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By Kanaan Hanna

Abandoned mines pose a serious threat to public health and safety, as well as the environment. When active workings approach old mine workings, miners could encounter significant hazards. Additionally, the presence of mined-out areas or voids underlying residential/commercial properties, transportation, and infrastructure systems can cause subsidence issues ranging from differential settlements and sinkholes to catastrophic collapse. All these features can result in adverse impacts in terms of cost and public safety. Traditionally, mine void detection and related engineering investigations for mine subsidence risk assessment and mitigation have been greatly dependent on systematic drilling and grouting backfill programs. The unknown location and condition of abandoned underground mines represent significant challenges to geologists and engineers in accurately evaluating subsidence hazards and developing appropriate mitigation measures. A comprehensive, multi-phase mine subsidence investigation was conducted at an abandoned western U.S. mine site. The mine operated in an approximately 8-ft (2.4-m) -thick coal seam using room-and-pillar extraction from 1919 to 1922. The mine lies at depths ranging from 50 to 100 ft (15 to 30 m), and it is overlain by critical pipeline and utility corridors. Any future subsidence/ sinkhole that occurs beneath or adjacent to the corridors could pose a significant threat, specifically to the structural integrity of the pipeline corridor. Therefore, the engineering geophysical investigation was initiated to determine the potential subsidence risk. This paper describes a recent success achieved by using integrated engineering geophysics in subsidence risk evaluation. A summary of significant results with emphasis on subsurface characterization, mine workings and voids delineation, modeling analysis, and high risk zones identification and mitigation strategy is presented.

Jan 1, 2011

By J. Marc Coolen

Marrowbone Development Company operates a large drift mining complex in the central Appalachian coal field. In 1997, five continuous miner supersections produced close to 9 million tons of raw plant feed. All production is from the Coalburg seam, a 5 to 10 ft thick coal seam with multiple coal benches and shale binders. The immediate roof lithologies consist of laminated shales, sandstones, or a mixture of sandstones and shales. The mine area is bisected by the regional Warfield anticline and associated Warfield fault. Mining conditions vary considerably due to frequent changes in seam and roof geology. The following geological categories can be distinguished: 1) presence of coal riders in the immediate shale roof, 2) excessive seam slopes around the Warfield fault, 3) splay faults associated with the Warfield fault, 4) zones of abundant slickensides, 5) abrupt roof lithology changes (sandstone - shale), 6) splitting and merging of different benches of the Coalburg seam, 7) sandstone washouts, 8) excessive roof dilution, 9) pillar sizing problems due to seam anomalies (soft fireclay binders). Several examples of these features are discussed in detail. Also, their relationship to roof fall occurrences is evaluated. Early recognition of potentially adverse geological conditions is of prime importance for the economic viability of the mining plans. Detailed core drilling in advance of the mining faces and coordination of the core data with underground mapping are the main forecasting tools.

Jan 1, 1999

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 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

By André C. Zingano

The domino effect in a violent pillar collapse occurs due to the redistribution of stress in the remaining pillars near the collapse, and this stress transfer occurs in the immediate and main roofs. The remaining pillars do not have enough load capacity to support this overstress and also fail, and the domino effect propagates further to the entire panel. The domino effect does not occur only due to low load capacity of the pillars, but also the roof rock mass quality, because the stress is transferred by the roof. In high competent roof without discontinuity (fault), like massive sandstone seam, the deformation of the roof is the cause of pillar collapse extension and propagation. Depending on how close it is approaching the collapse limit; the immediate and main roofs behave differently. When it is close to the collapse limit roof convergence and pillars compression occur. Conversely when it is far from the collapse limit divergence and pillars decompression occur. This is called the arc-effect. This paper studies this behavior of the roof (immediate and main roofs) using numerical modeling to simulate the roof deformation and stress distribution in the roof and pillars. This simulation represents the progression of pillar collapse and shows the zone of convergence and divergence of the roof, and also the effect of this behavior on pillar confinement. Two-dimensional plain strain model was applied to simulate the arc-effect. The simulation results were compared to those of the instrumentation that was installed before the collapse. Keywords: pillar collapse, room and pillar, numerical modeling

Jan 1, 2005

By Mao Bai

Mining in situations where there exists a potential hazard from inundation has always existed in coal mining, and often reserves were left unmined in favor of reserves with no such hazard. However, increased pressure on expanding the minable reserves and on producing low-sulfur coal, has made mining under water hazards necessary in many parts of the world. Recent joint efforts by the authors to develop case-history data bases of mining under water hazards has allowed the bringing together of Chinese and United States experience and guidelines for such mining. In China, mining under water hazards has been successfully accomplished for a number of years, allowing the development of a good data base from which to draw guidelines. In the United States, much more limited experience, and the longer delay in adopting longwall mining compared to the rest of the world, has forced reliance on a more limited data base and European experience. In the United States (and Canada), the data base is continuing to increase as more operations start-up. Developing a generally-applicable set of guidelines for international use, based upon extensive Chinese and other experience, appears feasible. The Chinese and United States experiences are detailed by case histories.

Jan 1, 1995

By Christopher Mark

The Bureau of Mines is conducting research to assess the effectiveness of different chain pillar designs in maintaining gate entry stability. The study described in this paper was performed in a 1200 ft long, experimental headgate section which contained three pillar designs: - a three-entry, yield-abutment pillar' system, - a four-entry, yield-yield-abutment pillar system, and - a five-entry, all-yield pillar system. As the longwall mined by the experimental section, Bureau engineers monitored entry convergence, roof sag, and changes in roof quality. The results of the study indicate that the all-yield system performed at least as well as, and probably better than, the two abutment pillar systems.

Jan 1, 1988

By Anthony Lannacchione

There has been a persistent need to forecast roof falls so that miner's exposure to hazardous underground environments can be minimized. Several monitoring techniques have been developed and are used today with varying levels of acceptance in the mining industry. This paper examines the potential for monitoring microseismic emissions activity as a means of forecasting roof falls. The use of this activity to forecast roof falls has drawn only limited attention, resulting in a lack of published field performance data. This deficiency is being partially addressed by analyzing data obtained during longwall mining at Moonee Colliery in 1998. The Moonee data base contains a wealth of information concerning roof fall forecast parameters and the seismic alarm criteria used to develop these forecasts. Four seismic alarm criteria were developed and used by Moonee with varying degrees of success. Roof falls were forecasted 73% of the time with average forecast times of 54 minutes. Ninety percent of the predicted roof falls had a warning time, i.e., the time between warning and roof fall event, of greater than 1 minute. The fraction of forecasted roof falls decayed logarithmically as a function of warning time, until only 20% of events were predicted more than 100 minutes prior to roof falls. False alarms occurred in 50% of the warnings. Many of the false alarms were quickly followed by a cessation in mining which could have temporarily halted the on-going failure process. If mining had continued immediately after the false alarms, a roof fall may have occurred soon after. The microseismic activity collected .from Moonee Colliery demonstrates that techniques to forecast roof rock instabilities in underground mines are possible.

Jan 1, 2005

By Jinsheng Chen

For underground openings, many factors can cause roof failure and thus roof falls. The most common factors related to Mother Nature are weak roof lithology, geological structures, and an abnormal in-situ stress field. Theoretically, any underground opening can be successfully supported under any geological and mining conditions if the opening width is limited and the roof supporting system is well designed. However, roof control cost must be taken into account. In practice, to minimize the roof control cost, different roof control plans have to be considered to take care of the uncertainty or variation in geological and mining conditions. For most underground mines, it is unsafe or economically unrealistic to use one fixed roof control plan for the entire mine. Therefore, local, abnormal geological and mining conditions must be identified and a site- or area-specific roof control plan must be employed. This study tries to identify the major factors that have significant impact on entry roof stability by analyzing the historical roof fall data at Rofomex mine. Finally, the appropriate roof control plans or roof bolting systems for different geological and mining conditions are recommended.

Jan 1, 2011

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 Kanaan Hanna

The integrity factor approach was developed by the Bureau of nines to assess the stability of mine pillars and has been applied primarily to longwall chain pillars. Recently this approach was applied to a room-and-pillar mine in the Illinois Coal Basin. A special configuration of hydraulic borehole pressure cells was installed in a pillar to determine the in situ strength-and- stress relationship of the coal pillar and to verify the applicability of the integrity factor approach. Results indicate that the integrity factor approach is applicable to the assessment of pillar st6b:lity and nay bs a realistic and effective approach for room-and-pillar mine design.

Jan 1, 1988

By Russell C. Frith

This paper presents recent findings concerning longwall caving mechanisms relating to Australian mining conditions and their effect upon longwall support requirements and behaviour. The findings arc based on a program of intensive hydraulic support monitoring in a variety of Australian mining conditions coupled with geotechnical observations. Results from the study illustrate periodic weighting cycles of varying severity and these are related to the overlying geology in terms of the strength of individual strata units, their thickness and proximity to the longwall extraction. Several different caving mechanisms are identified which contribute to periodic weighting cycles. The importance of recognising weighting cycles in relation to the operation of the longwall face will be discussed in detail. Longwall support design methods are critically examined in relation to the case histories of the monitoring program. The applicability of the said design methods to Australian coal mining conditions is discussed and an outline of design considerations to facilitate productive longwalling is presented. Overall, the paper only "scratches the surface" of the work currently being undertaken by ACIRL in this area, and the findings of that work.

Jan 1, 1992

By M. Bouteldja

A finite element modelling approach is developed for the evaluation of the mechanical performance of cable bolts supports in underground mine structures. The modelling approach permits the simulation of any arbitrary number or pattern of cable bolts, that can be either partially or fully grouted, with or without end plates. The modelling technique used treats each cable bolt as a set of bar elements representing the steel cable. The bond stiffness characteristics of the grout bolt interface are represented by shear springs uniformly distributed along the cable length. Coupled with a simple linear elastic analysis, this technique can serve as a dedicated numerical model for cable bolted excavations. The application of the model is illustrated with two case studies: one for a hard rock mine and another for a coal mine.

Jan 1, 1999

By Hanjie Chen

A mine panel collapse may occur when pillar sizes are too small or the surrounding rock strata (roof or floor) yields. After a small pillar fails, its loading is rapidly transferred to adjacent pillars that in turn fail. If the surrounding rock strata yield, the loading on pillars will redistribute. which may result in some pillars under high stress. This paper analyzes the causes of two cases of mine panels failures in two room and pillar mines. One case of massive pillar failure occurred in a phosphate mine and the other, mine panels failure, is in a coal mine. In the phosphate mine, both the roof and floor strata were a thick competent shale stratum. The finite element analysis shows that the major reason for the massive pillar failure is that the pillar sizes were too small in a panel. In addition, the high cliff topography has some influence on the failure. In the coal mine, the failed area was about 1,000 ft long by 760 ft wide (covering more than 3 panels) involving 390 pillars. The harrier pillars between the panels were punched on retreat into small chain pillars and the extraction ratio in this failed area was more than 80%. The roof remained intact, and pillars were crushed and punched into the floor. Based on the stress analysis, it is found that multiple panel collapse occurred because the barrier pillars were punched into small pillars and the loading on panel edges increased due to floor failure.

Jan 1, 1999

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

By Andrew P. Schissler

Cyprus Shoshone is an underground coal mine in southern Wyoming owned by Cyprus Coal Company, a subsidiary of Cyprus Minerals Company. To date, seven longwall panels have been extracted from the pitching seam under increasing depth of cover from 150 ft to 1200 ft. The purpose of this paper is to describe the geology, rock mechanics needs identification, design, testing and successful application of the current headgate/tailgate cribbing system. The immediate roof of the Shoshone is unique in that the first 50 ft of lithology is interbedded mudstones. The mudstone is slickensided, highly friable and has characteristics of unconsolidated fill relative to self-spanning and beaming abilities. Finite element modelling analysis of the overall geomechanical environment showed the mudstone to yield under applied longwall abutment stress. Roof bolt system design methodology also pinpointed the relative weakness of the mudstone roof. In-mine entry vertical convergence and weight response from the mudstone identified the need to design a crib which had superior weight capacity over a wide range of convergence. Design of a structure ensued to achieve the superior strength properties of wood parallel to the grain and the displacement properties of wood perpendicular to the grain. Testing of the prototype structure along with other alternative wooden cribs was done at the U.S. Bureau of Reclamation facility in the Denver Federal Center, Colorado. The test results verified the design. Crib placement pattern was chosen to maximize support with a confined core crib under the high convergence area. The design has been successfully installed in over 5,000 ft of headgate entry in the 11 left panel.

Jan 1, 1993

By John Stankus

Seam interaction in multiple seam mining has significant effect on entry stability. The remaining pillar in an old working usually creates a high stress concentration zone while the gob creates a stress release zone. A good panel layout should take advantage of the stress release effect created by old workings. This paper presents two case studies of multiple seam mining. The Jennmar Finite Element Analysis System (JFEA) is utilized to analyze the seam interaction effect and identify the mess concentration and relief zones Panel layouts are designed bad on the results of the finite element analysis Appropriate roof support system are also designed for different seam interaction areas.

Jan 1, 1999

By Zhang Hongzhen

The fast convergence of boundary is a major problem of probability integral method, especially for the prediction of mining under thick alluvium, abandoned mine goaf, etc. To solve this problem and improve the accuracy of probability integral method, a modified model is constructed for surface movement and deformation based on theoretical analysis. In which, three weighted values i.e. 1, -1/t1, 1/t2 are set for the mining movement and deformation of three different major influence radius i.e. r1, R1, R2, respectively, then the parameters are selected as following: (1) Parameters are inversed based on the observation data, in which the subsidence factor q, deviation of inflection point s, displacement factor b are considered as generic parameters, the tangent of major influence angle tanß is modified as tanß2, that is the major influence radius r is modified as r2. (2) Inversion parameters of tangent of major influenceangle tanß or major influence radius r, based on the observation station edge data which beyond major influenc eradius distance from work-face mining boundary. The inversed tanß and r will be used as tangent of major influence angle tanß1 and major influence radius r1, respectively. (3) Other relevant parameters i.e. t1, t2, R1, R2, are calculated with the algorithm of this paper. A comparison experiment between the traditional and modified models has been conducted using the observation data from a observation station, the results show that in its central area, 58.6 percent of observation points accuracy is improved and 36.6 percent is decreased, while in its edge area, 61.5 percent is improved and 15.4 percent is decreased, totally, 68.6 percent is improved and 30 percent is decreased. Therefore, we can draw the conclusion that our modified model can improve the prediction predication accuracy, in particular, the boundary region.

Jan 1, 2014

By Shuangsuo Yang

Coal ribs can roughly be divided into three types: (1) roof and floor rocks are similar to ribs, (2) roof and floor rocks are stronger than ribs, and (3) roof and floor rocks are weaker than ribs. Different types of roof/floor and ribs will exhibit different types of ground pressure around an entry. Most coal mines belong to the 2" type with ribs fractured or even broken. In this case the mine ground pressure depends on the occurrence and development of deformation and fracture of the ribs. If it's not properly controlled, the following deteriorating process will follow: the roof bent downward > the ribs squeezed and broken > rib sloughing and collapse > rib support to the roof reduced > the roof bent downward further > the ribs broken further. In this paper rib instability is divided into four types: compression/shear sliding rib fall, gravity sliding rib fall, cross arch rib fall, and cleavage rib fall. Since failure in thin walls is largely of tension type whereas that of thick walls by compression or shear, a concept is proposed here to utilize a thick bolted rib to stabilize entry ribs, i.e. the thickness to height ratio of the thick bolted ribs

Jan 1, 2005

By W. J. Guo

Overburden movement as a result of longwall mining can be classified into, in ascending order, caving zone, fractured zone, and bending zone. Operation experience and laboratory research have demonstrated that there are bed separations in the bending zone due to differential movement and rock mechanics properties between adjacent strata. In china several coal mines have injected flyash into bed separations to prevent strata movement and control surface subsidence. This method is very effective because it can reduce surface subsidence by 35-55%. Combining the field survey results and laboratory simulation research, the author has investigated bed separation and its mechanisms and effect of backfilling the bed separation and its mechanisms. Finally backfilling application in three mines are introduced, including grout flow chart, grouting pressure, and radius.

Jan 1, 1994