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By Jeff Whyatt

Deformation of underground salt, trona and potash mines is generally time dependent, providing for gradual adjustment of strata to mining induced stresses. Time dependence can allow for higher extraction ratios provided eventual failure can be tolerated. However, this eventual failure can be violent if creep deformation can shift stress and potential energy to strong, brittle geologic units. The mine failure case studies reviewed here illustrate this process. Yield pillars and defects in bridging strata figure prominently in these cases. Yield pillars provide local and temporary support to the roof, temporarily delaying the cave; and allowing extraction ratios and overburden spans to increase beyond the long term capacity of overlying strata. Defects (faults, voids, thinning) of strong overburden strata reduce the critical span, sometimes to less than panel width. Analyses of many of these cases have focused on a cascading pillar failure mechanism, but recent work and this review point to failure of strong overburden strata as the essential element. The suddenness of failure and attendant seismic events pose hazards to miners and, in some cases, to those on the surface. Characterizing these failures is a first step towards recognizing and managing the risk of catastrophic collapse in underground mines.

Jan 1, 2008

By Axel Preusse

Due to a political decision, German hard coal mining will come to an end in the year 2018. By then, there will have been more than 150 years of industrial grade mining activities. Located in the western part of Germany is one of the most densely populated areas of Europe. Multiple coal seams have embossed the earth?s surface up to 20 meters (21.87 yards), as mining has subsided. The aftercare of hard coal mining will consist of three tasks: groundwater management, polder management, and groundwater treatment. The Ewigkeitslasten (cost in perpetuity) at the current groundwater level has been determined in the past. The actual cash value is calculated to be about 12.4 billion euros (approximately 16.5 billion dollars). The cost of groundwater management is directly related to the groundwater level. Rising groundwater levels will reduce the cost of pumping water. Known effects of rising groundwater are uplift of the surface, emission of methane gas at the surface, higher risk of cave-ins, and contamination of drinking water horizons. Each of those processes produces cost in order to prevent negative consequences (e.g., uncontrolled methane emissions) or to handle reparations (e.g., damages caused by surface uplifts). In the long run, it is the goal to minimize the Ewigkeitslast. As a first step, all cost-generating processes are specified, and a cost type based characterization is performed. The objective of this case study is to identify key parameters to determine the quality of a cost-generating process (proportional cost, fixed amount cost). Afterwards, it is intended to specify the quantitative impacts of each parameter and to set up an optimization model to minimize the overall cost. The quantitatively specified parameters will form the base out of which the optimization model is created.

Jan 1, 2013

By Barry Voight

The law [ ] where [ ] is a measurable quantity such as strain and A and cu are empirical constants, describes the behavior of materials in terminal stages of failure under conditions of approximately constant stress and temperature. Applicable to intact rock and rock masses, the relation may be extended to conditions of variable stress and cyclic stress, and may be used to predict time to failure. Applications in rock mechanics include slope and excavation stability and various topics in ground control.

Jan 1, 1989

By Kazem Orace, Navid Hosseini, Mehran Gholinejad

"Selection of the optimum support system for underground openings such as tunnels is a complex process. In this paper a new approach, based on a combination of the Analytical Hierarchy Process (AHP) the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) and the Preference Ranking Organization METHod for Enrichment Evaluations (PROMETHEE), is introduced. For this purpose, the selection process is assumed to be a multi criteria decision-making problem. First different support systems using FLAC3D numerical code, based on technical safety and stability parameters of the tunnel are specified. Then taking economic and performance parameters as the decision criteria by using the combination of Al-IP, TOPSIS, and PROMETHEE the optimum support system is selected. As a real mine case study this approach is used in the main access entry to the C, coal scam of Tabas collieries. Results clearly demonstrate that the proposed support system selection method is more advantageous than other alternatives.INTROD CTIONGenerally, the support system for a tunnel is selected primarily with the aid of design engineers experience. In other words personal judgment instead of intellectual and scientific criteria is mostly the main basis (Oracc. 2005). However since various support systems can be applied in any particular situation, accurate selection of the optimum support method depends on the integration of many technical, economical and performance parameters, and also on the analysis of the intensity of influence by each of the criteria.In this paper an applicable approach for se lection of tunnel support systems based on Multi Attribute Decision Making (MADM) techniques including: Al IP (Orace et al.. 2009a; Saaty, 1980), TOPSIS (Hwang and Yoon, 1981: Yoon and Hwang, 1995), and PROMETHEE (Brans and Vincke. 1985; Brans and Mareschal. 1992) is introduced. As a field study, this approach was applied to tunnel C1 as one of the main entries in the Tabas coal mine which is the major collieries in the central pan of Iran (Hosseini, 2008).Due to the coal seam conditions coal is mined by mechanized longwall mining (Hosseini 2008). This method requires the 58 excavation of several entries some of which will be used for many years and even for the entire life of the mine (Peng, 2006). Therefore the selection of the support system in these entries is·very important in mining design."

Jan 1, 2010

By L. R. Stace

Arguably, one of the most significant technological breakthroughs that had been experienced during the period when British Coal was the major coalmine operating company in the UK, the use of rockbolts as principal support in underground roadways brought with it the need to re-assess the technical knowledge, management controls and legislative frameworks associated with the support of mine workings. Following a major roof fall in a roadway supported principally using rockbolts in which fatalities occurred, the process of the design of rockbolted support and the systems and procedures that had been put in place to control its use in the UK were further reviewed. The paper discusses the outcome of these reviews, and how the incident initiated a major new interest in strata control research in the UK against the background of the fragmentation of the ownership of the mines following the sale of the former British Coal assets at the end of 1994, together with the proposed introduction of new strata control legislation in 1996.

Jan 1, 1996

By Khaled Morsy

Most of the available numerical codes simulating the reinforcing effect of fully grouted bolt are accomplished by a series of bar elements which are connected along their length to the host rock by shear springs and sliders. The spring-slider system represents the shear bonding effect between the bolt and the host rock. This paper describes a three dimensional finite element modeling technique for the 3D simulation of fully grouted bolt reinforcement. The proposed technique introduced a physical simulation for the fully grouted bolts. The bolt simulation is composed of a series of beam elements (representing the steel rebar) embedded in brick elements (representing the grout). Because of the complexity of the grout/rock interface, ABAQUS explicit code was used. A modification of the ABAQUS default friction model was conducted to consider the cohesion of the grout/rock interface. The interaction between the grout and the host rock is simulated by the modified friction model. To verify the proposed technique, a simulation of the pull out test we conducted. The results from the simulation were compared with former studies

Jan 1, 2004

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 James H. Fletcher

Much data and experience has been accumulated, especially in the last 5 or 6 years, to show that modern roof trusses, both of the Birmingham type and the bolt-and-channel type, perform well to support certain kinds of coal mine roof. For some time now, roof trusses of the Birmingham type have been effective in controlling thick shale roof, which tends to sag and fall over a period of about eight weeks. These falls leave dame-shaped cavities which are reminiscent of the pressure arch theory of roof failure. Lately there has also been significant interest in supporting some longwall panel entries with channel and-bolt systems. These systems utilize two vertical center bolts and two 45 degree end bolts through a heavy gage channel. Both types of truss systems have long bolts, which require 8 to 9 Foot holes on a 45 degree angle, and point anchor resin bolts, which utilize a 2 or 3 foot rebar grouted in a thin annulus of polyester resin. Consequently, many coal mining people have been impressed with the very good anchorage characteristics and tensionability features of these systems. However, the same people have been proportionately dismayed to find that very often the most difficult part of introducing a trussing program to a mine is deciding on the machine specifications which determine the truss installation rate. The purpose of this paper is to indicate aspects of truss bolter design and use which are not shared by roofbolter design and its vertical pattern of bolts. Of the two types of drill units conventionally accepted, arm feed units and mast units, only one 15 judged suitable. While the arm feed unit offers low collapsed height with a long drill feed length, a great attribute for low seam use, it has not lent itself to being rolled over to a 45 degree angle. One reason for this inability is that many items which are conventionally mounted on the arm feed unit cannot be allowed to rotate with the unit. These items include drilling controls, tool Cray, lights, canopy or safety post, panic bar, and stab jack(s). If these items were independent of the arm feed drilling unit, they would have to be out of the way of the boom arms, links, and feed cylinder as the unit moves. To ease this problem, the arm feed unit can be made very short and even mounted across the machine. However, the resulting short feed length is undesirable. Several short drill steel extensions are required to drill a relatively long hole, making this procedure time-consuming as well as increasing the likelihood of drilling a crooked hole. Another reason for abandonment of the arm feed unit is that a very stiff drill guide (or "centralizer") is required to start the hole. Using the mast tilt control, the roof bolter operator will force the bit against the roof and will rotate the bit without feeding to cut sideways into the roof to form a hole face. Omission of this step often results in the skidding of the bit toward the rib. To date, no such sturdy drill guide has been designed for an arm feed drill unit. The mast style unit has been the most successful design for angle drilling. It is easily rolled over to a 45 degree angle or to 90 degrees and, on single head models, often has been made to roll through 180 degrees. The mast style is compact, in plain view and is easily fitted with heavy duty manual or hydraulically operated drill guides. It can be mounted directly on a machine chassis, or onto a swinging boom, or onto a slider arrangement in a broad variety of configurations which will accommodate controls, tool trays, stab and roof support jacks, lights, canopies, and panic bars. The only significant difficulty with mast drilling units occurs in lower mining heights when desirable low mast height is accompanied by undesirable short feed length. While the lead extension in the drill string can be as large as the mining height will allow, a short feed length will permit only the use of shorter middle extensions and drivers, and perhaps will. require the use of two different length wrenches. The associated problem is the relatively short distance from the drillhead to the drillguide. This distance is further shortened when raising the drillhead to get the bit to the roof. One might keep longer lead extensions on the machine; drill a vertical pocket for the bit, starting the angle hole at this pocket; or modify the mast. If the mast is made so that the distance from the drillhead to the drillguide remains constant while the mast extends, after which the drillhead travels up the mast, the shorter mast can be made to start the hole more easily. However, short teed length will still result in longer drilling time. It is obvious from these points that a proper choice of mast is simply the highest one which will be moved easily in the given mining height. This, of course, goes against the grain of most experienced coal mining people who realize that a relatively small decrease in mining height can create a problem correctable only with the purchase and installation of shorter masts. If the mining

Jan 1, 1982

Tailgate pillar and tailgate-face corner humps have been a major threat to longwall coal mines where hump-prone conditions exist. The yield pillar design concept, in which the longwall chain pillars are designed in such way that they can yield in a nonviolent or controlled manner during longwall retreat, has been used in many longwall mines to prevent tailgate pillar humps. However, this practice often shifts the side abutment load onto the tailgate-face corner especially when a massive and strong sandstone member is present within the caving zone or in the vicinity of the coal seam. Therefore, the abutment pillar design concept has also been used in some hump-prone coal mines to minimize the stress concentration on the tailgate panel rib for the control of tailgate-race corner humps. The disadvantage of using the abutment pillar design concept is that it is more costly and puts additional pressure on mine operations for gate entry development and significantly reduces the recovery of coal reserve. In addition to pillar size, panel layouts. such as the panel width and panel orientation also play a critical role in pillar or face humps and should he taken into account. Based Upon the Cyprus mine design and operational experiences at Willow Creek and case studies at other bump-prone longwall nines in the U.S., the authors attempt to identify the critical factors causing coal mine bumps and to introduce some guidelines and criteria for the design of both yield and abutment pillar systems and panel layouts.

Jan 1, 1999

By Robert G. Harris

During the last five years there have been significant improvements in face support monitoring techniques particularly in respect of face pressure. The measurement of support loading pressures was originally conceived as a maintenance aid for the hydraulic systems but recent work has also included geological loading displays of the complete face as well as individual supports. This paper briefly examines the means of obtaining this information, together with some examples of actual face data. Also included is a summary of the benefits of such monitoring and an outline of future development work.

Jan 1, 1993

By Robert P. Kudlawiec

The use of Mobile Roof Supports (MRS) in pillar retreat mining has been a fairly recent occurrence in coal mining operations in the United States (1988). Determining the pillar block configuration, the crosscut and entry centers, the number of entries developed, the method of ventilation, and the type and mixture of equipment used arc all factors in establishing a successful pillar retreat operation. This paper will evaluate and report on the history of one coal company's relationship with MRS usage for pillar retreat mining and focus on the company's use of horizontal pillars (inverted-blocks) for the panel configuration.

Jan 1, 2000

By Alan A. Campoli

Fosroc continues as a leader in the development of cementitious technology for the mining industry. The considerable expertise accumulated in the development of the Fm cement line has been employed in the development of two pumpable, supplemental roof support systems. The Tekprop system is a unique combination of steel and concrete, specifically designed as a viable alternative to conventional wooden supplementd support. The second system is the Fosroc mineable mi which is a cost competitive variable yield system designed to support longwall ventilation and predriven recovery room entries. Two, separate, ongoing, full scale applications of these systems will be discussed during the inference presentation

Jan 1, 1997

By Wen H. Su

Multiple seam mining and its associated problems are very serious in Southern West Virginia where poor planning or lack of knowledge in seam interaction often results in complete loss of coal properties. Several measures have been proposed to alleviate the interaction prob¬lems under various multiple seam conditions, but few of them are spec¬ific in terms of mining plan. Two parallel approaches are adopted for this paper. One is field investigation which consists mainly of mine site visits and gathering of data. The other is the two-dimensional finite element simulation of mine workings using the data gathered as input. Two typical cases which represent the past and present multi¬ple seam mining practice in West Virginia were investigated. Much attention is focused on the interaction problems experienced and the important parameters controlling interaction. Several key parameters controlling interaction in multiple seam mining were analyzed. Proper mining plans, by which the interaction problems can be minimized, are proposed for the individual cases after a better understanding of the interaction mechanism was realized through finite element analyses.

Jan 1, 1984

By P. Tsang

In order to deal with ground control problems such as roof falls, floor heaves and pillar failures in underground coal mines, a new pillar design method based on the "yield pillar" concept is proposed. Using the finite element model simulation, the functions and mechanisms of the "yield pillar" are studied. The important variables that affect the stability of longwall entry-pillar system are identified. To simplify the geological and mining conditions, a new pillar design method classifies the in-situ roof and floor conditions into four categories: 1) strong roof and strong floor, 2) strong roof and weak floor, 3) weak roof and strong floor, and 4) weak roof and weak floor conditions. The design formulae are derived based on the finite element model simulation, stability study and nonlinear regression analysis. The finite element models used consider the plastic failure properties and time-dependent behaviors of rock. To better organize the finite element model simulation, the orthogonal experiment design is used to arrange the finite element models and proved to be very effective. An application example is used to illustrate the basic design procedures involved in the new pillar design method.

Jan 1, 1993

By Joe F. T. Agapito

High stresses and adverse geology in deep coal mines in the state of Utah, USA, have caused numerous bumps. The larger bumps have been associated with seismic events with Richter magnitudes of 3.6 and greater, and in some cases have filled openings for lengths of 150 m. A better understanding of the mechanisms and stress levels involved in bumping is needed to help develop improved stress control design and bump mitigation methods. The geology of the area is notoriously bump-prone. The coal has poorly developed cleating and occurs in multiple seams that are often bounded by very strong roof and floor sandstone/siltstone beds. The overburden is formed by thick, competent strata with numerous sandstone channels This geology and deep cover are the major source of high stresses, causing bumps. This paper evaluates live common stress factors responsible for bump problems: depth, sandstone channels, arching, faults, and coal thickness. It uses ease study data from longwall panels with two-entry/yield pillar systems typical of deeper Utah mines. Results illustrate the importance of analyzing stress factor experience to allow a better understanding of the problem.

Jan 1, 2000

By John L. Edwards

The underground coal mines in the Lake Macquarie area of the Newcastle Coalfield, New South Wales, Australia are constrained by surface subsidence restrictions as low as 150 mm. The mining environment is shallow, generally between 100 m and 200 m, often multi-seam and commonly overlain by residential development or tidal waters. Most mines use partial or panel and pillar extraction techniques to meet the subsidence restrictions and allow maximum resource recovery. This paper gives an overview of the results of a recent two year government funded research project aimed at developing a mechanistic, geomechanically based mine design criteria for subsidence control. Three extraction layouts were monitored in detail to assess the relationship between mining geometry, pillar behaviour and surface subsidence. Pillars at each site were instrumented with stress cells, convergence monitors and extensometers. Field data was used to initially calibrate, and subsequently, verify stress and displacement predictions made by the three-dimensional displacement discontinuity code, SUBSOL. Back-analysis of the modelling results indicate SUBSOL's capability as a tool to satisfactorily predict surface subsidence magnitudes and pillar loads for alternative pillar and panel layouts.

Jan 1, 1993

By Mostafa Sharifzadeh, Ebrahim Ghasemi, Kourosh Shahriar

"The Tabas Central Mine (TCM) is one of the underground coal mines located in the Tabas coal region, in the mid-eastern part of Iran. This mine is the first mechanized coal mine in Iran that uses the room and pillar method. Pillar design is one of the most important issues in underground coal mines, especially in room and pillar mining. In this paper, a new method for coal pillar design in the main panel of TCM is presented. This method is able to estimate the magnitude of the various loads (both development and abutment loads) that pillars might experience throughout the mining process. Also, the method reduces the pillar failure risk. Based on this method, proper pillar sizes in TCM are obtained (11.6 x 11.6 m (38.l x 38.1 ft)).INTRODUCTIONPillar design and stability are two of the most complicated and extensive mining problems related to rock mechanics and ground control subjects. Although these problems have been investigated for a long time, to date only a limited understanding of the subject has been gained. Prior to this, the dimensions of a pillar were largely determined by experience based on trial and error, intuition, or established rules of thumb; any research available tended to be isolated and sporadic. But nowadays, various pillar design formulas have been developed based upon laboratory testing, fullscale pillar testing, and back-analysis of failed and successful case histqries. In 1980, a number of ambitious field studies conducted by the U.S. Bureau of Mines developed the ""classic"" pillar design methodology. It consisted of three steps (Mark, 1999, 2006):1. Estimating the pillar load using tributary area theory2. Estimating the pillar strength using a pillar strength formula3. Calculating the pillar safety factorAlso, Bieniawski (1981) presented a step-by-step method to determine coal pillar dimensions in room and pillar mines. In this method, similar to the classic method, pillar load is estimated by using tributary area theory.Currently, in mqst of the room and pillar mines, remnant pillars in panels are recovered by retreat mining (pillar recovery). Hence, the above mentioned methods are not appropriate for pillar design, because in these methods pillar load is estimated by the tributary area theory, and the effects of abutment loads, which result from retreat mining and the creation of a mined out gob, are ignored. Abutment loads affect the pillars in the adjacent of pillar line and a load greater than what is estimated by tributary area theory is applied on the pillar. So, pillar design without considering the abutment loads leads to pillar failures during retreat mining. Pillar failures continue to be one of the greatest single hazards faced by underground coal miners. Pillar failures are responsible for unsatisfactory conditions, which include the following (Mark et al., 2003):"

Jan 1, 2010

By Stephen C. Tadolini

Cable supports offer several advantages over traditional secondary support methods. Cable supports enhance stress redistribution to pillars and gob areas, minimize or eliminate timbers and cribs which reduce ventilation capabilities, eradicate material handling injuries related to the placement of crib supports, and provide a cost-effective alternative to secondary support. The U.S. Bureau of Mines has designed and installed cable supports to improve the stability of tailgate entries in an underground western coal mine. Two entries closest to the Iongwall panel were solely supported with high-strength cables installed with cement grout. Basically, with modifications in the intersections, 360 ft of roadway nearest the longwall panel was supported with two rows of 22-ft-long cables installed on a 7-ft spacing. Approximately 250 ft of a second bleeder entry was supported with two rows of 16-ft¬long cables installed on a 7-ft spacing. Individual cable loads were monitored with hydraulic U-cells and pressure pads. The roof behavior was simultaneously monitored with differential magnetic sag stations and closure meters on 25-ft spacing during longwall panel retreat to evaluate the response of the immediate and main roofs. Cable supported gateroads appear to be a viable alternative in secondary support when compared to wood timbers and cribs. This paper describes the theory, application, and advantages of cable supports and presents the results of the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1993

By W. M. Pytel

The applicability of the beam theory in analysis of roof- pillar-weak floor interaction in partial extraction room-and-pillar mining is presented. The mine structure is modeled as an equivalent multi-indeterminate overburden elastic beam supported by elasto-plastic pillars; resting on a viscoelastic layer of immediate weak floor strata underlain by a competent rock mass. The developed analytical model was initially utilized to conduct sensitivity analyses of different variables affecting the mining system, such as the deformability of coal and weak floor strata, thickness of weak floor strata, number of pillars in a panel, width of pillars, width of panel etc. These analyses were then extended to three overburden strata - coal pillar - floor strata lithologies typical of active coal mining areas in Illinois.

Jan 1, 1989

By R. J. Morrell

The Bureau of Mines developed a ground support system for use with an underground coal auger. The system consists of inflatable airbags that are inserted in the holes as they are drilled, and then gradually deflated to provide controlled roof caving. This paper presents results of laboratory and field tests of the system that show it to be technically feasible.

Jan 1, 1988