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By Shosei Serata

The application of conventional rock mechanics methods to complex ground is very difficult. Yet. In most underground mining, the ground is complex. Therefore. conventional methods are not only difficult to apply but are also generally ineffective in improving underground operations. Similarly, conventional rock mechanics instrumentation has not significantly contributed to improvement of mining practice. Rock mechanics instrumentation in underground mining operations is often considered to be" merely an expensive nuisance. A new system of mine instrumentation is changing this traditional view. The new instrumentation method, called the SPDR method, was originally devised by Dr. Shosei Serata to meet the specific requirement of establishing the Stress Control Method of mining upon a solid quantitative foundation. This instrumentation method has contributed substantially to the widespread success of Stress Control mining under widely varying ground conditions in different types of underground mines (1.2.3.4). The instrumentation method provides an effective means for optimizing Stress Control mining patterns, simultaneously achieving reduction in production costs and substantial improvements in safety (5). The principles and methods of the new instrumentation method are described below, with field examples.

Jan 1, 1986

By Christopher C. Woomer

Cyprus Shoshone Mine uses the longwall method to extract a deep, thick, pitching coal seam in the Hanna Basin of South Central Wyoming. The immediate, and main roof rock consists of weak, thinly-bedded, silty mudstones with weak, interbedded fine-to medium-grained sandstone. Tailgate ground control has been a critical factor impacting productivity at the mine. A gateroad condition mapping program for the 11 left longwall gateroads indicated potentially severe ground control problems for the tailgate. It was predicted that the existing, secondary support pattern of wood cribs would not provide adequate support capacity. As predicted, severe deterioration of the tailgate occurred in the form of a roof cutter, sloughing ribs, and failing wood cribs. Productivity was affected and additional support was required. Shoshone personnel evaluated several options for providing the additional support needed in the tailgate. The additional support system and placement method had to satisfy several requirements with respect to access, materials handling, placement method, set-up time and support capacity. Approximately 6,000-ft. of deteriorating tailgate had to be supported. The initial wood crib support system left a 6-ft. walkway for access and construction of the additional support. This access was further reduced as the roof loaded the wood cribs. This eliminated the possibility of mechanized material handling to the construction locations. All materials would have to be transported by hand. The additional support system had to be placed quickly to stay ahead of, and keep pace with, the required longwall advance rate. It had to incorporate the existing wood cribs and provide at least 500 tons of extra support capacity. And most critically, the system could not affect longwall ventilation. Longwall coordinators and engineers made the decision to use a low density, pumpable cement known to the industry as TeksealTM, to provide the system required. A 200 psi ultimate strength mix was decided on to provide the required load capacity. The existing cribs were formed with 1-in. by 6-in. boards and brattice cloth to provide the containment. To overcome the access limitations, three boreholes were drilled from the surface to the tailgate on 2,000-ft centers. A mobile pumping station was established on the surface and the TeksealTM was pumped 900-ft. down the boreholes through a 1.5-in. steel pipe, then as much as 1,800- ft. along the tailgate entry through 1.25-in. miner spray hose. The materials required for the TeksealTM supports could all be carried into the construction locations by hand. As a direct result of incorporating relatively new methods of pumping high yield, low density, cementitious grouts, the Shoshone Mine reduced downtime due to tailgate ground control problems by approximately 70% in comparison with previous Longwall panels. The longwall set three monthly production records while mining the 11 left longwall under the deepest cover, steepest pitch, and most extreme ground control conditions ever encountered at the mine.

Jan 1, 1995

By Clive Chadbourne

Investigations of world wide coal face strata control practice, the study of theoretical calculations and detailed engineering analysis have been combined to design and select a powered roof support suitable for use in conjunction with the extraction of 4.5-m (14.7- ft) of the 7.5-m (24.6-ft) thick Warwickshire Thick Coal Seam. The interface between geotechnical criteria and engineering design has been closely considered along with the following 3 important factors: Firstly, the need to further improve strata control and operational characteristics whilst producing a wall integrated overall face design. Secondly, to enable the supports to be rapidly transported from face to face in roadways driven at the minimum size for operational requirements. Finally, to provide hydraulic power, automatic control and monitoring systems which will improve operations reduce manpower requirements and improve management information. Clear design requirements were determined in order to allow selection on a commercial basis. A method of tendering is described with the subsequent analysis and selection process. In the event oversupply of coal in the UK resulted in the order not being placed. Brief consideration has been given to supports suitable for reserves to be mined In a few year's time where there is potential for extraction well in excess of 4.5-m (1 4.74.

Jan 1, 1994

By Mao Bai

In predicting the integrity of under-mined structures, surface horizontal displacements and curvatures are frequently of greater importance than the more homogeneous vertical displacements. Accurate evaluation of mining induced subsidence and horizontal displacement further requires that the predictive model adequately represents the subsurface geology, mining geometry and extraction sequencing. The following documents an extension of the Subsidence Prediction and System Identification method (SPASID) to the evaluation of horizontal strains. Comparisons are made between predictions from SPASID and two other empirical methods against measured data from a case study. The system identification method is illustrated to predict horizontal displacements and strains most consistently with the in situ data.

Jan 1, 1989

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 Mikhail P. Nesterov

The paper presents the results of the research that can forecast the development of subsidences and deformations of the earth surface with time in the permanent marginal areas of the movement troughs arising when mining sylvinite beds in the Upper Kama deposit of potash ores where the rock formation support is made by means of interchamber (rib) pillars of various yieldability.

Jan 1, 1997

By V. Bondarenko

The forecasting of floating earth inrush into the working is to a great extent connected with the investigation of the rock mass stressed-deformed state around the working. The aim of such investigations is to estimate the carrier ability of the protective layer (which is stabilized by the physico-chemical mode of fixing) and to determine it's thickness, which is to be sufficient to present the floating earth inrush into the working. We consider the protective layer to have exhausted it's carrier ability if the rocks of this layer reached the overlimit phase of deformation and it's greater part (section), which adjoins the working. is destroyed up to the limit of the remaining strength. So, the aim of investigations is to determine the size of the rocks inelastic deformation zone in the roof of the working. It is necessary to take into account the presence of water saturated seam in the roof and the overlimit characteristics of the rocks, their ability to lower the carrier capacity up to the level of the remaining strength.

Jan 1, 1996

By John Feddock

A recent increase in energy demand has resulted in the rapid growth of coal consumption in the People?s Republic of China (PRC), which is the largest coal producer and consumer in the world. Reportedly, the safety issues in Chinese coal mines have been improving, but the number of fatalities and serious injuries continues to be alarmingly high. The number of fatalities from catastrophic accidents have prompted researchers to review the Chinese mine safety situation and to look for potential solutions. Marshall Miller & Associates, Inc. (MM&A), along with various collaborators, has investigated these issues as part of its work on various projects in the PRC and has discussed the concerns with relevant Chinese officials. Preliminary investigations by MM&A of coal mine accidents during 2001?2006 in the PRC indicate that 65%-78% of the total fatalities were the result of gas explosions and roof falls in underground mines. Even though the drainage of methane has been practiced for decades, current practices do not always produce satisfactory results due to technical constraints and equipment limitations. Based on prior experience of in-seam degasification at the Daning Coal Mine located in Shanxi province, MM&A believes that in-seam directional drilling technology can be a viable solution to degas the low-permeability Chinese coal deposits. This paper 1) reviews current Chinese coal mine safety conditions, 2) analyzes the advantages/disadvantages of current degasification practices in China, and 3) presents the pre-mining degasification practices for the longwall panel at the Daning Coal Mine.

Jan 1, 2007

By A. T. Iannacchione

Underground stone mining represents an emerging sector for the U.S. mining industry. As this expansion takes mines under deeper cover and as more efficient mining methods are utilized, adequate stone pillar design methods will become more important. It is the purpose of this paper to examine current design practices and to discuss issues for safe mine layouts so that a rational first approach towards balancing the demands for increased production can be weighed against increased risk to worker safety from rib instabilities and pillar failures. Seventy-two stone mine pillar designs were examined. Pillars with width-to- height ratios below 1.5 and subjected to excessive stress levels appear more likely to fail. When width-to- height ratios fall below 1 .0, defects in the pillars, such as through-going discontinuities, can have a significant influence on stability. Discontinuity persistence, dip, material properties, and orientation are important factors controlling pillar strength. The influence of discontinuity dip, a characteristic easily identified in the field, was examined so that its presence could be accounted for in developing generalized guidelines for pillar design.

Jan 1, 1999

By Najdat I. Aziz

There are different schools of thoughts about the influence of gas pressure on coal strength. Researchers have reached contradictory results with regard to the effect of confining gas pressure on tensile strength of coal. Accordingly, this paper re-examines the influence of gas pressure on the strength of coal. A specially designed precision drill capable of drilling in a pressurised chamber was used to drill coal samples under both atmospheric and increased gas pressure environment. The drill cuttings were examined for particle size distribution using a laser controlled Malvern Mastersizer S particle size analyser. Petrographic analysis of the sample examined the effect of gas pressures on the maceral structure and the composition of coal. The paper demonstrates that there is a definite variation in the range of particle size distribution with particles obtained from drilling coal samples not subjected to gas pressures, in comparison with those obtained from samples drilled under confined gas pressure. Also observed were the changes in the rate of drilling in soft coal obtained from geologically-disturbed zones compared with other coal samples. Some variation on the maceral compostion of coals were also observed in relation to increased gas pressures and gas type.

Jan 1, 1996

By G. J. Hasenfus

This paper presents the results of an extensive hydrological and geomechanical monitoring program which Gas conducted at a longwall coal mine in W. Va. The field program included monitoring of groundwater levels. overburden hydraulic conductivity, overburden movement, and surface subsidence relative to the passage of the longwall. Overburden geology and rock strength characteristics were evaluated prior to mining. A postmining corehole was drilled to evaluate main roof fracturing. An overburden response model is proposed based on the correlations between hydrological and geomechanical data.

Jan 1, 1988

By John C. Stankus

At the 13th Conference on Ground Control in Mining, a paper was presented detailing a successful test in which a new truss system was utilized, in lieu of wood cribs, for tailgate support in the Pittsburgh seam (Stankus et al., 1994). Since that time, additional tests using this same truss system in the Pittsburgh and other seams are now complete. Also, several mines are now using trusses for full panel tailgate support with no cribs. Through an extensive instrumentation program, much data and new information has been gained from these tests and the full panel usage. From this data, not only have new concepts pertaining to tailgate support been developed, but also for headgate, pillar configuration, primary and supplemental support. In this paper, an update will be presented summarizing the results of these various tailgates.

Jan 1, 1995

By Karsten Zimmermann

The paper presents a study on the influence of face retreat rate in longwall coal mining on surface subsidence. It shows possibilities to influence dynamic ground movement by using a specific and controlled winning regime. For this, a dynamic stochastic prediction model used by DMT in Germany will be proved and adjusted on the specific character of geology in the Northern Appalachian coal field. Occurring ground movements such as inclination, curvature and deformation can be analyzed under static and dynamic points of view. It is theoretically shown that the face retreat rate has only a small influence on the static deformation values, but directly affects rate and acceleration of subsidence and deformation processes. The study shows that because of low mining depth and high mined thickness, static deformation values have the largest impact to surface objects. This implies that high face retreat rates don?t affect the impact on surface objects.

Jan 1, 2007

By Yajie Wang

High horizontal stress and its adverse effects on longwall gate entries have been witnessed in many US coal mines for the past years. It can cause cutter roofs and leads to roof falls either in the headgate or tailgate entries depending on the relative orientation of the panel to the maximum principal horizontal stress. Underground observations have shown that if the maximum principal horizontal stress is perpendicular to the panel entries, the roof is in the worst condition and if the maximum principal horizontal stress is parallel to the entries, the entry is in the best condition. However, little has been done about the high horizontal stress effects on panel entries subject to different orientations of high horizontal stress. In order to design a longwall panel and control the roof effectively, the redistribution and concentration of high horizontal stress field must be fully understood. Using 3-D finite element analysis, this paper presents the high horizontal stress redistribution and concentration for a retreat longwall face with different orientations of high horizontal stress. Based on the results, possible roof failures for different orientations of high horizontal stress are analyzed. A panel design criterion is recommended.

Jan 1, 1996

By Yemi Akinkugbe

Underground coal mining under massive sandstone can present significant challenges if the roof behavior is not well understood. This paper presents a classic cantilever bending/flexing behavior of a longwall tailgate roof caused by the weighting of structurally massive sandstone strata. Nineteen crosscuts of a longwall tailgate under massive sandstone were instrumented with position transducers connected to a data-logger to monitor roof convergence as the longwall face approached. During the monitoring program, an unusual tailgate roof divergence (uplift) was recorded. This roof behavior contradicts expected norm of roof convergence when the longwall is influenced by periodic weighting. The recorded roof movement data indicated that the roof divergence started well ahead of the longwall face. The most inby position transducer started recording progressive roof divergence when the longwall face was approximately 643 feet away. Very gradual roof convergence did not begin until the longwall face got to within 300 feet of the transducer. Recording of accelerated convergence only started after a massive caving event with the face within 280 feet of the recording transducer.

Jan 1, 2007

By Ted Klemetti

Roof deterioration in weak moisture-sensitive rock leads to roof falls in coal mines. An observation site was set up in intake air at an underground coal mine in central Ohio to evaluate the time-dependent deterioration of moisture-sensitive roof rock. By periodically collecting and weighing the rock falling from the roof, the National Institute for Occupational Safety and Health (NIOSH) was able to quantify the deterioration of the moisture-sensitive shale roof. The measurements showed that more roof rock is falling in the dry winter months. Movements in an intersection were monitored in order to determine if deformation correlated with seasonal changes in temperature and humidity. The rate of intersection roof-to-floor convergence was found to be cyclical and related to seasonal changes, with more convergence during the months of April through September. At the study site intersection, the roof beam did not show deflection indicating movement most likely due to mine floor deformation. The bolt load also increased in the warmer months before shedding load, possibly due to rock failure. Temperature and relative humidity sensors, placed in holes in the roof show that mine air does not mix with the air in the holes. Consistent saturation levels of relative humidity were measured in the holes and it appears that high levels of moisture may have migrated into the bolt holes from roof shales. A roof sealant applied to a portion of the test roof appears to be preserving the roof and reducing the amount of rock fall into the entry, while the screened area virtually eliminated rock falling from the roof.

Jan 1, 2009

By W. G. Pariseau

A crown pillar cave occurred at the Ropes Mine in December, 1987, that resulted in closure of the mine. Production was resumed soon afterwards with the formulation of a rock mechanics plan designed to address several issues of safety and stability. The plan proved to be successful and shows what can be accomplished in the realm of applied rock mechanics where a cooperative atmosphere exists.

Jan 1, 1989

By Frank E. Chase

Clay veins, also referred to as clay elastic dikes, have been responsible for numerous underground injuries and fatalities. These hazardous structures are also responsible for increased production costs during all phases of mining. For these reasons, the Bureau of Nines has investigated the physical characteristics of and roof instability problems associated with clay veins in order to make support and other mining recommendations. In addition the occurrence and origins of clay veins were examined to determine whether or not the structures could be projected into unmined portions of the coalbed. This investigation included portions of the Arkoma, Illinois, Northern Appalachian, Southern Appalachian, and Uarrior Coal Basins. Virtually a11 clay veins observed or documented occurred in the Illinois and Northern Appalachian Basins. Observed clay veins ranged in sire from 1 inch to 16 feet In cross-section and were found under shale, siltstone, sandstone, and limestone roof rock. Individual clay veins were predominantly composed of claystone; however, limestone. siltstone. and sandstone infilled clay veins do occur. Clay vein formation has been associated with the infilling of fissures which result from compressive, tensile, or shear ground failures. These gaping fissures were propagated by compactional processes and/or tectonic stresses active during and subsequent to coal iff cation. Associated fault, fracture, and slickenside planes commonly parallel clay veins and disrupt the lateral continuity of the immediate and, sometimes, main roof. When clay veins parallel or subparallel the direction of face advance the roof is segmented into cantilever beams causing unstable conditions. Therefore, the strata on either side of the clay vein must be bolted and strapped together to form a beam.

Jan 1, 1984

By Francis S. Kendorski

Over the past 25 years, the present author has written with several co-authors a series of papers beginning with the landmark paper in the 1st International Conference on Ground Control in Mining in 1981, in turn, based upon U. S. Bureau of Mines (USBM) funded contract research managed by the author with a final report issued in 1979, all concerning this paper?s title subject, Effect of Full-Extraction Underground Mining on Ground and Surface Waters. Initially the work was a summary of British, Russian, and Hungarian experience tailored to United States strata conditions, but has evolved into a consistent and well-documented model of the behavior of strata influenced by full-extraction underground mining such as longwall coal mining. The several strata zones recognized in these works, Surface Fracture Zone, Constrained or Aquiclude Zone, Dilated Zone, Fractured Zone, and Caved Zone, have been observed by several workers in the field. The concepts presented initially 25 years ago have been adopted, rightly or wrongly, by state regulators, ground and surface water researchers, and mining practitioners. The development and utility of these concepts and recent findings will be summarized.

Jan 1, 2006

By Christopher Mark

Pillar recovery continues to be one of the more hazardous activities in underground coal mining. Safety requires that the roof above the intersection remain stable until after the pillar has been extracted. Artificial supports (timbers and Mobile Roof Supports) arc essential to roof stability, but so is the final remnant stump or pushout. Traditional mining practices usually called for the complete extraction of the final stump, but the recent trend (both in the U.S. and internationally) seems to be towards mining plans that leave a remnant stump. For this study, a sample of roof control plans from the Mine Health and Safety Administration (MSHA) Coal Districts were analyzed to determine current pillar recovery practices. Both full- and partial- extraction plans were included. Special attention was paid to whether plans require that a final stump be left, and whether requirements regarding the dimensions of the remnant stump are included. Foreign experience with final stumps is also summarized. It seems that the risk of major pillar falls can often be reduced by leaving final stumps that are large enough to protect the intersection, but small enough that they do not inhibit the caving of the gob. Because final stumps are often irregular in shape, a new approach for estimating their strength is described. Analyses were conducted to assess the effect of seam height and depth of cover on the potential variation in the size of remnant stumps.

Jan 1, 2001