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By B. T. Wells

This work concerns the stress fields encountered around mining tunnels. A critical review is undertaken of the various analytical assessments of the support requirements for mining tunnels. Comments are made regarding degree and type of support together with method and timing of installation. The results have been used to indicate the type of support required in different mining and geological conditions.

Jan 1, 1981

By Mario G. Karfakis

Mining related subsidence is a major concern over abandoned shallow coal mines. Many of the subsidence prone areas are presently used or will be used in the future for residential housing development. Concern over mine subsidence is inhibiting the development of surface land uses on previously mined area. This concern prompted public officials to realize the need to develop guidelines which can be used to assess and reduce the risk of mine subsidence damage to building and other improvements. Subsidence and the associated ground movements must be defined before one can relate them to building damage. Surface movements resulting from underground coal mining occurs in two phases termed as: active subsidence and residual subsidence. Active subsidence refers to all surface movements occur in^ simultaneously with the mining operations, while residual subsidence is-that part of the surface deformation which occurs following the cessation of mining. Residual subsidence is the major concern over abandoned mine lands. The duration of residual subsidence is of particular importance from the standpoint of damage at the surface to structures built after the mine has been abandoned. Time spans during which surface subsidence may occur, vary markedly with the mining method being used. In total extraction methods, such as longwalls, subsidence is induced rapidly, beginning almost immediately after mining. Subsidence concurrent with coal extraction is desirable for a safe longwall mining operation. Usually subsidence is complete within a relatively short time. The remaining settlement occurs as residual subsidence due to the gradual compaction of the subsided ground. The relative proportion of the residual subsidence movements is dependent upon the presence of remnant pillars and roadside packs. rate of face advance and depth of workings. In partial extraction methods, such as with room-and-pillar systems. major occurrences of surface subsidence may be delayed for decades until the support pillars and/or the roof have substantially deteriorated and collapsed. The actual time involved depends on a number of factors such as the strength of coal, roof, and floor, extent of fracturing. presence of water, depth of workings, pillar size and percentage extraction. Consequently residual subsidence is more likely to occur over abandoned room-and-pillar mines. The following sections define subsidence over abandoned mines, review the proposed mechanisms, identify the controlling factors and examine the prediction methods.

Jan 1, 1987

By John C. Stankus

In this paper, an innovative concept in longwall tailgate support is discussed. This new concept utilizes a new high capacity roof truss system in lieu of conventional wooden crib block. The first field test was conducted at Bailey Mine and was very successful. Similar tests have been installed in other mines and seams. An extensive instrumentation plan was designed and implemented in the truss test area. The test data has shown that the dynamic roof truss system effectively controls the various abutment pressures caused by longwall mining thus maintaining a stable tailgate entry.

Jan 1, 1994

By David A. Newman

Strength and physical property testing of coal is conducted regularly by the Arch Mineral Corporation as part of reserve exploration. Pillar design and mine layout are the primary applications for the coal strength data. Distinct differences in pillar behavior were observed during longwall mining at a mine in Southeastern Kentucky. Horizontal and vertical variation in both triaxial and uniaxial compressive strength was observed within the four (4) benches of the Harlan seam. Similarly, geographical variations in coal strength were noted across the Harlan seam reserve. During the past three decades substantial research efforts have been focused upon coal pillar behavior and the refinement of pillar design methodologies. Ironically, little attention was directed to integrating the variability of coal strength into pillar design. The authors examine, through the presentation of case history data, i) typical industrial practice with respect to establishing a database of coal strength for use in pillar design and stability analysis, ii) approaches in which the observed variation in strength in the Harlan seam is integrated into pillar design and stability analyses, and to iii) propose standards for coal cube testing

Jan 1, 1993

By Ed Mchugh

Recent studies by researchers from the National Institute for Occupational Safety and Health indicate that shear loading contributes significantly to failure of bolts used for rock reinforcement in coal mine roofs. Laboratory tests on 17 bolts were conducted to study the behavior of roof bolts subjected to shear loading over a range of axial bolt loads. Fourteen strain gauges were attached to each bolt to measure axial and bending loads at seven locations along its length. The instrumented bolts were grouted through two high-strength concrete blocks, and axial tension was applied to as much as 75% of yield strength. With the block interface acting as a failure plane, shear loads were applied to the blocks at a constant rate of displacement to the ultimate strength of the bolts. The tests characterized the relationship of axial bolt loads to shear forces across a rock bedding plane and measured the distribution of axial and bending strain along the length of the bolts. These results will improve the selection of roof reinforcement in mine areas where high shear stresses are present, thus improving the safety of miners working in these areas.

Jan 1, 1999

By J. S. Oram

Maltby Colliery, operated by RJB Mining (UK) Ltd is successfully mining the Parkgate seam in Yorkshire at a depth of 1000m using longwall retreat. Extraction in the Parkgate seam is influenced by interaction from previous workings in the Swallow Wood Seam 160m above. The paper describes in-situ stress measurements and simple gate road convergence measurements carried out to assess the extent of the interactive influence. The stress measurements, obtained using the stress relief overcore technique, were carried out from a maingate development roadway in an area where the stress redistribution under an overlying pillar and adjacent panel goat could be measured. The measured vertical stress distribution confirmed a significant interactive influence with high imposed vertical stresses under the overlying panel edges. Simple roadway convergence monitoring stations established in the gateroads of a retreating panel and in the development gateroads of the adjacent panel, confirmed higher rates of roadway deformation relating to overlying panel edges and pillars, particularly in the areas where a vertical stress window developed during panel retreat. In addition to highlighting a close relationship between closure rate and the measured vertical stresses, the convergence monitoring proved valuable in establishing characteristic deformation behaviour under specific interactive influences and also in defining the optimum location, extent and time of installation u£ additional roadway support and reinforcement. The information from the measurement programme has enabled the Mine Management to gain a better understanding of the interactive influences on roadway behaviour and was instrumental in designing the layout of panels in an area of the North East district of the mine where previous experience had indicated the combined influence of high interactive stresses and weak roof lithology resulted in difficult strata conditions both at development and on panel retreat. The planned location of panels and gateroads took into account both mining operational considerations and the need to avoid potentially high risk interaction zones as highlighted from analysis of the monitoring results. To obtain additional data on parameters influencing roadway deformation, extended roadway convergence monitoring has been established.

Jan 1, 1997

By Victor V. Nazimko

Longwall mining technology without interpanel pillars is feasible from a geomechanical point of view. A set of pillarless longwall layouts are proposed including tailentry pillar recovery, headentry pillar recovery and drivage of a new tailentry, partial headentry pillar recovery and combination mining in which advancing panel entries are used for retreating panels. The following mitigative measures are proposed in order to enhance the stability of entries near the panel: driving crosscuts obliquely to the longwall face line, advance support of entries and crosscuts, localized destressing of the chain pillars by horizontal slotting, and partial stowing of the gobs behind the pillars being recovered. A new technology of recovering barrier pillars on the setup entry side is also proposed. This technology uses the same regular longwall face for interpanel pillars extraction.

Jan 1, 1994

By Syd S. Peng

Full-face shield leg pressures in a longwall panel for a three-month period were collected and analyzed. The pressure data were presented in the following forms: full- panel full-face shield pressure, individual shield pressure, full-face shield pressure, full-panel time-weighted-average pressure, and relationships among setting pressure, final pressure and pressure increment. Although shield pressure varies from shield to shield and from cycle to cycle, the setting pressure and face advance rate do control the 6nal pressure and time-weighted average pressure.

Jan 1, 1997

By Edward S. Custer

Recent investigations of mine roof falls have demonstrated that roof failures are generally associated with geologic conditions. The roof falls can be predicted by using the techniques of depositional modeling for the coal bed and roof rocks, and by lineament analysis. The ability to predict potential roof failure areas will allow the engineer and mine planner to develop a mine plan to avoid the roof failure areas and thus reduce overall mining costs. The ability to avoid roof failure areas also will permit maximum recovery of the coal resources.

Jan 1, 1981

By Holger Witthaus

The conditions of great mining depth and multi-seam¬longwalling require a special method of rockbolting, It can be described as a control loop which is based on the elements of planning, drivage and measurement. Different planning elements such as the results of the observation of physical models, numerical programmes for calculation of stress-distribution and support success, lab and underground support testing and the knowledge of strata behaviour underground including geotechnical classification are used to get best results. The increasing geotechnical information collected during drivage of roadways are used to estimate and minimise the risks of support-overload. The repeated plan/acutal comparison during the use of roadway and continuously monitoring of support and roadways provided the opportunity of optimising support design during the process of drivage and if necessary in front and after of the longwall face.

Jan 1, 2000

By P. M. Lin

The problem of abandoned mine land (AML) subsidence is getting more severe because abandoned mine lands which used to be located in the remote areas have been gradually developed into suburbs or even urban areas. Abandoned mine subsidence is also becoming more common than before because the time-dependent effect of exposed underground materials. These materials will deteriorate gradually and cause subsidence as time passes. Subsurface conditions of abandoned mines are seldom known to the public. Mine maps are not always available. If the mine mans are available, their accuracy are very - often questionable. In addition, the most common damages to surface structure and ground surface due to mine subsidence can also be created by other non-mining related geological hazards. These factors make the study of abandoned mine subsidence significantly complicated, tedious, and inconclusive. For instance, the cause, time, and amount of surface subsidence can be predicted with relative accuracy during active mining. However, it will be a considerable challenge to do so on abandoned mine subsidence. If abandoned mine subsidence can be predicted in advance, then the damages can be held to a minimum. However, this prediction technique is not yet well developed. Therefore, remedial measures require more attention. In order to have effective remedial measures, the real cause of the damage should be determined first. This paper describes the approach which was employed for the determination of abandoned mine subsidence. A site investigation was made first. According to the information obtained from the site investigation, a hypothesis was set up. Then the instrumentation was used to prove the hypothesis. By going through the procedures for several cases, a guideline which can be used by the inspectors to inspect abandoned mine subsidence will be established.

Jan 1, 1987

Effective strata control, utilising fully encapsulated roof bolts, is dependent on the installed quality of the reinforcement elements. One mechanism by which fully encapsulated roof bolts may become less than fully effective is by gloving (glove fingering) and un-mixing of the resin. Following some small roof failures containing gloved bolts, investigations have been undertaken in some New Zealand coal mines to determine the extent of, and mechanisms involved in, the gloving of fully encapsulated roof bolts for a range of roof types and installation methods. Gloving, in this context, refers to the resin cartridge partially or completely enveloping a length of the bolt, typically with unmixed resin filler and catalyst/hardener remaining within the cartridge. Results have shown that gloving and un-mixing is a systematic and widespread phenomena, occurring across a range of resin and/or bolt manufacturers, and across a variety of roof types. Gloving was found in bolts installed using: hand held pneumatic and continuous miner mounted hydraulic methods, under run of mine (ROM) conditions by mine workers, and under controlled "best practice" conditions. The roofs bolts investigated are all 1800mm (6ft) long, and have shown that, on average, 485mm (-19") of the bolt length is gloved and/or unmixed. This equates to a 27 percent reduction in effective bolt length. The effected length of bolt is sufficient to impact on the load transfer/reinforcement capabilities of the bolt system, and hence impacts on the design assumptions and subsequent stability assessments. Furthermore, there is a direct relationship between the mechanism for the gloving of bolts and an increased occurrence of resin loss and reduction in encapsulations length in soft (less than 10 MPa) and/or highly fractured rock. Investigations have concentrated on developing an understanding of the mechanisms involved. This has been undertaken using both field installations and recovered bolts, as well as test bench trials. The mechanisms involved have been confirmed as being: the development of a pressure front as the bolt encounters the resin and is spun up the hole, which, in turn, leads to over-pressurisation and radial expansion of the resin cartridge, resulting in an increase in diameter of the plastic resin cartridge, which allows the bolt to be spun up into the cartridge without making sufficient contact to shred the cartridge or the hardener tube, resulting in an unmixed portion of resin.

Jan 1, 2003

By David C. Johnson

Coal mining in Illinois has resulted in many structures being exposed to subsidence damage. Damage mitigation techniques (DMT) applied in the foundation design/construction have proven successful in minimizing damage in several test cases. To determine the practicality of these techniques, the Costs of their application are investigated. The AMT costs are compared to foundation and total house costs for various classes and sizes of houses. An economical and effective subsidence resistant foundation construction is proposed.

Jan 1, 1993

By Collin L. Stewart

A summary of the results of a limited subsidence monitoring program at Cyprus Amax Coal Company's Shoshone Mine No. 1 in the Hanna Basin of south central Wyoming is presented. The monitoring program was designed to meet regulatory requirements for confirmation of subsidence predictions made in the mining permit. The longwall panels were 380 ft to 600-ft wide. Mining heights varied from 11 ft to 13.5 ft. Gate road development included three-entry and two-entry head and tail gates. Overburden depth varied from 150 to 945 ft. The coal seam and overburden dip at approximately 11 degrees. The overburden is very low to low strength mudstone and carbonaceous shale with low to moderate strength, silty sandstone interbeds. Panel W/h ratios varied from subcritical to supercritical. Smax was 0.9 of the mining height. The subsidence ratio was about 0.6 for subcritical width. The draw angle on the up-dip side averaged 20 deg. The draw angle on the down-dip side averaged 12 deg.

Jan 1, 1995

By Changshou Sun

Excessive deformation occurred in the underground openings of Jinchuan Nickel Mine, the largest nonferrous underground mine in China. Developed in weak rock formations at great depth, the underground openings are under great in-situ stresses with the horizontal stress being twice as high as the vertical one. Based on in-mine observations and in-situ testing results at the mine, time-dependent mite element simulations with coupled creep and plasticity constitutive models were conducted in this study to analyze the stability of underground openings In the study, the Burgers creep model and the Drucker-Prager plastic model were coupled to simulate underground drifts and cut-and-fill stopes. Two-dimensional finite element models were developed with ABAQUS - a comprehensive finite element simulation package The study investigated the behavior of underground drifts at great depth with high horizontal stress and the behavior of cut-and-fill stopes with various stope heights and fill conditions. The numerical simulations revealed important characteristics of opening deformations as a function of time after excavation and location relative to the wall of opening The results had good agreement with field monitoring data, They provided useful information to aid mine engineers in the design of mine openings, the selection of opening supports and the determination of support installation procedures.

Jan 1, 1999

By George B. Quaye

Conventionally, roadways in South African collieries are 6 to 7 m wide. This dimension is chosen so as to allow maneuvrability of appropriate mine machinery and equipment, to meet production requirements and to ensure roof stability. Over the last two decades, a great deal of research effort has been expended to understand the behaviour of roadways in different strata within this width range. The results have improved the understanding of the strata behaviour to such an extent that 6 m wide roadways are developed with a high degree of confidence. With the advancement of technology and the quest to mine at a lower cost, some mines in South Africa have started considering the benefits of other types of mine machinery that will require much wider roadways to ensure optimum productivity. The roadways required will have to be about 10 - 14 m wide. The challenge confronting these coal mining companies is the potential instability that might be associated with such wide roadways. This dilemma is aggravated by the fact that, apart from theory, nothing is known about the expected behaviour of such wide roadways. This led to experimentation at a selected site in a South African coal mine where 10 m and 14 m wide roadways were developed and monitored. This paper highlights the experimental procedure adopted, discusses the results obtained, compares the prediction of elastic theory to reality and set the precedence towards understanding the behaviour of wider roadways in South Africa.

Jan 1, 2001

By Li-min Liu

The Semi-Analytical method can be used to solve a variety of problems in geomechanics and civil engineering. When it comes to layered rock strata and rock masses, it is superior to all other analytical methods. This paper presents the application of a 3-D semi- analytical method to 3-D surface subsidence and stress analysis caused by mining. In this semi-analytical method the layered elements and triangular elements ate used alternatively. Their displacement pattern, strain mahix, elastic matrix, stiffiness matrix, load matrix and stress matrix are presented. A numerical simulation program based on the semi-analytical method bas been developed using Fortran 90. This program is suitable for subsidence prediction and stress analysis of mom and pillar mining, strip longwall and mechanized longwall mining methods. It is applicable to all geological conditions and mining methods. Comparing with the finite method, this method reduces significantly the number of simultaneous equations to be solved and the amount of data preparation. It not only reduces the computer memory, but also runs faster and cost less. Key Words Semi-analytical method, surface subsidence, stress analysis, layered element, triangular prism element

Jan 1, 2002

By Khaled Morsy

This study presents a typical stress-strain model for coal material using the available in-situ and laboratory tests data conducted on coal. This model can be used as a basic coal model in finite element applications The effects of pillar width-to-height ratio and the confining pressure have been considered in this model. The extended Drucker-Prager model with mobilized friction angle was found to be very successful in simulating the proposed model.

Jan 1, 2001

By Jeffrey K. Whyatt

Estimation of the initial vertical stress carried in a coal seam is an important first step in virtually all methods of evaluating the required size of pillars in coal mines. Such estimates are a trivial exercise in coal fields overlain by gently undulating topography. The LaModel program is easily and routinely applied in such cases to efficiently estimate mining-induced stresses throughout the coal seam. However, application of LaModel to coal mines operating under rugged topography and/or in tectonically disturbed regions is more difficult. LaModel estimates stresses in a twostep process. Topographic and multi-seam mining effects on insitu stresses are estimated in an initial step, and then mininginduced stresses are estimated in an independent second step. A LaModel program function has recently been introduced that decouples these two steps, allowing in-situ stresses to be directly entered from a file rather than calculated. One application of this function is to streamline sensitivity studies by avoiding repeated recalculation of in-situ stress. The function also allows importation of in-situ stress estimates developed by other means. There are no constraints on the imported stress field, which may include significant stress perturbations due to topography, faults, etc. Hence, the range of geologic environments that can be examined by LaModel is significantly increased. This study uses the new decoupling function in an exploration of in-situ stress estimation in mountainous terrain. One option is to use the topographic stress estimation feature included within the LaModel program, and the influence of important parameters in this routine is explored. Another option explored is importation of results from a detailed volume element model. Results of this study suggest that the method used to estimate in-situ stress, and the details of its execution, can have a significant impact for mines in mountainous terrain with strong overburden. More generally, decoupling is an efficient and flexible function that should be considered for most design studies.

Jan 1, 2011

This paper describes the experiences with the application of the Coal Mining Roof Rating (CMRR) System to South African strata conditions. Roof classification studies using the CMRR system were undertaken as part of a SIMRAC investigation into falls of ground in South Mean collieries. Colliery roof rating exercises were undertaking in a range of geotechnical conditions from shallow level coal seams under flat topography to workings with cover depths of greater than 300m A comparison is made between the CMRR and other Rock Mass Rating systems used to classify different roof geotechnical conditions. The need for geological discontinuity orientation, blasting and rockmass stress adjustments to the CMRR is discussed. The effects of acid mine water on strata weatherability and the need to incorporate immersion tests using acid mine water are also detailed. In addition limited correlation is given between CMRR values and other geotechnical factors such as joint trace length, joint shear strength, joint orientation, weatherability and roof damage due to horizontal stress. Taking cognizance of these observations, a concept of stress induced roof damage triggering is discussed.

Jan 1, 2001