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By Kot F. Unrug

The shale response to moisture changes and the major finding of the appropriate research concerning this subject are briefly reported in this paper. Also described are mine ambient atmospheric conditions which are responsible for moisture changes in mines. The need for a quantitative measure of the weathering potential of roof. rock is demonstrated in relation to its influence on safety and the potential for increasing the cost of mine operations. The weatherability test presented in this paper has been developed and used in many mines. It imitates the natural process in a mine by subjecting the rock sample to alternating wet and dry cycles. The natural changes are thus accelerated, but are similar to the natural conditions to which the surrounding excavation rock will be exposed. The measure of the weatherability index is the percentage of the initial sample mass remaining after the test as the largest fragment of the sample. Thus, the index changes, from 100% for rock affected by moisture changes which disintegrates completely, to 0% for rock totally unaffected. In this paper, the apparatus and testing procedures are described. Also presented are examples of test results. with a discussion of the design implication to he considered by mine planners.

Jan 1, 1997

By Brian White

Two examples of en echelon mining-induced fractures seen in hard¬rock mines provided a basis for inferring that fracture zones and bedding plane separations immediately surrounding mine openings are promoted by oblique shear into the openings. It is hypothesized that initial fractures or separations form at the comers of openings as a result of high stress and physical constraint on the rock's ability to deform elastically toward the opening. These conditions result in a locally preferred direction of shearing. The shearing, in turn, generates tensile stress that initiates a progression of systematically offset fractures approximately parallel to the direction of greatest compressive stress. The fractures or bedding separations create tabular rock layers that amplify shearing displacement through bending and dilation. Such shearing effectively reduces and redistributes the compressive stress, but significant dilation is an inevitable consequence. The combination of dilation and shearing and the progressive development of fracture zones have important implications with respect to ground support. The concept of mining-induced fractures forming as a result of shear is illustrated by two examples from coal mines. First, frac¬tures seen at longwall faces probably result from shear associated with subsidence. The fracture zone that develops approximates or possibly defines the draw angle of subsidence. As the face advances, fractures extend downward along the lower edge of the fracture zone, while upper extensions of the fractures are pressed closed. Fracture zones in entry roofs provide a second practical example. Here, mining-induced fractures typically follow bedding planes. The shear zone model suggests that the first bedding separations develop near the edges of the roof and successive separations progress upward and toward the center. However, if the direction of greatest stress is inclined with respect to the roof, a fracture or bedding separation zone may propagate from one side only and also extend higher. Because coal ahead of the face provides some support against lateral shear deformation, bedding separation is inhibited near the face. Rock bolts installed close to the face ultimately become more strained and bent than bolts installed a few meters from the face, and bolts installed through the more remote part of a separation zone may ultimately experience the greatest tensile and bending strains. This model is supported by field data documenting progressive bolt failures that rapidly propagated downward across the roof during face advance.

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 John M. Karhnak

The Bureau of Mines has a long history of research in Ground Control. For many years, this work was done at Bureau facilities by Bureau researchers. As the tiles changed, however, the area of research interest broadened, and additional methods of implementation, such as con- tracts, were added. 'Ibis paper outlines the organization, goals, and methods of implementation of the Bureau Ground Control Program, along with procedures for project selection. This program organization is followed by several specific examples of Bureau research that will be soon ready for use in the mines.

Jan 1, 1981

By C. Babcock

This Bureau of Mines report considers some equations for mine pillar design from 1833 to the present. Most of the equations are of essentially the same kind, using pillar width to height relationships. The effects of end constraint and test volume size, that were implied in some of these early equations have received special emphasis in recent years. Experimental testing in the laboratory throughout the period given has been on prisms or cubes of coal or rock. Most of the present concepts have been known since about 1912. Wilson has recently emphasized the effect of end constraint in his confined core concept, but a number of other earlier relationships have indicated an awareness of the effects of constraint as well.

Jan 1, 1981

By Ismet Canbulat

The cut out distance in the continuous miner sections in South African collieries is limited to 12 m. However, this distance can be extended to 24 m by the permission of the Department of Mineral and Energy (DME). This paper investigates the risk of roof Calls associated with the extended cut out distances permitted by the DME. Extensive underground experiments have been conducted using sonic probe extensometer under various geological conditions to identify the effect of cutting distance and tinge on the stability of the workings and the performance of the support. The results showed that that the most critical parameter with respect to ground control aspects of extended cut out distance is the bord width, which determines the amount of deformation that will take place in the roof. This was confirmed by the numerical modelling. The results also showed that a significant portion of the maximum deformation in the roof takes place before the ratio of cut out distance to bord width exceeds two. This indicates that once the face is extended more than twice the bord width, the roof will stabilise, assuming that no geological discontinuity is present and that there will be no further geometrical changes in the area as a result of for example the development of an intersection.

Jan 1, 2000

By David Bartsch

The Ohio Valley Coal Company (Ohio Valley) has operated the Powhatan No. 6 Mine in Belmont County, Ohio since 1972. Ohio University owns a 455-acre research farm over a portion of the Ohio Valley reserve containing 51 acres of old-growth forest known as Dysart Woods. In 1988, when Ohio Valley applied for a new mining area for its first longwall located several miles away from Dysart Woods, Ohio University claimed that mining at that distance would lower the regional water table and kill the old trees. In response, Ohio Valley began to study potential impacts from longwall mining on hydrology, soil moisture, and tree growth. Early studies examined the impacts of longwall mining on hydrology, soil moisture, and tree growth directly over mined areas compared to woodlots that were not undermined. A subsequent study examined tree-ring growth for a five-year period before and after longwall mining for several woodlots, some directly over panels and others at distances of up to 2000 ft away. No significant differences in growth rate associated with longwall mining were observed. New research was begun that compared longwall-mining impacts on groundwater hydrology, available soil moisture, and tree growth in two stands of mature trees located near Dysart Woods in order to confirm the earlier results. Using a test technique known as "Before-and-After-Controlled-Impact-Pairs," data was collected from one woodlot (test) that was undermined and from another (control) that was not undermined. Data collection began several years prior to longwall mining and has continued for several years after mining. The data for the test site shows that although the shallow and deep groundwater levels were affected by longwall mining, the soil moisture, vigor rating, or the growth rate of trees did not decline significantly compared to the control site where there were hydrologic impacts. The study shows that there are no known significant impacts to trees from longwall mining. The results of these studies were used by Ohio Valley to obtain State approval to longwall to within 300 ft of Dysart Woods

Jan 1, 2005

By Yang Li

For the study of overburden movement in longwall mining of shallow coal seams, most scholars merely consider the thickness of bedrock and top soils. In this paper, the overburden movement in longwall mining of shallow coal seams with thin top soils and key strata group was studied. By using numerical simulation, the development of fractures in the overburden was analyzed for different ratios of panel width to depth (W/D) and depth to mining height. The results show that the overburden movement changed with the variation of mining height, panel width and depth under the specific geological conditions. It also explain how different overburden disturbance structures, such as ?three zones? (caved, fractured, and continuous deformation zones), ?two zones?(caved and fractured zones) or ?steps-like collapse? are formed under different mining conditions. The results broaden the vision for research on overburden movement in longwall mining of shallow coal seams.

Jan 1, 2014

By Bolger Witthaus

"High performance production under challenging geological conditions is a product of several parameters. Main factors are the technical equipment and the knowledge in processing. The design and the support system of maingates must also be taken into consideration. The extraction of seams in German underground hard coal mines of RAG in a range of heights between approximately 1.1 and 4.0 m ( 4 and 13 ft) is done by plow and shearer technology. A complete system for quality management of production is required. The process examination of processes begins with procurement of the equipment. Education and training of the engineers and specialists, as well as for the miners are requirements for success as well.This report gives an overview of RAG measures for achieving high performance. Starting with examination of the processes accompanying procurement of equipment, the following important elements are described. Standards in education and training, technical design and elements of automated processing using stateof- the-art in control systems are critical to this system.The application of automation plays a decisive role in modem coal mines. The latest development in German coal mines is the fully-automated shearer system for longwalls. This helps to enhance safety, particularly in difficult ground conditions, by removing the shearer and shield operators from the immediate mining activity.INTRODUCTIONLongwall mining in carboniferous strata in Germany faces various challenges. Different longwall systems are used to mine up to 24 seams, each with a thickness between 1.1 m (3.6 ft) and 4.0 m (13 ft). High stresses due to a depth of up to 1,400 m (4,593 ft) requiring pillars between the panels, require special systems for roadway development, roadway support and reinforcemen.t as well as for longwall advance operations. Two main types of equipment for extraction are shearer and plow systems. While shearers are utilized for seam thicknesses above approximately 1.8 m (6 ft), plow systems can be used for smaller seam thicknesses between 1.8 m (6 ft) and approximately 1.0 m (3 ft)."

Jan 1, 2010

By Rafal Misa

In order to prevent the formation of mining damage or to reduce its reach, it is advisable to look for technical solutions which would enable effective protection of construction sites from the impacts of underground mining. So far, other authors have mainly focused on developing methods of securing construction sites in order to minimize the damage resulting from underground mining; the issue of geotechnical methods for protecting buildings against mining damage has not been thoroughly covered in those articles. It should be noted that there are relatively few publications directly related to this issue, nor are there practical examples of geotechnical protection. In this paper, a geotechnical solution available for securing building structures is discussed. Trenches presented in this research reduce the size and range of mining deformation. The results of numerical modelling for sample protective solutions, aimed at reducing the impact of mining activity on the terrain surface under various conditions, are presented. The calculations were performed with the aid of the Abaqus 6.10-2 software, based on the Finite Element Method.

Jan 1, 2014

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 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 Yongping Wu

Taking the longwall mining face area in a steeply dipping seam as the research object, the stability of an asymmetrical rock mass structure is analyzed using shell theory and masonry beam theory. The results show that an asymmetrical shell structure is formed at the top strata of the mining stope and its magnitude is mainly determined by the mechanical characteristic of the overburden strata of working face, mining space size and so on. Localized instability of the shell structure is the main factor that leads to the overall structural instability in the stope. The instability modes includes: the shell roof instability dominated by tensile failure, the shell shoulder instability initialized by compression-shear failure, and shell bases composite instability occurs in tensile and compression-shear modes. The shell structure stability dedicates rational strata control measures to choose, and the caving height of overburden strata is the key for shell structure instability.

Jan 1, 2013

By Andre Zingano, Anderson Weiss

"The objective of this paper is to study the behavior of a low thick and low depth coal seam and the overburden rock mass. The mining method is room and pillar in retreat and partial pillar recovery. The excavation method is conventional drill and blast because of the small production. The partial pillar recovery is about 30% of the previous pillar size, 7m x 7m. The roof displacement was monitored during retreat operation; the surface movement was also monitored. The effect of the blasting vibration on the final pillar strength had been considered. Due to blasting, the pillar reduced about 20%. The consequence is more pillar deformation and roof vertical displacement. The pillar retreat and ground movement were simulated in a three-dimensional numerical model. This model was created to predict the surface subsidence and compare to the subsidence measured. This study showed that the remaining pillar and low seam reduce the subsidence that was predicted with conventional methods.INTRODUCTIONMining low coal seams (less than 1 meter) with the room-and-pillar mining method implies the excavation of part of the roof or floor to maintain adequate entry height for equipment and compliance with labor regulations with respect to ergonomic issues. The direct consequence of this is an increase in coal contamination and the increased cost of transportation and preparation at the plant. Traditional room-and-pillar mining only uses a development operation, even with a continuous miner or drill-and-blasting. Therefore, all the run of mine (ROM) coal is carried out with part of the roof or floor. However, if the retreat mining method were to be considered, the quality of the coal could increase because only the coal seam is excavated from the pillar recovery. The quality of ROM coal will increase, and the cost will decrease proportionally to the amount of coal produced from the retreat operation.However, the retreat mining with partial or total pillar recovery implies greater subsidence. This is particularly true if the coal seam depth is shallow, under 100m depth. Subsidence prediction in room-and-pillar retreat mining is complex; it depends upon the remaining pillar (stump pillar) size, as well as the pillar height, and the behavior of the pillar after retreat. This is related to the strength of the remaining pillar, which is related to the excavated method (drill-and-blast or mechanical)."

Jan 1, 2018

By Leigh J. Wardle

Significant coal reserves in New South Wales lie under areas with steep topography such as valley slopes and cliffs. Subsidence predictions are difficult to make for these locations as existing empirical databases cannot cover all permutations of topography and mining geometry. Two dimensional numerical model alternatives are of limited applicability. Consequently a novel three-dimensional numerical model is developed including the detailed panel layout, realistic rock mass properties including the full stratigraphy, goat behaviour and actual surface topography. Model results show the same trends as subsidence measurements from Baal Bone Colliery in the Western Coalfield of New South Wales

Jan 1, 1992

By Salah Badr

Longwall mine layouts with their entries, chain pillars, face support systems, advancing mining faces and compacting gobs represent a geomechanically complex mining operation. Geomechanical aspects are further complicated if mining occurs at depths exceeding 350 m, as stresses induced adjacent to the excavations exceed the unconfined strength of the coal and result in fracturing of the sidewalls of entries and chain pillars during development. Traditional methods of geomechanical analysis, whether empirical or numerical, are unable to represent such behavior satisfactorily. Achieving some meaningful results requires explicit representation of the three-dimensional boundary conditions and implementation of appropriate constitutive models for the rock mass. This paper describes a numerical modeling methodology for deep longwall mines operation using the FLAC3D finite difference de, which incorporates a strain softening constitutive law for modeling failure of coal. An important aspect of the modeling method includes accounting for stress relief provided to the pillars and entries due to the increasing load applied to the compacting gob.

Jan 1, 2003

By Asmaa Yassien

Finite element models were developed to study the effect of tensioned and fully-grouted bolts on the stability of the mine opening in a three-entry development system using a typical geological column for the Pittsburgh Coal Seam. The models were used to study the effect of each bolt type on the stress distribution in the immediate roof, yield zone development and the vertical deformations. Comparison of all of those features show that the effect of a tensioned bolt on the stress distribution in the immediate roof is restricted to limited areas, whereas the fully-grouted bolt has a much wider effect on the stress distribution. Both types of bolts can reduce the yielding and deformation in the immediate roof to different degrees. The fully-grouted bolt reduces the sliding along the bedding plane more than the tensioned bolt. Both types of bolts were able to maintain the bedding planes without separation. The induced forces in both types of bolts are analyzed. A rapid increase in the shear force and bending moment occurred in the fully¬grouted bolt due to the relative motion of bedding planes.

Jan 1, 2002

By Timothy J. Matthews, Timothy J. Batchler, Ted M. Klemetti

"From 2011–2016, there were 1,511 documented injuries, including 7 fatalities, from ground falls in underground coal mines in the United States. The majority of these ground-fall injuries were not caused by a major roof collapse, but from falls of smaller rocks from the immediate roof. Roof screen can significantly reduce the number of these injuries and has been widely used in underground coal mines for surface control. Because of the potential of reducing ground-fall injuries, the National Institute for Occupational Safety and Health (NIOSH) is further evaluating the performance characteristics of welded wire screen as used in underground coal mines by conducting a laboratory testing program using the Mine Roof Simulator (MRS) in Pittsburgh, PA.The load-displacement characteristics of an 8-ft x 12-ft panel of 8-gauge welded screen were evaluated using a newly designed, large laboratory screen test frame with multiple load pull locations. This screen was tested in a configuration that simulates current installation practices in U.S. coal mines. In this study, the effects of varying the number and position of the load pull location on the screen performance were evaluated. Ultimately, the type of information obtained in this and similar studies can be used to aid in developing wire roof screen design criteria and to assist mine operators in the selection and use of roof screen in underground mines.INTRODUCTIONThe large majority of ground-fall injuries are caused by falls of smaller rocks from the immediate roof. Various controls are currently being used in mines to control this surface rock, including the use of wire roof screen. In mines where wire roof screen has been installed, injuries from rock falls have been reduced dramatically (Robertson and Hinshaw, 2001). Roof screen has the potential to prevent hundreds of injuries caused by the fall of small rocks between permanent roof supports (Compton et al., 2007). Because of this potential for reducing ground-fall injuries, the National Institute for Occupational Safety and Health (NIOSH) is evaluating the performance characteristics of welded wire screen as used in underground coal mines by conducting a laboratory testing program in the Mine Roof Simulator (MRS) in Pittsburgh, PA."

Jan 1, 2018

By Greg Hendon

Longwall developments in the UK have historically consisted of single entry gate roads. Adjacent developments were separated from existing panels by large barrier pillars (designed of sufficient width to get away from the longwall abutments of the previous panel) or by small barriers driven in the shadow, or de-stressed zone, of the previous panel. Some 2nd panel tailgates were also driven skin to skin leaving no coal barrier between the newly driven entry and the heavily supported existing gateroad. With the developement and wide acceptance of fully bolted entries and the pressure to reduce production costs, alternatives to single entry drivage, particularly yield pillar developments, were examined. Through the Rock Mechanics Branch of British Coal, a cooperative study was begun with Jim Walter Resources, Inc., USA, (JWR) to look at the yield pillar alternative in detail. This study was to determine the feasibility of utilizing yield pillars in the UK and to determine, through monitoring, the possibility of reducing stable pillar widths at JWR. The study has included extensive monitoring of the yield-stable-yield pillar system at JWR No. 7 Mine and an underground trial of a two entry yield pillar test area at Welbeck Colliery in the UK. This paper describes results from the JWR study and the subsequent results of the first advancing yield pillar development in the UK at Welbeck Colliery.

Jan 1, 1995

By Xingkai Wang, Wenbing Xie

"To control the large deformation of soft rock around the roadway subjected to high stress, it is important to improve the stability of support structure, in addition to increasing support strength. In this study, the causes for the destabilization of U-steel frame and its loading characteristics were analyzed by theoretical analysis, laboratory experiments and field tests. The technology of constant support resistance, the principle of support structure compensation and the compensation technique are proposed. The results indicated that the main reasons for frame structure destabilization are caused by low strength of connector, irrational structure of connector, poor interaction between the surrounding rocks and support frame and structure, and unstable support structure. Thus, the corresponding technological measures, such as connector with high strength and rational structure are adopted. The bearing capacity of frame is unstable after slipping. In order to maintain the high support resistance when the frame is performing the support function, the frame connector should be retightened after slipping. Though the working resistance may be improved greatly by using the backfilling or back grouting of U-steel, the frame remains unstable because of the unstable frame structure. Therefore, according to the deformation characteristics of rocks surrounding the roadway and the structure instability of U- steel frame, the stable structure device is used to carry out the structural compensation of U-steel frame, and the whole bearing capacity of the frame and its structural stability are improved greatly.INTRODUCTIONThroughout the development of the roadway supports for coal mines in China, it can be seen that the supporting strength and supporting resistance increase continuously with increase in mining depth and supporting difficulties [1]. With respect to the ground control mechanism and supporting technology of rocks surrounding the soft rock roadways subjected to high stress, considerable research has been done both at home and abroad. The backfilling technology behind the high-resistance and yieldable U-steel wall, the new type of grout/bolt support technology and the bolting with high-strength mesh support technology were developed, and have been used widely in coal mines with notable economic benefits [2-7]. However, a large number of engineering practices show that in many cases the existing rock control technology of roadway may not control the large deformation of the soft rock roadways subjected to high stress. The reason that the large deformation of soft rock roadway occurs is mainly due to poor structure stability of existing frame bearing structure, not insufficient support strength."

Jan 1, 2015