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By Morgan M. Sears, Gamal Rashed, Jim Winfield, Brent Slaker

"Over the past decade, ground falls have resulted in five fatalities, representing over 40% of all fatalities occurring in underground stone mines in the United States. In conjunction with recent high-profile pillar collapses, the need for further ground control research in the nation’s stone mines is evident. Ground conditions are growing more difficult. Therefore, this research is focused on deep cover, steeply dipping, and multi-level underground nonmetal mines. The objective of this paper is to introduce a geomechanical study of pillar development in a steeply dipping limestone deposit. To accomplish this, a numerical model was developed using input from modified laboratory test data and in situ stress measurements provided by the mine to simulate the development of a pillar under these conditions. The model was then verified against field observations and stress mapping. The calibrated model was then used to calculate the estimated stresses in the pillar and immediate roof as development progresses around an instrumented pillar.INTRODUCTIONGround falls represent a significant hazard in the nation’s underground stone mines. In response to recent large-scale pillar collapses, as well as stakeholder interactions, researchers from the National Institute for Occupational Safety and Health (NIOSH) are currently conducting detailed investigations into the complex loading conditions at mines operating in challenging operational environments. These include deep-cover, steeply dipping, and multi-level mines where there is the potential for an increased risk of ground failure. The Pleasant Gap Mine, shown in Figure 1, operates in the Valentine Formation, which is typically light grey, extremely fine-grained limestone, and approximately 21.3 m (70 ft.) thick. The Centre Hall Formation makes up the immediate and main roof members and is approximately 9.1 m (30 ft.) thick, consisting of medium-dark-gray limestone (Murphy et al., 2018). Situated in a syncline, the top of the Valentine Formation dips at approximately 19° with a strike of approximately N54E in the area of interest (see Figure 2)."

Jan 1, 2018

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 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 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 Kun Li, Shuaishua Yue, Huigui Li, Yuan Ruifu, Yongxiang Xu, Huamin Li

"Coal seams in Tashan Mine of Datong Coal Group in China average 49.2 ft (15m) thick and have been mined by the top coal caving longwall mining method of large mining height. Mining height was 12.5 ft (3.8m) and the top coal caving height was 36.7 ft (11.2m). The gateroad pillar between panels was 124.6 ft (38m). During retreat mining, serious bumps occurred in the gateroads on both sides of the pillar affecting safety production. Therefore, pillarless mining was experimented.Using numerical modeling and comparative study of cases of similar mining condition, it was decided to employ a 19.7 ft (6m) wide pillar, rather than the previous 124.6 ft (38m) wide pillar. Support system for the gateroads was designed and implemented. During gateroad development, pillar failure conditions and entry deformation were monitored. Hydraulic fracturing method was employed to cut off the K3 sandstone along the entry rib so as to reduce the abutment pressure induced during retreat mining. Support reinforcement method combining grouting and advanced reinforcement methods was proposed to insure stable gateroad ahead of mining. Methane drainage and nitrogen injection were implemented to eliminate hazards associated with mine fire and spontaneous combustion.Since the development of gateroad has just completed, and retreat mining has not begun, the effectiveness of the proposed methods is unknown at this point. However, monitoring will continue until after mining. The results will be published in a separate paper.INTRODUCTIONGeneral descriptionTashan mine is located about 15 Km south of the city of Datong, Shanxi province, China (aure 1). The 2014 raw coal production was 25 million mt."

Jan 1, 2015

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 Naj Aziz, Peter Craig

"Double shearing tests were carried out on the 28 mm (1 in) hollow strand Jennmar TG cable bolt to gain a better understanding of the shearing behavior of the cable. The first test was limited to a 50 mm (2 in) travel on the testing machine and produced a shear load of 900 kN (92 t) at the maximum 50 mm (2 in) displacement. The axial load generated on the cable bolt was 238 kN (24.3 t). In the second test, the machine travel was increased to 75 mm (3 in), and first strand failure of the cable, due to shear loading, occurred at 1,354 kN (138 t) vertical load at a vertical displacement of 59 mm (2.3 in). The cable axial load was in the order of 385 kN (39.3 t). Analysis of the failure mode and loads achieved showed that the cable strands bent and the concrete crushed along the shear plane; the shear loading across the concrete and grouted cable then reached the tensile strength of the steel wires.INTRODUCTIONCable bolts were introduced to the mining industry around 1970, initially to surface mining and underground metalliferous mining and then to coal mining in the early 1980s, mainly for roadway reinforcement as a secondary means of support. Cable bolts have since been used as both primary and secondary supports. As primary support, Fuller et al. (1994) described the application of cable bolts, known as FLEXIBOLT, for strata reinforcement in both Angus Place and Ellalong Collieries, in New South Wales (NSW), Australia. As secondary support, cable bolts have also been used as cable trusses, which act to support the immediate roof in a sling-like manner (Fabjanczyk and Tarrant, 1988; Fuller, et al., 1991; O'Grady and Fuller, 1992), and for reinforcement at higher stratification and beyond the rebar bolt length, mainly for anchoring lower strata layers immediately above the coal seam to the higher, competent bedding formation above. Initially cable bolt anchorage was by cementatious grouting and, since the 1990s, by chemical resin. The dominant type of cable bolts used for secondary support in Australian underground coal mines are 588 kN (60 t) capacity cables which are point anchored, pre-tensioned and post-grouted. Point anchored refers to a cable resin anchor. The cables are tensioned on the resin anchor; then the same cable is grouted by cementatious grout pumped to ·fin the annular gap around the cable from the collar of the hole to the resin anchor. Bulbs are located within the resin anchor section for mixing of the resin chemical components and efficient anchorage. The installation of these typically 8 m long cable bolts involves a lot of manual handling of the cables, lifting of heavy hydraulic tensioners, and exposure to resin grouts. Table 1 shows the specification of various cable bolts currently marketed and installed in Australian underground coal mines."

Jan 1, 2010

By Shengliyang Yang, Jinghu Yang, Jiachen Wang, Xiaomeng Li

"Safe and efficient mining of extremely inclined thick coal seams less than 20m thick is one of the new challenges in China coal industry. In recent years, the extremely inclined thick coal seams have been encountered in some old mines and western China, and currently the top coal caving longwall mining method has been successfully applied to this kind of coal seams. Accordingly, this paper studies the roof movement and its impact on shield supports of top-coal caving longwall mining in extremely inclined thick coal seams. The structure of main roof before failure was simulated by the fix-ended beam, and provided the solutions for determination of maximum compressive stress of the bottom beam end and maximum tensile stress of the top beam end. The failure mode on the upper end of working face is mainly a combination of compression and shearing, while that for the bottom end is tension and shearing. Roof fracture begins at the upper end of working and propagates downward. The shape of collapsed roof strata can be described as parabolic and divided into three segments from top to bottom, including no filling, uneven filling and densely filling segments. The roof loading conditions of no filling segment is more likely to have dynamic burst impacts.INTRODUCTIONExtremely inclined thick coal seam refers to that the dip angle of coal seam is greater than 45° and the thickness is greater than 3.5m (Wang and Zhang, 2015a; Wang and Zhang, 2015b; Wang, 2005; Xie, Gao, and Shangguan, 2005; Wu, et al., 2014; Deng and Wang, 2014; He, et al., 2015). This type of coal seams can be divided into three categories: 1. The extremely inclined thick coal seam, of which the thickness is less than 5m and the dip angle is less than 60°, is usually mined by fully-mechanized longwall mining method. 2. The extremely inclined thick coal seam, of which the thickness is 5 m-20m, is usually mined by top coal caving longwall method. And 3. When the thickness of extremely inclined thick coal seam is greater than 20m, it is mined by the horizontal section top coal caving longwall method. For example, the average angle ofWudong coal mine in Xinjiang province is 45°, and the thickness is 27m. Wudong coal mine is applying the horizontal section top coal caving longwall mining, producing 3 million tons annually per working face (Figure 1)."

Jan 1, 2016

By David R. Burkhard

"Comparative roof beam performance, as measured by delamination within the bolted horizon, was used to evaluate two types of roof bolt drill hole geometries in a development setting. Short encapsulation pull tests had demonstrated a higher grip factor in drill holes featuring helical grooves in the side walls in comparison to drill holes featuring continuous smooth to slightly rough walls. The study incorporated the two geometries during development mining in which fully grouted mechanically anchored bolts were installed as primary roof support.bolts were installed as primary roof support.The study site, in an entry adjacent to a longwall panel, was instrumented with extensometers and data collected during longwall retreat. Statistical analysis was conducted to validate null and alternative hypotheses used to compare drill hole geometries. Analysis yielded no statistically significant difference between drill hole geometries. Inspection of the data does reveal less delamination within the smooth walled site in comparison to the site containing grooved hole geometry.INTRODUCTIONFrom their introduction as a means of ground control over sixty years ago, roof bolts today are the primary mechanism of roof support utilized throughout mining. Whether through suspension, keying, or beam building, roof bolts are employed to reinforce the inherent strength of the strata comprising the immediate roof with the goal of creating a competent bolted horizon of sufficient load bearing capacity to be self-supporting for a given mine's design life, stress regime, and geomechanical properties.For grouted bolts in sedimentary strata, axial loading due to vertical rock movement and bending due to horizontal rock movement are the principal means of loading (Signer, 2000). Regardless of the type of bolt and extent of grouting employed, the success of any roof bolting design is determined, to a large extent, by the effectiveness of load transfer between the rock, grout, and bolt.At San Juan Mine, an underground longwall mine located in the northwestern corner of New Mexico, a number of different roof bolt types, grades, and lengths have been used over the years in an effort to optimize primary roof support. One common denominator shared throughout has been the use of full length grout. Load transfer for full length grout is achieved primarily through mechanical bonding between the grout and rock. It is this interaction that is the focus of the study described in this paper. The study assessed and compared two types of bolt hole geometries employed during gate road development at San Juan. Underground instrumentation and evaluation of the bolted roof provided the means of assessment and comparison."

Jan 1, 2016

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 Fernando O. L. Xavier

"The Barro Branco coal mine was located in the southern region of Brazil, in the state of Santa Catarina. In 2002, an incident occurred at the mine, leading to a violent, massive pillar failure, resulting in almost 100 failed pillars in less than 3 hours, totaling nearly 700 collapsed pillars after a period of 6 weeks. The paper outlines the geological and operational conditions before the accident and analyzes the primary factors leading up to the event. This paper also discusses and examines the unusual characteristic of violent, uncontrolled pillar bursts that occur over a longer timespan, in comparison to other equally violent but rapid events, such as those that occurred at the Crandall Canyon Mine, at a Book Cliffs District mine, and at the Solvay Mine. INTRODUCTION The southern region of Brazil holds 100% of the coal reserves for the country, and half of it is located within the state of Santa Catarina. Almost the entire Brazil’s coal production, around 8 million tonnes/year, is consumed for electricity generation. Most of the coal mines in Santa Catarina are underground room-and-pillar mines, and most are shallow mines, with depths of cover ranging from 20 to 400 m. The Barro Branco mine, near the city of Criciuma, experienced a violent mine collapse at the end of March 2003, which was analyzed and discussed by Zingano, Koppe, and Costa (2004). The event consisted of violent failure of 100 pillars in less than 3 hours, and a subsequent failure of approximately 700 additional pillars over the next 6 weeks. After the panel collapsed, the mine was closed due to unsafe conditions as a consequence of the seismic event. A map of the collapsed area is shown in Figure 1. GEOLOGY AND MINING METHOD There are two coal seams mined at Barro Branco mine. The first is the 1.5 m to 2.3 m thick Barro Branco Seam, under about 75 m of cover. The second is the Bonito Seam, where the collapse occurred. The Bonito Seam is 2.5 to 5.5 m thick located at a depth of approximately 80 m, with an average dip of 0° to 15°. Various major faults and inherent joints are present in the area."

Jan 1, 2017

By Jiachen Wang

China is the largest coal producing country in the world. Coal is widely distributed in China and thus the mining conditions are complex and diverse. This paper summarizes the important achievements in the last 20 years in various mining methods including the extra thick seam with large mining height top coal caving, the thick seam with large mining height and top coal caving mining, the steep seam with top coal caving and fully-mechanized mining, the thin seam automated mining, and the backfill mining. At the same time, the rapid advance in the development of mining machinery and manufacturing has also contributed to the advancement of coal mining technology and the great improvement of coal mine safety. The future direction of China?s coal mining is to further improve mine safety as to improve the coal recovery rate, promote the pillar less mining technology, develop high-efficiency backfilling technology, develop steep seam high-efficiency mining technology for seams less than 10m thick, and high recovery mining techniques for extra thick seams, improve the reliability of mining equipment, and strengthen the protection and restoration of surface environments and water resources.

Jan 1, 2014

By N. P. Kripakov

This paper presents a brief overview of current bump mechanics theories and pillar design methodologies, and relates these concepts to experiences at two mines located in a north-central Utah coalfield where different pillar designs were used to control mountain bumps. Experiences gained at the first mine demonstrated the successful implementation of a two-entry, 9.8 m (32 ft)-wide, yield-pillar design. A U.S. Bureau of Mines field study quantified the timing of chain pillar yielding and resulting load transfer from the gateroad. In-mine pillar response, although apparently sensitive to site-specific conditions, compared favorably to estimates derived using two yield-pillar design methods. A second study conducted at another mine located in the same district, but subjected to different geologic conditions, documents the unsuccessful attempts to employ progressively narrower three-entry pillar designs based on successes achieved at the first mine site. This second mine never achieved a true yield-pillar design. Use of pillar widths ranging between 9.1 m (30 ft) and 27.4 m (90 ft) always resulted in violent bumps in the tailgate pillars. The pillars were either too large to yield, or too small to support peak operational loads. Hypothetical gateroad systems were evaluated using both analytical and empirical approaches for configurations comprised of two rows of conventional pillars, and systems incorporating both yield and abutment pillars. Analysis concluded that yield pillars less than 6.1 m (20 ft) wide and abutment pillars ranging between 30.5 m (100 ft) and 39.6 m (130 ft) square would be required to achieve a stable gateroad design. However, results of a field study conducted on a two-entry, 36.6 m (120 ft)-wide abutment pillar concluded that abutment pressures from the first panel overrode the pillar, and that a still larger pillar may be required to preclude bumps in the tailgate during second panel mining. Without the benefit of a demonstrated in-mine success, it is not clear which design would ensure elimination of gateroad bumps.

Jan 1, 1992

By Uwe Wyink, Guenther Thiele, Isabel Gollnick, Klaus Thyrock

"The application of a thin spray on liner was implemented in longwall T-junction sections in 2009 for the first time. The development of the material and the application is based on a technical cooperation between RAG Deutsche Steinkohle AG and BASF Corporation. The report gives an overview of the first trials. The underground test occurred in a seam with a height of 1.5 m (5 ft) in a depth of 1,100 m (3,609 ft). The roadway is dual-use entry and the roof strata is composed of shale and sandy clay. In some sections geological faults impact the surrounding strata. The poor conditions induce separating and loosening of roof rock. Several remedial actions have to be implemented for decreasing deformations at the face entrance.The application of thin spray on liner (TSL) provides simplification and cost reduction of supporting work at longwall T-junction. The installation of TSL to avoid roof and coal fall operations for lagging are omitted.INTRODUCTIONFor safe operation of face retreat in dual-use roadways, the processes of securing and stabilizing the face end area are very important in order to minimize deformation and stress. Avoiding roof and coal fall are the main concerns for safe operation. For that reason different methods of reinforcement are tested in underground operation. In this paper the application of TSL is presented.The specifications of TSL are described, followed by the case study results. The underground study is located in one mine of RAG Deutsche Steinkohle. The extraction of the hard coal in this mine is characterized by great depth and high stress. There are poor geological and geotechnical conditions. The roadway used for the underground application was headed with the support system combination type A, which will be described later in the paper. After the first face retreat, deformation of steel arch support and bagging destroyed the roadway, so for the second face retreat remedial actions had to be undertaken. For this a new system of reinforcement, simplifying the supporting work at the longwall T-junction was tested. The reinforcement consisted of different parts that are described in the following report. Furthermore two different versions were applied. One method is the operation with TSL and the other one is the use of concrete for sealing. Both methods are compared in the text."

Jan 1, 2010

By Boqiang Wu, Ganggang Xu, Xiaodong Wang, Hai Wang, Shibin Zhu

"Using a paste-like slurry as a filling from the ground surface to the goaf, artificial pillars can be built in the cavity goaf to support the goaf roof to prevent roof collapse and ground subsidence. If the accumulation laws of paste-like slurry are known well, the filling technology can be optimized. The laws can establish the foundation for finding out the best methods for building artificial pillars and filling technology. In order to understand the accumulation laws of paste-like slurry in the cavity goaf, Aeolian sand with wide distribution in northwestern China was selected as aggregate, and Portland cement was used as cementitious material to prepare a paste-like slurry. A test model of the cavity goaf was built to simulate the filling process by pump from the ground surface to the goaf to study accumulation laws of the paste-like slurry. The results show that the whole process of expansion and accumulation of the paste-like slurry could be divided into stages: free expansion, fill tightening expansion, and stable expansion. The angle of repose of the accumulation body of the paste-like slurry is constant in the stage of stable expansion, and it is the final angle of repose of the accumulation body. The angle of repose of the accumulation body is highly relevant to the slump of the paste-like slurry, and it is unrelated to the distance between the floor and roof. Under certain conditions of the cavity goaf, reducing the slump of paste-like slurry is one of the effective ways to obtain cost-effective pillars.INTRODUCTIONA large number of cavity goafs of coal mines were left in northwestern China; they were formed in the mining process by room-pillar or strip mining. Safety coal pillars were used to support the roof to keep long-term stability in the goaf, so the ground subsidence was not obvious (Fu, Deng, and Zhang, 2011; Xie, 2014). With gradual weathering, destruction of coal pillars, or additional load from ground surface buildings and structures, there could be a risk of instantaneous collapse of the roof in the goaf. This would affect the safety of existing structures on the ground and adjacent mines (Qu et al., 2017; Li, Xiang, and Jia, 2011; Tan, Guo, and Zhao, 2016; Tan et al., 2017). For disaster prevention of the falling cavity goaf, the current common practice is full filling (Hu et al., 2008; Liu, Zhu, and Huang, 2015; Pan and Wu, 2010). However, for buildings and structures with lower deformation requirements, full filling is excessive. For example, it is acceptable for goaf treatment under general roads and industrial sites to control the maximum deformation and keep the deformation continuous (Liu, 2011; Liu, 2012)."

Jan 1, 2018

By Jonathan P. Furniss

The paper describes a range of laboratory tests undertaken on full column resin anchored glass fibre rockbolt systems, to assess all aspects of their performance. Laboratory instrumentation has been developed for the accurate study of strain transferral from the Glass Fibre Reinforced Plastic (GRF') bolt, through the surrounding resin annulus, and into the rock. Experience in laboratory testing to the new British Standard on glass fibre rockbolting is discussed. A critical comparison is made of the influencing parameters affecting . underground rockbolted roadway performance to the parameters assessed by BS 7861 : Part 1 The work has been co-funded by MA1 Systems UK Ltd. who supply GRP rockbolt components to the British coal mining industry, mainly for the reinforcement of coal ribsides. In rockbolting, glass fibre has a number of useful * qualities which are discussed in the paper. These properties, make the product more suitable than steel in certain applications, both in mines, and other engineering structures such as tunnels, soil stabilisation, and coastal defences.

Jan 1, 1997

By Zhou Ying

Based on the mining condition and roof lithology of the 2-3 coal seam at Gengcun Mine, the simulated materials modeling method was used to study the movement process and characteristics of overburden strata movement and to analyze the movement laws of overburden strata during top coal caving mining. Based on the modeling results, variation of the peak abutment pressure and its effect on the top coal's crushing and caving in the mining process was discussed.

Jan 1, 2001

In recent years, improvements made to the longwall system have accelerated at a faster pace than those made to the gate entry development system. As a consequence, unless planned years ahead of time, extending the panel length is nearly impossible without some innovative approach. At Cyprus Amax Coal's Emerald mine the G panel length was extended by 3,500 ft- by developing three entries across it to facilitate development of its headgate and tailgate entries simultaneously when the adjoining F panel was in operation. After evaluating alternative supporting methods and associated risks, determination was made to backfill these entries and crosscuts using a low strength flyash cement mixture prior to mining thru. Instrumentation was also used to monitor the stress change or front abutment stress in both the coal and the backfilled material ahead of the face. Longwall mining thru these three entries took about four eight-hour shifts and proved to be very successful. The designed flyash cement mixture provided adequate roof stability and caused no difficulty in the shearer's cutting while eliminating an in-panel longwall face move and minimizing the risk of roof or floor failure in the in-panel openings, or in the worst scenario loss of the face during the longwall cutting thru. This approach provided the intended result of extending G panel and recovered about one million tons of coal. This paper summarizes the engineering design of the cutting-thru project and flyash cement mixture, front abutment stress changes within the longwall coal block, backfilled material, and coal pillars ahead of the longwall face, and our underground observations

Jan 1, 1997

By Thomas M. Barczak

Roof support systems are necessary to provide stable mine openings and much research has been conducted to design a variety of roof support systems that will function in various manners to ensure that stable ground conditions are achieved. Despite these advancements in technology, mistakes continue to be made in the evaluation and/or installation that significantly degrade the support capability or lead to erroneous determinations of support expectations. The purpose of this paper is to discuss misconceptions about how roof supports perform and factors that impact their performance. The goal is to present practical information that will assist mine operators and engineers in selecting, installing, and evaluating roof support systems properly, and help them to avoid mistakes that can lead to erroneous expectations and potentially catastrophic results that may lead to roof falls. The paper is limited to a discussion of secondary roof support systems and powered roof supports such as longwall shields.

Jan 1, 2001

By David P. Conover, Jake Bain, Christopher M. Ross

"Highwall mining of thick (up to 100 ft) steeply dipping (20° or more) coal seams provides many challenges, both geotechnically and operationally because seam dips near or in excess of highwall mining machine capabilities are encountered. Maximizing coal recovery while maintaining highwall stability requires innovative techniques with regard to web and barrier pillar layout, depth of penetration, and choice of mining horizon within the seam. Stability of highwall mining slopes, openings, and pillars are typically analyzed using the ARMPS-HWM program, as well as LAMODEL, UDEC, and Slope-W modeling. Highwall stability can be maintained and highwall mining production optimized by applying design criteria in creative ways, including alternating miner penetration depths and initiating mining of thick seams toward the bottom of the seam. Highwall mining of thick, steeply dipping coal requires careful planning and execution, including close cooperation between geotechnical design engineers, the mining company, and the highwall mining contractor. This paper describes the application of creative design techniques to a specific pit arrangement at the Westmoreland Kemmerer Mine, Kemmerer, Wyoming. Highwall mining was accomplished by UGM ADDCAR Systems, LLC, on a contract basis. INTRODUCTION Highwall mining is a technique for attaining additional coal recovery after the economic strip limit is reached in surface mining. It involves remote deployment of a continuous miner in openings beneath the final highwall, with no personnel entry. Many candidate areas for highwall mining have thick and steeply dipping seams. Mining downdip presents challenges related to the maximum pulling capacity of the machine, traction of the cutting head, and material conveying, all of which limit penetration depth. Maximum penetration is greater for flatter slopes and decreases for slopes nearing the threshold of the maximum machine operating angle. Most highwall mining operations are relatively flat with slight undulations within the seam; therefore, the highwall mining pillar design criteria applies fairly equally to the entire mining area. However, in steeply dipping deposits, design criteria based on higher overburden loads at the far end of the penetration are excessively conservative for the shallower portions of the openings near the highwall."

Jan 1, 2017