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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 Gary B. Petersen

The author has developed a seven-step process called the Applied Rock Mechanics (ARM) Protocol that the author has used to solve numerous roof stability problems in mines throughout North America since 1975. It is a practical evaluation process using trained observation, innovative data collection instrumentation, video borehole cameras, analytical processes, and controlled testing methods that have become highly effective in solving ground control problems in every type of underground mine. This paper will describe each of the steps of the protocol and how the rock mechanics engineer applies them to resolve a typical ground control problem.

Jan 1, 2012

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 John H. Perry

Strata III is a full-spectrum roof control system which provides the mine operator the ability to upgrade his roof control in place. The procedure is easily adaptable with current roof bolting techniques. To use the system, the operator starts by installing a special hollow core mechanically anchored roof belt during the roof bolting cycle. This bolt can be upgraded to a fully grouted tensioned rebar bolt by injecting StrataGee G-300, a special polyurethane grout, around the bolt through an injection port in the roof plate. This same bolt can then be used to inject RokLok binders for roof consolidation by pumping the binders through the core of the bolt. This concept in roof control was developed to provide the mine operator a flexible way to maintain roof control in areas which deteriorate over time, or must be kept open for long periods of time. Possible locations of use would be: longwall set-up rooms, recovery rooms, headgates, intersections, and belt entries.

Jan 1, 1987

The basic objectives of mine roadways are to provide sufficient cross sections in order to meet the needs of accommodation of equipment, transport, personnel travel and ventilation. However, many roadways become damaged to the extent of needing maintenance, generally dinting, and in some cases requiring re-ripping. The damaging of a roadway is a result of several factors including geological properties of surrounding strata, method of roadway formation, type and spacing of support and so on. Among these factors, strata conditions play an important effect on the stability of roadway. Weak rocks cause excessive roadway closures and water softens some rocks and promotes the closure problem. In this paper, a scientific discussion on the effect of water on the stability of roadways is given on the basis of results obtained by means of in-situ investigation and laboratory rock tests.

Jan 1, 1996

By Khaled Morsy

In the past two decades, high horizontal stress has been attributed to many ground control failures. Most believe that entries parallel to the high horizontal stress is most stable and least stable when perpendicular to it. Entry stability decreases when the angle between entry and horizontal stress increases. The concept of 2-D stress shadow has been proposed for selection of proper panel orientation, often with mixed results. In this paper 3-D finite element modeling was used to analyze the stress distribution at the headgate of a longwall panel subject to high horizontal stress at various angles. It was found that the directions of induced principal stresses at the longwall face and headgate are independent of the panel orientation. The induced principal stresses concentrate or relief at headgate based on the panel orientation with respect to the directions of the induced principal stresses. Accordingly, only 3-D modeling can fully account for the effects of high horizontal stress on panel orientation. The paper illustrates in details the changes in orientation and magnitude of all three principal stresses around the longwall face and T-junction area as panel orientation changes.

Jan 1, 2006

By Murali M. Gadde

Perimeter mining is a special form of very productive partial extraction method practiced at some mines in the United States. In the Illinois Basin mines where all major coal seams have weak underclay in the immediate floor, experience shows that any long-term ground control issues will arise due to floor failures rather than instability of the coal pillar itself. While there is a significant amount of work done on floor stability associated with regular pillars, no work has been done so far on the stability of perimeter pillars. This paper reports the results of a comprehensive study that was conducted for the first time on floor stability during perimeter mining in the Illinois Basin mines. A special plate template, which mimicked a common perimeter pillar configuration was created to conduct in-situ bearing capacity tests at five mines in three states of the Basin. Additionally, because of the complex shape of pillars created in perimeter mining that defies any closed-form solution, floor bearing capacity was studied using three-dimensional nonlinear numerical modeling. Based on these in-situ and numerical studies, some guidelines are provided for perimeter pillar stability analysis.

Jan 1, 2009

By Andre Cezar Zingano

"The objective of this paper is to estimate the coal seam strength and the pillar safety factor for low coal seams using back analysis and rock mass characterization. The coal seam strength is very important to estimate both pillar capacity and mine section design. There are two approaches to estimating coal seam strength: (i) by geomechanics characterization of the boreholes cores; and (ii) by back analysis from pillars successes and failures, or some in-situ tests. For low coal seams (1.2 m, or 47 in.), the confinement of the center of the pillar is very important to defining global pillar strength because the low seam height increases the confinement of the pillar core, even in small pillars. We used in this case study one underground coal mine in the Parana State where the coal seam thickness is 0.8m the pillar height is 1.3m, the entry width is 5m and seam depth is 130m. A panel test was prepared to study the pillar behavior for use of the retreat mining method. The designed pillar size was 6x5m and 1.3m high with a predicted safety factor of 1.0 based on the Hoek and Brown failure criterion is 5.3 MPa. Once no pillar failure was observed, the pillar safety factor was assumed at 1.0 and the coal seam strength was backcalculated. The back analysis estimated the coal seam strength to be 6.2MPa. The confinement of the pillar center directly affects the coal pillar strength, and also the coal seam strength estimated by back calculation.IntroductionThe coal seam strength is very important to estimating pillar strength and mine section design. There are two approaches to estimating coal seam strength: (i) by geomechanics characterization of the coal seam using borehole cores; and (ii) by back analysis from pillar successes and failures, or some in-situ tests like panel test or ultimate pillar capacity (Salamon and Munro, 1967; Maleki, 1981; Peng and Dutta, 1992). For the first approach, lab tests (mainly triaxial tests) and borehole geomechanics descriptions (quality and quantity of the discontinuities) determine the coal seam strength by the Hoek and Brown failure criterion (Hoek and Brown, 1997). For the second approach, a panel test to study the pillar behavior can be built to define coal seam strength by back calculation. The safety factor for the pillars in the test is 1.0, based on the estimated coal strength from the geomechanics characterization. In this case, the probability of failure is 50%. If some pillar failure occurred, and the real pillar size was measured, the back analysis could be applied to define pillar strength and the coal seam strength."

Jan 1, 2015

By George S. Kalamaras

Coal strata strength is included as a vague term in most of the pillar design formulas, either as a questionable fraction of the intact coal strength or a size-dependent relationship for scaling down a laboratory value to the in situ strength. In order to introduce a coal mass strength criterion which would be compatible with current design trends, the Rock Mass Rating (RMR) system was revised to incorporate factors governing coal seam behavior under load. A number of adjustment factors have been proposed but, the most significant change, is the introduction of a parameter representing the heterogeneity of the seam. When developing a rock mass strength criterion, it is necessary to find the constants of appropriate intact rock strength criteria and to establish a correlation to scale down these constants as a function of the RMR. For four intact rock strength criteria, the constants for coal were found using the Nelder-Mead method. To correlate the constants of the strength criteria with the coal seam quality values (RMR), a procedure was developed based on finite element modeling of in situ large scale tests for which the RMR was known. A new coal strata strength criterion is proposed, in a principal stress formulation, for modeling coal pillar behavior when the stress field in a pillar is known. A linear relationship is also given, to be used when empirical pillar strength formulas are used to design pillars. The proposed criteria have been evaluated in a case study.

Jan 1, 1993

By Brian E. Travis

The recent development of polymer grids for the underground coal mining industry has created new alternatives for supplemental ground control practices. The grids utilize strong, lightweight polymers - usually special grades of polypropylene. High tensile strengths and resulting load support characteristics are achieved by molecular orientation of these polymers in the manufacturing process. One of the most important applications of polymer grids as supplemental ground control is in longwall shield recovery. When shields are moved from one face to another, the determining factor in the success of the recovery is the ground control provided by roof support structures along the old face. Whereas primary support is usually provided by roof bolts and cables which run the full width of the panel, supplemental support is often provided by metallic meshes of welded wire or chain-link fence. lightweight, high-strength polymer grids replace these heavy, cumbersome metallic meshes, giving the operation increased productivity by decreasing installation time and reducing injury downtime.

Jan 1, 1990

By Stephen C. Tadolini

The U.S. Bureau of Mines (USBM) is conducting research to provide alternatives for traditional secondary support methods. These cost-saving methods are proving to afford safer installation, improve entry stability, eradicate material-handling injuries associated with crib supports, and enhance ventilation. In cooperation with Mountain Coal Company, the USBM replaced timber cribbing with high- strength cable supports to provide stability in a gate road that was used for two longwall panels. The support design consisted of 2.1-m (7-ft) full-column resin-grouted bolts and 4.8-m- (16-ft-) long high-strength resin-grouted cable supports. Three support concepts were evaluated: passive, stiff, and tensioned systems. Cable loading and roof deformations were monitored to evaluate the behavior of the immediate and main roofs during panel extraction. The stress and loading histories for both panels and the abutment and yield pillars were monitored to assist in evaluating the stress transfer and pillar performance in conjunction with the roof behavior. The test results indicated that resin- grouted cable support systems successfully maintained the immediate and main roofs during the extraction of two longwall panels. This paper describes the theory, application, and advantages of resin-grouted cable supports and presents the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1994

By John K. Wood

The development of an expanded cement product represents a major advance in the technology of roof control systems. Such a product is gaining recognition as a valuable tool in today's cost- conscious coal mines. The National Coal Board's Bretby Research Lab, in conjunction with Polement Ltd., developed the first lightweight cement for use in the mines of the United Kingdom. The product, Aqualight, is a low-density (about 12.5 PCF) cement with a typical compressive strength of about 19 p.s.i. The final product increases in volume by approximately 15 times its dry powder volume. Aqualight gels in about 2 minutes, becomes self-supporting in 15 minutes, and fully cur& in 24 hours. When subjected to a load, the "cells" within its bubble structure collapse so that the product gains in density and compressive strength while retaining its integrity. Traditional methods to contain and support roof fall cavities through cribbing and blocking are both dangerous and labor-intensive. Aqualight represents a significant advantage over such traditional methods. It is non-toxic, non-cmbustible, and very easy to install. Aqualight can be pumped to the worksite from re- mote locations (as much as 600 feet), thereby reducing danger to men and equipment. Addition- ally, the strength of the cured product can be varied by adjusting the expansion ratio and resulting density. A compressive strength of 150 p.s.i. can be obtained in this manner. Although primarily designed to fill cavities resulting from roof falls. Aqualight has found a wide range of applications in mining industries around the world. In addition to cavity- filling, Aqualight has been used for ventilation stoppings, fire stoppings and emergency fire control, backfilling between arches and the roof and ribs, and backfilling of old workings. Since its recent introduction into the United States. Aqualight has been used by MICON SER- VICES, INC. to fill several roof fall cavities along headgate entries. In all cases, the cavities were contained and the longwall moved through without problems. Roof Pall cavities along main haulage entries and at shaft/slope bottoms have been successfully filled. MICON has also installed Aqualight as backfill material around arches. A current major project is the installation of trial ventilation seals using Aqualight as an alternative to traditional block and mortar construction. Pending MSHA approval, MICON anticipates the use of Aqualight in the construction of both ventilation seals and stoppings. In addition to new construction, Aqualight can be employed to repair stoppings damaged by roof convergence and floor heave. In labor savings alone, this is very attractive for remote stopping repair. Among the many other uses of Aqualight is the backfilling of old auger holes, which will support the highwall and allow access to coal re- serves which are presently sterilized.

Jan 1, 1986

By Stephen P. Signer

To improve the understanding of the support interaction mechanics between grouted bolts and coal mine roofs and to help lay the foundations for improved design and evaluation techniques, the 0.8. Bureau of Mines has conducted laboratory, field, and analytical studies of the load transfer mechanics of grouted roof bolts. Mechanisms by which load is transferred from a bolt through the grout to the rock mass are analyzed. Strain gauges were installed on over 70 bolts and tested to observe the rate of load transfer when a force was applied at the bolt head. Laboratory and field tests were conducted to determine bolt-grout- rock interaction behavior in both the elastic and postyield phases. Long-term laboratory tests were run to determine the time-dependent properties of both polyester resin- and-gypsum-grouted bolts. Numerical studies using finite- element techniques were compared to the experimental results. Elastic tests showed that 90 pct of the applied load was transferred within 24 in of bolt length. Long-term tests indicated that creep is more significant in gypsum-grouted bolts than in resin- grouted bolts. Plastic studies showed that yielding of steel will translate down the length of the bolt from 6 to 20 in.

Jan 1, 1988

By Nicholas P. Kripakov

The unpredictable massive collapse of roof in openings developed under apparently safe and stable roof conditions in coal mines of the United States has been of much concern for many years. One type of massive caving phenomenon is often attributed to cutter roof, a failure that initiates at one and/or both upper corners of an entry and propagates nearly vertically resulting in overall failure with little or no warning. This paper reviews current methods being used to control cutter roof and suggests new alternatives. The impact of basic parameters that include mine geometry, overburden geology, rock properties, and in situ stress conditions is discussed in the context of their relationship to this problem. The effects of various stress control measures are investigated with model analysis to gain insight into means of reducing the high level of stress known to exist at the entry corners of a West Virginia coal mine prone to cutter roof. The results are presented in the paper and the feasibility of their practical implementation is discussed.

Jan 1, 1982

By Yi Luo

Subsurface strata movements and deformations associated with underground mining activities could cause problems to subsurface structures and water bodies. By incorporating the methods for surface subsidence prediction with the subsidence parameters derived from collected subsurface subsidence data, methods have been proposed for the prediction of final and dynamic subsurface movements and deformations induced by underground longwall mining. Two new' types of strata deformations in the field of subsidence research, vertical and total strains that could he useful in assessing subsidence influences to the subsurface structures and water bodies, have peen introduced.

Jan 1, 2000

By Stephen C. Tadolini

A joint research program was designed and conducted at Lodestar Energy?s Baker Mine to investigate the performance of a high-capacity tensioned cable supported gateroad. The secondary system consisted of a 4.5 m (14.5 ft) 7 strand cables that were 23 mm (.9 in) and 18 mm (.? in) in diameter with a designed capacity of 515 kN (58 tons) and 355 kN (40 tons), respectively. The cables ware installed with an effective resin column length of 1.25 m (4 ft) and then pre-tensioned using a hydraulic jacking system. Roof separations, caving characteristics, and support performance was monitored and evaluated during the critical extraction of the adjacent panel, complex geological conditions of the west Kentucky No. 13 seam were mapped and the support performance recorded. This paper will describe the theory of tensioned cable systems and present an assessment of the support performance during the longwall retreat mining process.

Jan 1, 2000

By Daniel S. Mazzei

This paper will be a general discussion of surface flooding considerations as they affect mining facilities site selection, based primarily on experience with several such problems with which MSHA's Bruceton Technical Support staff has been involved. The detailed analytical procedures used in arriving at engineering solutions of these problems will not he presented here; the emphasis is on drawing attention to this additional aspect of mine design which hitherto has been largely overlooked due to insufficient awareness, information and criteria. The paper's purpose is to stimulate thought and offer guidance concerning the selection of sites for mining facilities in light of the potential for flooding problems caused by surface waters. Flooding by surface waters, would generally not be a serious consideration in areas of relatively low topographical relief, i.e. flat lands, although there are exceptions to this rule, for example, in Western areas where flash flooding is a major concern. Therefore, this discussion will mainly apply to areas where the severity of surface flooding would be amplified by the topography-large, well defined watersheds of considerable relief, i.e. mountainous areas. In these regions, steep valley walls cause any rainfall to flow quickly into the relatively narrow valley bottoms. These valley bottoms provide little flow area during storm conditions, so deep and rapidly flowing streams result.

Jan 1, 1982

By Leo Gilbride

Western U.S. longwall operators face increasing challenges with optimizing ground control and productivity as mines reach greater depths and coal bursting hazards increase. Some western U.S. mines, many known to be bump-prone, achieved a successful balance between ground control and productivity by transitioning to side-by-side longwall panel mining combined with a yield pillar gateroad system. With this design, development footages could be minimized and pillar bumping averted by controlled yielding at moderate depths, generally in the range of 450 to 600 m. Over the past four decades, the yield pillar system has won wide acceptance among western mines facing pillar bursting hazards, particularly those in the Wasatch Plateau and Book Cliff coal fields of central Utah. However, recent attempts among the deepest Utah operators to mine side¬by-side panels with yield pillars at depths in excess of 600 m have been met with mixed success and, in some circumstances, with serious difficulty. Challenges include violent face bursting and excessive tailgate convergence outby the face, which can be crippling to ventilation. The use of interpanel barriers, i.e., barriers left between longwall panels, offers one possible solution to mining under deep cover with bump-prone geology. Interpanel barriers have already been adopted by Utah's deepest longwall mine, and others are considering their use. The geomechanical implications of mining with and without interpanel barriers, and the competing tradeoffs between ground control and ventilation are discussed.

Jan 1, 2004

By John P. Dunford

Ground support continues to be the most important safety consideration in underground mining. The Bureau of Mines has developed a yielding steel post with a capacity of 45 tons and b c1osure allowance of 24 inches in a 6 to 7 foot seam height. The post mechanism in a three piece unit consisting of a top and bottom leg made from standard steel pipe and a separate foot bracket. It develops its load carrying ability when the lower pipe, with an attached interference ring, is forced into the larger top leg. The walls of the top leg are deformed radially and along the axis. Yield continues until the unit bottoms out. Other advantages of the post is that due to its simplicity it can be made at the mine shop level and its cost should be about one-hundred dollars. Prototype units were tested in the laboratory. Based on these results, modifications were made that would make the post perform better as a mine support. Two field tests were conducted. The first test used the units as supplemental support for the wood crib rows in a longwall tailgate. The second and larger field teat had 84 posts placed on salvaged track rails. They were installed on two foot centers in the middle of the entry. Overall the supports functioned well during use in the headgate entry, but problems with the quality of some of the pipe caused localized failures. However, these problems can be attributed to one manufacturer of pipe and through tighter specifications can be eliminated. The use of the posts with the rails is showing additional benefits as a ground support system. By providing continuous support between the posts a beam is created that has much more load bearing. The additional support can reduce floor heave and in some cases eliminate the need for the rib side crib line.

Jan 1, 1986

By Jay Hilary Kelley

This paper proposes a modification of longwall roof support to meet new difficult mining conditions that are anticipated in the future. A gob canopy is a movable appendage attached on the rear of a longwall shield to counterbalance the troublesome forward downward force couple (FDFC). The gob canopy can be swung in both vertical and horizontal directions, powered by hydraulic cylinders. Several functions and applications are de- scribed

Jan 1, 1997