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By Dennis Dolinar

In this National Institute for Occupational Safety and Health (NIOSH) investigation, numerical modeling is used to evaluate the degree the pillar strength is increased with a rectangular pillar over a square pillar. Three-dimensional models are developed with pillar sizes and geometries typical of underground limestone mines in the United States. Physical properties for the models are based on geotechnical surveys of nearly 30 underground limestone mines. To examine the effects of pillar length on pillar strength with the modeling, a parametric study was conducted where the pillar width was held constant while the length and height were varied resulting in width-to-height ratios ranging from 0.5 to 1.5 and width to length ratios ranging from 1 to 6. There are two aspects to pillar strength and failure. At relatively low stresses, pillars can fail as a result of geologic structure. At greater depths and sufficient extraction ratios, the pillars can also fail as a result of the vertical stresses exceeding the pillar strength. The effects of increasing pillar length regarding both strength aspects are presented in this study. Equations were developed for the increase in pillar strength with both height and length based on the model results. The results from these equations were compared to those from previously developed empirical equations for rectangular pillars. For the more slender limestone pillars with a width-to-height ratio below one, there is little increase in pillar strength for length-to-width ratios above 2 and almost no increase with length for a width-to-height ratio of 0.5. Only with the more squat pillars is there a significant improvement in strength with length. The gain in pillar strength with length for the pillars is explained by the change in the ratio of the area of the pillar affected by the ends to the area of the pillar. For the squattest pillar, the change in strength is also affected by an increase in pillar confinement along the central length of the pillar as the pillar transitions from a square to a long rectangular pillar. With geologic structure, added pillar length can significantly improve the pillar strength, depending on the strike of that structure with respect to the long pillar axis.

Jan 1, 2007

By Michael Alber

Coal mining by the longwall method typically causes seismic events. This paper describes the research on mining induced seismicity around two specific longwalls at 1100m depth under several old longwalls. A local network featuring 21 geophones has been installed, underground core drilling and an extensive rock testing program were executed. A rock mechanical model featuring a multi-layer strata was created for use with numerical analyses. Seismic events were localized and three different event types are described. Focal analyses yielded distinct modes of failure. Those types are explained by different failure mechanisms, namely fault activation, remnant pillar punching and beam failure. The use of rock mass classification, rock mass strength criteria and in situ stresses together with 2D and 3D numerical analyses allows evaluating the findings from seismology approaches. Conclusions are drawn that the orientation of longwall with respect to the major horizontal stress orientation plays an important role in mine induced seismicity.

Jan 1, 2008

By Hakan Gurgenli

Many coal seams have weak shale immediate roofs that cause ground control problems. Therefore, it is important to know the properties of these shales so that preventive measures can be developed in advance. However, it is sometimes hard to understand this rock type due to its unpredictable behavior. A good example of this situation is found in two shale coal mine roofs located in the Illinois coal basin (Herrin No. 6 seam). Although, these mines are located very close to each other, one of them has many roof falls, whereas the other has no measurable ground control problems. The aim of this paper is to explain why and how one shale roof behaves worse than the other. Laboratory tests, including point load strength index, slake durability index, moisture activity index, weatherability index, water content determination, swelling strain and x-ray diffraction tests were performed to analyze the shale properties. According to these analyses, some useful correlations were found between the engineering properties, moisture-sensitivity indices, and weathering properties. Finally, mine and support design considerations based on the test results were discussed and recommendations to prevent ground control problems were given.

Jan 1, 2006

By T. N. Singh

An evaluation was made on the cavability of the roof formation of Dishergarh seam including caving sequence, caving span, and load on powered support.

Jan 1, 1994

By Scott A. Shapkoff

High strength cable strand supports are predominately used as a supplemental means of ground control in longwall and development operations in the U.S. The advantages of cable strand supports have been documented and include flexibility, strength, and ease of installation. Since the majority of cable supports installed in the U.S. today are partially grouted, and this same segment of supports has no inherent installed tension, the supports are considered currently arbitrarily strictly supplemental and therefore, cannot currently be deemed primary support (in most mining districts). This paper details and discusses two distinctly different configurations and the technical advantages of active partially grouted cable supports for certain primary support applications. The evolution of the designs for both systems is highlighted. Field data illustrates the amount of active load in relationship to installed torque and the affects that component variables have on this load measurement. Roof control patterns using active cable supports are examined as alternatives for combination patterns using primary and supplemental systems. The importance of maintaining consistent installation procedures plays an important role in the level of performance of the system. Economics comparing the installation of active cables with other primary/supplemental patterns is also included.

Jan 1, 2006

By Wenquan Zhang

Based on the mining and geological conditions of Wadong Mine, Longkou City, Shandong Province, China, a viscoelastic model with proper creep parameters was established. The 3-D ANSYS was used to simulate surface subsidence, analyze overburden subsidence characteristics, and propose the rational mining and pillar widths for mining in weak rocks and under special mining conditions, and provide the policy makers scientific data for decision making on mining activities.

Jan 1, 2006

By Rob Thomas

Main gate roof control during longwall retreat is subject to a significant increase in horizontal stress and risk. The implementation and management of an appropriate roof support design is as a result, an imperative requirement for any longwall mine and must in addition to the likely consequences of a roof fall, also consider the variables of the geotechnical environment, associated support costs and practicalities. This paper will therefore summarise a number of recent advances in main gate roof control in the Australian coal industry. Points considered include; (i) the ?Longwall Acceleration? concept, (ii) macro-scale horizontal stress notching, (iii) micro-scale horizontal stress notching, (iv) bedded versus laminated roof types and (v) the type of cable reinforcement used.

Jan 1, 2006

By Qingxiang Huang

Shallow seam coal field has the largest coal reserve in China. Mining in shallow depth causes serious problems. One of them is subsurface dewatering. In this paper, the physical simulating model was performed to study overburden movement and aquiclude stability in the shallow seam mining of Yushuwan coal field, China. According to the characteristic of clay aquiclude in the overburden, the proper simulation materials and proportions of mixture for simulating the plastic clay aquiclude layers and brittle bedrock layers were determined by the stress-strain tests. The physical simulating model was carried out to simulate the failure and caving process of the roof and overburden. Based on the simulating tests, it was found that the movement of clay aquiclude follows the movement of the underlying bedrock layers. The basic caving mechanism of the overburden roof strata was also presented. The tests also showed that the best way to control the stability of aquiclude is to reduce the subsiding gradient Ts. This research provides a foundation for further study on mining dewatering and water conservation.

Jan 1, 2006

By Larry DeGraff

With the increase of new mine development in the past several years, many coal mines have decided to self- perform the design and construction for their slope development projects. The development process of determining the proper configuration is challenging, and a major economical consideration. Greenbrier Smokeless Minerals engineering staff and consultants, along with DSI/American Commercial, developed a support configuration that was best suited for their slope. The developmental process took into consideration the length of slope, depth of slope at the high-wall transition, economics of a single or multiple entries, waterproofing, stresses on the steel supports and lagging during the backfilling operation. This paper describes the developmental concept of using a single entry, side by side configuration, using steel ?Banjo String? supports with steel plank lagging, to develop a long term permanent installation in a cut and cover application.

Jan 1, 2009

The U.S. Bureau of Mines (USBM) has developed the coal Mine Roof Rating (CMRR) to aid in ground control. This paper describes the application of the CMRR at four coal mines operated by a major U.S. coal company. The mines are widely separated geographically, and cover a wide variety of geologic conditions and mining methods. The study found that the CMRR was a useful predictor of roof performance. In mines where the CMRR varied, and other parameters held constant, roof falls were associated with low CMRR values. Important similarities in roof performance were found in mines with similar CMRR values, despite radically different geologic environments. The analyses also indicated that statistics can be used to evaluate ground control performance. For these mines, a "roof fall probability" can be estimated from the CMRR and the intersection span. A spreadsheet program has been developed to facilitate treatment of CMRR data, and it is briefly describe

Jan 1, 1994

By Anthony T. Iannacchione

The Roof Fall Risk Index (RFRI) is a new method introduced by the National Institute for Occupational Safety and Health (NIOSH) to assist the underground stone mine operator in 1) assessing defects the mine strata and 2) rating the relative roof fall risk these defects pose. The RFRI utilizes observational techniques to identify defects in the roof strata caused by local geologic, stress, and mining conditions. Assessment values for ten defined defect categories provides the foundation for estimating a range of risk conditions between 0 and 100 that define the potential for a roof fall. This paper examines how the defect information is collected using two field verification sites and proposes methods to analyze the RFRI data. These examples provide information on how well the method replicates observed conditions and how it might be applied in practice.

Jan 1, 2006

By Rob Thomas

Pre-driven recovery roads are used to speed up the longwall recovery process by reducing bolt-up requirements and by improving ground conditions on the face during the removal of the powered supports. To lower the geotechnical risks associated with the use of pre-driven recovery roads, and so improve its overall acceptance in the industry as a viable alternative to more conventional, potentially difficult and costly longwall recoveries, this paper outlines (i) a geomechanical model for the use and design of pre-driven recovery roads, (ii) an updated version of the worldwide database on pre-driven recovery roads, (iii) a summary of the various methods of holing a longwall into a pre-driven recovery road and (iv) a series of relevant ground support and strata management guidelines.

Jan 1, 2008

By Thomas A. Bailey

Since the advent of longwall mining, various methods and materials have been used during relocation to control the intrusion of the gob in the shield/panline area. (figure 1) Controlling this gob is critical to ensuring a safe, speedy, and productive longwall move. An uncontrolled gob can cause a number of problems, from increased worker exposure to entrapment of equipment to lost production. With an eye toward worker safety and increased productivity, several different methods have been developed to minimize gob intrusion. This presentation will take a look at the history and development of longwall ?screens? and ?mesh?, from chain-link fencing to high-modulus polyester geogrids. The benefits and advantages of these developments will be defined. Since 1994, longwall coal mines have used polymer geogrids during longwall recovery to control intrusion of gob material into the shield/pan line area. As described by Brandon and Collins (1996) in their paper presented at the 15th Ground Control Conference in Golden, CO., mines had used various forms of support, such as welded wire mesh, chain-link fencing, wooden cribbing, props, and steel cable to accomplish this. These methods worked, but were labor-intensive, slow, and provided varying degrees of gob stoppage. Although a few mines still use chain-ink fencing to control the gob, longwall coal mines have generally settled on polyester geogrids to control gob and roof and these mines have been instrumental in providing input in order to develop and improve these geogrids products. he trend during the past twelve years has been to incorporate stronger grids and to eliminate auxiliary reinforcements, such as steel cables and nylon or polyester webbing. A job that once required heavy steel cables, props, and fencing is now accomplished with strong, lightweight, high-modulus polyester grids. When we discuss high-modulus grids, we refer to the ratio between load and strain, as illustrated in figure 2. These grids are woven or knitted from yarns that have been pre-stressed. When these grids come under load, they pick up the load more quickly. This allows less deformation in, in this case, the mine roof. The same principle applies when these grids are used to control the ribs. With less elongation, the grids will hold rib material tighter, thereby keeping travel ways more open.

Jan 1, 2006

By Richard C. Repsher

The U.S. Bureau of Mines has developed a method and device designed to detect lose rock material in underground mines. The technology is designed to be an aid to mine workers in detecting hazardous roof conditions in underground mines which can complement or replace the traditional roof sounding techniques where the miner relies on experience to determine whether rock conditions are sound. The leading cause of accidents and fatalities in underground mines is falls of lose rock pieces or rock slabs from the mine roof. The device is an electronic roof sounding device that acoustically determines the integrity of mine rock. In previous research the Bureau of Mines found that loose rock, when impacted, vibrates at a much lower frequency than intact rock material. A major problem in determining rock stability using this technique has been the repeatability of the matt signal. This difficulty baa been greatly reduced in the current design by measuring the power spectra contained in two separate frequency bands of the signal produced by striking the rock in question. The ratio of the energy contained in each band is computed. This process minimizes any striking force differences, producing accurate, repeatable results for solid rock as well as loose, drummy material. The prototype has been successfully- tested in a variety of underground environments including coal, uranium, molybdenum, silver and salt. The technology has been investigated by the U.S. Mine Health and Safety Administration and the Department of Energy for use in detecting detached tunnel lining areas in nuclear repositories. The paper will discuss the technique, applicable results, and future applications.

Jan 1, 1990

By D. Newman

The objective of this is paper is to review the current state of knowledge and practice in highwall mining (HWM). HWM has become a standard method in surface mining, commonly used alone or in conjunction with contour or slot mining. It provides 800-feet to 1,200-feet of additional recovery when the economic stripping ratio is reached in contour mining or in slot mining when surface access to a reserve is limited. A significant attribute of the highwall miner is its versatility. HWM has been used successfully to mine abandoned pre-reclamation law highwalls, points or ridges uneconomic to mine by underground or other surface methods, outcrop barriers left adjacent to underground mines, separate benches of the same seam where the parting thickness or quality differences between benches render complete extraction uneconomic, previously augered areas containing otherwise inaccessible additional reserves and close or widely spaced multiple seams. The theory and design methods to assess roof; pillar, and floor stability are presented followed by three case histories. Simple design charts for sizing HWM web and barrier pillars are also presented. A recommended web pillar width may be obtained from the design charts given the overburden depth, the HWM cut width, and the mining height. Given the depth and panel width for a set of HWM cuts, another set of charts gives a suggested barrier pillar width. The case histories, hm Northern and Southern Appalachia are used to illustrate the application of rock mechanics to quantify the stability of the highwall, roof, web pillars, and floor. The case histories involve 1) mining through a previously augered highwall, 2) mining under back-stacked spoil and 3) selective mining of closely spaced benches of the same seam. Because each site is unique, the appropriate pre-mining geotechnical analyses range from the calculation of roof, web pillar, and floor bearing capacity stability factors to detailed numerical modeling of the auger and underground mine workings. When operating in the vicinity of existing underground mine or auger workings, the determination of ground deformation and strains resulting from highwall mining is a necessary facet of a ground control investigation.

Jan 1, 2005

By Michael Alber

Resin grouted rock bolts are the major support measure in coal mining. They are rather easy and foolproof to install, take immediately load and are the least time consuming support measure for controlling gateroad deformation. The use of only resin grouted bolts allows fast gateroad development which is essential for economical resource exploitation. The design of reinforcement / support systems for coal mining typically bases on empirical methods, which work fine when encountering known or usual ground conditions. However, coal mining approaches depths where little experience exists and numerical modeling is often the only method to estimate ground behavior. This paper discusses the geotechnical properties relevant to numerical model and focuses particularly on input values for modeling resin grouted rock bolts for safe gate design.

Jan 1, 2006

By Changsheng Song

With increasing depth of coal mining in China, the number of coal and gas outbursts has been increasing for years. By means of destressing the protective seam, the gas in the protected seam is drained to reduce gas concentration in the high gassy seams, thus ultimately attaining simultaneous extraction of coal and gas. Base on the simulated material scale modeling and numerical simulation, the protective seam mining method was practiced at No. 8 coal mine in Pingdingshan city. After extracting the No. 15 seam, the overlying strata movement and the deformation of No. 9-10 protected seam were studied. The experiments showed that it was feasible to destress the protected seams with large interburden thickness. As the protective seam was being mined, the protected seam experienced dynamic movement process and the change in vertical stress in the No. 9-10 seam assumed ?W? shape. When the face had advanced 656 ft (200m) from the setup room, the No. 9-10 seam was fully destressed, resulting in easy gas drainage in the destressed zone. Recommendations on mining sequence of multiple seams mining in the Pingdingshan coal areas were made.

Jan 1, 2008

By Dao Hong Quang

This paper describes the numerical analysis of some geotechnical factors influencing the application of Longwall Top Coal Caving (LTCC) method. The study was aimed at comparing the differences in stress distribution in the chain pillars of the face ends and caving mechanism of the top coal in LTCC method between the flat and inclined thick coal seams. The outcomes from this analysis will benefit mine planners in designing the location of roadway development in the next longwall panel. In addition, results from the study will extend the understanding of caving mechanism in top coal section when applying the LTCC method (with the direction of the longwall face is down dip, retreating along strike) in inclined coal seams. The results demonstrate a clear effect of dipping factor on cavability of the top coal and on the stress distribution in the chain pillar of the face ends.

Jan 1, 2008

By S. K. Sharma

The present study attempts to critically review the pillar-design as required by Coal Mines Regulations of India vis-a-vis latest rockmechanics formulae, Equivalent material modelling was the approach adopted for testing the safety and stability of the pillar sizes predicted by various formulae. Two parameters, considered here for calculating pillar sizes was the stresses acting over the pillars and the strength of the pillars. Stresses acting on the pillars were calculated by the tributary area method. Based on the studies of Coates and Hustrulid, the stress obtained was reduced by 40 percent. For calculating the strength of the pillars, three formulae, namely Holland and Gaddy, Salamon and Munro and Central Mining Research Station, India, were used. The pillar widths were then determined by equating the product of recommended factor of safety and pillar stress with the pillar strength.

Jan 1, 1992

By Yongjie Yang

Experimental study on the strength and deformation characteristics of coal under cyclic loading provides guidance for designing coal pillar which is subjected to repeated mining influence during mining. This paper studies the strength, deformation and fatigue process of No.3 coal from Xinhe Colliery under cyclic loading by the MTS815.03 servo-controlled rock mechanics test system. The results indicate that coal is more susceptible to fatigue damage than other hard rocks. Under uniaxial cyclic loading, the fatigue failure ?threshold value? of coal is smaller than 78% of its uniaxial strength, and there is also some fatigue damage when cyclic loading and unloading experiment is carried out below the fatigue failure ?threshold value?. Axial deformation can be divided into three stages: Initial deformation, steady-state deformation and accelerating deformation. Transverse deformation can be divided into two stages: Stable deformation and accelerating deformation. When the transverse deformation begins to increase rapidly and the deformation recovers only slightly upon unloading, it?s a precursor to failure of coal. Under cyclic loading, fatigue failure develops through a process of compaction, strain hardening, and strain softening.

Jan 1, 2006