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By Ross Seedsman

Discussion on the design of roof support in tailgates has often been conducted without a clear statement of the stress and failure conditions acting. There is general agreement that in the tailgate the vertical stresses above the chain pillar increase. Within the stress 'bulb' associated with this vertical stress increase there will be an increase in horizontal stress. This does not mean that the horizontal stresses acting across the roof line increase. In retreat longwalling, the proximity of the tailgate to the adjacent goof results in a substantial reduction in the horizontal stress field acting in the immediate roof. Furthermore, at the face/tailgate corner the onset of significant compression of the chain pillar. if it is designed to yield, will result in a further reduction in the horizontal stresses in the immediate roof. It is suggested that this latter mechanism can cause the horizontal stresses in the immediate roof to go to zero. Whatever, the horizontal stress acting across the roof in the face/tailgate corner will be significantly lower than in the face/maingate corner. The implication of the model is that support design in tailgates should be based on the assumption of zero horizontal (in fact tensile) stresses. How the roof behaves in a tensile stress environment is a function of joint spacing and orientation compared to excavation width and direction. It is speculated that the empirical relationship between support density and roof rating results from the relationship between joint spacing and bedding spacing. The onset of stress reductions has major implications to the design of cable and bolt anchorages in the tailgate environment.

Jan 1, 2001

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

Coal mass strength properties must be estimated to obtain realistic estimates of coal pillar strength. The limitations of existing techniques for approximating these properties are discussed. The coal mass is anisotropic. The Hoek and Brown approach to rock mass property estimation is modified to allow for this. This approach requires the Rock Mass Rating (RMR) for the strata. In the normal case where bedding is sub-horizontal, these major planes of weakness have little effect on the strength of pillars. As a result of this the RMR must be measured parallel to bedding. This means that a significantly higher rating than that usually obtained for coal is observed. The use of mass strength property estimates in extant empirical relationships is outlined. At high width to height ratios, an acceleration in the rate of increase in pillar strength with increasing width to height ratio has been observed. The empirical relationship developed by Salamon (the so called squat pillar formula) takes account of this. This relationship was found to have significant limitations with respect to its universal application. Modifications arc made to Bieniawski's empirical formula to extend its use to large width to height ratio (squat) pillars. This new squat pillar formula has none of the disadvantages associated with Salamon's relationship. Strain softening was incorporated into non-linear finite difference numerical modelling to estimate pillar strengths. The technique outlined above is used to obtain peak coal mass properties. Modelled pillar strength was found to be sensitive to both peak and post peak mass strength properties. Post peak properties were found to be difficult to obtain and required some back analysis. Modelling results were used to estimate both the strength of squat pillars and parameters for use in a now squat pillar empirical strength relationship.

Jan 1, 1992

By Frank E. Chase

Sacrificial entries, roof slotting, and other tactics have been used to combat high horizontal stresses during roadway development in U.S. coal mines. In Australia, the "pillar extraction on the advance" mining method has been successfully employed as a horizontal stress control technique. The concept of advancing and relieving mine workings through pillar extraction developed from observations that some mines experience difficult ground conditions when advancing and retreating the first panel in a new reserve. However, conditions dramatically improve in subsequent panels. Apparently, the creation of a gob area stress relieves the adjacent panel workings. After much debate and close scrutiny, the first U.S. roof control plan to advance and relieve was approved in 1998, for the Sargent Hollow Mine in Wise County, VA. The major concern was that this mining practice places section personnel inby of the gob. The tentative approval permitted barrier pillar slabbing and removal of the adjacent development pillar during panel development. The intent of the plan was to provide a "stress shadow" for the advancing faces and outby workings to reduce the excessive number of roof falls. Sargent Hollow successfully extracted their first panel by pillaring on advance and retreat mining using posts. Mobile roof supports were employed to extract pillars while advancing the second panel. This paper summarizes Sargent Hollow Mine's experiences with the advance and relieve mining method.

Jan 1, 1999

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 K. Y. Haramy

Rock burst and coal mine bump research using static and dynamic rock mechanics instrumentation has been conducted for several decades. Research efforts typically have been conducted using static instrumentation such as pressure cells, convergence measurements, multipoint borehole extensometers, and shield loading pressure measurements; dynamic rock mechanics instrumentation, for example, microseismic monitoring, has been used to help provide precursors to those violent failures. Unfortunately, these different styles of study have never been used in tandem. We report in this study the unique opportunity to have both static and dynamic instrumentation available for monitoring a longwall panel in a deep western coal mine and show that the two methods used in tandem provide information to understand the causes of a major bump. Several static rock mechanics instrumentation sites were carefully chosen along the longwall panel in the coal seam, packwalls, roof, and floor to provide the stress history during mining, including the stress redistribution that occurred before and after the major failures. Monitoring of the shield pressure on selected shields of the longwall face revealed that the cyclical loading anticipated during normal caving processes was not occurring. In addition, by locating each microseismic event (rock noise), the areas that were not adjusting to the changing stress field were clearly delineated by a lack of microseismic activity. The area of the longwall panel that did not adjust to the changing conditions ultimately failed. The combination of static and dynamic rock mechanics instrumentation data analysis provided the necessary information to determine the interaction of the structural members and the cause of the major failures at this mine.

Jan 1, 1988

By Claude A. Goode

Large quantities of high grade coal exist in thick seams in the United States. Many of these thick seams are too deep to be surface mined and do not lend themselves readily to underground extraction. To develop the new mining technology required to effectively mine thick seams, the Department of Energy has entered into a Cooperative Agreement with Mid- Continent Resources, Inc. to demonstrate the multilift longwall mining technique in a 28 foot-thick seam at their L.S. Wood No. 3 mine near Redatone, Colorado. The seam will be extracted in two lifts, with a 5-foot natural partition of coal left between the upper and lower lifts. Proper selection of face supports is critical to the success of this project. Specifications were defined for the roof supports, and it was concluded that the supports selected must provide full coverage, yield at the legs, and have a 60-inch push- out cantilever that can be set at any position under full load. A Hemscheidt two-leg shield was judged as the best qualified candidate and was selected for the multi-lift trial.

Jan 1, 1981

By I. L. Follington

A breaker line support (BLS) monitoring system, BLSmon, has been designed and constructed by the CSIRO and installed and commissioned at Laleham No.1 Colliery, Queensland. This system was designed to record hydraulic leg pressure and canopy position measurements and relay these, in real time, to the surface. The system has shown itself to be capable of operating on the BLSs without any adverse affect on production, Analysis of the results of the monitoring exercise has shown that rate of change of leg pressures on the BLSs could be used as an aid in the prediction of adverse mining conditions. Under favourable mining conditions the supports were observed to initially unload from the set pressure, this unloading reduced with time and by the end of a typical lift cycle had either ceased or reversed slightly. This behaviour was considered to be due to one of four possible causes, these being; the compaction of floor debris, and/or compaction of the floor, and/or crushing of the roof and /or support creep. The actual cause can only be determined through detailed monitoring of both roof and support convergence. However observational data would suggest that compaction of debris and softened floor material beneath the supports was the most likely cause. The few positive loading rates recorded under these conditions were of short duration and occurred towards the end of individual lifts. Where mining conditions were unfavourable the negative rates of change of pressure, unloading of the support, were reduced throughout a lift, in both duration and magnitude, when compared with favourable mining conditions. Over the same period the positive rates of change were observed to be greater. The rate of change of loading and convergence can be utilised to identify the onset of instability in the lift area. However, before this facility can be utilised by the mine for the reliable prediction of instability more monitored data and sophisticated processing techniques are required. A mechanism has been postulated for the deformation behaviour of the roof and pillars in a mechanised pillar extraction operation from the continuously monitored data, observational data and results of previous studies. The 7 m wide fenders were observed to yield under abutment stresses before lifting commenced. The stability of these fenders throughout the lifting sequence was considered to be dependant upon the magnitude of their post peak load carrying capability. This is influenced by the post peak stiffness and the extent of roof lowering in the vicinity of the fender. Difficulties with fender extraction occurred where remnant coal left standing in the goaf caused uncaved goat to bridge to the mined fender. Difficulties were also experienced where sub-vertical jointing occurred in the roof of the split reducing it's ability to bridge effectively. Despite the relatively short monitoring period, the potential role and application of continuous monitoring in advancing understanding of the rock mass response to mechanised pillar extraction operations has been demonstrated. This understanding now requires enhancement and shows great potential for aiding the optimisation of mechanised pillar extraction operations.

Jan 1, 1992

By J. R. Koehler

The failure of yield pillar-based gate mad designs to provide adequate ground control performance is primarily related to the use of "critically" sized chain pillars. A "critical" pillar is one that falls into a range of pillar sizes that are too large to either yield nonviolently or yield before the roof and floor sustain permanent damage, but are too small to support full longwall abutment loads. To directly compare the in-mine performance of critical and yielding pillar designs, the U.S. Bureau of Mines recently completed a field study in a tapering gate road at the Sunnyside No. 1 Mine, Sunnyside, UT. Extreme pillar stresses and associated coal bumps characterize the response to first panel mining of a 16.8-m-wide critical design. Significantly lower pillar stresses, early yielding of the pillar and adjacent panel rib, and an absence of coal bumps suggest that a narrower 12.2-m-wide design more closely approaches proper yield pillar dimensions. Probehole drilling of several 10.6-m-wide pillars revealed low stress levels and substantial pillar and panel rib yielding prior to abutment onset. suggesting a properly functioning yield pillar design.

Jan 1, 1995

By Styles P.

To determine whether 130 felt earth tremors around Edwinstowe, Nottinghamshire U.K, which also experienced severe surface fissuring, were caused by coal extraction, a surface seismometer array was established around Thoresby Colliery. Over the next year, 785 microseismic events were detected. The spatio-temporal seismicity patterns are clearly associated with the commencement, continuing extraction and closure of faces. Of particular note are events which locate at the surface and appear to be related to the active fissuring. Events occur within days of commencement of production and cease when production finishes, with good correlation between face advance and hypocentral position. Naturally occurring microseismic events have also been detected up to 1 km ahead of active longwall faces in the Midlands using triaxial geophone packages grouted into the seam together with a surface seismometer in the top of the borehole. The quality of these data were very high and guided waves can clearly be seen with the dispersive characteristics associated with seam waves. In one experiment more than 2000 events were detected in only two days of monitoring even in a relatively noisy surface environment. This paper demonstrates how very accurate locations ( 5 10 metres) can be generated using three-component digital data from only one borehole. The surface seismometer and the borehole P-wave onset can give extra precision for the height of the event relative to the seam. The event distributions give a dynamic, three-dimensional image of the developing patterns of fracturing above, below and ahead of the longwall face with important implications for roof control, subsidence prediction and gas migration.

Jan 1, 1992

By L. Z. Wojno

Tunnels in South African gold mines are developed at depths down to 3 600 m below surface where the virgin rock stress approaches 100 MPa and, on occasions, through rock where the field stresses exceed 150 MPa. In addition, many tunnels are subjected to increases in field stress of over 50 MPa during their useful lifetime. As a consequence, the walls of most tunnels are fractured causing the rock to dilate into the tunnel. Quantitative data on the extent of this fractured zone are presented. The function of support in these circumstances is to reinforce and maintain the integrity of the fractured rock. In 80 per cent of the 800 km of tunnels developed annually, the support methods used are capable of maintaining the stability of the tunnels. However, the remaining 20 per cent of tunnels is subjected to severer conditions where dilation in excess of 250 mm occurs and/or where the probability of strong seismic ground motion is high. In these tunnels, additional support is required. The type and pattern of support for both normal and severe conditions is described. In spite of the improved support used, damage to tunnels may occur. To minimize this damage, the concept of a rock reinforcing tendon that will yield and do work, when subjected to quasi-static or rapid deformation, has been put forward. Specifications for such a tendon are presented and the development of certain yielding tendons are described.

Jan 1, 1987

By Frank E. Chase

Geologic structures have been responsible for numerous underground accidents and fatalities, and are a constant-source of down time. During - longwall mining, ground control problems associated with geologic anomalies are aggravated by the abutment pressures which develop as the panel is retreated. Therefore, it is imperative that these structures be recognized and properly supported in head- and tailgates. Fractures, faults, clay veins, igneous dikes, and ancient stream channels (paleochannels) disrupt the lateral continuity of the mine roof and can cause falls in gateroads. Horsebacks and kettle bottoms are also hazardous because abutment pressures tend to "pop out' these structures into the gateroads. This Bureau of Mines investigation describes the physical characteristics associated with hazardous geologic structures, identifies the coal fields where-they have been observed, and, where possible, suggests support strategies and other mine plan modifications to help minimize their effect.

Jan 1, 1990

By Bruce W. King, David A. Newman

"Underground mines have always been an integral part of the limestone industry. However, beginning in the 1990s, the number of underground limestone mines has increased for many reasons including• Stripping ratios at surface quarries have become uneconomic.• Existing surface quarries have reached the mineral boundaries, and the remaining stone reserves are underground.• Surface access to the entirety of the limestone reserve is limited by development or infrastructure.• The once-rural setting of the surface quarry is now surrounded by subdivisions and development. An underground mine is less intrusive and takes the mining operation out of the public eye.• For larger markets, an underground mine enables year-round production without the sequence of overburden stripping in the winter to expose mineable stone for summer production. This is common at many surface quarries.Multiple-level operations are a natural extension of the decision to mine underground. Most limestone reserves, for example, the Tyrone-Oregon-Camp Nelson, Hermitage-CartersLebanon- Ridley, and Salem Limestones, are sufficiently thick to accommodate multiple levels. Multiple-level mines should be planned by considering each level separately and then integrating the individual levels into a larger mine plan. Failure to plan all the levels of a multiple-level mine in concert can result in ground control (back, pillar, floor, and inter-level sill pillar) problems attributable to stress transfer and interaction between the levels or the necessity to leave a thicker sill pillar between adjacent levels.Multiple-level mine planning begins with geology. Once the upper level back horizon is identified, the geology, rock strength, and physical properties in each level are used to• identify the optimal back horizon• quantify whether back support is required and, if so, the type, spacing, and length of the back support• determine the pillar centers• determine the total mining height, including floor shots"

Jan 1, 2016

By Raj R. Kallu

Underground gold mines in Nevada are typically hosted in geologic zones with a wide range of geotechnical properties ranging from blocky competent rock to faulted and highly altered soil-like material. Geotechnical engineering design in weak rock utilizes rock mass classification systems as inputs to both empirical and numerical methods. The most common rock mass classification system utilized in the gold mines of Nevada is the easy-to-use Rock Mass Rating (RMR) system; however, in practice it is difficult to apply in weak to very weak rock masses. This paper attempts to improve the sensitivity of RMR in weak rock masses by utilizing W-RMR?a modified procedure for calculating RMR?and evaluates its performance against other methods of determining RMR for predicting in situ moduli. In-situ rock mass moduli were determined using a portable plate loading device (PPLD), and these data were used to quantify the improved sensitivity of the modified RMR versus existing classification systems. The PPLD was designed for performing rapid and economical in-situ deformability tests in a mining environment. Tests were performed at eight sites yielding thirteen rock mass moduli measurements. Statistical analyses indicate Detailed-Discontinuity RMR-89 and W-RMR provide the best correlations to the in-situ moduli compared to other methods of calculating RMR.

Jan 1, 2014

By Gexin Sun

This paper presents the physical modelling results of subsurface fracture development associated with longwall mining operations and an application of image processing techniques to interpretion of the results. Physical modelling results have been obtained by employing a large sand and plaster model loaded purely by gravity as a means of fracturing the subsurface strata above the longwall excavation. The results have shown fracture development and crack propagation as a longwall face advances and demonstrated that the strength of stratified rocks, the presence and position of a weak band-, and the extraction thickness, have significant effects upon the overall fracture patterns above the excavation. Binary images of fracture patterns were scanned from the black and white photographs of the tested models using an image analyser coupled to a video camera. These images were manipulated by a custom written C program which allows the images to be processed in both vertical and horizontal direction. The images were filtered to quantitatively differentiate between vertical and horizontal fracture intensities. The vertical fracture intensity is particularly important as vertical fractures are considered to provide the main avenues for surface and subsurface water inflow into the excavation. The use of the image analysis techniques has shown a promising improvement in image enhancement and in model results interpretation in a quantitative manner. The numerical nature of the image analysis results will allow further statistical comparison with other modelling techniques to be undertaken and significantly increase the scope for use of the modelling technique results.

Jan 1, 1992

By E. Bauer

During 1987 and 1988, the Eighty Four Complex Mine of BethEnergy Mines, Inc. used the open entry longwall recovery method to recover one partial and three complete longwall faces. Various cement- concrete supports were utilized to support the open recovery entry until the approaching longwall face cut into this entry. This recovery method and the supports used were studied by the mine personnel and the Bureau of Mines. This paper discusses the findings, results and conclusions of that study.

Jan 1, 1988

By S. N. Zhow

China has at least 200 collieries suffering from the problem of instantaneous outbursts of coal and gas. The strong outbursts often occurred in active tectonic lanes and areas of high stress concentration. In this paper, the author describes some characteristics of the strong outbursts and analyses the effects of ground stress, gas pressure and the properties of coal in outbursts.

Jan 1, 1987

By Dieter Wittenberg

The use of thc ultrasound technique for monitoring rockbolt support has at last provided support engineers with a measunng system capable of pinpointing fracture failuns in fully-grouted rockbolts. The measurement data are based on the operatmg characteristics of the bolt type, which are determined both under test-bench conditions (DIN 21521) as well as in underground trials. Practical experience acquired to date suggests that this new measuring system is both effective and reliable.

Jan 1, 2001

By R. Karl Zipf

Multiple seam mining interactions caused by full extraction mining, whether due to undermining or overmining, frequently involve tensile failure of the affected mine roof. The adverse ground control conditions may prevent mining for both safety and economic reasons. Prior researchers have identified the geometric, geologic and mining factors controlling multiple seam mining interactions. This numerical study examines the mechanics of these interactions using a modeling procedure that 1) incorporates the essential constitutive behavior of the rock such as strain- softening of the intact rock and shear and tensile failure along bedding planes and 2) captures the geologic variability of the rock especially the layering of weak and strong rocks and weak bedding planes. Specifically, the numerical study considered the effect of vertical stress, interburden thickness, and the immediate roof quality of the affected seam in both undermining and overmining situations. The models show that for overburden-to-interburden thickness (OBAB) ratios of less than 5, interactions do not occur, and that for OBAB more than 50, extreme interaction is a certainty. In between, the possibility of an interaction was found to depend on gob width-to-interburden thickness ratio, site specific geology and horizontal stress to rock strength ratio in addition to the OBIIB ratio. The models also showed that horizontal stress was profoundly altered well above or below a full extraction area and that these changes are likely to influence the success or failure of multiple seam mining. The role of horizontal stress in multiple seam mining interactions has received little attention in prior investigations. Four factors control the mechanics of multiple seam mining interactions: 1. vertical stress concentration 2. horizontal stress concentration 3. stress re-direction 4. bedding plane slip bands A combination of vertical and horizontal stress increase and high stress gradients in the vicinity of full extraction areas re-orients principal stresses into a very unfavorable direction. This seemingly small stress re-orientation has a profound adverse effect on bedded rock.

Jan 1, 2005

By Paul A. Lu

This paper proposes an unconventional, simple, and practical method of measuring mining-induced around stresses with hydraulic borehole pressure cells. The method is a direct ground stress measuring technique. It does not require the value of deformation modulus for back-calculation of stresses; instead, it directly measures the ground stress in situ. The applicability and practicality of the technique are shown by two case studies. The technique may be applied to monitoring a stress build-up that may lead to mine roof failure or to rock and coal outbursts.

Jan 1, 1984