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By Behdeen Oraee-Mirzamani

Since the advent of New Austrian Tunneling Method (NATM), shotcrete as a primary means of support in tunnels has been widely applied. It?s most important features are durability, speed of application and cost effectiveness. This paper introduces some tables that provide guidelines for the thickness of shotcrete required in some common situations of mine roadways. In order to devise such tables, two different arch sections, together with three different overburden types, were considered. Geotechnical parameters such as apparent cohesion and angle of internal friction of surrounding rocks were chosen, based on the five-category classification of Bieniawski. Two K0 factors (the ratio of horizontal stress to vertical stress) and an average rock density were utilized. Using numerical methods, 60 models were then devised in this way. By applying interaction diagrams of axial force and the bending moment for different thicknesses of shotcrete, appropriate shotcrete thickness for these models were calculated. The results of this research, as well as the methodology applied, can be used in mining roadway support design and all types of civil engineering tunnels.

Jan 1, 2011

By Venkata S. Vutukuri

Compressive strength of cylindrical specimens of a model material and sandstone with various diameter/height (d/h) ratios was determined in unconfined compression tests. It was observed that the compressive strength increases with increase in d/h ratio. Triaxial compression tests were also conducted on the specimens of the same materials with a d/h ratio of about 0.47. It was observed that the axial stress at failure increases with increase in confining pressure. A very interesting correlation was found between the effect of d/h ratio on compressive strength in unconfined compression tests and the effect of confining pressure on axial stress at failure in triaxial tests. The relationships between d/h ratio and confining pressure, s3 in MPa are:- For model material - s3 = -0.95 + 1.7957*(d/h) 0.7883. For sandstone - s- (-0.4554 + 0.9888* (d/h) 1.0355. The correlation coefficients for these equations are 0.9997 and 1. These equations are valid for d/h ratios of up to 10.

Jan 1, 1993

By Song Yong Jin

The Jurassic coals at Datong are characterised by their strong, massive sandstone and conglomerate roofs which fail suddenly over large areas, crushing pillars and creating destructive windblasts, collapsed areas have exceeded 180,000m2 and windblasts have extended over one km underground and 200m on the surface and have been accompanied by subsidence troughs on the surface. Scientific analysis has researched premonitory signs of caving and the nature of windblasts leading to considerations of alternative mining methods and ways of dissipating the energy of windblasts to reduce their damage and danger.

Jan 1, 1992

By Hamid Maleki

This study presents a historic overview of the role of mobile roof support (MRS) technologies in improving stability and worker safety and presents the results of recent field evaluations of the MRS load rate monitoring device and other remote deformation-monitoring techniques. Field studies were implemented at two sites in cooperation among researchers from the National Institute for Occupational Safety and Health (NIOSH), Maleki Technologies, Inc., and J. H. Fletcher & Co. The objective of the field programs were to (l) study the interaction between MRS's and coal mine strata and (2) develop and test suitable monitoring systems for assessing roof and pillar stability. An MRS consists of a roof canopy, four hydraulic cylinders, a caving shield canopy, and associated electromechanical systems mounted on crawler tracks. The machines are controlled by radio from a remote location and operate on self-contained power units. Typically, MRS's have capacities of 5,340 and 7,120 kN (600 and 800 tons). In comparison to posts, an MRS is capable of maintaining the yield load after significant amounts of roof-floor deformation. Because the mining cycle is accelerated, MRS's help reduce the potential for time-dependent roof falls. MRS performance has been monitored in the laboratory under controlled static loading conditions and in the field under deep, two-scam mining conditions. Laboratory studies have quantified support capacity and system stiffness as a function of machine height. Field investigations have focused on determination of optimum operating conditions and development of warning systems that indicate excessive load on the machine and/or impending roof-pillar stability problems. Analyses of field data show that roof instabilities are influenced by (1) pillar failure, (2) pillar yielding, (3) mine seismicity, (4) geologic structures, and (5) panel layout designs and mining practice. Pillar yielding and failure (unloading) and seismicity can be conveniently monitored by the load rate monitoring device, but for consistent detection of roof falls, additional deformation measurements directly within the cuts ate needed.

Jan 1, 2001

By Stephen C. Tadolini

The risk of injury caused by minor roof falls continues to increase in underground mining. Most ground control injuries result from small rock falls that occur in a supported area, but do not involve the failure of the support system. In the roof, these are the "skin failures" that occur between previously installed primary or secondary bolting systems. To minimize skin failures and the associated accidents, several devices were designed and tested in the laboratory to maximize stiffness, minimize material thickness and weight, and ease material storage and handling on roof bolting machines. The goal of this project was to design and evaluate devices that can be installed with traditional primary or secondary roof supports, and that arc easy to handle/install under temporary or previously supported roof.

Jan 1, 2001

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 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 Daniel W. H. Su

This paper presents a retrospective assessment of ground control research and its successful application by CONSOL Energy over the past 25 years. Recognizing the need to improve the understanding of coal pillar design mechanics under shallow, medium, and deep covers, an extensive underground coal pillar instrumentation program was initiated in the mid 1980s. Combined with sophisticated numerical modeling technique and laboratory tests, results from this instrumentation program were used to advance the concept of coal pillar design and to demonstrate that numerical modeling can be used to predict in situ coal pillar strength, especially under non-ideal conditions where interface friction and roof and floor deformation are the primary controlling factors. To evaluate the impact of longwall mining, a 10-year subsidence research program was initiated in 1986 that encompassed surface, subsurface, and underground measurements, as well as a sophisticated numerical modeling technique. Results from the research revealed that interface friction and shearing between rock layers within the overburden had a profound effect on surface subsidence development and the resulting strain. Recognizing the impact of horizontal stresses on CONSOL?s mining operations, extensive horizontal stress measurements were conducted using the USBM Borehole Deformation Gage and the CSIRO Mininfrac system in the mid to late 80s. Roof extensometers and CSIRO HI Cells were used to measure roof displacement and roof stress concentration in longwall headgates and tailgates, which resulted in the definition of right-hand and left-hand longwall panels. Pillar design modifications, combined with cable bolt application and innovative standing support installation, were then successfully employed to mitigate horizontal stress impact on the right-hand longwall headgates and left-hand longwall tailgates. An in-house floor bearing capacity test apparatus was fabricated and employed underground to determine mine-specific floor bearing capacities which, along with other geotechnical parameters, were then used in the mine-specific shield design. To promote caving of thick, massive sandstone immediately behind longwall face supports and to improve face conditions, the hydraulic fracturing technique was successfully employed by CONSOL to pre-fracture the sandstone prior to longwall mining. Face monitoring and numerical modeling confirmed the effectiveness of the hydraulic fracturing technique. A roof geology reconnaissance program was initiated by CONSOL in the early 1990s to improve roof control, safety, and production. Information obtained from the reconnaissance program was used successfully to guide mine planning decisions and daily roof support practices.

Jan 1, 2011

By John P. Dunford

Ground support continues to be the most important safety consideration in underground mining. The Bureau of Mines has developed a yielding steel post with a capacity of 45 tons and b c1osure allowance of 24 inches in a 6 to 7 foot seam height. The post mechanism in a three piece unit consisting of a top and bottom leg made from standard steel pipe and a separate foot bracket. It develops its load carrying ability when the lower pipe, with an attached interference ring, is forced into the larger top leg. The walls of the top leg are deformed radially and along the axis. Yield continues until the unit bottoms out. Other advantages of the post is that due to its simplicity it can be made at the mine shop level and its cost should be about one-hundred dollars. Prototype units were tested in the laboratory. Based on these results, modifications were made that would make the post perform better as a mine support. Two field tests were conducted. The first test used the units as supplemental support for the wood crib rows in a longwall tailgate. The second and larger field teat had 84 posts placed on salvaged track rails. They were installed on two foot centers in the middle of the entry. Overall the supports functioned well during use in the headgate entry, but problems with the quality of some of the pipe caused localized failures. However, these problems can be attributed to one manufacturer of pipe and through tighter specifications can be eliminated. The use of the posts with the rails is showing additional benefits as a ground support system. By providing continuous support between the posts a beam is created that has much more load bearing. The additional support can reduce floor heave and in some cases eliminate the need for the rib side crib line.

Jan 1, 1986

By Ben Fahrman

The classical approach to engineering design involves the purely deterministic calculation of the factor of safety, the ratio of strength to stress. The factor of safety is an easy calculation to perform in many circumstances, and it is very easy to interpret the results. The simplicity of the method is both a strength and a weakness. The results of the deterministic factor of safety calculation are simple to determine and interpret, but they have no ability to account for the uncertainty present in the inputs or outputs. Uncertainty is prevalent in geotechnical engineering applications. This uncertainty can stem from spatial variability, poor sampling, a dynamically changing environment, etc. Probability analyses allow input parameters to take the form of a distribution rather than a single value, making them uniquely equipped to handle the uncertainty inherent in geotechnical engineering design considerations. A stochastic approach also has the benefit of providing results as a probability of failure, which is more meaningful than a factor of safety. Though stochastic modeling is becoming more common in many geotechnical applications, it is still not widely used for coal pillar design. The objectives of this study are to justify the concept of using a stochastic approach to coal pillar design and to compare the results of a simple stochastic analysis to those of standard industry practice. A simple synthetic study was performed to compare the results of a probabilistic analysis of coal pillar design to the results obtained from running ARMPS, which is published by NIOSH. Normal distributions were assigned to each input parameter that is required to determine the factor of safety of a single coal pillar. The probability of failure resulting from the synthetic study was found to be more conservative than the ARMPS stability factor. A probabilistic approach to coal pillar design is discussed in comparison to the traditional deterministic approach. The greatest strengths of the probabilistic approach were determined to be the effective handling of uncertainty and the increased meaning of the inputs and outputs inherent to a probabilistic approach.

Jan 1, 2013

By Yi Luo

The potential influences of mining subsidence on structural stability, integrity and functionality of guyed tower structures have not been systematically studied before. This paper presents a ease study where a guyed steel tower of a telecommunication company was mined under by a longwall operation. Prior to mining, every aspect of the subsidence influences to this structure was studied. The pre-mining assessment indicated that the structure could experience vibration problem in the second half of the dynamic subsidence process. Bawd on the study, a simple mitigation measure was recommended and implemented and the structure was successfully protected.

Jan 1, 2003

By Kenneth L. Johnson

The effects of underground longwall coal mining on flow in perennial streams within the Pittsburgh seam basin were evaluated. Four streams above four different longwall mines were studied. Overburden depths at the study sites ranged from 500 to 600 feet. At all four locations, premining streamfow measurements were made to establish baseline flow conditions. Measurements were then repeated at each location after the longwall face passed beneath the stream. At one location, in which the longwall panel axis was roughly parallel to the stream, the streamflow was monitored as the longwall face advanced. Measurements at this location showed that temporary flow reduction occurred as the face advanced underneath; however, after the tensile portion of the traveling subsidence profile passed, this effect was lessened. At two of the sites in which the stream was roughly perpendicular to the panel axis, postmining streamfow increased across the panel and decreased over the development headings, such that the net lateral change in streamflow across the fully subsided panel and associated development headings was small compared to the measured flow.

Jan 1, 1997

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 Buddhima Indraratna

The paper highlights the conceptual developments in rock mechanics applied to quantify the effect of bolts in the stabilization of joints and yielding soft rocks.

Jan 1, 1992

Excessive roadway closure in the gateroads impedes the passage of equipment, supplies, personnel and ventilation air in longwall panels. This pager describes several factors that influence the gateroad stability and presents some measures to alleviate the problem.

Jan 1, 1994

By M. E. Duncan Fama

The aim of the paper was to model numerically some aspects of the extraction of coal pillars by split and fendering. A two-dimensional elasto-plastic plane strain model with strain softening and interface modelling was used. The model was able to simulate the improvement of conditions experienced in lifting from the fender, when the roof adjacent to the fender has caved. In contrast, when a stook was left, providing support to a span of uncaved roof, the model predicted the fender would suffer considerably more deformation and strain.

Jan 1, 1992

By Sun Henghu

This paper analyses problems associated with cement based technique in metal mines, and provides a new mining technique of packing the tailing sand and water into solidified material. In the new technique, the cementing agent is a solidifying material containing a high percentage of water and tailing sand. The sand and water solidify into the packed body at the same time. The slurry of the packed body varies widely in concentration and suitability for different mining conditions. It consists of two liquid materials, each composed of a cementing agent, tailing sand and water. The thick liquids are transported to the packing place at the same time. Their pumping property is good; they do not solidify or block up the pipe during transportation. At the packing place, the two liquids are evenly mixed. The mixed liquid flattens out well and its solidifying speed is so fast that it solidifies into a packed body within a very short period of time. The compressed strength of the packed body can reach 1.2-5MPa in one day, which satisfies the requirements of different mining conditions. By using the new mining technique, all the tailing sand can be used again in the packed body and it is low in cost and investment. The mechanisation is easy and thus the working efficiency is increased and the use of labour decreased. Contamination problems caused by excess water drainage under the ground are resolved.

Jan 1, 1992

By Douglas Morrison

By applying modern 3D numerical modelling techniques to the analysis of a pillar cascade failure in a room and pillar mine, it has been possible to simulate the collapse and develop an understanding of the role of low rock strength areas within the mine. The cascade collapse occurred in a normal strength area but would not have occurred without the influence of a low strength rockmass in one section of the mine. The numerical analysis made it possible to develop a realistic assessment of the residual pillar strength. With the introduction of regular empirical pillar performance assessments, critical pillars or groups of pillars can now be identified proactively. This has led to an aggressive extraction ratio of 90% that manages pillar failure, avoids pillar and roof collapse and prevents surface subsidence. Since the mine also has significant waste products, the use of backfill for ground support has made it possible to significantly improve the performance of strategically located pillars, and so achieve even higher extraction ratios.

Jan 1, 2003

By Takashi Sasaoka, Pinyo Meechumna, Hiroshi Takamoto, Pipat Laowattanabandit, Akihiro Hamanaka, Kikuo Matsui, Hideki Shimada

"The EGAT Mae Moh coal mine is an open-cut coal mine in Thailand that produces about 16 million tons of lignite annually to generate 2,400 mw of electricity. The surface mine pit covers is 4 by 7 km (2 by 4 mi) with mining depths ranging up to 260 m (853 ft). A large pit slope is formed with the progression of the mining operation. However, massive slides in the pit slope often occur due to weak planes, such as faults and bedding, and the weak mechanical properties of the rocks. Hence, a 200-300 m (656-984 ft) boundaries of rock block including coal seams is left in front of faults in order to prevent slides and maintain the stability of the pit slope. As a result, there are significant coal reserves beneath the abandoned area along the pit slope.The focus of this paper is on the applicability and design of the highwall mining for recovering much of the coal left along the pit slope.INTRODUCTIONThe EGAT Mae Moh Mine is located about 630 km (186 mi) north of Bangkok, Thailand (see Figure 1). It is the largest coal mine in Thailand and produces about 16 million tons of lignite annually from open-cut mining (70 % of the total coal produced in Thailand). The coal is used to generate 2,400 mw of electricity at the Mae Moh power plant, which is situated near the mining area. The mine pit covers an area of 4 by 7 km (2 by 4 mi) at varied depths up to 260 m (583 ft) as shown in Figure 2. As the mining operation progresses, the pit becomes to be deeper and deeper. In the year 2035, the bottom of the pit is expected to reach depths of 500-600 m and the open-cut mining operation will be finished. The possibility of an underground mining operation is being investigated.A large pit slope is formed with the progression of open-cut mining. However, massive slides often occur in the pit slope along weak planes including faults and bedding planes due to the weak nature of the rock. As a result, 200 to 300 m (656 to 984 ft) boundaries of rock including coal seams are left in place in front of faults to prevent slides and to maintain the stability of pit slope. Moreover, the dip of the slope is less than that of the planned design. As a result, there significant coal reserves abandoned along the pit slope."

Jan 1, 2010

By Michael Alber

Pullout tests on resin grouted rock bolts have been executed in coal mines and surface test sites. The goal of this study was to provide some qualitative information about the rock mass quality from mandatory tests with a bit little more effort. Movements of rock bolt head and steel plate at different load levels were analyzed and were found to yield information about the rock mass deformability. The tests were interpreted similar to plate bearing tests and a rock mass compressibility index C was assigned. This index reflects the complex interactions between the rock mass, resin grout and bolt under load. Although further research is necessary it may be concluded that those tests may aid in recognizing geological conditions.

Jan 1, 2003