Search Documents

By Wm. Mark Hart

Under a specific geological condition, roof supporting method, and pillar-entry system, an entry convergence concept may be the most effective means that can be used to effectively indicate the longwall entry stability (i.e. pillar bumps, roof falls, or floor beaver This concept may also be used to verify the effectiveness of the current pillar-entry system and roof supporting method. Furthermore, convergence measured under different mining phases may lead to a better understanding of the longwall mining-induced abutment loads and may be used to estimate stress change in pillars and thus pillar stability. In this paper, a remote entry activity monitoring system was designed; a comprehensive entry convergence monitoring strategy was proposed; thee abutment loading phases in longwall mining were defined; the relationship between roof deflection and stress change in a pillar was derived, and finally the extent and magnitude of the mining-induced abutment loads as well as their impact on entry convergence were discussed using the field data obtained so far.

Jan 1, 1995

By Gregory M. Molinda

The U.S. Bureau of Mines has developed a rock mass classification system for coal mine roof control. The Coal Mine Roof Rating System (CMRR) evaluates the structural competence of mine roof using simple field tests and observations obtained from underground exposures in roof falls or overcasts. The basis of the WRR is a weighing system which converts descriptive geologic roof rock data to a numerical value, facilitating its use in engineering design. A full suite of data collection and hand calculation sheets accompanies the CHRR A data base is currently being collected from all major coal basins in the United States to validate the CMW. To date, CHRR values are available from 96 roof exposures in 59 coal mines representing these basins. The system can be used to help solve many practical ground control problems, including longwall gate road design, roof support selection, and decisions regarding extended cut length.

Jan 1, 1993

By Greg Hendon

Longwall developments in the UK have historically consisted of single entry gate roads. Adjacent developments were separated from existing panels by large barrier pillars (designed of sufficient width to get away from the longwall abutments of the previous panel) or by small barriers driven in the shadow, or de-stressed zone, of the previous panel. Some 2nd panel tailgates were also driven skin to skin leaving no coal barrier between the newly driven entry and the heavily supported existing gateroad. With the developement and wide acceptance of fully bolted entries and the pressure to reduce production costs, alternatives to single entry drivage, particularly yield pillar developments, were examined. Through the Rock Mechanics Branch of British Coal, a cooperative study was begun with Jim Walter Resources, Inc., USA, (JWR) to look at the yield pillar alternative in detail. This study was to determine the feasibility of utilizing yield pillars in the UK and to determine, through monitoring, the possibility of reducing stable pillar widths at JWR. The study has included extensive monitoring of the yield-stable-yield pillar system at JWR No. 7 Mine and an underground trial of a two entry yield pillar test area at Welbeck Colliery in the UK. This paper describes results from the JWR study and the subsequent results of the first advancing yield pillar development in the UK at Welbeck Colliery.

Jan 1, 1995

By C. J. H. Brest van Kempen

The techniques developed should provide a useful tool, not only during the initial formulation of a suitable bolting plan for a new section, but also for periodic check¬ing on the utilization factor of hardware installed in the roof. Indications are that with present practice, utilization seldom exceeds 507.. Retro-fittable bolter equipment now available (FCBS?) allows doubling of the utilization factor in many cases. We are also introducing a computer pro¬gram based on the material outlined in this paper. This program can save a great deal of time in examining the real effect of implementing alternate bolting plans or using different installation techniques. It allows fine-tuning of bolt lengths, spacing and installation tension to mini¬mize roof support cost, while maximizing safe life of the mine opening. We have developed a procedure which allows easy, site-specific quantification of the need for reinforcement of a mine roof. The concept is based on the forma¬tion of a direct link between theory and empirical data to custom-fit each mine section. In its simplest form, the proce¬dure requires only recording of roof bolt torques in a sample working place (16 to 25 bolts) several times during the first week or so after initial bolting. The way in which the torque pattern changes during this time can reveal whether or not the bolts carry enough tension to maintain stability in that specific roof. From the same data, the nature of the anchorage horizon is derived. Specifically, shale or limestone can be distinguished from sandstone. If the procedure is repeated a few times (within the same mine section) with different bolt installation tensions, we can numerically characterize the roof in terms of one single value: an "effective angle of internal friction". This number can then serve to calculate trade-offs in bolting plan design, such as bolt length, bolt spacing and optimum bolt tension. Additional refinement is possible if a sagmeter is installed in the sample work¬ing place being monitored and if bolt anchorage capacity is determined. The lat¬ter can be done automatically on each bolt as it is being installed, using a system we developed and patented. Alternately, a number of pull tests could be performed. Using the sagmeter, bolt torque and anchor¬age data, it is possible to project the safe life of the supported opening. The details of the procedures described apply specifically to tensioned roof bolting, but a brief discussion paints the way for a similar analysis of full-column resin bolting also.

Jan 1, 1986

By S. Jayanthu

This paper presents analysis of strata behaviour with special reference to convergence and stress variation during an experimental trial of extraction of pillars with cable bolts as major support system in 6.5 - 8.0 m thick coal seam. Critical span for roof falls was estimated through empirical models and also evaluated by numerical models. In situ strata behaviour studies during 1992-96 for about 70 local/major falls revealed inadequacy of the available literature for interpretation and prediction of the roof falls. Convergence acceleration and variation of induced stress based on continuous monitoring data showed distinct anomalies and potential for better understanding of strata mechanics during pillar extraction.

Jan 1, 1998

By S. S. Peng

This paper presents a convenient way for the preliminary design of the shield supports. The locus equations which are applicable for all kids of shield supports are derived. These equations facilitate the statics analysis of the shields. An indirect method in determining the coefficient of friction, the external toads and their locations are also developed.

Jan 1, 1982

By Lorraine Kent

Comparison of in-situ stress overcore measurement results with those determined from laboratory evaluation of Acoustic Emission (AE) testing on oriented sub-core samples has been carried out for a variety of mining environments. These include UK and German Coal mines at moderate depth, shallow-level South African Coal mines and deep-level South African gold mines. Results show that the `Kaiser Effect' or 'Stress Memory-Damage is readily determinable throughout the brittle deformation process, although particular care is required in interpretation of results during the 'bedding-in' phase of testing and the region associated with unstable crack growth prior to failure. The results suggest that AE provides a useful complimentary method for comparison with existing methods of stress determination. The success of the method can, however, be dependent on both rock type and the likely magnitude of in-situ stress in comparison to the strength of the rock. Keywords: in-situ stress, acoustic emission, Kaiser Effect

Jan 1, 2002

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 R. J. Morrell

The Bureau of Mines developed a ground support system for use with an underground coal auger. The system consists of inflatable airbags that are inserted in the holes as they are drilled, and then gradually deflated to provide controlled roof caving. This paper presents results of laboratory and field tests of the system that show it to be technically feasible.

Jan 1, 1988

By Lewis A. Martin

Instrumented cable bolts developed at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health were used in conjunction with existing ground control systems to monitor rock mass loads at Tg Soda Ash's trona mine, Granger, WY. Axial and shear loads were determined to levels of strain gauge accuracy of ±5 N or ±5 microstrain. These gauges were embedded in a remanufactured king wire that replaced the conventional king wire. Cable bolt performance, quality of grout, and installation techniques were also assessed. By using instrumented cables, a mine operator can determine axial load along the cable at predefined gauge locations. These data provide necessary input for an operator to design a safer working environment for miners.

Jan 1, 2001

By Thomas Barczak

A new generation of hydraulic mine support prestressing devices has been developed. These thin-walled steel shells are machine-welded and can be inflated with water or any liquid to provide prestressing for a wide variety of roof support products ranging from Can supports to props and cable bolts. With an expansion capability of several inches, depending on the geometry and size of the unit, they can also establish roof contact eliminating the need for wooden wedges or crib blocks that are commonly employed with several standing roof support products. Capable of withstanding pressures from 1,000 to 6,000 psi, these cells are able to transfer roof loading into the support structure without rupturing. A recent development has been the addition of an inexpensive yield valve that provides control of the maximum pressure and load development on the support. This paper examines the performance capabilities of these inflatable prestressing units and the impact they have on the performance of various support systems, including an evaluation of the overall stiffness of the support system and the load control during yielding of the prestressing unit. Recommendations are made regarding the design requirements of the prestressing unit to optimize the support performance. A summary of mine applications using this technology is provided with general comments regarding their impact on ground control.

Jan 1, 2004

By Gregory M. Molinda

An investigation was conducted to determine the nature and frequency of coal mine roof failure beneath valleys. A mechanism for this failure, and suggestions for controlling this problem are presented. Hazardous roof conditions identified in a number of mines were positively correlated with mining activities beneath stream valleys. Mine maps with overlays of unstable roof and locations of stream valleys show that 52% of the instances of all unstable roof in the surveyed mines occurred directly beneath the bottom-most part of the valley. The survey also showed that broad, flat-bottomed valleys were more likely to be sites of hazardous roof than narrow-bottomed valleys. Evidence of valley stress relief was found beneath a number of valleys in the form of bedding-plane faults and low-angle thrust faults. This type of failure, previously believed to be only a shallow phenomenon, was found at mining depths as great as 300 ft. In situ horizontal stress measurements in a mine beneath a valley and the adjacent hillsides confirmed valley stress re1ief. Underground mapping showed that roof falls in excess of 100 ft in length and up to 25 ft high closely aligned with the valley axis. Numerical analysis of 13 valleys overlying one mine property showed the effect of the valley excavation on horizontal and vertical stress. Thickness of cover, valley shape, and the orientation of the valley relative to the maximum regional horizontal stress all influence the "valley effect" on roof stability.

Jan 1, 1990

By Leslie Carver

The effects of longwall mine subsidence on water resources were studied at a West Virginia coal mine (mine Z) by Carver (1994). Data were obtained for 137 domestic supply wells and springs and 12 baseflow streams. Subsidence caused short-term dewatering (of a few days to weeks) of wells located on lower hillslopes and valleys, and long-term dewatering of hillslope and hilltop springs (with no recovery yet). New or increased spring discharge was observed at lower elevations. Unsubsided water supplies experienced no dewatering beyond a nearby panel's 27°-38° angle of dewatering influence zone. Subsidence from longwall mining beneath streams commonly reduced discharge when panel ages were less than two to three years. However, "normal" discharge was typically observed over panels greater than three years in age; such streams exhibited lower baseflow recession constants, moderated winter baseflow, and increased summer baseflow compared to unsubsided streams.

Jan 1, 1994

By John Cassie

The condition of roadways in the Deep Main seam at Asfordby mine, operated by RJB Mining (UK) Ltd, is strongly dependant on drivage direction relative to the maximum horizontal stress. The mine is laid out with gate roads driven approximately parallel to the maximum horizontal stress giving favourable drivage conditions. This has led to particulary difficult conditions for face installation headings which have to be driven to an increased width and at a high angle to the horizontal stress. A solution to the formation of face headings is described making use of stress relief with the face heading being driven in the zone of reduced stress surrounding a previous heading. In implementing this solution use was made of results from computer modelling and detailed monitoring of roadway behaviour. High reinforcement densities including cable bolts were used in supporting the headings. The experience provides an example of a traditional technique being applied more scientifically. The results obtained demonstrate the effect of stress relief and reinforcement methods in altering drivage conditions and suggest possible alternative strategies for the formation of future face headings in these conditions.

Jan 1, 1997

By Patrick S. Artrip

A deep mining operation in the Lower Kelly (Imboden) seam in Wise County, Virginia experienced severe ground control conditions in its attempt to develop reserves situated under increasingly higher cover conditions. Ultimately one of only three mines capable of providing access to a large reserve area had to be abandoned. A thorough geotechnical exploration program was carried out to allow characterization of the critical roof and floor rock strata in accordance with the modified geomechanics classification system. The principal roof and floor rock stratigraphic sequence types were defined and mapped throughout the reserve area along with other critical geological factors. A ground control study based on the geotechnical findings provided a quantitative analysis of the causes of previous entry instability. Further analysis of pillar and entry stability identified configurations which would remain stable for the variety of strata types and range of overburden depths within the remainder of the reserve. Mine designs based on the geotechnical evaluation have, to-date, provided the stability required for long-term future development - of the reserve area. However, some areas of difficult ground were encountered which could not be anticipated with the drill data available at the time of the original study.

Jan 1, 1993

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 Nikolaos Polysos

Besides considering the given geomechanical conditions, the geological/geotechnical logging of drill cores and heading faces during the driving of parallel entries and the resulting rock classification form the basis of the assessment of the proneness to caving of the roof strata in the actual mining phase. Both types of separation planes, parallel to the stratification as well as intersecting the bedding, will be defined and assessed within the immediate roof strata depending on the lithological structure. These data may hold the potential to plan the winning and design its operational organization in a way that - to a major extent - future damage of the workings by increased caving or cavities can be predicted in advance. Following the general description of the separation plane structure, the combined influence of the petrographic and structural geological roof properties on the operating cycle of the planned working face will be presented and illustrated.

Jan 1, 2003

By Keith Heasley

An eastern longwall mine was required to mine through a set of three entries located midway down the length of a panel. Of course, the mine desired to design a successful support system for the mine-through area at the least cost. To further complicate the design process, the mine-through entries were located directly under the end of a previously mined, overlying longwall panel and the entries were close enough to be subject to stress interactions from the overlying seam. In order to accurately evaluate the multiple-seam stresses and modify the mine-through support system appropriately, the boundary element program LaModel was used to model the stress and displacements in the two mines. Using the results of the model's stress analysis, the support density was systematically varied throughout the mine- through entries for optimum sup@ efficiency and economics. During the actual mining of the cross panel entries, the longwall experienced few, if any, additional delays, and overall, the support and mine through of the cross-panel en- tries at Harris #1 was very successful, saving the mine confineable time and money.

Jan 1, 2003

By Brian Clifford

Thoresby Colliery is in the process of recovering an old roadway that was flooded following the closure of an adjacent mine. The aim is to re-use the roadway as a gate road for a longwall retreat panel and it is therefore important to establish the condition of the existing rockbolt support during the recovery operation. Two NDT methods, ultrasonics and RMT's newly developed RF (Radio Frequency) system were used in combination to interrogate the rockbolts to determine their condition. This paper describes the condition of the rockbolt supports, the NDT methods used and the problems that had to be overcome to successfully apply the two methods.

Jan 1, 2002