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By Anthony T. Iannacchione

Longwall mining produces surface subsidence basins that affect streams by water diminution and contamination. These effects can ultimately impair the biological character of the plants and animal communities living in the streams. The combination of flow, water quality, and ecological effects threaten the environmental sustainability of longwall mining and, ultimately its social acceptability. Pennsylvania has developed new standards, released in 2005 and implemented in 2007, to help protect the commonwealth?s valuable streams from the detrimental effects of subsidence. These standards are aimed at eliminating environmental problems and will therefore help to make underground bituminous coal mining more sustainable. The engineering solutions necessary to comply with these standards are fundamentally affected by the geologic nature of coal-bearing strata, the existing in-situ stress conditions, and the ability of selected species to survive and adapt to changes in their environment caused by longwall coal mining subsidence. Needless to say, geologists, biologists, and engineers are all playing a pivotal role in developing prevention controls and recovery measures necessary to protect streams and sustain underground coal mining. The challenges facing scientists and engineers in sustaining the development of underground coal mining in Pennsylvania are examined through case studies. These challenges all arise from the subsidence effects of longwall mining. The key effects of subsidence, discussed through case studies, are: a) tension cracks, b) springs in headwater stream areas, c) stream gradients and subsidence basins, and d) stream compression ruptures and horizontal stress. All of these potential threats demonstrate the need for high-level scientific knowledge and innovative engineering solutions. One could argue that Pennsylvania?s stringent stream protection standards put the coal mining industry at a competitive disadvantage compared to other coal producing states. It is equally likely that compliance with these standards help to mitigate the effect of subsidence, thereby helping the coal industry achieve a sustainable future.

Jan 1, 2011

By Mietek Rataj

Installation of pre-tensioned Strand Bolts i.e. resin grouted cable bolts is very common now in Australian coal mines. A short historical development of this type of rock reinforcement is outlined Various systems have been developed to apply 20 to 25 tonnes of pre-load to these bolts but there are still some technical problems which have to be overcome. One of the major problems encountered is bleed off tension which is highly undesirable as required "over loading" rocks the immediate roof thus inducing fractures iii weak strata. The major reason for the loss of tension apart from the design of the tensioning jack, is the design of the conventionally used barrels and wedges. Due to a roof plate tilting during tensioning the cable moves out of the barrel centre which impedes proper positioning of the wedges and subsequent excessive return of the cable once load of the jack is released. This problem has been largely resolved by the design of a special barrel having a centralizing rim allowing for significant reduction of the toss of tension. KEYWORDS: Pre-tensioned Strand Bolts, Cable Bolts, Bleed off Tension, Tensioning Jack, Loss of Tension

Jan 1, 2002

By Mictck Rataj

Methods of rock reinforcement in Australian coal mines are being advanced progressively through the better understanding of rock failure mechanisms around roadways and the-development of improved bolting hardware and installation techniques. It has been recognised that to improve the performance of rock reinforcement systems, the stiffness of the system as a whole has to be improved. New style bolts and accessories together with high pre-tensioned roof and cable bolts are being developed as a result. A number of the new techniques used to stabilise the roof, especially in weak roof/high horizontal stress areas have been site proven and are now currently used on a routine basis as a means of improving development rates and gate road stability during longwall retreat.

Jan 1, 1997

By Hamid Maleki

The advance-and-relieve method benefits from lateral destressing associated with mining in laminated rocks and a high horizontal stress regime. This stress control method is based on measurements showing that occurrence of rock failure in the roof and floor of an entry (or caving of roof strata in a panel gob) results in redistribution of stresses in adjacent entries. By locating other entries within the shadow zone of the first entry (or gob), improvements in stability can be achieved. Numerical modeling proved useful in studying the basic mechanics of lateral relief while investigating the sensitivity of results to different geologic and mining parameters using controlled experiments. It was shown that failure of rocks near an entry results in redistribution of horizontal stress and shifting of the stress to higher horizons. Measurements from two mines are consistent in showing significant horizontal stress reductions in comparison with the far-field stress regime within the destressed zone. Although the far-field stress regime is very anisotropic, these measurements show near-equal secondary principal horizontal stresses, or perhaps a switch in orientation, as a result of destressing. Stress relief is achieved through lateral movement and relaxation of rocks along weak bedding planes toward adjacent caved zones (or softened zone). Because of cave geometry in the advancing panel, horizontal stress concentrations occur near the cave line both in front of the face and to the sides. The horizontal stress concentration reaches 1.7 times the far-field stress ahead of the face. This stress increase is significant and may cause structural damage in this zone benefiting from additional support. In the next advancing panel located within the shadow zone of the gob, horizontal stresses are significantly reduced in the roof (by 50%). Thus, the stability of future advancing panel can be improved through prudent layout designs and sequencing. The width of the relief zone is significantly influenced by the height of the softened (or cave zone) and rock mass properties.

Jan 1, 2003

By Robert C. Garson

The Mine Roof Simulator at the DOE'S Mining Equipment Test Facility is described. Its use in the research and development on the behavior and response of longwall roof supports to simulated mine environment is investigated. Advanced technology analysis methods such as finite element modeling, strain gage instrumentation, and photoelastic techniques are investigated for application in roof support design two case studies are presented of test data and results.

Jan 1, 1982

By Paul Cartwright

A rockbolted roadway risk assessment procedure has been developed specifically for the Parkgate seam at Thoresby Colliery by Rock Mechanics Technology and colliery engineers. The purpose of this assessment is to identify any areas of significant risk in relation to roof instability due to potential failure or overloading of the rockbolt support system. This allows appropriate additional support to be installed in sections of roadway considered to be at increased risk. The assessment of risk in rockbolted coal mine roadways, in terms of fall of material, has to be based on an understanding of the rock mass behaviour at that site. The development of an appropriate risk assessment system is therefore site specific. A consistent approach to developing such a system can, however, be applied by adopting the same general logic to each individual site. The use of risk assessment to categorise areas with increased risk of roof instability has been undertaken for four longwall panels at Thoresby. The procedures used for these assessments are discussed and the operational benefits examined. The approach successfully used at Thoresby has been developed into a routine tool to assist mine personnel in assessing areas of potential roof instability in rockbolted roadways and as a means of communicating the position of these areas to both supervisors and workmen. The system provides a rational and objective interpretation of the current underground conditions based upon a logical, consistent and structured approach.

Jan 1, 1999

By Ross Seedaman

The specification of roof support needs to consider rock strength, imposed ground stresses, and more importantly the bedding and joint structure of the immediate roof rocks. A database structure has been developed that allows these considerations to be incorporated into modern mine planning software so that better predictions of mining rates can be made. Rock strength is estimated from core and/or geophysical logs (especially the sonic velocity log) and ground stresses are estimated from depth of cover and knowledge of the regional stress field. The database is unique in the way it addresses bedding structure. Only 10 roof lithologies are allowed and these are allocated a numerical value that reflects an increase in the average spacing of bedding partings. These are referred to as 'Geotechnical Ferm Units' (GFU). The locations of all bedding discontinuities logged in the core are entered into the database and referenced to the GFUs. It has been found that, within each GFU, bedding spacing follows a negative exponential distribution and so it is a simple matter to predict the probability of the occurrence of any roof beam thickness. Output from the database can include plans of roof strength, the onset of overstressing, and the statistics of the distribution of beam thickness in the immediate roof.

Jan 1, 2003

By Daniel W. H. Su

This paper presents the results of an extensive geomechanical testing and monitoring program conducted at a longwall panel. The field program included the monitoring of roof caving sequence and mechanism, surface subsidence, change of pillar stress, roof displacement and entry convergence, and strength characterization of the overburden rocks. The response of eight headgate pillars and the adjacent entries was monitored as the longwall face approached and passed the instrumented locations. Overburden response to longwall mining was monitored using time domain reflectometry (TDR) technology; and, the height of the highly fractured zone above the gob was determined by postmining drilling. Good correlation was found between the height of the highly-fractured zone and the TDR, subsidence, and pillar stress data. The results provide a detailed picture of the response of overburden strata and chain pillars to longwall mining. A preliminary model representing the overburden strata behavior at the test site in response to the longwall mining is proposed.

Jan 1, 1987

By Lisa Burke

In underground coal mines, concrete block stoppings are widely used to control mine ventilation. Although stoppings are not intended as roof support, they are subjected to roof to floor convergence, and may fail as a result of this loading. Premature failure of stoppings can significantly affect mine ventilation, limiting the amount of fresh air reaching the working faces and increasing the risk for methane explosions. Softening materials are often used in stoppings to reduce the damaging effects of roof to floor convergence. However, to date, research has not focused on the behavior of stopping construction materials subjected to roof or floor loading, leaving the design process to trial and error. A combination of numerical simulations and large scale physical tests were employed to develop a scientific understanding of stopping performance. The first series of physical models were constructed using standard CMU (concrete masonry unit) blocks and tested in a load frame. The behavior of these walls was simulated with a distinct element model. A key finding was that very small variations in block size and irregularities in block shape significantly affect wall performance. These variations, which are inherent to the type of blocks typically used underground, create stress concentrations that initiate the failure of stopping walls. A second series of models employed concrete stoppings that incorporated woos planks, a shredded wood material, and foam planks like those commonly used underground. Numerical models were again able to match the physical test data. Analysis of the softening layers used in stopping construction indicated that the strength and stiffness of the material relative to that of the wall is crucial in determining the amount of roof to floor convergence the wall can withstand prior to failure. The product of this study is a numerical model that can be used to evaluate the performance of stopping materials and different wall geometries in a controlled environment. Testing was done in a laboratory setting with walls constructed on flat, parallel surfaces and subjected to uniform loading at a constant rate. Parametric studies with the model indicate that in this ideal environment it is possible to control the amount of vertical displacement the stopping can withstand by adjusting the number of softening layers built into the wall and the thickness of the layers. Additionally, under these conditions, the position of softening layers within the wall appears to have little effect on the amount of displacement walls can withstand prior to failure.

Jan 1, 2004

By Jim Galvin

4 and 6 point timber chock constructions are used extensively on both a systematic and spot basis by Australian longwall operators to support tailgate roadways. In 1996, the School of Mining Engineering at UNSW published design and performance criteria for timber chocks constructed from Australian hardwoods. The research was funded by the Australian Coal Association Research Program (ACARP). In 1997, ACARP awarded funding for a stage 2 testing program. This testing focussed on examining the effect of using thinner timber elements in chock constructions to improve OH&S and comparing the full-scale performance of traditional 4 point chocks to Link-n-Lock constructions. It was established that chocks constructed from thinner elements have failure loads much less than thicker elements. However up to the failure loads of the thinner elements there was no significant difference in the response to convergence of chocks constructed from 25, 50 or 100mm thick elements. Chocks constructed from 150mm thick elements have a similar ultimate strength to the 100mm thick elements. However, they have a significantly lower resistance to load, thereby permitting more convergence to occur. There is more potential for chocks comprised of thin elements to be constructed 'out of plumb' and so suffer eccentric loading. The full scale testing program established that at any given level of convergence up to 10% strain, the timber in a Link-n-Lock chock generates twice the support resistance of the same quantity of the same timber in an equivalent size 4 pointer chock. On the basis of support resistance per cubic metre of timber, there is only a marginal difference in the performance of 1.0m Link-n-Lock chocks and 1.2m Link-n-Lock chocks constructed from select Blue Gum. The cost per tonne of support provided by a Link-n-Lock chock is effectively half of that provided by a 4 pointer chock of equivalent dimension and timber. Up to a strain level of 4%. Link-n-Lock chocks constructed from 1.2m long elements are slightly more cost effective than the same chocks constructed from 1.0m long elements. Thereafter, there is no significant difference in cost per tonne of support resistance. Beyond a strain level of about 4%, Link-n¬Lock chocks constructed from landscape grade Blue Gum are just as cost effective as Link-n-Lock chocks constructed from structural grade 4 Blue Gum.

Jan 1, 2000

By M. E. Duncan Fama

The aim of the paper was to model numerically some aspects of the extraction of coal pillars by split and fendering. A two-dimensional elasto-plastic plane strain model with strain softening and interface modelling was used. The model was able to simulate the improvement of conditions experienced in lifting from the fender, when the roof adjacent to the fender has caved. In contrast, when a stook was left, providing support to a span of uncaved roof, the model predicted the fender would suffer considerably more deformation and strain.

Jan 1, 1992

By Michael Kelly

Current research in longwall mining in CSIRO Exploration & Mining is focused within five major projects and several minor projects. The major projects are: 1 3D Aspects of Longwall Geomechanics. This was reported to the conference last year and a final report will be available by mid-2001. 2 Mine Gas Control. This just completed project centred on Dartbrook Colliery which has a longwall gas make of more than 8 cubic metres per second. 3 Rapid Roadway Development This includes the construction of an autonomous conveying and bolting module (ACBM) that sits between the miner and haulage system. 4 Top Coal Caving In conjunction with the YanKuang group in China, CSIRO and the University of NSW are conducting a prefeasibility study of the Top Coal Caving method in Australian conditions. 5 Longwall Automation. In partnership with the Cooperative Research Centre for Mining Technology and Equipment (CMTE), the CSIRO has conducted a scoping study on this issue and will commence an initial three-year demonstration project in July this year. This paper describes these protects in more detail with particular focus on the last three in the above list.

Jan 1, 2001

By Changshou Sun

The geotechnical properties of mining zones at Leeville Underground mine (or simply called Leeville mine through this paper) are different from one zone to another. The typical ground condition is poor to very poor. Therefore, an appropriate ground control practice is the key for the successful and smooth ore production at Leeville mine. Based on the available geotechnical data, the empirical and numerical methods are used to design the right ground support systems. The main ground control practice at Leeville mine includes geotechnical data collection, evaluation of the stiffness of the support system, selection of rock bolt types, design bolt length and bolting patterns, development of the ground support standards, and performance of quality controls on bolt installation to ensure proper rock anchorage, and strength of shotcrete and backfill. Through all these activities, an appropriate ground support system has been established and applied at Leeville mine for the past four years. So far, a good result related to successful ground control and smooth ore production has been achieved, although the system is still being improved. The authors of this paper will summarize the ground control practice conducted in the last four years and will discuss the procedures and standards of establishing the appropriate ground support systems for Leeville mine.

Jan 1, 2013

By Dieter Wittenberg

The successful extension of the applications in which rockbolts can be used in the German coalmining industry is, because of the very great raining depth combined with a high degree of stress in the native rock, only possible on the basis of a comprehensive quality management system. The quality assurance system consists of material tests on the individual component parts of a rockbolt, of test rig trials of installed rockbolts, of suitability tests underground and of measures for quality assurance during roadway drivage. Only on the basis of exact knowledge of the way in which installed rockbolts function in the underground and possible sources of error and faults can quality assurance be guaranteed during roadway drivage.

Jan 1, 2000

By J. M. Girard

Ground control problems at surface mining operations can occur for a variety of reasons. Stress, gravity loading, rock strength, geology, pore pressure, weather effects, underground workings, and many other factors contribute to slope instabilities that range from small rock falls to massive slides of material. While some of these failures can he predicted or controlled by preventive measures, each year many completely unexpected failures occur. Current methods for monitoring generally involve measuring displacements at a few, selected points in and around the suspected area of instability. While most of the displacements along these points will be in a downslope direction, freeze-thaw cycles of water-filled joints, horizontal stresses or pressure, buoyancy in saturated soils, human measurement errors, or other situations can produce deformation in almost any direction, even without any instability in the slope. Determination of which, if any of the observed movements represents a potential hazard is essential.

Jan 1, 1998

By Radoslaw L. Michalowski

The operation of mobile mining equipment near the crest of stockpiles and wastedumps often results in localized slope failures. The Bureau of Mines has developed a computer code based on a rigorous limit plasticity approach which determines the admissible total weight for a concentrated loading near the crest of these otherwise stable structures. The program is based on a three- dimensional analysis and provides an upper bound solution to the load that can be applied to the top of the operating distances for mobile equipment from the slope edge. This presentation will discuss the application of limit plasticity to the development of the local three-dimensional collapse mechanism, and the use of the computer code based on this approach for mining applications.

Jan 1, 1990

By Vasundhara

A chance detection of a seismic event of unexplained origin in the mining district of Appin in New South Wales. Australia, led to a detailed investigation of recording and analyzing seismic events occurring in the area. Multiple events were recorded each day on the seismometer stations. The larger events were strong enough to be recorded on a number of stations spread out in the district. Background investigation of seismicity stimulated due to engineering activities revealed that similar cases of mining triggered seismicity have been reported previously from many mining districts around the world. If the tectonic stresses are close to failure in a region then a small perturbation of the stress state may lead to instability in the region which may manifest itself as seismicity. The parameters that control the occurrence of such siesmicity include tectonic stress magnitudes and orientations. geological weaknesses or structural features and the amount of stress contribution due to mining. This paper reports on the seismic investigative study conducted in the Appin area and considers its association with the geological layout of the region and the mining activity.

Jan 1, 2001

The purpose and current practice of cable trusses in underground coal mines is briefly outlined and details of various cable truss designs are presented. The main part of the paper describes the in-situ performance of cable trusses installed as secondary supports in gateroads and recovery roads of longwall coalfaces. The performance analysis is based on the results of in-situ monitoring carried out at a number of sites at two underground coal mines within Australia. Truss performance has been assessed as load developed in relation to roof convergence, face position and time. Additionally, the influence of strata conditions, installation pattern/density, roadway layout and geometry have been taken into account. An analytical model has been developed to predict truss loads. The principal variables are listed and the basis of the model is described. Based upon behaviour observed in the field the model has been progressively upgraded and validated against measured performance. Guidelines on truss performance and factors which influence it are presented and the implications of these to the use of cable trusses is discussed. Possible methods for improving truss performance in certain strata conditions are outlined and some comments are made on the value of cable trussing compared to cable bolting in these situations,

Jan 1, 1992

By Nikolaos Polysos

For the planning and driving of gateroads supported by roof bolts a practical concept has been developed in order to determine the geological and rock mechanical parameters derived from exploration drillholes. Based on this, an assessment scheme for the design of roof bolting has been established. The new methods of investigation comprise of geotechnical logging of drill cores for the determination of structural-geological rock characteristics and complementary geophysical well-logging which enables the evaluation of petro-physical and rock mechanical properties. The tools and their measurements will be briefly presented. The relationship between the geophysical log readings and the geotechnical parameters will be illustrated. The aim of the data acquisition is to improve roof bolting design which is based on the assessment scheme. This will be adjusted to the prevailing rock mechanical conditions and stands up to the required stability of the gate roads. In order to ensure consistent data acquisition, a method of core logging has been standardized for the respective demands. To achieve this it was necessary to introduce specific parameters and new methods. For the recording of linear and planar structural elements at the drill core, a special system for the documentation of the angles in relation to each other has been set up. The rock mechanical parameters and their acquisition will be explained. In order to carry out an objective analysis as fast as possible, software has been developed for the acquisition, management and presentation of the data. These programs and their functions will also be presented. The software is characterized by being user-friendly and the flexibility of its interfaces, Different options for the combined presentation of input and evaluated data give a more objective basis for the planning of roof bolt design. Examples will illustrate the operational implementation of the results during the planning and driving phases.

Jan 1, 2001

By James M. Kramer

This paper introduces an analytical approach for the determination of the vertical stress in the chain pillar system between two adjacent longwall panels. The development of this approach is based on the principles of fracture mechanics and superposition. Some empirical formulae have also been incorpomted into the development of the mathematical model. The mine entries and longwall gobs are treated as Mode I cracks in an infinite plate while the resistance of gob material and the characteristics of the pillar yield zone are considered as the distributed forces on the surface of the cracks. An user- friendly computer program has been developed to facilitate the calculation involved in this task. This approach is as accurate as the numerical analysis methods but requires much less expertise and computer hardware and software to apply

Jan 1, 1998