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By Maynard O. Serbousek

The mechanics of load transfer of fully-grouted roof bolts in coal mines has been investigated in laboratory and analytical studies. The purpose of this research is to increase the understanding and better interpret data from pull tests which are routinely performed in the field. Laboratory work includes variation of bolt length, hole diameter and grout type in pull tests. Four-foot bolts were tested in holes of 1- and 1-3/8-inch diameters. Both polyester resins and inorganic grouts were used as the anchorage medium. The bolts were strain gauged at six locations to observe the rate of load transfer from the bolt to the rock mass when a force was applied at the bolt end using standard Jack pull testing procedures and equipment. Analytical studies using closed form solutions and finite-element techniques were compared to experimental results. The models are for the elastic regime only but do include slip elements. A detailed analysis of the load transfer mechanisms from the bolts through the grout to the rock mass is presented.

Jan 1, 1984

By Quanzhong Gu

Stability of a pre-exnacted recovery room plays a vital role when longwall face approaches and during a longwall face move. In an attempt to extend the application of pumpable cribs into recovery room support and to limit roof bolt installation when a longwall face approaches the recovery room, yieldable and cuttable pumpable cribs (YCPC) were employed in the recovery room at the northern West Virginia mine. The YCPC consists of two layers of materials with different strengths. This allows considerable deformation from the floor and/or roof by sacrificing the soft layer while maintaining relatively high load capacity under mining¬induced front abutment loads. The thickness and stiffness of the soft layer, installation and quality control are the key factors when using the Yieldable and Cuttable Pumpable Crib to stabilize longwall recovery room- Underground observation and monitoring results show that the Yieldable and Cuttable Pumpable Crib is a viable solution to insure stability of the recovery room and can save at least two days for a longwall face move at the mine.

Jan 1, 2004

By Ross W. Seedsman

The in situ behaviour of claystone floors associated with the Wallarah, Great Northern and Fassifem Seams has been studied using a comprehensive suite of stress and displacement monitors. The instrumentation was supplemented by in situ and laboratory testing of the claystone materials. The instrumentation demonstrated that the presence of claystone floors is associated with floor heave, extra rib spall, and settlement of the pillar into the floor. Horizontal floor extensometers showed that the depth of extrusion of clay was limited to 1-2 in. A pore pressure transducer installed in a claystone floor demonstrated the possible existence of short-term undrained behaviour of the claystone. Long term swelling of destressed claystone was also documented. Settlement of pillars into claystone floors can be analysed using simple elastic theory. The assumption of plane strain is valid for most claystone thickness/pillar width aspect ratios. Floor heave can be analysed using bearing capacity concepts. However, it is considered that heave is the result of local bearing capacity failure in the rib spall zone. The advantage of this model is that the bearing capacity equation can be applied without resort to unrealistically high factors of safety.

Jan 1, 1992

By Jinsheng Chen

For underground openings, many factors can cause roof failure and thus roof falls. The most common factors related to Mother Nature are weak roof lithology, geological structures, and an abnormal in-situ stress field. Theoretically, any underground opening can be successfully supported under any geological and mining conditions if the opening width is limited and the roof supporting system is well designed. However, roof control cost must be taken into account. In practice, to minimize the roof control cost, different roof control plans have to be considered to take care of the uncertainty or variation in geological and mining conditions. For most underground mines, it is unsafe or economically unrealistic to use one fixed roof control plan for the entire mine. Therefore, local, abnormal geological and mining conditions must be identified and a site- or area-specific roof control plan must be employed. This study tries to identify the major factors that have significant impact on entry roof stability by analyzing the historical roof fall data at Rofomex mine. Finally, the appropriate roof control plans or roof bolting systems for different geological and mining conditions are recommended.

Jan 1, 2011

By H. N. Maleki

Tension-torque relationships were investigated *n a coal mine for bath standard 5/8 in. (16 mm) bolts and a vibrating wire instrumented bolt, using conventional and controlled installation techniques. The instrumented bolts differed from the standard bolts by their head size and hole preparation procedures. The controlled installation technique developed by the U. S. Bureau of Nines (USBN) involved using hardened washers and lubrication in the bolt assembly, and maintaining a minimum thrust level during bolt installations. The object of this study was to determine the tension to torque ratio (lb/ft-lbs; for standard bolts and instrumented bolts for both conventional and controlled installation techniques, and to modify the bolt assembly so that the ratios would be equal for the instrumented and standard bolts for both installation techniques. It was shown that the instrumented bolts installed in faced holes had a similar tension torque ratio td that of standard 5/8 in. (16 mm) bolts installed in unfaced holes for each installation technique, and thus they were representative of other uninstrumentea bolts. This was achieved by evaluating the influence of several factors oh the relationship, including: facing the hale surface, introducing washers and lubrication in the bolt assembly, and applying different levels of thrust at installation due to bolter operator practice. The tension to torque ratio fur conventional bolts was only 21 (lb/ft-lb) for the coal mine, resulting in an installation tension of 4000 lbs (17.8 kg) with the typical installation torque of 150 to 400 ft-lb. This tension level was approximately 50 percent of the required bolt tension, and thus, it was concluded that torque standards are not sufficient to assure adequate bolt tension. .he low tension levels were due to the particular wait hardware in use, operator practice, and bolt fencing.

Jan 1, 1984

By William D. Gallant

This paper describes a joint cooperative project between the CANMET Cape Breton Coal Research Laboratory and the Cape Breton Development Corporation to conduct a trial of split set cable slings for remedial support in longwall gateroads. Slings were installed in two test sections of the 2 West panel, Phalen Mine which exhibited deformation and the potential for excessive deterioration when subjected to the front abutment of the wallface. Roadway support behaviour was monitored at the trial sites, including lengths of roadway with the standard support of steel sets without remedial support. Observations were made on convergence, loading of the steel supports, loading of the slings, and wallface performance. Comparisons are made of the results obtained from the areas with remedial support to that without, which demonstrates that the slings provided effective remedial support limiting further deterioration of the roof. Improved face end conditions have contributed to increases in safety and productivity of the longwall panel.

Jan 1, 1990

By David H. Tang

Similar massive pillar failures were observed in two underground coal mines with different configurations of mine workings and overburden depths. Finite element models were used to analyze the causes and mechanisms of the pillar failure. The results of the analyses confirms the underground observation. For Case No. 1 the failure of coal pillar is mainly caused by a combined effect of smaller coal pillars under a large overburden and the interaction of multiple seam mining. For Case No. 2, the pillar failure was initiated by the weak roof rock but the ultimate pillar failure was caused by the splitting of pillars into very small size.

Jan 1, 1984

By Dennis R. Dolinar

The U.S. Bureau of Mines has modeled yield pillars to determine the effects of reinforcement techniques on pillar performance. Model pillars with width-to-height (w/h) ratios ranging from one to four were reinforced using advanced lateral support systems. To evaluate reinforcement and pillar performance, the model pillars were strained up to 18.6 pct in compression. The results show that the reinforcement had a significant effect on the postfailure characteristics of the pillar but had only a small effect on it's maximum failure strength. Pillar residual strengths increased up to 10 times, while the energy capacity increased by a factor of three depending on the reinforcement method. At maximum deformation, the residual strengths of several types of reinforced specimens exceeded the maximum compressive strength up to 2.5 times and resulted in an increasing positive postfailure stiffness. Test results, data analysis, and application of the results to underground mining are discussed.

Jan 1, 1993

By Tom Denton

The bolt design criteria, since it was first published by Dr. John Stankus (Stankus, 1997), has been recognized as an important tool in ground control design. This concept is based on the Optimum Beam Building Effect (OBE), which is defined as the roof beam that has no separation above or within the bolted range and having the shortest bolt possible. This paper will present an application of this concept at the American Coal Company. Galatia mine. This mine previously used a 6' and 7' length bolt. However, since seam thickness has gotten smaller in the Southern area of the nine, a two-piece bolt is required. A two-piece bolt increases ground control costs and delays production. Therefore, a one-piece, shorter bolt, if feasible, can reduce costs and expedite development. In this analysis, Computer modeling is first conducted to determine bolt length and installed load. Second, a special bolt is developed, that can achieve the proper installed load as determined by the computer modeling. Third, underground bolt pull and load cell tests were conducted to evaluate bolt performance. Finally, an underground "bench" test was conducted. The test results have shown that entry convergence in the short bolt (5) test area is less than the 6' bolt area. Overall roof condition in the test area was as good as the 6' bolt area. Based on this analysis, the mine has converted to a shorter bolt in the low seam area and it is working well.

Jan 1, 2000

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 Jinsheng Chen

The major functions of longwall headgate entries are to provide access ways to the outby submains or mains for coal transportation and to the longwall face for fresh air and for transporting men and material during longwall operations. Any roof control problems such as cutter roofs or roof falls in the outby headgate entries could result in significant production delays to the longwall face and may be a threat to miners' safety Under a thinly laminated immediate roof and a high horizontal stress field, even a longwall headgate entry that has an angle of less than 45 degree to the maximum horizontal stress may experience roof deterioration in the outby headgate if the longwall panel mining sequence and retreat direction are not properly considered. This paper attempts to analyze the causes of the roof control problems experienced in the longwall gate entries at our Cumberland and Emerald mines in Pennsylvania and to determine the optimal longwall panel mining sequence and retreat direction for the future mining area at Cumberland

Jan 1, 1998

By Herryal Anwar

The application of roof bolting technology to control the stability of roadways in coal mining within weak strata or under high ground pressure has improved progressively. One of several factors that affect the design of rock bolting is the immediate roof characteristics of the roadway. On the competent rocks, the mining conditions may indicate a high level of stability and a low density of bolting patterns may be adequate to control the ground stability. However, at a weak strata, the roadway will be affected by both horizontal and vertical stresses that may generate a high level of deformation in the immediate roof. This sensitivity of the roadway will require a mine plan design which consider the pillar sizes and sequences of extraction as well as a conservative reinforcement to maintain the integrity of the deforming weak roof strata. Rock bolting technology has been used to maintain the stability of roadways in Indonesia and Japan for a few years. In this paper the overview of the application of rock bolting reinforcement in Ombilin Coal Mine, Indonesia and Miike Coal Mine, Japan is given.

Jan 1, 1999

By S. S. Peng

The hydraulic powered supports have been employed in U. S. longwall mining for the past two decades. Yet the sciences of roof control at the powered supported faces remain largely unknown. In order to improve longwall roof control and increase production, a series of questions in longwall roof control should be studied and answered: what is the basic rules of roof behavior in longwall coal faces equipped with the hydraulic powered supports? What is the interaction between the roof, supports, and the floor What is the contact condition of the canopy with the roof, and consequently the load distribution on the canopy? Des the periodic roof weighting really exist in the powered supported face and if so, how does it affect the face area? This paper presents the preliminary analysis of underground observations and instrumentations in a longwall panel with 4-leg shield supports designed to address the questions mentioned above.

Jan 1, 1982

By Swapan Bhattacharya

Section 516 of Public Law (PL) 95-87, commonly known as the Surface Mining Control and Reclamation Act of 1977 (SMCRA), requires that underground coal mine operators must 'adopt measures consistent with known technology in order to prevent subsidence causing material damage to the ex- tent technologically and economically feasible, maximize mine stability, and maintain the value and reasonable foreseeable use of such surface lands, except in those instances where the mining technology used requires planned subsidence in a predict- able and controlled manner..." Implied in the constraints of Section 516 (b) (1 of SMCRA is a determination of what constitutes "material damage" to surface structures and renewable resource lands, and the capacity for sequential land use following underground mining. This Study was undertaken in an attempt to provide quantitative guidelines for estimating material damage to a wide variety of surface structures and renewable resource lands. Surface structures studied included buildings, bridges, railroads, highways. pipe- lines, d~, embankments, and water reservoirs. Damage criteria for buildings developed in this study along with details of the adopted methodology have been presented elsewhere (Bhattacharya et al.) (1). and will not be repeated here. Similar guidelines for renewable resource lands have also been discussed (Singh and Bhattacharya) (2)

Jan 1, 1984

By A. Mahyera

BETTER ROOF SUPPORT During the last decade, resin-grouted roof bolts have taken a substantial foothold in the coal mining industry - now accounting for about one-third of all bolts installed annually, due to the improved roof support. However, the high cost, toxicity and flammability of the resin cartridges appear to be a limiting factor in even greater use of such bolts. Terra Tek, Inc. has, over the past few years, been developing a novel roof bolting system to mix/inject a very fast-setting and high strength inorganic cement into bolt holes and install point-anchored or full-column grouted rebars - all automatically and at substantial cost savings. This grouting system utilizes a unique method to mix the grout ?in-the-hole?. It has been integrated with a conventional bolter, retrofitted with an indexing-head mechanism and an on-board pneumatic computer to sequence and Control the drilling, grouting and bolting. Cycle times are the same or better than for a resin system. Following laboratory testing/development, the prototype system has undergone field trials in a Utah coal mine. A description of the bolting system, results of pull-tests in the laboratory and field, and economics and productivity gains with this system in comparison to a conventional resin-cartridge bolting method are presented. The relationship of this technique to improved roof control is also discussed.

Jan 1, 1984

By K. Itakura

Information about geostructure for effective rock bolting must be obtained before setting the bolt into rock in a coal mine roadway. Especially, rock type variation and distribution of discontinuities such as layer boundaries, and separation of strata and cracks all represent indispensable knowledge. To achieve in-situ evaluation of roof rock, we developed an MWD (measurement while drilling) system using mechanical data from a drilling machine for rockbolt. System hardware detects torque, thrust, revolution, and stroke of the machine, and software analyzes the mechanical data log and displays locations of discontinuities using Neural Network techniques. Also, this system is able to estimate roof rock 3-D geostructure for varied rock types and discontinuity distribution in cases of a drill hole array arrangement in roof rock. We conducted a feasibility study for it using hydraulic drilling at roadways of the Taiheiyo Coal Mine, Japan. Simultaneously, some core drillings were also performed to confirm system analysis data. For core samples, distribution of cracks was observed and the borehole wall was investigated by OFS (optical fiber scope). Must discontinuity locations estimated from roof logging were conformed with data obtained from core inspection. However, the core sample contained a number of cracks which were not found in the mechanical data log. This indicates that additional cracks were generated during and/or after core drilling by the influence of rotational force of the machine and water supply. Furthermore, in comparison with the OFS image, resolution of the discontinuity's log estimated by this system was clarified; that is, it is difficult to detect small size cracks (hairline cracks) from a mechanical data log and the core inspection by OFS. This paper outlines the new hardware system for hydraulic drilling machine and compares the discontinuity map obtained from the software system with drilling core inspection.

Jan 1, 2001

By Laxminarayan Holla

There exist many formulae for designing coal pillars. However, when applied to a given set of mining parameters, they lead to different pillar sizes and factors of safety. From the subsidence point of view, the pillars designed as protection pillars not only have to be stable, but also have to limit subsidence over them to a level acceptable to the structure being protected. The subsidence over a pillar of given width to height ratio depends upon the mining depth. A pillar may be stable, but the subsidence over it may be quite large and unacceptable for the structure. In many cases even though pillars may yield, they support the undermined strata and reduce surface subsidence. In addition, the yielding pillars eliminate peaks and troughs in the surface subsidence profile and reduce ground strains. In a given situation, whether a pillar yields or not depends primarily upon its width in relation to mining depth.

Jan 1, 1992

By A. Wahab Khair

This paper presents results of the experiments carried out in Beth Energy No. 33 Mine for the purpose of alleviating problems associated with cutter roof. Cutter roof problems have delayed the advance of the entry development considerably and maintenance cost of these entries was very high due to the requirement of very extensive and heavy artificial supports. The study includes assessment of the problem, investigation of the geology and lithology of the mines, study of in-situ stresses and corrective measures to combat the problem. Two approaches were selected as the corrective measures: 1) implementing yield pillar concept in order to reduce the effects of in-situ horizontal stresses, 2) designing an effective roof reinforcement system and application procedure. Results of this study revealed that roof strata is weak and in-situ stresses are very high at the problem area which results in development of cutter roof. Implementation of yield pillar concept was found to be ineffective because cutter roof often developed at the entry face concurrent to the development. However, proper design and implementation of roof reinforcement system stabilized the roof and eliminated roof caving during both entry development and retreat of longwall face, despite existence of the cutter roof, at a much lower cost.

Jan 1, 1992

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 Ken Mills

This paper describes the first successful use of hydraulic fracturing to induce caving events "on demand" in Australia. Moonee Colliery operate a longwall immediately below a thick conglomerate strata. This strata temporarily bridges across the extracted longwall panel to create a large area of standing goal' When this standing goat' eventually collapses, the windblast generated presents a significant hazard to men working on and around the longwall face. Hydraulic Fracturing has been successfully introduced to take control of the timing of these caving events so as to eliminate the risk of windblast injury, The longwall face area is completely evacuated during the treatment. Water is pumped into an injection point located in the conglomerate strata above the standing goat. A horizontal fracture is generated and grows outward from the in point, separating the conglomerate strata below the fracture horizon_ At some point the strata can no longer span and a goat fall is initiated. After a treatment, mining can be recommenced with the windblast hazard eliminated.

Jan 1, 2000