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By Hamid Maleki

Results of geologic investigations, in-mine instrumentation, and numerical modeling are presented for two longwall mines in which pillars with large width-to-height ratios (8 to 9) are used. These investigations revealed that wide pillars unload when subjected to sufficient stress and that pillar unloading is influenced by geologic conditions. Field measurements in mine 1 clearly indicated load transfer from the longwall area toward the gate roads, resulting in pillar failure near the margin of a fluvial channel. Loads were shown to transfer to a harrier pillar (width height ratio equal to 9), contributing to progressive failure of this pillar. Unloading of the barrier pillar was associated with significant load transfer to the two-seam submains to the north of the harrier, contributing to roof falls, floor heave, and rib spalling within the submains. Stress analysis results addressed pillar peak strength and unloading behavior that was in agreement with the measurements and underground observations. Underground observations and stress analyses were completed before and after longwall mining near a six-entry submain in mine 2. This submain was intended to he used as a temporary bleeder and tailgate for a small longwall block consisting of one to two panels, depending on the development schedule in other areas of the trine and observed ground conditions. Numerical modeling using both elastic and inelastic pillar behavior for submain pillars with a width-to-height ratio of 8 was completed before mining began. Results indicated that pillar unloading was likely. During retreat of the first longwall panel, pillars unloaded. as evidenced by progressive rib spalling along the cleats, This behavior was mostly confined to the immediate face area, an observation that was in agreement with the modeling results,

Jan 1, 2000

By John Magyar

Special foamed cements were used to stabilize and secure caving ground in a newly-reamed 12.5-ft diameter underground ventilation raise. Two zones of very poor Rock Mass Rating (RMR) material were encountered during reaming of a ventilation raise at Placer Dome's Turquoise Ridge Mime in Northern Nevada These zones caved to large cavities, continued failing, and could not be safely secured using standard rock bolting, wire mesh, and shotcrete procedures. In stable sections of the raise, the ground control plan called for the contractor to install 8-ft and 124 length resin grouted No. 7 Dywidag bolts, chain-link fence mesh and shotcrete. This work was conducted on three decks installed on the raise bore reamer. The first of the caved areas was approximately 1,500 cubic yards in volume and extended out to 40 ft beyond the wall of the raise. Placer Dome considered several options for structural fill to repair the area including pumped shotcrete, concrete, and placing and grouting aggregate. A safe, cost effective, and expedient method of stabilizing the raise with foaming grout and bolted liner plate was selected and successfully executed. The special fast-setting, foaming cement grouts used were Tekfoam and Tekseal® manufactured by Minova. The raise was quickly placed in service. Factors considered in selecting the foaming grout are discussed.

Jan 1, 2005

By Matthew J. DeMarco

Research personnel at the U.S. Bureau of Mines have conducted longwall gate stability studies at four western U.S. mines attempting to employ yield pillars for improved ground conditions. Located in the Book Cliffs and Wasatch Plateau coalfields of north-central Utah, these operations are generally characterized by multiple-seam mining, cover depths averaging 450 - 600 m (1,500 - 2,000 fl), occasionally extending beyond 760 m (2,500 ft), abrupt cover depth variation due to the overlying canyon/mesa topography, and massive, rigid sandstone roof units. Such conditions encourage severe gate and panel coal bumping, as well as various types of roof instabilities and occasional floor-heave problems. To alleviate these adverse conditions, several mines have employed yield pillars to-reduce hazard-inducing excessive gateroad stress concentrations. This paper describes the evolution of gateroad designs at four of these operations, the specific mining conditions involved, and the general stability performance attained by using yielding gate systems. Mine experiences are "compared" in terms of basic setting conditions, and the general concepts of yielding system applicability and success potential are summarized.

Jan 1, 1993

By A. W. Khair

This paper presents an analysis of ground movements recorded from longwall operations in the Northern Appalachian region in West Virginia. The site chosen for the investigation was selected due to varying heights of overburden and topography and was instrumented for both surface and subsurface measurements. The instrumentation areas consisted of two panels covering various sections with unique geologic and topographic features. Instrumentation included surveying monuments and shallow boreholes using an inclinometer for measuring horizontal strata displacements. Vertical strata deformation was monitored utilizing a unique design of the mechanical grouting method. Full length borehole profile extension was measured using Sondex displacement rings in the inclinometer installation. Subsurface displacements, i. e. caving zone, fractured zone, and highly deformed non-fractured zone were identified. The nature of surface disturbances with respect to varying geologic settings were compared, and an attempt was made to simulate final strata caving and bed separation using finite element analysis.

Jan 1, 1987

By Ihsan Berk Tulu

The complex interaction between multiple seam pillar extraction panels and variable topography can generate unexpected ground control problems. This case study describes stress-related damage in multiple seam workings that resulted in difficult operating conditions and compromised the safety of the support crews. The damage appeared to be caused by horizontal stress but the unexpected location of the damage baffled the authors. A 2D Finite element analysis was conducted of the mine layout and surface topography in an attempt to explain the observed failure. It was discovered that the topography resulted in the rotation of the major stress at the location of the current workings. The stress rotation caused asymmetrical interactions between the workings on the different coal beds, which can explain the unexpected response observed in the mine. Consequently the layout of the next panel was changed by locating the vulnerable entry 30 ft away from its originally planned position. The new location was selected to locate the entry in favorable stress conditions. The change in layout resulted in a significant improvement in conditions when the next panel was mined. At the time of writing this paper, a full panel has been developed using the modified layout without undue ground control problems. The case study demonstrates that numerical models can help engineers to interpret and solve complex multi-seam interaction problems.

Jan 1, 2014

By Douglas C. Peters

Part of the ground control research at the U.S. Bureau of Mines is involved with improving ground control data collection and analysis methods. A geographic information system (GIs) is being used within this research effort to standardize the data collected and provide a common graphical and tabular format and analysis system for researchers to access information on underground coal mines. In addition, GIS's can perform relational analyses on multiple map and tabular data sets which are not possible through map-creation or CAD packages or through data bases alone. In addition to standardizing ground control analyses, there is an increasing need for rapid analysis of ground control data in connection with increasing production rates at longwall mines. The examination of a large number of interrelated parameters and their impact on short-term mine safety and productivity under the time constraints imposed by rapid production requires computer-aided techniques. A GIs provides the means to quickly, quantitatively, and reproducibly evaluate mine-wide conditions. This overview of GI8 techniques for mine data integration and analysis, using a mine in Colorado as an example site, will cover the input of data into a GIs, including mine-wide monitoring system data and general data such as roof control plans and geology. The methods to geographically reference these data sets and retrieve and combine them also will be covered. Automation of such processes briefly will be reviewed to allow future users to automate their mine data analyses.

Jan 1, 1990

By W. C. Smith

Over the past several years, the Bureau of Mines has conducted studies on the performance of fully grouted resin bolts In underground mines. The effectiveness of resin-grouted bolts in maintaining a competent roof, even in weak rock, has been well documented in the literature. Recent Bureau studies have evaluated the affects of shortened grout columns on bolt behavior in a variety of rock types (1, 2, 3). The application of partial grout columns to primary roof support systems has been introduced as a viable alternative to fully grouted bolt systems in some mines. The point-anchored resin bolt and combination bolt represent such alternatives. While both systems usually employ some mechanical interlocking system, their primary difference lies in the length of resin column used. Point- anchor systems rely on an anchoring device that is a short length of resin column of 2 feet or leas. Combination bolts, on the other hand, often require 3 feat or more of resin column. Resin migration into voids and fractures intersecting the borebole wall is a major problem when installing fully grouted bolts in weak roof rock and can result in a reduced column length that provides reduced anchorage. The relationship between column length and anchorage integrity deserves closer study. The Denver Research Center of the Bureau of Mines has examined the load- carrying capabilities of resin-grouted bolts in weak strata. Pull tests were performed on about 900 resin-grouted, 4- foot-long bolts with column lengths ranging from 6 to 48 inches. The practical performances of 8 different column lengths were evaluated for 10 mines under in sit" conditions using data obtained from standard pull tests. In addition criteria are discussed, including engineering properties of the host rock for designing safer and more effective grouted bolt installations.

Jan 1, 1987

By Morgan M. Sears, Mark Van Dyke, Gamal Rashed, John Rusnak, Khaled Mohamed

"In 2016, room-and-pillar mining provided nearly 40% of underground coal production in the United States. Over the past decade, rib falls have resulted in 12 fatalities, representing 28% of the ground-fall fatalities in U.S. underground coal mines. Nine of these 12 fatalities (75%) have occurred in room-and-pillar mines. The objective of this research is to study the geomechanics of bench room-and-pillar mining and the associated response of high pillar ribs at overburden depths greater than 1,000 ft. This paper provides a definition of the bench technique, the pillar response due to loading, observational data for a case history, a calibrated numerical model of the observed rib response, and application of this calibrated model to a second site. INTRODUCTION Bench mining is a an underground mining technique typically applied to room-and-pillar mines where full seam extraction on development presents an unacceptably high risk of injury from high pillar ribs or where the mining equipment is not designed to extract the full seam thickness. To mitigate the increased risk associated with high ribs and slender pillars, a more modest thickness is extracted from the top of the seam on development. This is followed by grading the floor and recovering the bottom of the seam with or without extraction of the pillars during retreat mining. A mine located in Eastern Kentucky extracts two seams at depths ranging from outcrop to 2,000 ft. In some areas of the mine, Seam A and Seam B are close enough together to be mined simultaneously. Seam A (top) is mined on development 7 to 9 ft. high for the entire length of the panel. On retreat, as each pillar is extracted, the continuous miner is ramped down into the floor (Seam B), two pairs of Mobile Roof Support (MRS) units are set, and the pillars are extracted at the full mining height of 14+ ft. During the pillar recovery process, the floor is extracted in each entry, from a ramp initiated outby the retreat line, sequentially just prior to extracting the lifts from the pillars (see Figure 1)."

Jan 1, 2017

By Gregory M. Molinda

Difficult mining conditions in deep western U.S. Iongwalls have necessitated the use of novel tailgate support systems. Extreme ground movement in these yielding gate systems has caused operators to utilize secondary support systems which match yield and strength characteristics with expected ground reaction. Other benefits include ventilation and safety gains, as well as lower costs. Recently, the use of novel support has been tried at longwall mines in other U.S. coal basins. One of the main considerations is the capability of the support to maintain an adequate airway for gob ventilation in gassy eastern coal mines. In one large Pennsylvania mine working the Pittsburgh seam, a number of support configurations were monitored over a 2,300 ft tailgate test area. The test included 4-pt wood cribs, Link-N-Lock cribs, Propsetter yieldable posts, cable trusses, and vertical cable bolts. Ground reaction, and thus support performance, was documented by roof sag and convergence instruments, ventilation measurements, and photographs. Various combinations of standing support and cable truss and vertical cable bolts provided adequate support for ventilation and safe escape. A cribless configuration consisting of only cable trusses and vertical cable bolts was inadequate to provide inby gob ventilation to the next open crosscut. Instrument ground movement data and visual documentation are presented.

Jan 1, 1997

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

By Gregory J. Chekan

Developing a coal seam that has been influenced by previous mining in seams either above or below can result in severe ground control problems. In many instances, interactions between operations are inevitable. but improvements in ground stability can usually be achieved by-predicting potential problem areas in advance and adjusting the mining plans accordingly. This report investigates four case studies involving interactions between operations that used both room-and-pillar and longwall mining methods. In each case, geologic and mine design parameters are addressed, and those parameters found critical to the interaction are further defined in relation to observed ground problems.

Jan 1, 1990

By Lu Jiam Zhang

Coal and gas outburst is one of the serious natural disasters in the coal mines of China. Since 1950, more than 10,000 coal or gas outbursts have occurred in more than 200 underground coal mines of different types. The outburst prone coal mines are widely spread out. It occurs in different types of coal -ma and is very serious in some areas. Most outbursts occurred at the time of roadway driving in coal seam, serious outbursts occurred mainly when entering the coal seam during the cross-cut driving; of the outbursts, coal and gas outbursts We UP major proportion, while pressing out and pouring out ah account for a large proportion; outburst. occur under different operating conditions, but mostly occur during shotfiring. This paper introduces the prevention and control measures as practiced in China and illustrates the three types of outburst cla8sification. Both technology and management are equally important in outburst prevention and control. Measurement of the volume of drilling cuttings and the initial gas outflow rate fm- boreholes have proven to be effective. A special team and organization must be set up to deal with the problems.

Jan 1, 1987

By Takashi Sasaoka

EGAT Mae Moh Lignite Mine in Thailand produces about 16 million tons of lignite annually from open-cut mining. The coal is used to generate 2,400 MW of electricity. In the near future, however, the coal must be extracted from underground at 400? 500 m (1,312?1,640 ft) deep from the surface. The problem is the thickness of the coal seams. There are two coal seams that are about 25 m (82 ft) thick with 20?25 m (66?82 ft) of interburden between them. Up to now, many mining systems for thick coal seams have been used and improved in the world. However, where the thickness of coal seams is beyond the limitation of the current mining technology and system, an applicable mining system has to be introduced, considering influencing factors such as caving and subsidence. In order to investigate thick seam mining systems and propose a new mining system in ultra-thick coal seams in Thailand, a joint research group was established by Japanese and Thai researchers and mining engineers in early2009. This paper describes underground mining systems for thick coal seams and proposes an innovative underground mining system for thick coal seam, ?eco-friendly underground mining system?, considering geological, geotechnical, and environmental conditions in this mine. This paper discusses its applicability to this mine by means of numerical analysis from the geotechnical point of view.

Jan 1, 2011

By John L. Hill

Over 50 mines of the Appalachian and Illinois Basins are presently experiencing poor ground conditions believed to be caused by overlying stream valleys. The Bureau of Mines is conducting research into the causes of this problem with the aim of developing a predictive method for determining the safe limits of mining beneath stream valleys. In addition, the factors that control instability in these regions will be analyzed to provide design guidelines for adjusting support requirements. This paper presents the theorized causes of poor ground conditions beneath stream valleys and compares the theories with the results of a Bureau study of the Birchfield Mine No. 1 and a preliminary investigation of the Davidson Mine, both of southern West Virginia. The Birchfield Mine excavated four main entries beneath a 50-ft wide stream with only 50 ft of overburden. The valley flood plain is over 500 ft wide, flanked by valley wall slopes of 25 deg and relief of over 950 ft. In-mine monitoring of strata movement and detailed geologic mapping revealed that roof rock strength was greatest under high cover and weakest along outcrops and beneath the valley, with localized areas of decreases in rock mass quality. To improve the rock mass quality beneath the stream and reduce the threat of inundation, an extensive drilling program was undertaken to inject grout into the overburden from the surface. As a result, mining beneath the stream was practically routine from a ground control perspective, and water was not encountered until mining had advanced to a point beneath the floodplain that had not been grouted. The only ground control problems which were encountered were entirely a result of localized poor rock mass quality. In contrast to the Birchfield Mine, the Davidson Mine experienced failure of intact roof rock at a depth of 300 ft beneath a V-notch shaped valley with no change in rock mass quality. Finite element analysis was conducted to study the premining in situ stress at the two sites. The results are discussed within this paper and provide insight into the causes for the types of failure seen beneath valleys of varying shapes and depths.

Jan 1, 1988

By A. Wahab Khair

This paper presents results of the experiments carried out in Beth Energy No. 33 Mine for the purpose of alleviating problems associated with cutter roof. Cutter roof problems have delayed the advance of the entry development considerably and maintenance cost of these entries was very high due to the requirement of very extensive and heavy artificial supports. The study includes assessment of the problem, investigation of the geology and lithology of the mines, study of in-situ stresses and corrective measures to combat the problem. Two approaches were selected as the corrective measures: 1) implementing yield pillar concept in order to reduce the effects of in-situ horizontal stresses, 2) designing an effective roof reinforcement system and application procedure. Results of this study revealed that roof strata is weak and in-situ stresses are very high at the problem area which results in development of cutter roof. Implementation of yield pillar concept was found to be ineffective because cutter roof often developed at the entry face concurrent to the development. However, proper design and implementation of roof reinforcement system stabilized the roof and eliminated roof caving during both entry development and retreat of longwall face, despite existence of the cutter roof, at a much lower cost.

Jan 1, 1992

By Anil Mahyera

Under contract to the Bureau of Mines, a roofbolting machine has been developed which will: control torque on the bolthead to within 4 percent of a set point, control thrust to a low level (150 lbf), and implement a tightening method to minimize variability due to bolt tension decay of a group of bolts caused by differing anchor creep rates. This bolter was used to determine bolthead and thread friction factors, and to determine the effect of adding a hardened washer to the bolt assembly on the uniformity of bolt tension in the laboratory and field. Recommendations for torquing speed, bolt lubrication, use of hardened washers, and thrust level on the bolthead are made. The relationship of this system to Improved roof control is discussed, along with additional work that remains to be performed. Patent Notice Patents are pending which protect the hardware and methods of controlling tension decay as discussed in this paper.

Jan 1, 1981

By Stephen Tadolini

Reduction in the time required and improvements in safety during a longwall face move have compelled coal mine operators to examine and use pre-driven longwall recovery rooms. While this concept is not without controversy and risk, the rewards can result in substantial reductions in longwall face move/set-up times and provide safer working conditions. Both of these factors can reduce mining costs and enhance profitability in a competitive market. This project was even more complex because the longwall system operates under a weak roof condition. To examine the feasibility of this concept, a 16-ft wide by 170-ft long experimental pre-driven recovery room was designed using analytical, empirical and 3-dimensional finite element modeling techniques. Based on the projected loading conditions, immediate and main roof geological and strength characteristics, a variety of roof bolt types were selected and installed in the recovery room and adjacent chutes. To supplement the intrinsic supports, high-capacity roof-to¬floor cuttable cribs were strategically placed to reduce the potential of a weighting failure before the longwall shields safely entered the room. To evaluate the behavior and final support performance, the recovery room and chutes were instrumented to monitor the physical response during various loading phases. Additionally, the cuttable cribs were instrumented to evaluate the loading and stiffness characteristics during the extraction process. This paper describes the successful design process, support methodology, monitoring results, final observations and recommendations.

Jan 1, 2002

By Vladimir Adamek

Two existing predictive methods were chosen for evaluation; an influence function (Gals Theory) and a profile function (hyperbolic). These were applied to several field measured subsidence pro¬files over United States coal mines. The effect of homogeneous versus non-homogeneous overburden on surface subsidence is demonstrated and the utilization of Bals Theory as a subsidence predictive method for both types of overburden is examined. The major factor affecting subsidence profile characteristics is the migration of the inflection point toward the centerline of the longwall panel. A computer program was written to aid in the analysis.

Jan 1, 1981

By George Gardner

The rapid growth of highwall mining in the Appalachian coalfields has resulted in unique safety concerns. Due to the concentration of activity at the base of the highwall and the potentially destabilizing influence of the mining process, proper mine planning and continual monitoring of the ground conditions is essential to ensure a safe and productive operation. This will also help to minimize the need to recover equipment that has become entrapped underground, and the subsequent hazards associated with a recovery operation. Adequate geotechnical site investigation and rational design methods should enable mine planners to properly account for the site-specific conditions that influence web and abutment pillar, roof, floor, and highwall stability. This paper examines the growth of highwall mining, provides a general overview of the various safety concerns, and reviews the modes of highwall failure as they relate to a highwall mining operation. Gcotechnical considerations that should be taken into account before and throughout the mining operation are emphasized. The importance of closely examining and monitoring highwall conditions for subtle signs of highwall instability will be discussed.

Jan 1, 2002

By K. Y. Haramy

Strong roof helps minimize roof fall problems in coal mine entries. However, the Inability of strong roof to cave readily may contribute to major ground control problems In Iongwall and retreat mining operations. The concern expressed by mine operators prompted this Bureau of Mines study to analyze the effects of strong roof members on ground stability around Iongwall openings. The problem was approached from three directions: development of a theory to analyze the strain energy conditions In the coal and surrounding strata, modeling the effects of different thicknesses and strengths of Coal and roof strata, and field Instrumentation of powered supports In two Iongwall mines. Results from the theoretical and modeling analysis confirm the accepted belief that a strong roof does, In fact, play a major role in high stress concentration problems In and around the Iongwall panel. Analysis of the effect of different roof overhang dimensions and properties on relative strain energy buildup In the coal and roof Is presented. Shield pressure data Indicate possible methods for detecting noncaving roof behind the shields. Strategies to monitor roof overhang and control dangerous conditions caused by the noncaving roof are also presented.

Jan 1, 1988