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By Murali Gadde

Among different types of ground control problems associated with underground coal mining, those related to in situ stresses are the most common ones affecting the safety and economy of a mining operation. As a result, this has been the focus of many ground control research works done in the last three decades. This paper summarizes a recent work done on in situ stress related ground control issues of underground coal mine development workings. The approach used in this study was three¬-dimensional finite element modeling by an implicit code, ABAQUS/Standard. In the models, roof stability evaluations were made using Hoek-Brown rock mass failure criterion. Influence of the in situ maximum horizontal stress angle on the stability of both entry and intersection was examined to help determine the best development layout orientation. Also, the effect of in situ stress ratios on the stability of development openings was studied. To make the work more general, both 'low' and 'high' in situ stress fields were considered In general, the modeling results indicated that entries oriented in the direction of maximum horizontal stress were in the best condition while those oriented at 90° had the least stability. For intersections, the maximum and the minimum stability were seen at 0°/90° and 45°, respectively. However, under some combinations of input conditions, the best or the worst conditions were noticed at same other orientations as well. Based on these findings, layout design charts were prepared for both entries and intersections. It was also found that the change in ground conditions with change in layout orientation was more significant for 'low' in situ stress fields than for the higher ones. Change in the average safety factor with change in the ratio of in situ maximum horizontal to vertical stress resembled a lognormal distribution curve. However, the effect of the ratio of in situ maximum to minimum horizontal stress lacked such a clear trend for different input combinations considered.

Jan 1, 2004

By Jialin Xu

Bed separations position, size, development characteristics and their influence factors have been analyzed in-depth by experimental and theoretical methods. The results show that the key strata determine the creation, development and space-time distribution of the overburden bed separations and fractures. Based on the dynamic growth principle during the two-stage development of bed separations, the theory of section-grouting for the overburden bed separation space and the O-ring theory for the pressure-relieved methane drainage are proposed. The technique has been applied in some coal mines in China to reduce surface subsidence and for methane drainage. Keywords: rock strata movements; key stratum; bed separation; bed separation grouting; pressure-relieved methane drainage

Jan 1, 2003

By Robert Trueman

This paper details the results of an assessment aimed at understanding the shield loading mechanisms associated with strata-related issues on a longwall face. Shield load cycle analysis theories developed by the authors were used to quantify the interaction between the shields and strata. Shield pressure data was back-analyzed using a version of the Longwall Visual Analysis (LVA) software that has been extended to enable efficient load cycle analysis to be undertaken. The software outputs enabled the shield strata interactions to be characterized. The results of the analyses indicated that the major cause of the roof falls was high-level periodic weighting, resulting in periodic shield overload. Exploration borehole data was analysed to identify and characterize the overburden unit that was the likely cause of the shield overloading.

Jan 1, 2011

By Larry Howe

Second, or retreat mhng with Mobile Roof Supports (MRS) has now been part of coal mining in the United States for a decade Their utilization has evolved into mining applications which vary in seam heights from 38 inches to 170 inches, and overburden depths from outcrop to beyond 1200 feet This paper will detail the pillaring plans, lift sequences, and mnchky utilized in these operations and the rational for the plans A comparison of the current productivities achieved will be outlined

Jan 1, 1998

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 Chris Mark

The Coal Mine Root Rating (CMRR) was first presented at this Conference nine years ago. Since Its Introduction, the CMRR has been incorporated into many aspects of mine planning. including longwall pillar design, rout support selection, feasibility studies. and extended cut evaluation. It has also become truly international, with involvement in mine designs and funded research projects iii South Africa. Canada. and Australia. The purpose of this paper is to bring the minim community Up to date with recent improvements and applications of the CMRR. The must important new development is a streamlined process for determining the CMRR from exploratory drill core. Just three types of information are Jaw required: Fracture spacing (or RQD) Uniaxial compressive strength for axial Point Load Testing) Diametral Point Load Testing The moisture sensitivity adjustment has also been changed, and new research has related the immersion test to slake durability. Most recently, the CMRR has been implemented in a computer program. which can be obtained from NIOSH free of charge. The program facilitates calculation of the CMRR firm either underground or drill core data. Values from ninny locations can he saved in a single file. and an interface with Autocad allows CMRR contour plots to he integrated 11110 mine planning.

Jan 1, 2002

By A. Grenoble

The mining of seams in close proximity can greatly accentuate interaction problems. At distances of less than 110 feet vertically interaction can occur for both over and under mining. Research into ground control problems resulting from undermining a previously mined seam has been carried out using finite element and body-loaded photoelastic modeling methods in conjunction with numerous case studies. Analysis of field data was used to validate and expand the model results. Research findings demonstrate situations under which the worst ground control conditions can be expected to occur for a specified geologic environment, depth, innerburden spacing and mining geometry. Special emphasis has been placed on the effects of pillar load transfer on the lower seam in terms of pillar and floor stability and roof control. Roof and floor condition indexes are used to summarize structural and lithologic conditions in the affected seam. The effects of joint orientation and continuity are incorporated in the analysis and the research results have been compiled into a software package for application by the field engineer.

Jan 1, 1984

By Deno M. Pappas

The purpose of this U.S. Bureau of Mines study was to determine the material stiffness properties of the gob for use in numerical models of rock mass response to longwall mining. Photographs of actual mine gob were used to approximate particle size gradations of gob material. The gradation curve was shifted down to a laboratory scale, and 20 uniaxial compression tests were conducted on simulated gob material from 3 different rock types. The study found that the nonlinear stress-strain curves obtained from the three materials were similar, and could be represented in terms of moduli as follows: Es = 2.36s + 1,360 (1) Et = 0.00181s2 + 9.33s + 294 (2) where Es = secant modulus, psi Et = tangent modulus, psi and, s = stress level (due to overburden), psi. These curves correlated well with the theoretical solution curves developed by Salamon. Other effects on gob behavior were attributable to these studied factors including void ratio, thickness-to-width shape ratio, and rock compressive strength. The generated equations should provide numerical modelers with a practical means of estimating gob modulus as a function of stress level.

Jan 1, 1993

By Stephen C. Tadolini

A new innovation in coal mine roof support has been developed by the Bureau of Mines at the Denver Research Center. The technique, called "Thrust Bolting,"1 converts a traditional passive roof support system, a full or partial column rebar bolt, into an active support system by following a specific installation process. The thrust bolting system provides two advantages from a ground control standpoint. First, because the system is active, no roof movement is necessary to put the bolt into tension. This eliminates the possibility of strata separations in sedimentary roots, minimizing progressive-type roof failures. Secondly, the tension placed on the system is independent of mechanical devices or torques. This eliminates the highly variable frictional losses that occur between the bearing plate and the bolt head and in the threaded portions of traditional active support systems. These two improvements create a safer working environment for mining personnel. An additional advantage of this improved support system is the reduced cost of installation and maintenance. The initial support cost is reduced by a minimum of 30 percent per bolt when compared to similar conventional support systems. The labor intensive and potentially hazardous torque¬tension test, required for most tensionable active support systems, is not required for thrust bolts. The potential for improved support performance and cost reduction of thrust bolting has generated a considerable amount of interest from both industry and MSHA. This paper will describe, in detail, the theory and application of thrust bolting and present the results of two underground coal mine case studies.

Jan 1, 1990

By X. L. Yao

This paper analyses the effects of mining extraction geometry (width/depth ratio) and dip of seam on subsidence parameters based on a large Database, containing 120 longwall mining cases all within the UK coalfields. The subsidence parameters evaluated in this paper include the following: the maximum subsidence value with different goaf-treatments, the position of maximum subsidence, the position of half-subsidence, subsidence value over the rib-side, positions of maximum tensile strains, positions of inflection points and the magnitude of maximum tensile strains on both rise and dip sides. Additionally, the maximum compressive strain and two limit angles are also studied in this paper. Based on a comprehensive Database analysis, several equations are derived relating these parameters to the width/depth ratio and the dip of seam. The results should assist engineers to design new structures, or take measures to protect existing structures from strain effects. The findings of the research assist in extension of the SEH to inclined seams with greater certainty.

Jan 1, 1990

By R. W. Hill

Multiseam mining has been carried out at many collieries in South Africa. It the seams are In close proximity difficult and dangerous mining conditions may arise. In the Witbank area multiseam bord and pillar mining has been carried out, while in Natal where the seams are thinner and of a higher quality, total extraction methods are common. Several combinations of multiseam mining have been carried out. The paper highlights a number of case studies and the strata control problems that can arise.

Jan 1, 1995

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 David W. Evans

"Induced hydraulic fracture of strata during roof bolt installation is a potentially prevalent, but masked phenomenon within the underground coal industry. Previously reported resin testing programs (McTyer et al., 2014) examined the relationship between resin mixing effectiveness and varying bore hole diameter. The methodology employed within this earlier test program facilitated a further critical area of research – the measurement of back pressures generated within the bore hole during standard rock bolt installation practices. Experimental data has indicated that fluid resin can be pressurised to levels where it exceeds the compressive strength of the strata, inducing hydraulic fracture within the immediate area of the bolting horizon. The routine cycle of roof bolting serves to propagate this effect, progressively fracturing and delaminating the roof during mine advancement. This masked phenomenon can lead to a perception of difficult ground conditions - mining efficiencies and costs are therefore affected, with increased needs for additional support subsequently required to restabilise the inadvertently damaged roof.Further analysis of the parameters associated with resin bolt installation has now been conducted, assisting in the development of an empirical relationship between bore hole pressure, bore hole diameter and bolt insertion times. This relationship has been analysed for 15:1 ratio resins and 2:1 ratioresins, within 28mm and 30mm boreholes. Further to this, load transfer performance has been comparatively assessed for both 28mm and 30mm boreholes, suggesting that for 2:1 resins, acceptable resin mixing and load transfer can be obtained within a 30mm bore hole. The combination of 2:1 resins, utilised within a 30mm bore hole, may well provide the optimal solution to reduce the risk of hydraulic fracture in weaker strata during resin bolt installation.INTRODUCTIONA growing number of industry papers have previously investigated areas of concern associated with the performance of cartridge style resins in roof bolting, predominantly focussing on the effects of plastic film gloving, inadequate resin mixing and the pressurisation of resin within the bore hole. These three effects have a critical influence on the load transfer of the steel bolt element, through the cured resin and into the surrounding strata.Gloving occurs due to the plastic film of the resin cartridge partially encasing or wrapping around the steel roof bolt element – a known phenomenon over many years (Pettibone, 1987). The plastic film creates regions of discontinuity between the bolt, cured resin and borehole, reducing effective load transfer into the strata. Experimentation conducted into the effects of bolt ends fully encased by plastic film (Pastars and MacGregor, 2005) revealed that load transfer can be reduced by 85 to 90% in worstcase occurrences. It is also known that the aggregate filler size used within the resin can assist in shredding and breaking up the plastic film – this effect was observed in a previous study on resin mixing effectiveness (McTyer et al., 2014)."

Jan 1, 2015

By Paul C. Hagan, Jianhang Chen, Serkan Saydam

"Modified cable bolts are commonly used in underground mines due to their superior performance in preventing bed separation when compared with plain strands. To better test the axial performance of a wide range of cable bolts, a new Laboratory Short Encapsulation Pull Test (LSEPT) facility was developed. The facility simulates the interaction between cable bolts and surrounding rock mass, using artificial rock cylinders with a diameter of 300 mm in which the cable bolt is grouted. Furthermore, the joint where the load is applied is left unconstrained to allow shear slippage at the cable/grout or grout/ rock interface. Based on this apparatus, a series of pull tests were undertaken using the MW9 modified bulb cable bolt. Various parameters including embedment length, test material strength and borehole size were evaluated. It was found that within a limited range of 360 mm, there is a linear relationship between the maximum bearing capacity of the cable bolt and embedment length. Beyond 360 mm, the peak capacity continues to rise but with a much lower slope. When the MW9 cable bolt was grouted in a weak test material, failure always took place along the grout/ rock interface. Interestingly, increasing the borehole diameter from 42 mm to 52 mm in weak test material altered the failure mode from grout/rock interface to cable/grout interface and improved the performance in terms of both peak and residual capacity.INTRODUCTIONFully grouted cable bolts have been used in underground mining industry for many decades. Initially, plain cable bolts manufactured by winding seven flexible steel wires together were commonly used (Thorne and Muller, 1964). Laboratory and field tests lead to a better understanding of the transfer process between grouted cable bolts and the surrounding rock mass, which is beneficial to improve the reinforcing performance of cable bolts in engineering practice (Fuller and Cox, 1975; Cox and Fuller, 1977; Schmuck, 1979; Stillborg, 1984). Nevertheless, as in many cases the initial cable was sourced from discarded steel with a smooth surface along the strand, plain cable bolts did not always meet performance expectations (Windsor, 1992). Over time, it was realised that cable surface geometry plays an important role in determining anchorage performance. Consequently, modified forms of cable bolts were developed. For instance, twin-strands which were made by using spacers to combine two plain cables together improved anchorage capacity two-fold (Matthews, Tillmann and Worotnicki, 1983). However, this required a large borehole to be drilled. Epoxycoated strands had remarkable performance in practice but were difficult to install without damaging the epoxy layer (Dorsten, Hunt and Kent, 1984). In the 1990s, there was a rapid development in the cable bolt design. A typical example was birdcaged cable bolts. They were initially suggested by Hutchins et al. (1990) via unwrapping traditional seven-wire strands and generating a number of nodes together with antinodes along the strand. But it was difficult to make birdcaged cable bolts that could be coiled for transporting."

Jan 1, 2015

By C. Babcock

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

Jan 1, 1981

By Yi Luo

The severity of disturbances caused by longwall mining subsidence to various residential structures can be re¬duced. The success of such mitigation efforts depends on accurate subsidence prediction, correct assessments of the subsidence influences, and careful design and implementa¬tion of the proper mitigation measures. This paper presents the systematic approach involved in the subsidence mitiga¬tion efforts with a case demonstration.

Jan 1, 2003

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 Eric Westman

Ground control problems caused by multiple-seam interaction frequently sterilize coal reserves. Interaction mechanisms differ for overmining and undermining and are controlled by mining, spatial. and geologic conditions at each site. A multiple-seam interaction analysis software program, UGLY (Uppersearn Gateroad Longwall Stability), has been used to evaluate sterilization of coal reserves due to overmining at three locations. Four case histories were reviewed to determine the effectiveness of improved ground control in mitigating interaction effects.

Jan 1, 1997

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 Andrew Starkey

Ground anchorage systems in the form of rock bolts are used extensively throughout the world as supporting devices for civil engineering structures such as bridges and tunnels. It is widely recognized that non-destructive testing methods for ground anchorages need to be developed as a high priority [1 ], with only a small proportion of anchorages currently being monitored in service [2]. The condition monitoring of anchorages is a relatively new area of research, with the objective being a wholly automated or semi-automated condition monitoring system capable of repeatable and accurate diagnosis of faults and anchorage post tension levels. The GRANIT tm system developed by the University of Aberdeen is capable of measuring the integrity of ground anchorages and has won two major UK Awards - the National John Logic Baird Award for Innovation in 1997 and a Design Council Millennium Product Award in 1999. The normal procedure with the GRANIT tm system [3,4] is to train a neural network on dynamic response data that has been sampled from an anchorage over a range of post-tension levels. Further data is needed in order to test the neural network. The paper presents neural networks that have been trained on data taken from a number of anchorages on different sites, and the results of the diagnosis of these neural networks on new test data is presented. The results show that the cross-diagnosis of anchorages is possible, where a neural network trained on one anchorage is capable of successfully diagnosing the post-tension level for an anchorage of similar construction, and the high level of accuracy that can be attained with the GRANIT tm technique is shown. The paper also introduces a mathematical model of the GRANIT tm process.

Jan 1, 2001