunately, the average rib fatality rate has been about 1.3 per year between 1996 and 2013 (Jones et al., 2014). In an effort to better understand coal rib behavior, photogrammetry is being investigated in this study as a measurement tool for surveying ribs in an underground coal mine environment."

Jan 1, 2016

By Q. F. Wang

Spontaneous combustion hazard of loose coal in the top-coal caving region of entry affects the safety of the entry and top- caving longwall face seriously. In this paper, by combining on- site temperature tests with lab tests and numerical simulation, the characteristics of microcirculation close to the top-coal caving region are analyzed. A model for the temperature fields of the microcirculation in loose coal is presented. Based on this model, the temperature distribution and possible self-ignition region can be predicted. Further, the heat and mass transfers in microcirculation are studied. Finally, the mechanism of oxygenation during the spontaneous combustion of microcirculation in loose coal of the top-coal caving region of entries is put forward. The research results in this papa may provide ideas for the prevention and putting out of the fire caused by spontaneous combustion in the caving zone.

Jan 1, 2004

By Shuren Wang, Wenbing Guo, Paul C. Hagan, Yanhai Zhao

"The overlying strata is often destroyed in large scale during shallow coal seam mining, and the sliding instability of the caved roof seriously threatens the safety of the mining field. The typical shallow coal mining in Shendong Mining Area in China was used as engineering background; the load distribution characteristics of the overlying strata were analyzed based on the monitoring data of the support resistance. Through theoretical analysis and numerical simulation, the formation and changing process of the pressure-arch in the overlying strata were studied, and the instability mechanism of the overlying strata was revealed by focusing on the accumulation and release law of the elastic energy in the pressure-arch. The results show that the continuous pressure-arch was formed when the horizontal stress exceeded the primary vertical stress of the mining field, and the elastic energy of the roof was released by the mining unloading effect. The elastic energy was accumulated in the pressure-arch, and the energy arrived the highest at the front arch foot. The accumulated energy at the arch foot was released by coal mining, and the shear zone could be formed. Therefore, the sliding of the caved zone along the shear zone induced strong roof weighting. The concentrated stress and the released energy during each mining increased with the panel advancing, and the height of the shear zone also increased. The conclusions obtained in the study are of important theoretical value to direct similar engineering practice.INTRODUCTIONThe instability of overlying strata during shallow coal mining, such as the large-scale roof falling and step-like ground subsidence, is a key problem that can restrict the safety mining in the mines (Ju and Xu, 2015). The self-bearing structure of the pressure-arch can form in the overlying strata after coal mining, and this structure can support the load of the upper strata and soil layer, so the weighting intensity of the panel is determined by the caved rock in the unloading zone under the inner boundary of the pressure-arch. A large amount of elastic energy is accumulated in the pressure-arch under the concentrated stress, and the released energy for mining is the internal cause of rock failure (Wang et al., 2017). It is an important problem to reveal the distribution characteristics of the stress and energy fields in the mine and to analyze the stability of the overlying strata during shallow coal mining based on the evolution characteristics of the pressure-arch."

Jan 1, 2018

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