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By H. N. Maleki

Extensive measurements and underground observations in three Western U. S. coal mines are integrated in this paper to determine in-situ pillar load-deformation characteristics for narrow (30 it wide, 80 ft long) pillars in two-entry gate road systems. In spite of similarities in the regional geology, coal pillar laboratory mechanical properties and gate pillar geometries, the pillar peak strength, post-failure behavior, and failure mechanism were shown to be significantly different. Pillar peak strength was shown to be dependent on depth at one site, approaching burst-prone stress levels of 4000 psi. At another site, the pillar peak strength was lower because of lower confinement; this was related to the higher frequency of cleats and the lower friction- al properties of the roof/floor and coal contact. Two failure mechanisms were identified - one in the pillar, and the other in the mine floor. The roof stability was good at all three sites because of the thick-bedded nature of the roof strata and limited total gate span in a two-entry system. Existing pillar design techniques were shown to be inadequate for design, requiring adjustments for depth of cover, cleat frequency, and roof /floor frictional properties.

Jan 1, 1988

By Gustavo Herrera Rico

As part of a general effort to increase safety, efficiency, and profitability of its mining operations, MICARE personnel with the support of NorWest Mine Services, Inc. determined that the key to improving development mining was the implementation of roof bolting as the means of primary roof support for both main entry and longwall gate road development. Until mid-1994, MICARE had relied exclusively upon primary roof support systems composed of heavy steel beams and wooden posts. Support systems were designed, implemented, and evaluated that were successful in controlling the mine roof under extreme geological conditions. This paper describes the geological challenges, support selection criteria, and presents the instrumentation and 'mine measurements made to assess the bolting performance during the development and retreat mining process of longwall mining.

Jan 1, 1997

By Danny J. Van Roosendaal

An investigation was undertaken to determine hydrogeological effects of subsidence over a longwall coal mine in the Illinois Basin. At this mine, approximately 200 ft of bedrock overburden is overlain by up to 150 ft of glacial till and localized water-bearing glacial sediments within a buried bedrock valley that overlies the longwall panel. A finite-element mechanical model of the subsided longwall panel shows sufficient strain to enhance permeability throughout the bedrock portion of the overburden. However, examination of the actual surface subsidence profile indicates that the overburden deformation is localized in narrow zones at the panel edges. The aquifer units at the base of the glacial sediments are confined by glacial clays (above) and weathered shales (below). Elevated water levels were observed in the glacial aquifers during subsidence, which indicates that the clays and shales maintained confinement as the overburden subsided and deformed. A three-dimensional ground-water flow model was used to simulate the effects of subsidence on the hydrogeologic system and to estimate inflows into the mine. Longwall retreat was simulated by several model runs, each representing a new longwall face position. Simulated heads and mine inflows were calibrated with recorded water levels and observed inflow conditions by modifying the hydraulic conductivity of particular model layers. Model calibration indicates that the permeability of certain lithologies, such as plastic underclays and weathered shales, is not significantly enhanced by subsidence. A worst-case scenario was simulated by inserting a deeply incised, sandfilled valley into the model, which had only minimal impact on the simulated mine inflows.

Jan 1, 1995

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 Daniel W. H. Su

Surface subsidence induced by multiple-panel coal extraction was calculated with finite element stress analysis. The use of nonlinear material behavior and GAP elements, which provide a realistic representation of shearing along existing planes of weakness, allows the finite element program to accurately reproduce observed subsidence profiles. A reduction factor of 1/6 is applied to the measured rock strength and modulus for use in the finite element model. The models satisfactorily predict a significant change in maximum subsidence and subsidence profile associated with an increase from subcritical to near critical panel width. Modeled shearing along planes of weakness is in good agreement with available time domain reflectometry field observations and is demon¬strated to have a very profound effect on the maximum subsidence and the shape of the subsidence profile. When both material nonlinearity and planes of weakness are incorporated, the finite element model appears to possess great potential for accurately predicting surface subsidence and subsurface strata deformation in environments where little subsidence information is available.

Jan 1, 1990

By Kent D. Mctyer

Mining commenced at Tasman Mine in late 2006. The current method of mining is bord and pillar using continuous minerbolters and shuttle cars for first workings and secondary extraction using mobile roof supports. The two stage process was chosen to accommodate irregularly shaped coal deposits, allowing adjustments to be made to extraction ratios for better management of subsidence and to maximize the efficiency of the operation. Following the completion of the first three full/partial extraction panels, the mining method was changed due to variable caving. Another partial extraction method, referred to as the Duncan Method was adopted; it involves stripping the developed square pillars on four-sides on retreat to leave a load-bearing remnant coal pillar. The system of partial extraction has been successful in meeting safety, productivity, and subsidence targets.

Jan 1, 2011

By Mohummad Akram

Statistical analysis of multi-seam mining case studies demonstrated that geologic structure and lithological type and sequencing were predominant factors in entry failures. Finite element analysis was used to evaluate the effects of stress field changes on entry instability under differing geologic conditions. Analysis showed the roof to be particularly vulnerable to shear with the location of the failure site changing to meet loading and geologic conditions. Effects of external support on roof stability were examined. Results were correlated with field data to determine stress levels at which failure could be expected to occur. Based on this research design, criteria were developed for roof stability evaluation under specified interactive loading conditions.

Jan 1, 1993

By Qiang Fu

ABSTRACT This paper aims to identify the main issues concerning top-coal loss and the optimum drawing interval of LTCC (Longwall Top-coal Caving) mining by underground tests and DEM (Distinct Element Method) modeling. By extracting a new roadway in the goaf of the mined LTCC Panel No.4318 in Wang Zhuang coal mine of Lu-an CMA (Coal Mining Administration), and underground observation of the distribution of loss coal and broken rock in different sections of the roadway, this paper analyses the composition and presents a 3-D distribution map of the loss coal in the exploration roadway. This study indicates that the DEM modeling of broken top-coal is reasonable and scientific as it can simulate the movement of broken coal and waste rocks and determine the optimum drawing interval of top coal.

Jan 1, 2002

By William Monaghan

Over the period 2000 to 2003, roof falls have accounted for 4 to 14% of the fatalities in underground mining operations. The National Institute for Occupational Safety and Health (NIOSH) is conducting research to reduce the frequency, exposure, and risk of these events through an ongoing program of field and laboratory studies. One area of research involves the evaluation of polyurethane grouting technology that is commonly used to stabilize fractured mine roof strata. Concurrently, NIOSH is conducting work to evaluate the application of ground penetrating radar for advanced delineation of problematic mining areas. In this study, NIOSH partnered with Sub-Technical, Inc. who injected a polyurethane grout (also known as glue) into a roof area of NIOSH's Safety Research Coal Mine. The mine roof area was scanned using ground penetrating radar technology before and after grout injection in an attempt to determine the extent of grout infiltration. A comparison of the pre-and post-grouting radar records showed a significant change at a mine roof depth of about four to five feet. The interpreted radar records were then compared with drill core information, borescope evaluation of roof-bolt holes in the study area, and underground observations. At this site, the interpretations of the radar records correlated with data obtained from the core holes, borescope evaluations and underground observations. This study showed that ground penetrating radar technology can be a useful tool for detecting changes in mine roof due to the injection of the grout.

Jan 1, 2004

By Gregory Molinda

Extremely difficult ground conditions were encountered in the main belt entry in a room and pillar mine in West Virginia. Several generations of supplemental support, including cable bolts, cribbing, and Heintzmann jacks and beams failed to halt roof falls that threatened workers safety and the life of the mine. Polyurethane (PUR) was injected into 27 intersections in the roof and the results were monitored by borehole video camera. The mine had averaged 2-3 falls in the beltway per year and, since the roof injection, no roof falls have occurred. Pre- and post injection monitoring of test holes showed that stability can be achieved without filling all of the existing void spaces. Video logging showed that the project goal of building a beam from 2-6 ft into the roof was successful. Pre-injection monitoring of roof fractures uphole can delineate the zones to target for reinforcement, and avoid pumping PUR into large voids to little effect. Multiple-¬injection zones can be identified, and the entire intersection drill hole array can be optimized based on knowledge of the fractured zones. In addition, pre-monitoring can prevent excessive hydrofracturing in solid zones. Polyurethane injection is a proven method of rock stabilization in even the weakest, most broken ground. Optimization of the injection design by pre-injection video diagnostics can greatly contribute to the successful and efficient roof stabilization.

Jan 1, 2004

By David A. Newman

Barrier pillars created during mine development provide additional reserves during mine closure and can be the sole access to reserves previously abandoned because of mining height or out of seam dilution limitations. The safe and successful mining of barrier pillars requires a thorough knowledge of the magnitude and distribution of the vertical stress field in addition to the strength and physical properties of the coal, immediate roof, and immediate floor strata. Quantifying the vertical stress field is complicated by the combination of the overburden stress, abutment stresses transferred onto the barrier pillar from adjacent gob areas, and in multiple seam mining, stresses transferred from abandoned subjacent mines. These variables make the extraction and development of barrier pillar reserves a site specific consideration. Three case histories are presented in which barrier pillar development and extraction provides; i) an access point and the means to ventilate a large area of low seam reserves left by a prior operator after developing only the thicker areas of a multiple split reserve, ii) the means to increase total recovery of a deep, thick seam longwall reserve, and iii) opportunities for the development and retreat mining of a two mile long barrier pillar in thick seam reserves.

Jan 1, 1995

By D. M. Shu

A two-dimensional finite element modelling technique is applied to predict the subsidence movements on a sloping ground surface above a completely mined panel. The modelling is carried out for the surface both directly and using a combination of an equivalent horizontal surface and a rays projection method and the result compared. The rays projection method provides the subsidence components on a sloping surface from the corresponding ones on an assumed equivalent horizontal surface through the point of mean elevation of the relevant part of the sloping surface. This method is used next to compare the subsidences, horizontal displacements and horizontal strains from the mining of a given panel at the surface for different inclinations. The results show that the magnitudes of the subsidence components are greater on the dip side of the slope from the point of the mean elevation, than the corresponding ones on the rise side and the asymmetry increases with the inclination of the surface. The findings are also compared with those of other research workers.

Jan 1, 1992

The effects of corrosion upon the load bearing capacity and ultimate tensile strength of galvanised friction bolts were determined. Corrosion rates were measured by overcoring of the reinforcement elements in situ. Overcoring was achieved at several sites using a specifically designed overcoring drill rig. This rig has the capacity to drill up to 3.0m long 140- diameter core in any orientation and up to a collar height of 7.0m. The recovered core and friction bolt elements were pull tested in the rock mechanics laboratory at the WA School of Mines (WASM) in order to determine their load-displacement characteristics for various embedment lengths. The friction bolt elements were then analysed for rates of corrosion and tested for their ultimate tensile strength. Relationships were formed between these data and the environmental conditions to which the friction bolts were exposed, in order to determine the major factors affecting the corrosion process and how they affect bolt performance.

Jan 1, 2005

By G. I. Gritsko

In this paper we consider the elements of a distributed information geomechanic monitoring system which is used in computerized mine planning and mining operations control. The automated system is a technical means of geomechanic monitoring, as a complex system of regulated observations, estimation and prediction of geomechanic state of rock mass and workings during underground mining of coal deposits. Geomechanic monitoring at coal mines and the automated system execute data collection, analysis and calculations of geomechanic situation in the panel during planning, development and carrying-out of mining operations for ensuring geotechnical stability of underground mining.

Jan 1, 1996

During the last decade mechanical rock excavation has become increasingly widespread in the mining industry and, more in general, in excavation engineering. This technology, especially the tools and their operating mode, has developed according to empirical procedures and for this reason their validity should be ascertained by systematically re-examining the rock breakage processes. This topic has been approached in the framework of a joint research project involving the Universities of Cagliari (Italy) and Wollongong (Australia). The investigation has been carried out experimentally as well as with the methods of numerical modelling which is based on fracture mechanics principles. The first results of this integrated research led to the proposal of an interpretation of rock breakage by means of drag tools and to the identification of a basic correlation between the rake angle of the tool and the grain size of the rock fragments produced and, therefore, the energy efficiency of the operation.

Jan 1, 1990

By Yajie Wang

Traditionally, roof falls are usually attributed to bad roof strata and high vertical stress related to the overburden depth. However, roof falls under shallow overburden depth are oflen caused by a high horizontal stress field. Recent studies have proven that the high horizontal stress field, if existing, plays a major role in roof stability. Also the roof conditions are related to the entry diredions under a horizontal stress field. Questions are: How to orient the entry to a favorable direction to prevent the roof falls? If the entry cannot be oriented, how to support it? Through a case study in mine A, this paper analyzes the horizontal stress effect on roof stability. The guideline for the entry orientation is given based on the results of analysis. Additionally, suggestions are made for those entries that cannot be oriented to a favorable direction.

Jan 1, 1998

By Yi Luo

Regulating the face advance rate in longwall mining operation can be an effective means to reduce the disturbance potential to surface structures associated with a longwall subsidence process. However, there are two seemingly different views about the relation between the face advance rate and the disturbance potential associated with dynamic subsidence process. The US mining industry prefers a faster but a constant face advance rate while the German counter- part is forced to select a slower rate due to its mining conditions, especially while undermining sensitive objects. In order to gain a better understanding about these two different approaches, the experiences in longwall mining subsidence in both Germany and US are presented in this paper with the fundamental differences in geological conditions and mining practices.

Jan 1, 2001

By Dwayne C. Kicker

This paper introduces the concept of design theory and methodology as applied to rock mechanics for more innovative and efficient design. This is a "frontier" research area in rock mechanics because although design is a fundamental foundation to all engineering branches, very little attention has been paid to this aspect in mining. In discussing the theoretical premise of design, this paper identifies the principles which form the basis of design activity. Also, from a practical viewpoint, the findings from a series of interviews with mine design engineers are presented. These findings lead to the identification of needs arising from overlooked design methodology in practice.

Jan 1, 1989

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 Eric Westman

Ground control problems caused by multiple-seam interaction frequently sterilize coal reserves. Interaction mechanisms differ for overmining and undermining and are controlled by mining, spatial. and geologic conditions at each site. A multiple-seam interaction analysis software program, UGLY (Uppersearn Gateroad Longwall Stability), has been used to evaluate sterilization of coal reserves due to overmining at three locations. Four case histories were reviewed to determine the effectiveness of improved ground control in mitigating interaction effects.

Jan 1, 1997