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By Shengli Yang

Solid backfill mining, as a green mining technology, has the advantages of a higher resource recovery, less damage to the environment and the ability to dispose of waste underground. It has been applied successfully in the Xingtai and Xinwen mining areas in China. Strata behavior and the movement of overburden strata in this mining method are very different from fully mechanized longwall mining, which results in a natural caving, because the gob is filled with waste rock, fly ash and other backfill material. The results of field measurements showed that the periodic weighting interval of the main roof is longer; the magnitude of periodic weighting is smaller; the peak and zone of influence of the front abutment pressure are smaller than those in fully mechanized mining; that major deformation of mine roadways occurs within the influence zones of the abutment pressures; that the falling space of immediate roof is smaller, with the lower portion falling more sufficiently than the upper; that the main roof subsides slowly; and that the falling height of the overlying strata is greatly reduced.

Jan 1, 2011

By Richard E. Thill

Two Cross-borehole systems were developed by the Bureau of Mines, one operating at a central frequency of 20 kHz for close-in spacing between boreholes (2-20m) and another operating at a dominant frequency between 1-2 kHz for more distant range (40-300+m) between boreholes. The systems are specially designed for the expected ranges and high resolution required in mining applications. Potential applications of the crosshole systems are presented for a variety of geophysical measurements related to underground mining problems.

Jan 1, 1989

By Andre Zingano

The variation in pillar behavior at underground coal mines working in the Bonito coal seam is observed in both research and mining company case studies. There is no consistency in pillar behavior, as it varies from mine to mine. In some cases, it is possible to find rib yields for both excavation modes and pillar sizes. This paper?s objective is to investigate pillar behavior for different load conditions and pillar sizes. Three-dimensional numerical simulation models considering these factors were built. A matrix was then assembled to compare pillar behavior between loading conditions, as well as between different pillar sizes with the same loading condition. The results showed some pillar rib plasticity due to vertical loading and stress relief independent of the pillar size and load. For the same load conditions, the yielding thickness is the same for different pillar sizes.

Jan 1, 2012

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

By Christopher Mark

"Many problems in rock mechanics are limited by our imperfect knowledge of the material properties and failure mechanics of rock masses. Mining problems are somewhat unique, however, in that plenty of real world experience is generally available and can be turned into valuable experimental data. Every pillar that is developed, or stope that is mined, represents a full-scale test of a rock mechanics design. By harvesting these data, and then using the appropriate statistical techniques to interpret them, mining engineers have developed powerful design techniques that are widely used around the world. Successful empirical methods are readily accepted because they are simple, transparent, practical, and firmly tethered to reality.The author has been intimately associated with empirical design for his entire career. But where his past papers have described the application of individual techniques to specific problems, the focus of this paper is the process used to develop a successful empirical method. A six-stage process is described:1. Identification of the problem, and of the end users of the final product2. Development of a conceptual rock mechanics model, and identification of the key parameters in that model3. Identification of measures for each of the key parameters, and the development of new measures (such as rating scales) where necessary4. Data sources and data collection5. Statistical analysis6. Packaging of the final productEach of these stages has its own potential rewards and pitfalls, which will be illustrated by incidents from the author’s own experience. The ultimate goal of this paper is to provide a new and deeper appreciation for empirical techniques, as well as some guidelines and opportunities for future developers."

Jan 1, 2015

By Andy L. Shaffer

A novel liquid settlement system (LSS) was installed in advance of mining at a western United States (U.S.) longwall mine to measure total roof sag from the time of cutting to days after mining during gate road development. The system was used to compare differences in roof sag between conventional place-change mining using a Joy continuous miner and a Fletcher roof bolter and mining using a miner-bolter. The LSS was installed with an intrinsically safe, miniature data acquisition system (MIDAS) to record roof response at four locations in the same gate road. The system utilized pressure transducers to measure roof sag by the principle of falling head (fluid pressure) in fluid-filletubes installed in the immediate roof. Details of the LSS design, installation, data collection, post-processing, and future applications are discussed.

Jan 1, 2014

By A. W. Khair

This paper deals with the analysis of pillar failure in a room and pillar mining operation of a deep underground coal mine. The incident here involved the failure of more than 100 pillars. The mechanism of pillar failure (bump/squeeze) has been studied in detail. The analysis consists of inmine eyewitness observations, laboratory experiments and analytical study of the pillar failure area using finite element technique. Results indicated that the mechanical properties of coal, roof and floor, geologic condition, and the method of operation have affected the mode of pillar failure. Hard roof and floor prevented roof caving and floor heaving in the gob area creating an inhomgeneous stress field on the pillar. This condition developed a yield zone and a core zone in the pillar. Progressive increase of vertical stress further increased the yield zone of the pillar and reduced the core pillar leading to a total squeeze of the pillar. However, the analysis indicated that the pillars were liable to bump provided any sudden shift of load occurred on the core pillar due to some dynamic shock imparted by a local failure of the immediate roof (slickenslided area) or during splitting.

Jan 1, 1986

By C. A. Vining, J. R. Nopola

"Evaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. Evaporites are subject to continual creep and no amount of ground support can successfully arrest this process. Common ground-support techniques that are useful for most rock types may reduce the effective life of bolts in evaporite mines and can create unseen hazards. A different approach to ground support must be applied to provide long-term support in areas where high creep rates are present. The goal of effective ground-support designs in these situations is not to contain the rock and prevent it from moving; rather, the ground-support design must be able to move along with the rock and contain any fully detached slabs until additional remediation measures can be undertaken. The most appropriate ground-support designs will consider both the expected movement of the rock and the duration that the area needs to remain accessible.INTRODUCTIONEvaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. The factors that must be considered in design stem from the deformation behavior of salt, which can fail in a brittle manner, akin to most rocks, but can also deform in a slow, ductile manner, resembling the stretching and elongation of steel. The deformation behavior of salt has been well established in technical circles where it has been studied for decades (Hardy and Langer, 1984). Much of the research in salt mechanics is related to repository science, specifically at the Waste Isolation Pilot Plant (WIPP) in southwest New Mexico. RESPEC has been continually involved in the WIPP from the initial design considerations (Gnirk, Grams, and Zeller, 1977) to the current day (Keffeler, 2016) and has been able to leverage cutting-edge research of salt mechanics into practical solutions for mining operators. While a detailed description of salt mechanics is not provided in this document, Fossum and Fredrich (2002) include a fine overview of salt mechanics. The following list paraphrases their chapter on basic facts about salt to provide some context for the reader unfamiliar with salt mechanics:"

Jan 1, 2018

By P. M. Lin

A subsidence monitoring program over a longwall panel was established (1) to explore the impacts of dynamic subsidence on the ground surface and structures and (2) to correlate the movements between the structures and their corresponding points on the ground. The program consisted of a transverse line survey and a series of survey points around the structures on the ground surface and their corresponding survey points on the exterior walls of the structures. The vertical displacement of each point can be expressed as: where Si is subsidence at i point; Sif is the final subsidence at i point; F is face location in terms of distance from the i point and SD is seam depth. The development of subsidence can be divided into five stages. Damage to the structures starts at the beginning of the second stage, The maximum damage developed in the middle of the third stage. The relationship of the vertical displacement between ground surface and structures can be expressed as: G = aH + b where G is ground displacement, H is structures displacement, and a and b are constants. The relationship is linear. However, the strain is non-linear and more complicated because of the interaction of the ground surface and structure.

Jan 1, 1989

By Shugang Li

The layout and efficiency of the degasification system is heavily dependent on the distribution of and the methane migrating mechanism in the mining induced fractures in the overburden strata In this paper, physical modeling and numerical simulation are employed to study the mining-induced fracture zones and the stress distribution in these zones. The physical models show that both the fracture and compaction zones form a parabolic shape in the over- burden strata along the panel transverse direction. When this is generalized to the panel longitudinal direction, the shape of the mining induced fracture zone over the longwall gob can be represented by a 3-D elliptic paraboloid function. Through the observation of bed separation, fracture density and distribution pattern in the physical models, it is believed that the development of the parabolic fracture zone can be divided into three distinguishable phases. The expansion characteristics and sizes of the fracture zones can be estimated. The numerical stimulation studies can be used to estimate the extent of the stress release zones. The stress release zones can be also divided into a high compressive stress - fracture closure zone, stress relieved - fracture expansion zone, and intact zone. The methane desorption model is then used to analyze the methane migrating pattern in the fracture zone. The developed model has been verified by the results of a longwall gob degasification tests at a coal mine in Yangquan, China with a fair agreement. Such studies show that the model can provide a reasonable accurate tool for designing the gob-well degasification system for longwall mining operations.

Jan 1, 2005

By John A. Rusnak

"This paper discusses the variation of coal strength based on the megascopic coal lithotypes for Central Appalachian high-volatile A coal beds. The composition of coal beds is complex and variable, depending on the depositional environment associated with original plant material, inflow of clastic material, and water level fluctuations in the original swamp/mire environment. Thus, different types of coal and sediment are formed within each coal bed. These differences in coal composition were first classified by Marie Stopes, a paleobotanist, in 1919. The classic coal megascopic lithotypes are vitrain, clarain, and durain, which correspond to bright, bright banded, and dull coal, respectively. Stopes also identified that the lithotype fusain, which is mineral charcoal, has no inherent strength and can act as a plane of discontinuity. In this study, two other coal-bed lithologies are also considered—bone and carbonaceous shale. This makes for a comprehensive suite of coal-bed stratigraphic components apart from rock partings. A total of 1,000 uniaxial compressive strength (UCS) and 440 indirect tensile strength (ITS) tests were conducted on cores from southern West Virginia that were specifically logged and described using the megascopic coal lithotype nomenclature. Statistical analysis is presented showing the results for each lithotype group with mechanical properties correlated to the lithotype. Previous studies have analyzed the mechanical breakage properties associated with coal lithotypes, which correlate well with these results. However, coal rank does appear to be a contributing factor, and this study is confined to only high-volatile A bituminous rank. Application of these results would be well suited to a rock mass classification system to address coal pillar rib stability. This would allow for rib support design and implementation to consider relative coal strength by visual observation of the seam stratigraphy through ongoing observations within a mine. Changes in seam composition can be readily identified, allowing the adjustment in rib support design. Another application would be for the determination of numerical modeling inputs based on detailed seam stratigraphy. However, these results, even with the standard strength reduction criteria, may not be applicable to currently accepted empirical coal pillar design procedures."

Jan 1, 2017

By David Conover

Technological advancements in electronic sensors and mine-wide monitoring systems have improved remote monitoring and analysis of underground mining information. The effective use of . these systems for ground control applications, however, remains a challenge facing the mining industry. The U.S. Department of Interior, Bureau of Mines is conducting research, utilizing real-time acquisition of rock mechanics data, to provide mine operators the means to rapidly identify and manage potentially hazardous ground conditions. The Bureau has installed a remote monitoring network in a Colorado coal mine to record Pressures from instruments installed in the roof and pi1lars of gateroads and from transducers on shield-legs along the face. The field data are being monitored at the Denver Research Center, 150 mi from the mine. A computer at the Center analyzes the field data and generates graphic displays, which permit detailed examination of mine-wide ground conditions in real-time. For the first time, Bureau researchers are able to perform real-time monitoring and analysis of rock mechanics data while mining is in progress. The data collection and display capabilities of the system were effectively used to identify and evaluate face stability problems at the mine.

Jan 1, 1990

By S. Paul Singh

The implications of blast damage are harmful in both economic and physical sense. To understand the mysterious nature of blast damage assessment and prediction, field work was done by driving an experimental tunnel in the bench of a quartzite quarry. On the basis of the blast vibration monitoring records, a model was developed to predict the PPV at different distances from a blasthole. Critical particle velocity for damage and the output of the prediction model were used to delineate the extent of damage for different explosives. The predictory model was tested by designing a special blast and the burdens for the back holes have been recommended. The relationship between the critical particle velocity for "Fall Off' and the critical particle velocity for damage has been established. The existing criteria for blast damage have also been renewed in this paper.

Jan 1, 1993

By David C. Oyler

Few studies have specifically measured and documented the large-scale loading behavior and durability of ventilation stoppings to mining induced movements, particularly from longwalls. Ventilation stoppings are more commonly the concern of ventilation engineers, and ventilation, topping response to ground movement has only been documented in an incidental manner. This paper presents the investigations of underground measurements that have been conducted to determine the loading response of stoppings constructed from lightweight aggregate concrete masonry units (CMU). These investigations have produced some interesting results that may prove beneficial, not only for developing and assessing alternative stopping construction techniques, but also for designing and selecting standard construction methods for use in varying mining conditions. For instance, the interface friction that results from wedging a CMU stopping during construction plays a substantial role in its ability to resist both lateral and vertical loads. Although they are not intended for ground support, the study showed that block stoppings can resist vertical loads of at least 2,700 to 3,000 kN. Asymmetric loading of a stopping may result in localized failure of blocks within a stopping, which, depending on the severity, can be a precursor of impending stopping failure. In addition to measurement results and implications, the paper will also present details of field measurement methods used to assess stopping response. Stopping stiffness and material strength characteristics will also be presented,

Jan 1, 2001

By H. Gene Wilson

Mobile Roof Supports (MRS) look similar to a longwall shield on crawler tracks. The MRS units offer a safe, productive method of extracting the 40% to 60% product left normally unrecovered in room and pillar mining. The units feature radio control, self-contained power units, crawler track drive, and a 600 ton yield. Cribbing and setting posts in retreat mining has been a necessary burden to production. The MRS reduces this burden. Long blocks of coal can be developed to further reduce move and tram time, thus increasing production. The MRS units have successful safety records. The workers are removed from the immediate fall area, and we feel back injuries have been reduced by the workers not setting posts. While MRS units have been damaged in falls, the other capital equipment has been unharmed. The MRS product line has undergone normal product improvement and evolution. Product evolution is seen in our development of machines for particular mine requirements. Through the continuing development of the MRS, Fletcher presents the coal industry with a safe, productive method of retreat mining with potential to develop new mining methods.

Jan 1, 1990

By J. G. Stewart

Polyurethane injection for the consolidation and control of broken strata is a common practice in the deep mines in Europe. The technology has gained wide acceptance in the coal mines of America since the introduction of RokLok polyurethane binder by Mobay Chemical Corporation in 1977. A popular and growing use for polyurethane consolidation grouting is to control the roof and prevent gob flushing during recovery of longwall face equipment. The technique provides many time and performance-related benefits compared to more conventional methods using wire mesh. Kitt Energy Corporation installed the nation's first 1000-feat longwall face in April and May of 1984. The face completed the first panel in October, 1984, and recovery was begun. Roof Strata consolidation for the recovery was accomplished by a RokLok polyurethane binder infection program designed and supervised by Halbach & Braun Industries. The application provided the most rigorous and demanding test to date of this method's performance and cost-effectiveness relative to bolts and wire screen.

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 Nicholas Chlumecky

One of the oldest mine supports in use today is timber cribbing. In many mining operations material, installation and maintenance costs for timber cribbings are substantial. Their performance characteristics also add to mining costs. During the 1970'6, the U.S. Bureau of Mines recognized many of the deficiencies of timber cribbing and very successfully initiated the development of steel-fiber- reinforced (SFR) concrete blocks. Since the early 1980's this SFR concrete cribbing has been manufactured as a high quality product which has found increasing acceptance by mine operators. The rapid acceptance of the SFR concrete block cribbing has proven that its use is eliminating many of the deficiencies of timber cribbing while providing superior load carrying capability. The use of SPR concrete blocks to build the cribbing is based on the same principle as wood. That is, stacking longitudinal, rectangular blocks in layers in a crisscross manner. The blocks are positioned at right angles to the blocks of the layer below. With this situation, the possibility of misalignment exists when the concrete blocks at the base are not placed exactly parallel or when the blocks of the base are subject to unequal settlement. Such misalignments creates localized, high stress areas which reduces the load capacity of the cribbing made from SFR concrete blocks. Recognizing the problem of misalignment, the Underground Supports Division of Commercial Shearing, Inc., conceived a circular, one-piece, steel-fiber-reinforced concrete cribbing which is being marketed under the name - Donut Cribbing. This concept eliminates the possibility of misalignment and provides 100% bearing between layers of stacked cribbing as well as other advantages. The purpose of this paper is to discuss the performance characteristics, installation speed and economical factors of each of the three cribbing types -- timber cribbing, SFR concrete block cribbing and SFR concrete Donut Cribbing.

Jan 1, 1986

By William R. Kohl, Craig B. Clemmens

"Since 2010, North Central Resources, Bridgeport, WV, has been using the sonic log to estimate the rockmechanics properties of the coal and adjacent strata in their exploration drilling program. In 68 out of the 85 boreholes they have drilled since 2010, they have conducted shear wave analysis on sonic logs run in these boreholes. By incorporating the rock mechanics logs into the geological model of the coal seam, the engineers can develop a better understanding of the support requirements for the mine.WellCAD Borehole processing software provides vastly improved efficiency in analyzing shear waves from borehole sonic data than previously available methods. The calculation of most rock mechanics parameters requires knowledge of the density and both the compressional wave velocity and the shear wave velocity. Density logs are well known and an industry standard. Sonic logging is equally well known, although less frequently used, as an accurate source of compressional wave velocity. Shear wave velocity is difficult to measure in a geophysical log, but can be estimated from digitally recorded wave form records. This is a time consuming manual procedure which requires the visual examination of thousands of individual wave forms.WellCAD allows the viewing all the wave forms on a VDL display. The observer can create a pair of false logs by manually tracing the interference points on the display for each receiver. He can check himself by observing wave forms in the traditional format. He can then create a shear wave transit time log or “Delta-S” log by subtracting the false log from the near receiver from that of the far receiver. This can be plugged into standard formulae to generate curves for most standard rock mechanic parameters, including bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio.While these parameters do not entirely substitute for destructive testing, they are extremely cost effective. An entire borehole can be analyzed for the cost of a single destructive test. This allows for a more judicious use of destructive testing while permitting the extension of test data into the rock mass with much greater confidence."

Jan 1, 2015

By Andre Zingano

The objective of this paper is to study the effect of annulus thickness on the quality of resin mixture for fully grouted resin bolt. Pullout and mixture tests were made in underground coal mines for the following arrangements: (a) borehole diameter 1.14 in. (29 mm), and roof bolt diameter 0.75 in. (19 mm) with steel wire around the roof bolt; (b) borehole diameter 1.14 in. (29 mm), and roof bolt diameter 0.75 in. (19 mm) with no wire around the roof bolt; and (c) borehole diameter 0.95 in. (24 mm) and a roof bolt diameter 0.75 in. (19 mm) with no wire around the roof bolt. The mixture test in steel pipe showed that the best resin mixture is for borehole with 0.95 in. (24 mm). The arrangement (b) produced big voids around the bolt. But for the 0.95 in. (24 mm) hole, arrangement c, the mixture was the best. It was corroborated with the pullout tests results where there is a difference in grout stiffness: the 0.95 in. (24 mm) borehole is about 10 times higher than a borehole with diameter 1.14 in. (29 mm). On the other hand, for the cohesive strength the difference is not significant. Numerical models were built to simulate the pullout tests, and also to simulate the roof support at an intersection. Convergence at two underground intersections, consisting of an immediate roof with laminated sandstone, was monitored to evaluate the performance of the roof supports, using the 1.14 and 0.95 in. (29 and 24 mm) diameter boreholes (i.e. arrangements ?a? and ?b?). The convergence readings showed that the 0.95 in. (24 mm) borehole diameter resulted in a smaller, although it was not significantly different.

Jan 1, 2008