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By Yi Luo

Accurate mechanical and geological information of the roof strata is vital for roof bolting design in underground mines. In order to obtain such information in a timely manner, a research has been under way to develop a technology for mapping the roof geology using the drilling parameters acquired during the roof bolting operations. In this paper, a systematic approach has been proposed for determining the rock strengths using the acquired real-time drilling parameters. A computer simulation program based on the mathematic model has been developed to verify the approach and to study its sensitivity. The method has been used to interpret the acquired drilling data.

Jan 1, 2002

By Karl H. Brandt

In the Auguste Victoria mine a bolted rectangular roadway has been kept open for the first time in the seam D/C after retreat. The target of the project was to gain a basis for planning and experience for future mining layouts in the same scam in view of the reuse of boated rectangular roadways. For this the roadway was additionally secured in sections with longbolts before the influence of the retreating panel had taken place. On retreat a concrete sidepack was concurrently put in place on the face side. The installed monitoring systems consisted of endoscopic devices, multiple extensometers (Tell-Tales), shear-tales, extensometers and convergence measurements. The experience that was gained was carried over into a physical and numerical model.

Jan 1, 2001

By Rao Pothini

Measurement of pressure distribution under the base plate of the Hemscheidt 2-leg 800-ton shield was performed underground for two shifts using 12 pressure cells installed in two rows, one each under each split base plate. Pressure distribution was uneven depending on contact condition. In general the highest base pressure occurred at 1/3 of the base plate contact length from the rear edge. The resultant was near vertical, and the pressure under the tail side split base plate was always higher than the head side. For comparison, pressure distribution on the canopy and under the base plate was also measured under controlled conditions at the USBM's Mine Simulator Testing Facilities in Pittsburgh, PA. A total of 45 tests were conducted under several simulated contact conditions. The results showed that the actual contact area is highly dependent on the compliance of the immediate roof and floor due to the sloped canopy tip and a base lifting device under the base tip. The loading distribution tends to change in magnitude and profile as the leg pressure increases. The base lifting device exaggerates the high toe loading on the base plate characteristics of this type of shield support. This paper describes in detail the testing methods, data analysis results obtained, and recommended changes.

Jan 1, 1992

By Andre Zingano, Anderson Weiss

"The objective of this paper is to study the behavior of a low thick and low depth coal seam and the overburden rock mass. The mining method is room and pillar in retreat and partial pillar recovery. The excavation method is conventional drill and blast because of the small production. The partial pillar recovery is about 30% of the previous pillar size, 7m x 7m. The roof displacement was monitored during retreat operation; the surface movement was also monitored. The effect of the blasting vibration on the final pillar strength had been considered. Due to blasting, the pillar reduced about 20%. The consequence is more pillar deformation and roof vertical displacement. The pillar retreat and ground movement were simulated in a three-dimensional numerical model. This model was created to predict the surface subsidence and compare to the subsidence measured. This study showed that the remaining pillar and low seam reduce the subsidence that was predicted with conventional methods.INTRODUCTIONMining low coal seams (less than 1 meter) with the room-and-pillar mining method implies the excavation of part of the roof or floor to maintain adequate entry height for equipment and compliance with labor regulations with respect to ergonomic issues. The direct consequence of this is an increase in coal contamination and the increased cost of transportation and preparation at the plant. Traditional room-and-pillar mining only uses a development operation, even with a continuous miner or drill-and-blasting. Therefore, all the run of mine (ROM) coal is carried out with part of the roof or floor. However, if the retreat mining method were to be considered, the quality of the coal could increase because only the coal seam is excavated from the pillar recovery. The quality of ROM coal will increase, and the cost will decrease proportionally to the amount of coal produced from the retreat operation.However, the retreat mining with partial or total pillar recovery implies greater subsidence. This is particularly true if the coal seam depth is shallow, under 100m depth. Subsidence prediction in room-and-pillar retreat mining is complex; it depends upon the remaining pillar (stump pillar) size, as well as the pillar height, and the behavior of the pillar after retreat. This is related to the strength of the remaining pillar, which is related to the excavated method (drill-and-blast or mechanical)."

Jan 1, 2018

By Kikuo Matsui

Standing supports such as steel arches or square work supports along with the necessary struts and lagging have been traditionally employed in the longwall gateroads of the Japanese underground coal mines. Miike Colliery has newly introduced the interpanel-pillar mining system and the strata bolting system in gateroads to raise productivity levels and reduce operating costs. The Department of Mining Engineering, Kyushu University, in cooperation with the Miike Colliery and the Coal Mining Research Center, Japan (CMRCJ), has been conducting research into the optimum design of strata bolting as a primary support system in longwall gateroads. This paper presents a brief overview of the current state of strata bolting technique being applied in longwall gateroads at the Miike Colliery and discusses some problems in employing bolting.

Jan 1, 1993

By Xun Luo

In this paper we report on the use of microseismic monitoring techniques to investigate the fracturing process associated with longwall mining. The study was carried out at three mines in Australia and concentrated on some major concerns of the mines, such as failure patterns and growth rate associated with mining progress, their implications to water ingress and gas emissions, caving processes in the goaf and precursors of roof falls. The results have shown that a zone of shear fracturing has occurred further ahead of the longwall face than predicted using conventional longwall geomechanics theory. Failure patterns in the roof and floor vary from mine to mine and this appears to be dominated by geological structures. The reaction of faults, the formation of fractures which allow gas emissions and precursors roof falls have also observed. Results from this work have opened up new understanding of longwall caving processes and proved that microseismic monitoring is a very useful technique for studies of longwall geomechanics and hazard controls.

Jan 1, 1998

By D. F. Howarth

Two innovative ground reinforcement products have been developed for use in the mining industry. One of these products is a high strength polyethylene (HOPE) bar for use as a rib bolt in underground coal mining and as a corrosion proof tendon in open pit mines. The other product is a cable bolt (Ultrastrand) which demonstrates enhanced load transfer and is available in continuous lengths. Ultrastrand's exceptional in¬situ performance and versatility makes it ideal for mechanised installation in both underground and surface mining operations. Details of these products are presented and results from labortaory and field trials are discussed.

Jan 1, 1992

By Thomas Lautsch

After a short description of geology and gateroad roof support technology in the German coalfields the development of roof bolting as primary roof support is presented. Based on geotechnical parameters, different strategies used for roof support at depths of 3,300-4,600 ft. are explained. Strict quality management and a strict monitoring program are part of the entry development. A number of recently finished developments are described such as the introduction of standardized bolts and accelerated resin systems. An outlook of future goals is presented. The paper concludes with a comparison of roof control systems at RAG's mines in the US and Germany.

Jan 1, 2000

By Erdal Akyol

The rockbolting method of support is employed widely in several countries in different of mines. Hard rock mines use this method extensively. Soft rock mines, such as coal mines, have employed standing supports (steel sets) as the main method, although some countries such as the U.S.A. and Australia have preferred rockbolting. There is increasing interest in extending the rockbolting method in coal mines. The application of rockbolting techniques in an industry which traditionally has not applied such techniques, has required the development of a systematic design approach. This design approach uses extensive monitoring, to asses the viability of the support system in differing conditions. Currently at a number of new and existing mine developments in the U.K., rockbolting is being successfully applied. Although, inevitably some localised geotechnical problems have been encountered and have needed to be taken into account. The effect of faulting on roadway conditions is discussed in this paper and a design approach has been applied using a numerical analysis method, namely the Finite Element Method. The results presented in the paper serve as a useful guide to roadway support designers, and in particular the role played by different bolts in a range of conditions.

Jan 1, 1992

By Kot Unrug

For the purposes of underground mine design, twelve cores of the Springfield (V) Coal (Pennsylvanian) roof and floor were retrieved. Each core was photographed, described fresh in the barrel, and the rock quality designation (RQD) was measured. Cores were wrapped in plastic and delivered to the rock mechanics lab. At the lab RQD was remeasured and new photographs were taken before running geotechnical tests including weatherability. Significant differences in barrel RQD's vs. lab RQD's have been noticed in spite of careful handling. The drill site RQD is generally used to calculate geotechnical results, but in some types of coal measure rocks, further fragmentation of core takes place with slight changes of moisture content, which occur even in well wrapped core. This creates a dilemma which RQD should be used for support design. Rocks around excavations are exposed to the significant changes in moisture content due to the seasonal weather changes and related fluctuation of humidity of air ventilating a mine. Therefore, it appears to be more realistic using laboratory RQD which corresponds closer to the actual rock condition around the excavation. Weatherability test, developed for characterization of time dependent behavior of rock exposed to fluctuations in moisture content, is thought to be related to the above-mentioned changes of RQD. In weak shales, mudstones and claystones which weather rapidly, it is essential to characterize their future behavior in the pre-mining stage. This can allow for a realistic design of primary, and particularly secondary support in coal mines, especially in coal fields where such geologic conditions exist.

Jan 1, 2003

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 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 P. Forrest

Based on the results of mathematical analysis, photoelastic modelling, and case studies, the mechanisms of longwall under- mining interaction has been thoroughly investigated. The effects of using yield pillars in combination with rigid pillars in lower seam mine design have been found to provide significant improvement in ground conditions during undermining. Off- setting of solemnized longwall panel entries relative to upper-seam remnant structures has proven to be effective in alleviating adverse stress conditions.

Jan 1, 1988

By Guy Reed, Russell Frith

"Current coal pillar design is the epitome of suspension design. A defined weight of potentially unstable overburden material is estimated, and the dimensions of the pillars left behind are based on holding up that material to a prescribed or user-defined Factor of Safety. In principle, this is seemingly no different to early roadway roof support design. However, for the most part, roadway roof stabilisation has progressed to reinforcement, whereby the roof strata is assisted in supporting itself. This is now the mainstay of efficient and effective underground coal production. Suspension and reinforcement are fundamentally different in their roadway roof stabilisation approach and, importantly, lead to substantially different requirements in terms of roof support hardware characteristics and their application. In suspension design, the primary focus is the total load-bearing capacity of the installed support to ensure that it is securely anchored outside of the potentially unstable roof mass. In contrast, reinforcement recognises that roof de-stabilisation is a gradational process with an ever-increasing roof displacement magnitude leading to ever-reducing stability. In a reinforcing situation, key roof support characteristics relate such design issues as system stiffness, the location and pattern of support elements within the roadway, and mobilising a defined thickness of the immediate roof to create (or build) some form of stabilising strata beam. The objective is to ensure that horizontal stress acts across the roof of the roadway and is maintained at a level that prevents mass roof collapse. This paper presents a prototype coal pillar and overburden system representation where reinforcement, rather than suspension, of the overburden is the stabilising mechanism via the action of in situ horizontal stresses within the overburden, the suspension problem potentially being an exception rather than the rule, as is also the case in roadway roof stability. Established principles relating to roadway roof reinforcement can potentially be applied to coal pillar design under this representation. The merit of this assertion is evaluated according to documented failed pillar cases in a range of mining applications and industries found in a series of published databases. Based on the various findings, a series of coal pillar system design considerations and suggestions for bord and pillar type mine workings are provided. This potentially allows a more flexible and informed approach to coal pillar sizing within workable mining layouts, as compared to common industry practices of a single design Factor of Safety (FoS) under defined overburden dead-loading to the exclusion of other potentially relevant overburden stabilising influences."

Jan 1, 2017

By Marek J. Mrugala

Coal strength based on scaled uniaxial compressive strength from the laboratory and back calculations from in-mine observations of pillar stability indicate that material strength scaling rules are open to debate. Analyses presented indicate that the application of scaling factors lower than 0.5 provide a better correlation with field observations of pillar stability.

Jan 1, 1989

By Thomas Rymer

Mathematical modelling of mine subsidence measured at 21 locations in Pennsylvania and northern West Virginia allows for the refinement of current methods of predicting maximum land subsidence associated with longwall coal mining. This new method (RYBAD model) is an extension of that provided by Tandanand and Powell (1984; TP Model). The RYBAD model accomplishes more accurate subsidence predictions (error typically less than 10 percent) for a greater range of mining and geological parameters through the recognition of the following: 1) subsidence increases at a decreasing rate (curvilinear relationship) with greater overburden thickness up to a critical value beyond which subsidence decreases; 2) subsidence is strongly influenced by the stratigraphic proximity of strong rocks to the coal (size of potential caving zone); and 3) subsidence index is related to overburden thickness by a family of curves, each for incremental variations in the effective percentage of strong rock in the overburden.

Jan 1, 1988

By David B. Plumeau

The Cayuga Mine, a deep rock salt mining, operation, changed from a standard room and pillar mine design to a yielding pillar/pressure arch design five years ago. In general the roof conditions improved. However, entries adjacent to the abutment pillars continued to fail. It was noticed that where abutment pillars were accidentally "notched" (by driving cross- cuts one cut into the pillars), the abutment entries had better roof conditions than where there were no "notches". Experiments with intentionally notched abutment pillars showed that notching did improve abutment entry conditions. During the past four years notch configuration has been varied to suit local ground conditions. Recently, "pre-cutting" of notches has proven itself as a means of protecting abutment entries under severe ground conditions. As an added bonus, the notches improve productivity by providing more places to work and reduce costs because it is not necessary to bolt the notches.

Jan 1, 1981

By Chris T. Franke

The Waste Isolation Pilot Plant is a deep underground facility mined in bedded salt interspersed with persistent clay inter- beds. The clay seams delineate beams of salt above and below the excavations which strongly influence excavation performance. This paper reviews available rock mechanics information relevant to improving drift stability by removing the roof beam. Conceptual deformation mechanisms, relevant field data, and numerical analyses were used to investigate the effects of mining the roof of the excavations along a clay seam. Finally, a case study that applies the results of this investigation is presented.

Jan 1, 1997

By Craig Byington

The stress-field orientation mapping and analysis (SOMA) technique for determining the operative stress field near mine workings and its relationship to various fracture sets is described using Dickenson-Russell Coal Company's Laurel Mountain mine as an example. Carried to its practical application, this technique ultimately defines the optimal orientation for the mine workings wherein pillars, rather than rock bolts or other roof support methods, dominantly shoulder support of the highest-probability, potential, roof-fall blocks. It also provides a predictive tool describing local variability in rock deformation and in secondary roof support methods. Fracture discontinuities within a rock mass, either as pre¬existing natural fractures or as mining-induced fractures, are uniquely necessary for every brittle-deformation ground failure including roof falls. Prediction and mitigation of roof falls requires quantifying the interaction between the fracture sets and the operative principal-compressive¬ stress-axis orientation (olo). The SOMA technique utilizes the interaction between fractures and 61o, specifically their dilation or closure, to calculate the orientation of 610, and to estimate the orientations of 62o and o3o. A stereonet program is used to organize and statistically analyze the fracture data, and then to determine the optimal orientation for the mine workings. The final step in the SOMA technique involves modeling the Laurel Mountain mine using a 2-D, finite-element, modeling program. This assures that the interpretations regarding the optimal working orientation closely match the deformation features observed in the mine. It also provides a predictive tool describing highly variable stress and strain partitioning, seemingly erratic rock-bolt failures and the interactions between different sets of mine workings within different coal seams. The critical importance of including the fracture sets is well illustrated in the Laurel Mountain mine where the effects of earlier mining of an underlying seam is contrasted in both a fractured and otherwise identical non-fractured model to illustrate the consequences of ignoring fractures in a predictive roof-fall model.

Jan 1, 2004

By John Stankus

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

Jan 1, 1999