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By Al Campoli

The majority of the 100 million roof bolts installed each year in the United States are 5/8 inch deformed rebar fully grouted into a one inch borehole. Commonly referred to as B-Series or No. 5 bar, this system is proven effective and cost efficient in coal mines with average to above average roof conditions. One of the critical variables affecting resin bolt performance is the size of the resin annulus, calculated as the difference between the hole and bolt diameters. The optimum annulus has been proven to be '/4 inch, which provides for effective mixing of the two polyester resin components and incorporation of the plastic film packaging within the cured resin. Glove fingering occurs when the plastic film is forced against the borehole wall potentially interfering with the mechanical interlock of the resin and the rock mass. Glove fingering is not a problem with the optimum '/n inch annulus but has been know to occur with the B-Series 3/8 inch annulus. Laboratory testing correlated the degree of Glove fingering with resin formulation and the bolt head to shaft axes alignment. Glove fingering occurred over 45 % of the grouted length of standard B¬Series bolts. The newly formulated Eclipse B-Series resin when combined with a 1/8 inch offset between the bolt head and shaft axes reduced the glove fingering 70%. The Eclipse system allows the 3/8 inch B-Series annulus to achieve the positive mixing characteristics associated with the / inch optimal annulus systems.

Jan 1, 2002

By Rob Robinson

Following a massive rib wall failure along the reclaim belt of the large underground surge bunker at CONSOL?s Bailey Mine, a multifaceted, cleanup and rehabilitation effort was initiated to rapidly stabilize the bunker rib walls and place the bunker back into production. Rehabilitation of the bunker included infilling and re-mining of the failed excavation area, installing a system of horizontal tie-back cable bolts, constructing a large knee wall at the base of the rib, reducing water inflow, and providing drainage for rib water. As a result of the coordinated effort, the bunker was safely placed back into production within 38 days. The 7500-ton surge bunker, an engineering feat in itself, was constructed in 2003 to accommodate production surges from two longwalls. In 2005, 14.8 million raw tons passed through the bunker. The 22-foot wide by 75-foot high surge bunker excavation extends as much a 52 feet below the base of the Pittsburgh seam. The bunker excavation measures 325 feet in length, with feeder and reclaim belt excavations extending the bunker excavation by approximately 500 feet. The massive rib roll, which occurred near the bottom of the reclaim belt slope, measured as much as 45 feet high by 60 feet long. Failure of the rib was attributed to progressive failure of a thick claystone unit present near the slope bottom area. Saturation of the claystone by water inflow, high insitu horizontal stress, and pre-existing rock joints were considered contributing factors. Immediately after rib failure, mine production was temporarily rerouted to a parallel belt. Two bulkheads were then constructed 175 feet apart along the reclaim belt slope, and a customized fly ash-cement mixture was pumped from the surface into the unstable reclaim slope area. A tunnel was then mined through the backfill along the original reclaim slope contour; the new excavation supported by a system of customized steel sets. To assure stability of the adjacent bunker rib wall, a pattern of 70-foot long cable bolts were installed downward from the adjacent entries to effectively tie back the rib. Drainage holes and a 12-foot high knee wall were installed. Adjacent gas wells were also plugged to help reduce inflow. Periodic inspections and continuous monitoring are being used to help assure bunker stability.

Jan 1, 2007

By Keith Heasley, Christopher Newman, Zach Agioutantis

"Previously, in 2013, a laboratory version of the computer code ARMPS-LAM, introduced at the 32""d International Conference of Ground Control in Mining, successfully integrated the laminated overburden model of LaModel into an ARMPS-style pillar stability analysis. The ARMPS-LAM analysis has now been fully integrated into the ARMPS program allowing users to more accurately classify the stability of a given mine design utilizing a laminated overburden model for the determination of pillar loading within theAMZ. ,This paper introduces the new Windows-based ARMPS-LAM program. The program's ease of use, output result generation, and limitations are highlighted through the parametric analysis of three hypothetical case studies where traditional ARMPS output is compared to output generated by LaModel. As mining operations continue to produce at greater depths and in more complex geometric and geological conditions, flexible and efficient analyses of mine stability for underground room and pillar mining has become more and more essential. The ARMPS-LAM program provides the mining industry with an empirically backed, numerical design tool for the evaluation of underground room and pillar mine plans.INTRODUCTIONIn the Southern Appalachian coal fields of the United States, retreat room-and-pillar methods have allowed mining operations to maintain high rates of production in the face of adverse geologic conditions and shrinking coal reserves. Over the years, research has been dedicated to understanding and mitigating mining-related dangers due to second mining practices, such as: pillar squeezes, floor heave, roof falls, pillar bumps, etc. However, as mining operations continue at deeper depths and with more complex geometric and geologic conditions, there is an inherent industry need for more accurate, flexible, and faster evaluations of stability for underground room-and-pillar mining."

Jan 1, 2016

By Ali Benbia

The paper shows results from the physical modelling of longwall excavations concentrating on the fracture pattern development above longwall panels. The modelling technique is briefly described and the detailed analysis of the fracture pattern is discussed. The physical modelling is particularly useful in visualising the caving process above longwall panels. This advantage has been exploited using the image analysis technique. Black and white pictures have been taken while the models are tested in successive stages and then enhanced and stored as binary images by scanning using an image analyser coupled with a video camera. These images are then processed using a custom C program which allows the discretization and characterisation of the fractures. This provides a valuable data base for further numerical and statistical analysis which subsequently may allow the assessment of the water inflow hazard to the workings. In this paper a sequence of different extraction thickness have been analysed using the technique mentioned above to show the effect of the extraction thickness on the fracture distribution and intensity.

Jan 1, 1993

By Daniel W. H. Su

"This paper presents results from a recent study conducted to evaluate the effects of longwall-induced stresses and deformations on the mechanical integrity of shale gas wells drilled over a longwall abutment pillar. The primary objective is to demonstrate that a properly constructed gas well in a standard longwall mining abutment pillar can maintain mechanical integrity during and after mining operations. Successful designs would allow for the safe recovery of both coal and gas reserves. Major elements of this comprehensive study include: (1) modeling of rock movements to predict impacts on gas wells, (2) measurements of actual rock movements at the surface and in the subsurface, (3) measurements of actual impacts on test wells designed similarly to real shale gas wells, ( 4) comparison of modeling results to actual rock movements to validate the model, and (5) employing the validated models to evaluate various casing and cementing designs to minimize casing deformations and absorb longwall-induced strains along the intermediate and production casings.A study site was selected over a southwestern Pennsylvania coal mine, which extracts 1,500-ft-wide longwall faces under about 600 feet of cover. Four test wells and four monitoring wells were drilled and installed over an abutment pillar with 125-ft by 275-ft centers. The four test wells were drilled to a depth of 642 ft, or 42 ft below the Pittsburgh seam, and were designed like a real shale gas well, except that only three casings (namely the surface, water protection and coal protection casings) were installed in each well. Casing steel grades, wellbore diameters and annular spaces were purposely varied in each of the four test wells to evaluate the effect of different well designs. The four monitoring wells - one vertical extensometer and three in-place inclinometers (IPI) - were drilled to different depths to measure the vertical settlement and horizontal displacements as a result of longwall mining on both sides of the abutment pillar. In addition to the test wells and monitoring wells, surface subsidence measurements and underground coal pillar pressure measurements were conducted as the 1,500-ft-wide longwall panels on the south and north sides of the abutment pillar mined by."

Jan 1, 2016

By Archie Richardson

Periodic weighting of longwall supports occurs during retreat ofthe face under certain geologic conditions. Strong stratain the immediate and main roof tend to cantilever over the gob, weighting the supports periodically. The distance between weighting peaks, as well as the intensity ofperiodic weighting, is determined by the suength and thickness of roof members, their location relative to the seam, the frequency of joinling, and characteristics of the gob. In this paper the effects of varying several of these variables ate investigated using block modeling. The models are calibrated against a published case history of periodic weighting in the Pittsburgh Seam. It is concluded that block modeling is a useful tool for correlating measured support pressure histories with expected modes of strata behavior in the roof, with potential for predicting shield behavior in new geologic enviromnents.

Jan 1, 1998

By Rossen A. Halatchev

This study introduces a new probabilistic approach to seismic stability analysis of embankments and slopes. The approach is based on Sarma's solution, which is modified through presentation of the seismic force by its horizontal and vertical components; i.e. the force may have an arbitrary inclination. The probabilistic modelling includes Monte Carlo simulation of the shear strength parameters of the soil mass, which are assumed to have a normal distribution. The probability of failure is determined based on the seismic co-efficient, which is treated as a random quantity. The probabilistic approach is illustrated through a solution of a numerical example from an opencast coal mine.

Jan 1, 1992

By Thomas W. McClain

This paper describes the importance of considering ground control measures when importing U.S. continuous mining technology in foreign countries. Important ground control factors are discussed. Each year the USA exports a significant amount of continuous miner equipment (i.e. continuous miners, roof bolters, coal haulers, shuttle cars, continuous haulages, scoops and feeder-breakers). International customers see the opportunity to significantly improve their productivity and increase their longwall gateroad development rates by implementing the room-and-pillar (place changing) mining systems used throughout the USA. However, a number of issues are often overlooked or not thoroughly evaluated when selecting the equipment, often resulting in lower than expected productivity. Ground control is one of the critical factors that must be addressed. Since 1993, the author has assisted numerous international customers and manufacturers with implementing or improving the performance of continuous miner and room-and-pillar systems in Australia, India, Mexico, South Africa, and the United Kingdom. An essential first step is to conduct a thorough ground control study, and then integrate its findings into mine layout, equipment selection and section operating plans. Based on actual case histories, the author will present a number of examples and "Do's and Don'ts." Another key to success is the involvement of manufacturers and suppliers with customers when implementing new systems and technology. The importance of allowing for cultural evolution by both management and workers for acceptance and successful implementation of any new system is also discussed. Over the past ten years, the global coal industry has gone through some very dramatic changes. Countries that previously experienced a freight advantage, such as Australia shipping coal to Japan and Korea versus the United States, are finding they are now in a world coal market similar to other commodities and products. Today, coal prices arc stated F.O.B. at the consumer, and the origin is often irrelevant. Also, previously state-run coal industries have been privatized, such as the United Kingdom, and others such as Poland and India are currently going through the same transition from state-run to the private sector. In all cases, the key to continued viability, or survival, depends largely on reducing costs in order to compete for a market share. Two ways exist to reduce costs. First, almost everyone tries to reduce expenditures. Depending on the circumstances at a given mine or in a specific country, this helps to some point. However, most find that secondly, production must be increased to achieve the necessary cost reductions to compete in today's global coal market. As a result, over the past ten years, foreign companies and countries have looked to the USA for new mining methods and systems to significantly increase productivity. Thus, a major portion of American equipment manufacturers and suppliers sales have been exports, which has played a major role in helping to "keep-the-doors-open" for a number of companies through the lean 1990's, when American coal companies were postponing capital expenditures and expansions. As foreign companies and countries have looked to the USA for ways to improve productivity, they have become particularly interested in continuous miner equipment systems and most often "room-and-pillar," "bord-and-pillar" or "place changing" mining systems for four reasons. First, in countries such as the United Kingdom and Germany, who have been longwall mining for generations, there remain numerous remnant small pockets of reserves in existing mines that are either too small or too difficult to longwall mine, and are conducive to the flexibility of continuous miner systems. Second, for countries such as Australia, who have experienced the constant problem of developing longwall gateroads in a timely manner to avoid longwall move delays, new or alternate methods are required. Third, companies and countries using roadheader machines on coal production are recognizing the increased productivity potential of continuous miner equipment. Fourth, countries such as South Africa and India, where a substantial amount of the production comes from conventional mining methods, "drill-and-blast", companies are recognizing the opportunity to improve by implementing continuous miner technology. Colliers & Associates, Inc. has found that over the past eight years too often foreign companies or countries purchase American continuous miner equipment without first conducting a thorough geologic, geotechnical and ground control study for the reserves to be mined. The problems are compounded by not developing mine plans, and submitting them to the regulatory authorities for approval prior to purchasing and even implementing the equipment.

Jan 1, 2001

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 Raymond M. Stateham

The Bureau of Mines has developed a portable, lightweight device for determining the integrity of resin-grouted roof bolts. The instrument functions by sending a known pulse of ulstrasonic energy into the bolt and comparing the amount of energy reflected hack with the original measured pulse. Where the bolt is properly bonded or coupled with the rock, the energy passes through the bolt-resin-rock interface and dissipates in the rock. Where the bolt is not properly coupled, the energy is reflected back along the bolt to the instrument. The ratio of reflected energy to the pulse approximates the fraction of the bolt's surface that is improperly bonded. The instrument is easy to use. The operator holds the sensor head of the device firmly over the head of the bolt to be tested, causing the pulse of ultrasonic energy to be imparted to the bolt. The device then listens for reflected energy. This cycle is repeated about 200 times per second until an evaluation of the bolt installation is made and indicated by colored lights in the display window. Bolts having 50% or less of their surface bonded are considered had and cause a red light to he illuminated. Bolts with 50% to 75% of their surface bonded cause a yellow light to be lit and are considered marginal. Bolts having more than 75% of their surface bonded are considered good and cause a green light to be lit. The indicator lamp remains on for about 4 seconds and then goes off, indicating that the instrument is ready for another test.

Jan 1, 1982

By John C. Stankus

At the 19th International Conference on Ground Control in Mining, a paper was presented detailing new types of Jennmar INSTáL tensionable cable bolts (Stankus and Taylor, 2000). Since that time, significant testing and trials of the various types of tensionable cable bolts have been conducted. Several MSHA districts and state regulatory agencies have now approved these systems as primary support This paper will detail the use of tensionable cable bolts as primary support at two longwall mines in Virginia (MSHA District 5).

Jan 1, 2001

By A. K. Bhattacharyya

The excessive displacement of the seam floor in headings (i.e. roadways) called 'floor-heave' is often experienced in the underground mines of the Southern Coalfield of New South Wales. Apart from interfering with the mining operations, the phenomenon could be detrimental to the stability of the pillars left for support leading to an unpredictable increase in the subsidence of the overlying strata. The latter could pose a serious problem, particularly in the design of partial extraction layouts of mining for areas where subsidence is to be controlled. The susceptibility of the floor of different rock materials and thicknesses in a given 'panel and pillar' partial extraction geometry was examined by two-dimensional Finite Element Modelling of the displacements, stresses and failure zones around the excavations. The modelling, assuming elastic behaviour of the materials, indicated that the maximum vertical displacements of the floor increased substantially as the Young's Modulus of the material decreased to less than 7GPa and the thickness of the layer increased. The results also discount the possibility of a properly designed panel and pillar geometry in the Southern Coalfield of New South Wales being unable to adequately control surface subsidence through the pillar, or its core independently of the yielded ribs, penetrating the floor.

Jan 1, 1992

At any given lime, the tidal force subjects massive regions to a spatially uniform load. The deformation of any large structure due to these forces is proportional to the elastic properties of the structure. In this aspect, the tidal force applies a load in a manner similar to that of a large-scale testing machine. Any anomalies in the measured tidal response must reflect alterations in the average elastic response of the massive structure. In the present work, tidal tilt measurements obtained in the wall of a large dam are used to infer changes in the response of the dam to applied loads. In particular, it is found that the dam response has changed by approximately 13 percent, on average, for two monitoring periods separated by 5 years. It is possible that these changes were produced by the different volumes of water in the dam for the monitoring periods. The tidal measurement technique can also be applied to monitor changes in the average structural response of large underground and surface masses relevant to coal and metal mines.

Jan 1, 1992

By Kenneth L. Johnson

The effects of underground longwall coal mining on flow in perennial streams within the Pittsburgh seam basin were evaluated. Four streams above four different longwall mines were studied. Overburden depths at the study sites ranged from 500 to 600 feet. At all four locations, premining streamfow measurements were made to establish baseline flow conditions. Measurements were then repeated at each location after the longwall face passed beneath the stream. At one location, in which the longwall panel axis was roughly parallel to the stream, the streamflow was monitored as the longwall face advanced. Measurements at this location showed that temporary flow reduction occurred as the face advanced underneath; however, after the tensile portion of the traveling subsidence profile passed, this effect was lessened. At two of the sites in which the stream was roughly perpendicular to the panel axis, postmining streamfow increased across the panel and decreased over the development headings, such that the net lateral change in streamflow across the fully subsided panel and associated development headings was small compared to the measured flow.

Jan 1, 1997

By Nengxiong Xu

The WTZ coal mine is located adjacent to an auxiliary dam in P. R. China. The mining-induced ground movements are calculated using a 3-D numerical procedure based on the finite difference method (FDM) to judge whether the extraction of the coal seam will have a negative impact on the dam. First, the values of the rock mass mechanical parameters in the numerical model are estimated using the available literature that relates intact rock and discontinuity properties to rock mass parameters. Then, based on available surface subsidence monitoring data for an area in WTZ mine, the main mechanical parameters of coal and rock masses are calibrated by a back analysis procedure that combines an experimental design technique with numerical simulations. Then the reliability of these rock mass parameters is verified. Finally, according to the planned mining sequence, predictions of mining induced surface subsidence are computed in steps, and the boundary of the ground movement area of each step is determined. The nearest distance from the boundary of the surface movement area to the edge of the dam foundation was found to be about 1347 m. The maximum surface subsidence within the mining area turned out to be 2.7 m, and the maximum settlement point was found to be close to west-south corner of A2 mined area. These findings led to the conclusion that the coal seam mining would not cause dam damage.

Jan 1, 2013

By Houquan Zhang, Yu Wu, Yanlong Chen, Zhenfu Luo, Aihong Lu

"A non-destructive detection scheme for bolt installation quality was designed by adopting the method of non-destructive detection on bolt installation quality. Based on the field measurements, a random non-destructive detection method for determining the characteristic length including anchoring length, free segment length and initial axial load of bolt was discussed, and the characteristics of increasing bolt axial load in the initial stable stage was obtained. The results show that the bolt axial load and deformation of surrounding rocks correlate each other. Compared with the traditional detection technique for bolt installation quality, the non-destructive detection method has obvious advantages such as random, accuracy, and economic. The method provides a reliable basis for bolt support optimization and stability control of surrounding rocks.INTRODUCTIONBolt support in roadway can make the surrounding rocks and bolt form a structure in the early stage of roadway development through the bolt pre-tension, which can maximize the self-bearing capacity of surrounding rocks. Thus it is a form of active support (Kang, 2007). In order to make the bolt play an active supporting role, it needs to ensure the installation quality of bolting. Thus the bolts can supply a high initial anchoring force and control the deformation of surrounding rocks (Shan, 2009; Wei, 2012). Moreover, bolt supporting is a concealed work. Once installation quality problems appear; it will be difficult for effective detection and screening (Xie, 2008).THE PROBLEMS OF BOLT INSTALLATION QUALITY IN COAL MINE AND ITS DETECTIONThe Problems of the In Situ Bolt InstallationDue to the problems in installation environment, machine, and technique, and workers proficiency (Li, 2003), there are many problems exist in bolt installation. If the borehole depth is too long, the anchoring agent will accumulate at the bottom of the bore, which reduces the anchoring length. Furthermore, if the borehole depth is short due to the poor geological condition, the exposed part of the bolt will be too Jong and the pallet cannot cling to the rock face, which prevent the pre-tension from being applied. The pre-tension is very important for bolt installation. If pre-tension is insufficient the rock bolt cannot control the deformation of surrounding rock efficiently and the bolt anchoring force will decrease considerably. The bolt length is another important parameter for anchoring quality, which will affect the control area of bolt supporting. In addition, the cleanness of drill hole has a great influence on the effective anchorage length of bolt. If the drill hole cleaning is not enough, it will directly affect the anchoring agent and the contact area of surrounding rocks, which will decrease the effective anchorage length and initial anchor-force."

Jan 1, 2016

By Buddhima Indraratna

The paper highlights the conceptual developments in rock mechanics applied to quantify the effect of bolts in the stabilization of joints and yielding soft rocks.

Jan 1, 1992

By K. Goshtasbi Goharizi

This paper discusses experimental results of physical scale models employed to study the stability of a pillar between rectangular roadways. A programme of laboratory investigations utilising a bi-axial test rig has been undertaken to examine the influence of various stressfields on the stability of pillars between mine roadways. The effect of pillar height/width ratio on the overall stability is studied. Observations of failure characteristics in and around pillars are discussed especially in relation to the various stressfields applied to the models.

Jan 1, 1992

By Axel Preusse

This paper shows basic considerations about the prognosis and prediction of surface subsidence and other ground movements in the German hardcoal industry. The following case studies describe experiences collected in projects between university and industry.

Jan 1, 2000

By Wenbing Guo

Surface subsidence for fully mechanized thick seam top coal caving was measured, and surface subsidence characteristics were then analyzed. The subsidence initiation distance from the set-up room and the advance influence distance ahead of the advancing longwall face were studied. The maximum surface subsidence velocity and the delay angle of the maximum subsidence velocity were determined. The results indicated that surface movement was intensive, the surface subsidence velocity was very high, the ground surface was badly damaged, and the subsidence initiation distance from the set-up room was smaller than that in general geological and mining condition.

Jan 1, 2011