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By John J. Bowders

The stability of surface slopes undergoing subsidence due to longwall mining was investigated to examine the effect the subsidence had on the stability of the slopes. Stability analyses were performed on all slopes along both longitudinal and transeverse sections of the longwall panel before and after subsidence. Both finite- and infinite-slope analyses were performed with both drained and undrained cases. The results indicated that the changes in the stability of slopes due to subsidence were insignificant for the cases analyzed. Although significant lateral ground movements were recorded, these were not attributed to slope failure. Explanations for these movements are presented in the paper.

Jan 1, 1988

By Gamal Rashed

This research is divided into two parts: the first part of this research focuses on understanding coal mine bump mechanisms, and the second part focuses on mitigating the violent failure associated with a bump. Many researchers believe that the coal bumps happen because of sudden loss of constraint between pillar/ roof and pillar/floor or the hammering effect due to the breaking of thick, massive strata above the coal seam, creating a sudden impact load on the pillars and causing them to bump. However, very little work has been done to explore and understand the consequences of these proposed mechanisms for coal bumps. In the first part of this research, computer simulation of coal specimen behavior have been used to explore two issues: 1) How would an instantaneous loss of constraint or a small sudden impact load cause violent failure? 2) How does the strength of the coal specimen control coal mine bumps? Eliminating or reducing the violent failure is the main objective of the second part of this research. The coal specimen needs to be softened either in the rib zone or the core zone and then its mode of failure studied to see whether it is violent or not. Instantaneous loss of constraint and the hammering effect are associated with sudden release of elastic energy, increase in the kinetic energy, and very rapid transient vertical stresses of the coal specimen. These are three of the major proposed mechanisms for bumps. The potential for violent failure increases with increasing the unconfined compressive strength, UCS, of the coal specimen. Failure of the rib zone is not the main factor for the violent failure of a coal specimen. The energy stored in the core zone is the key factor for the violent failure.

Jan 1, 2013

By Jinhua Wang

Based on engineering examples and theoretical analysis, the mechanism of chemical consolidation for fractured roadway surrounding rock was discussed. The chemical consolidating materials and three consolidating methods, that is, capsule, forced infusion and spraying, were introduced. Finally the application effects of the chemical consolidating technique to reinforce the fractured surrounding rock of roadways were analysed.

Jan 1, 1996

By David P. Conover, Jake Bain, Christopher M. Ross

"Highwall mining of thick (up to 100 ft) steeply dipping (20° or more) coal seams provides many challenges, both geotechnically and operationally because seam dips near or in excess of highwall mining machine capabilities are encountered. Maximizing coal recovery while maintaining highwall stability requires innovative techniques with regard to web and barrier pillar layout, depth of penetration, and choice of mining horizon within the seam. Stability of highwall mining slopes, openings, and pillars are typically analyzed using the ARMPS-HWM program, as well as LAMODEL, UDEC, and Slope-W modeling. Highwall stability can be maintained and highwall mining production optimized by applying design criteria in creative ways, including alternating miner penetration depths and initiating mining of thick seams toward the bottom of the seam. Highwall mining of thick, steeply dipping coal requires careful planning and execution, including close cooperation between geotechnical design engineers, the mining company, and the highwall mining contractor. This paper describes the application of creative design techniques to a specific pit arrangement at the Westmoreland Kemmerer Mine, Kemmerer, Wyoming. Highwall mining was accomplished by UGM ADDCAR Systems, LLC, on a contract basis. INTRODUCTION Highwall mining is a technique for attaining additional coal recovery after the economic strip limit is reached in surface mining. It involves remote deployment of a continuous miner in openings beneath the final highwall, with no personnel entry. Many candidate areas for highwall mining have thick and steeply dipping seams. Mining downdip presents challenges related to the maximum pulling capacity of the machine, traction of the cutting head, and material conveying, all of which limit penetration depth. Maximum penetration is greater for flatter slopes and decreases for slopes nearing the threshold of the maximum machine operating angle. Most highwall mining operations are relatively flat with slight undulations within the seam; therefore, the highwall mining pillar design criteria applies fairly equally to the entire mining area. However, in steeply dipping deposits, design criteria based on higher overburden loads at the far end of the penetration are excessively conservative for the shallower portions of the openings near the highwall."

Jan 1, 2017

By Joe McQuerrey, William Kendall, Gene Wilson

"Roof bolting is the standard for ground control in underground coal mining. Rib and truss bolting is just as important when conditions are less than desirable. As a manufacturer of roof drills, J.H. Fletcher & Co. (""Fletcher"") promotes a walkthrough design machine to protect the machine operators from rib hazards.When installing rib and brow bolts in high mined heights, the operator's natural desire is to step from under the canopy onto the drill feed mechanism. This is done to allow a better reach to the drilled hole in the rib or brow. This, of course, is not an approved or acceptable practice.The effort to improve the operators' ability to reach the rib and brow has resulted in a new boom design. The design was first shown in September 2008 at the MINExpo in Las Vegas, NV. The design incorporates a step out area towards the rib side of the machine. The step provides improved access to the mine rib and brow.Having a solid concept does not guarantee an accepted design. Operator acceptance is the goal. When the machine was first tried in the underground environment, the machine operators did exactly as requested: run the machine as supplied and suggest improvements.In the fast pace job of roof bolting, any ergonomic improvements are welcome. These improvements usually increase the operator's productivity and safety. Based on operator feedback and final evaluation with Fletcher, machine revisions were generated. When the machine revisions were completed, Fletcher again asked the operators for comments, giving the boom design an evolving path into our industry.INTRODUCTIONIt's human nature to find the most comfortable position in which to do a task, especially for repetitive tasks. Designing a machine that will withstand the rigors of underground mining is a challenge in itself. With stringent safety requirements and limited real estate, there is a constant evolution in design to converge safety with productivity. With the ongoing development of the ""Rib Access Boom,"" one more step in the evolution process has been taken."

Jan 1, 2010

By Lewis A. Martin

Instrumented cable bolts developed at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health were used in conjunction with existing ground control systems to monitor rock mass loads at Tg Soda Ash's trona mine, Granger, WY. Axial and shear loads were determined to levels of strain gauge accuracy of ±5 N or ±5 microstrain. These gauges were embedded in a remanufactured king wire that replaced the conventional king wire. Cable bolt performance, quality of grout, and installation techniques were also assessed. By using instrumented cables, a mine operator can determine axial load along the cable at predefined gauge locations. These data provide necessary input for an operator to design a safer working environment for miners.

Jan 1, 2001

By Jaco J. van Vuuren

Modikwa Platinum Mine, in the Eastern Bushveld of South Africa's Limpopo Province, is a new operation employing a hybrid mining method where mechanised methods are used for the main developments and the narrow, 60cm reef is conventionally stoped using a down-dip conventional mining method. Throughout the mine, the hanging wall immediately above the reef comprises 3 jointed and laminated layers of varying thickness, commonly known as the triplets, below a non-cohesive leuconorite parting plane. At North shaft, using pre-stressed standing support and mechanically anchored rockbolts, it was found difficult and hazardous to hold all three triplets in the hanging wall, however bringing them down caused a significant increase in dilution and therefore cost of mining. A novel solution was conceived whereby two of the triplets are kept up using short, stiff resin anchored rockbolts installed systematically within 0.5 m of the face. This resulted in both safety and productivity benefits. The rockbolts create a stable and safe hanging wall and allow the mine to extract UG2 reef more economically. New drilling equipment, developed locally from available drilling technology, can be operated from a safe position, and in situ tests and measurements are made to confirm effective support. The technology and underlying concepts were successfully tested in a trial stope prior to rolling out the new support system across all the stopes at North Shaft. This has involved an extensive training programme to ensure that the rockbolt system is installed correctly and that the conditions are appropriate for this technology. The paper describes the motivation behind the project, the underlying technology and its application to Modikwa, and the benefits now being realised.

Jan 1, 2004

By Li-min Liu

The Semi-Analytical method can be used to solve a variety of problems in geomechanics and civil engineering. When it comes to layered rock strata and rock masses, it is superior to all other analytical methods. This paper presents the application of a 3-D semi- analytical method to 3-D surface subsidence and stress analysis caused by mining. In this semi-analytical method the layered elements and triangular elements ate used alternatively. Their displacement pattern, strain mahix, elastic matrix, stiffiness matrix, load matrix and stress matrix are presented. A numerical simulation program based on the semi-analytical method bas been developed using Fortran 90. This program is suitable for subsidence prediction and stress analysis of mom and pillar mining, strip longwall and mechanized longwall mining methods. It is applicable to all geological conditions and mining methods. Comparing with the finite method, this method reduces significantly the number of simultaneous equations to be solved and the amount of data preparation. It not only reduces the computer memory, but also runs faster and cost less. Key Words Semi-analytical method, surface subsidence, stress analysis, layered element, triangular prism element

Jan 1, 2002

By Syd S. Peng

Full-face shield leg pressures in a longwall panel for a three-month period were collected and analyzed. The pressure data were presented in the following forms: full- panel full-face shield pressure, individual shield pressure, full-face shield pressure, full-panel time-weighted-average pressure, and relationships among setting pressure, final pressure and pressure increment. Although shield pressure varies from shield to shield and from cycle to cycle, the setting pressure and face advance rate do control the 6nal pressure and time-weighted average pressure.

Jan 1, 1997

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 John K. Wood

The development of an expanded cement product represents a major advance in the technology of roof control systems. Such a product is gaining recognition as a valuable tool in today's cost- conscious coal mines. The National Coal Board's Bretby Research Lab, in conjunction with Polement Ltd., developed the first lightweight cement for use in the mines of the United Kingdom. The product, Aqualight, is a low-density (about 12.5 PCF) cement with a typical compressive strength of about 19 p.s.i. The final product increases in volume by approximately 15 times its dry powder volume. Aqualight gels in about 2 minutes, becomes self-supporting in 15 minutes, and fully cur& in 24 hours. When subjected to a load, the "cells" within its bubble structure collapse so that the product gains in density and compressive strength while retaining its integrity. Traditional methods to contain and support roof fall cavities through cribbing and blocking are both dangerous and labor-intensive. Aqualight represents a significant advantage over such traditional methods. It is non-toxic, non-cmbustible, and very easy to install. Aqualight can be pumped to the worksite from re- mote locations (as much as 600 feet), thereby reducing danger to men and equipment. Addition- ally, the strength of the cured product can be varied by adjusting the expansion ratio and resulting density. A compressive strength of 150 p.s.i. can be obtained in this manner. Although primarily designed to fill cavities resulting from roof falls. Aqualight has found a wide range of applications in mining industries around the world. In addition to cavity- filling, Aqualight has been used for ventilation stoppings, fire stoppings and emergency fire control, backfilling between arches and the roof and ribs, and backfilling of old workings. Since its recent introduction into the United States. Aqualight has been used by MICON SER- VICES, INC. to fill several roof fall cavities along headgate entries. In all cases, the cavities were contained and the longwall moved through without problems. Roof Pall cavities along main haulage entries and at shaft/slope bottoms have been successfully filled. MICON has also installed Aqualight as backfill material around arches. A current major project is the installation of trial ventilation seals using Aqualight as an alternative to traditional block and mortar construction. Pending MSHA approval, MICON anticipates the use of Aqualight in the construction of both ventilation seals and stoppings. In addition to new construction, Aqualight can be employed to repair stoppings damaged by roof convergence and floor heave. In labor savings alone, this is very attractive for remote stopping repair. Among the many other uses of Aqualight is the backfilling of old auger holes, which will support the highwall and allow access to coal re- serves which are presently sterilized.

Jan 1, 1986

By Phillip E. Gramlich

Through extensive practical experience and multiple studies, tensioned roof control systems have proven beneficial in difficult mining conditions. Due to increased mining of less desirable seams, tensioned roof control systems have claimed a progressively larger share of the total number of roof bolts installed in the US coal industry. Each tensioned roof control system has its benefits, but also has problems concerning cost, cycle time, stress concentration, load retention, and protrusion into the mine opening. For example, resin-assisted mechanically anchored bolts (also known as resin point anchor bolts) have a fast cycle time and the potential for high tension loading, but the stress from the mechanical shell is concentrated in a small area and can create concerns with certain roof structures (Zhang and Peng, 2001). Resin point anchor bolts can be difficult to fully grout. The cost of resin point anchor bolts can be an issue; a two-piece resin point anchor bolt has a large number of parts (10) and manufacturing steps (9), adding to its higher cost. Tension rebar bolts can achieve high loads and are easier to fully grout but have longer cycle times. In addition, a high percentage of the installed load can ?bleed off? if the resin mixing time or the ?hold time? before tensioning is short-circuited. Tension rebar bolts can be notorious for protruding into the mine opening, creating an obstruction for man and machine (Figure 1), in particular, with reduced mining heights. A new tensioned roof control system has been developed that doesn?t have moving parts, is cost effective, provides fast cycle times, has a low profile, and is simple t install.

Jan 1, 2014

By Li Yang, Andre Cezar Zingano, Alberto Fronza

"Immediate roof control is very important to the security and stability of entries in coal mining. It is also the highest cost in the coal mining operation, and roof spalling is directly responsible for coal contamination. The immediate roof in the coal mines in the State of Santa Catarina varies in lithology and in the presence of structures, such as bedding planes, vertical fractures that cut lamination, sandstone, and siltstone boulders. The presence and crossing of these structures should influence the decision of the specification of the roof support. Furthermore, the variation of rock layer and quality of the roof must be immediately followed by the variation of support specification. This work aims to study the behavior of the immediate roof under different geological and structural conditions during the excavation of entries, including the changing of the geometry of the entry as a function of time. This includes the effect of vertical fractures that cross the bedding planes of the roof and the position of those fractures on the roof. The effect of time between excavation and installing support to stabilize the roof is also examined. This study uses empirical methodologies for support design and numerical methods to study the behavior of the roof when anchored to the different scenarios proposed. The results of this work show the importance of quality characterization of the roof in terms of the type of rock and its thickness and the presence of vertical fractures that cut the bedding planes of the roof immediately.INTRODUCTION The immediate roof in a coal mine can varies in terms of typology and structural features, like bedding plane spacing and vertical joints. The roof support must be specified based on the features of the roof layer and its classification. Unal (1983) and Mark, Molinda, and Dolinar (2001) suggest that the roof support design should be based on the rock mass rating (RMR) and coal mining roof rating (CMRR), which look at rock mass classifications. These methods are based on the beam-building approach, and the bedding planes are tied together using fully grouted bolt. The bolt can be pre-tensioned or not depending the time of the curing of the resin."

Jan 1, 2017

By Joseph A. Cybulski

In response to the mining of reserves with increasingly adverse round conditions and the roof support recommendations of current rock mechanics theories, roof support systems have continued to improve. A few of these newer bolting systems. while giving improved performance, require a new method to check bolt installation. Previous1 , a torque wrench had been the sole means to check the installed torque of a roof bolt. However, due to the characteristics of soma of the newer bolting systems which can prevent a reliable manual torque check and a recent change in the federal roof control regulations, an alternative method to a manual torque check is gaining acceptance. This alternative method improves monitoring the hydraulic pressure of the roof bolting machine torque circuit and provides a safe, reliable and quick wav to check bolt installation in those instance when an accurate manual torque check cannot be made.

Jan 1, 1990

By Anthony T. Iannacchione

Underground bituminous coal mining is a large and significant industry within the Commonwealth of Pennsylvania, and it has a legacy of environmental consequence. For example, in 2008, 13.5 acres (5.5 hectares) of land were undermined by longwall mining every day. Laws and regulations have been promulgated requiring companies involved in this industry to remedy mining damage caused to homes, businesses, and land; replace impaired water supplies; and repair impacts to streams and wetlands. The regulations and technical standards set by the Pennsylvania Department of Environmental Protection (PA DEP) have been put in place to assure that coal operations comply with ACT 54 of the Bituminous Mine Subsidence & Land Conservation Act and other related legislation. These regulations and standards require operations to address impacts when a mining permit is submitted and as permits are modified and extended. One of the special provisions of ACT 54 is the requirement by the PA DEP to produce an assessment of the surface impacts of mining every five years. The University of Pittsburgh was contracted by the PA DEP to perform the 3rd assessment, which spanned from August 21, 2003, to August 20, 2008. This paper details the findings related to surface structure undermined during this period. A total of 3,735 structures were tracked. Four-hundred-fiftysix reported effects were found, and each required additional information and analysis. The majority of the reported effects were associated with eight longwall mines. Characteristics of the structures were analyzed and discussed, with special attention to those with reported effects. The number of undamaged and reportedly damaged structures (i.e., dwelling, barn, commercial building, etc.) undermined during the assessment period were listed. Lastly, the resolution status and type of resolution were analyzed for all structure reported effects.

Jan 1, 2011

By Stephen C. Tadolini, Minova USA Inc., Peter S. Mills, David R. Burkhard

"Fiber-reinforced spayed concrete has been used for several years in civil and tunneling operations. Research conducted to reduce cure times and increase compressive and flexural strengths resulted in the development of Tekcrete Fast®, a cementitious product capable of obtaining 41 MPa (6,000 psi) compressive strength and 8 MPa (1,200 psi) flexural strength in only 3 hours and reaching 7 day strengths of 62 MPa and 11.7 MPa (9,000 and 1,700 psi), respectively. A single bag product that uses conventional shotcrete and gunite application systems make it a natural crossover product for mining applications. The discovery of incredible adhesion properties and high resistance to chloride permeability helps ensure long-term stability and increases the ease of application. Project results from Disaster City® in Texas and the application for rehabilitating a coal mine belt entry are presented. The case study illustrates the effectiveness of the product in stabilizing a coal mine beltway and adjacent cross-cuts that were subjected to progressive sloughage due to humidity and cyclical loading. INTRODUCTION Original research produced a shotcrete formulation that extends beyond stabilization and can be used to restore the structural integrity of damaged infrastructure. The primary challenge was to develop a shotcrete or gunite that had structural strength within a few minutes to prevent parasitic loading, allowing ingress into damaged structures minutes after applying the mix formulation. This also applies to mining conditions where rapid support of progressive failures can minimize failure. An additional benefit was discovering that the product had extremely high adhesion properties in addition to high flexural strengths. The approach to product design was the development of “single bag” dry mixes that are entirely self-contained, can be rapidly transported anywhere in the field or underground, and can be deployed using simple equipment. Dry mix eliminates issues relevant to high shear water mixing because water is added in the form of a mist at the point of delivery; thus a low water-to-cement ratio can be achieved. However, since there is no onsite powder mixing, the product must have a high degree of inherent homogeneity. Minova offers a commercial version of this material, Tekcrete Fast®, a product that has been successfully deployed in the mining industry and is currently being used in civil, construction, and tunneling operations. The material can rapidly stabilize lose ground in metal, nonmetal, and coal mines. This paper described the technology development, product testing results, and applications and describes a case study to stabilize a coal mine beltway."

Jan 1, 2017

By Bruce Hebblewhite

An ACARP-funded industry research project in Australia recently investigated the problem of premature rock bolt failures in underground coal mines. The project carried out limited sampling from 9 different collieries in NSW and Queensland, followed by a more comprehensive, controlled sampling from one colliery. Over 50 broken bolts were collected and subjected to metallurgical analysis at UNSW to determine both the steel composition and to analyse the fracture surfaces. Prior to this study, there had been anecdotal evidence of similar bolt failures from a number of sites within the industry. The sampling and analysis conducted by this project - whilst being below a statistically representative sample of the entire industry bolt population - highlighted the fact that bolt failures were continuing to occur at a number of sites, in both older and recently installed bolts. The failures were almost entirely attributable to stress corrosion cracking (SCC). It appears that a combination of factors - including steel composition, bolt manufacturing process, loading environment, geological and hydrological environment, bacterial corrosion - can all contribute to this problem. There is no simple solution, however, as with similar research findings from the UK, it appears that steels with higher values of fracture toughness, or Charpy Index, are less susceptible to SCC. Testing to date has been limited to bolt failures below the resin encapsulation horizon. A real concern is the potential extent of the problem at higher horizons, within the encapsulation. These may require some form of non-destructive testing (NDT) for detection. Initial trials in Australia of the ultrasonic device developed by DMT in Germany have proved encouraging. Further testing using this device is anticipated in future research. This paper describes the outcomes of the project, completed in 2002.

Jan 1, 2003

By Rusty Mchenry, Craig Dickerson, Robin L. Oldham

"It has been proven that longwall faces can be moved safely and efficiently. However, abutment pressures and poor ground control conditions can halt operations and be hazardous to coal miners. Recently at a mine in Southwestern Pennsylvania, roof material collapsed above shields that created two large voids and caused major challenges for shield recovery. A unique, engineering solution was developed that utilized a modified concrete material to fill the voids, creating stability in the affected area. The many phases of this project included the construction phase, void pumping, cutting out, and bolting of the concrete material. This project eliminated the hazards associated with bolting the recovery face and removing shields in adverse conditions, making it possible for the mine operator to safely complete the longwall move.INTRODUCTIONFrequently in the mining process, especially during the recovery phase, voids above the longwall supports have occurred. These voids are simply roof cavities caused by falling rock strata above the longwall face. Most often, this condition slows down or halts mining operations and creates hazardous work conditions for coal miners. Numerous methods of supporting void areas on longwall faces have been attempted and utilized throughout the mining industry since the advent of the longwall mining system. These methods have utilized many different materials and processes and have included the installation of steel or wooden beams, crib blocks, and various types of roof bolts. However, these methods can be dangerous due to exposure of personnel to the mine voids, falling roof material, and the necessity to handle large amounts of heavy support materials.This paper describes a unique case study in Southwestern Pennsylvania where longwall recovery methods, through two void fills, were successfully completed, eliminating exposure of coal miners to the rock voids above. At the start of this endeavor, normal recovery operations were halted, and hazardous conditions became a concern when the voids were encountered. However, in the interest of safety, the decision was made to try a unique method to completely fill the void from the bottom of the coal seam to the top of the void with a cementitious material and then mine through it. This is in contrast to traditional methods where beams and support materials are used to stabilize the area above and in front of the mine shields, which can expose miners to the void above."

Jan 1, 2015

By Marek J. Mrugala

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

Jan 1, 1989

By C. Tom Blevins

The purpose of this paper is to discuss production and roof control problems associated with directional lateral stresses in an in situ stress field and the approach that Inland Steel Coal Company made in trying to cope with them. Before proceeding, a description of the mine location, product, geology, restraints, and mine history is in order. The mine is located in Hamilton County in Southeastern Illinois. It produces a high volatile metalurgical coal and is slated to produce 2.1 MTY. Product ion is in the Illinois No. 5 seam at a depth of 930 feet, approximately 490 feet below sea level. The scam height averages from 5.5 to 6.0 feet throughout the reserves with height increasing in the eastern reserve area to approximately feet and a 5 Foot average in the West. The scam tends to dip to the North, North East, but is locally very undulating. The Galatia channel washout outlines the eastern boundary of the reserves and there is a split coal area toward the central part of the property than runs south-southeast. The top above the #5 seam is laminated medium gray shale called Dykersburg. Individual shale bedding planes vary locally From ½ to 1" in thickness to a more normal thickness of 1 to 1 ½ with some areas being interlayed by thin carboncous material. The top normally must be considered as materially competent. Even though the mine is still relatively small in area with only 1.3 million tons of total production to (late, we have encountered dense zones of slicker sides and premining, stress cracks (joints). There are also areas of siltstone, that vary from a few inches to several feet thick, and areas of rider coal, clay instrusions, and rolls. We have crossed lineament concentrations that show strong evidence of corelation to localise top problems. These are being monitored closely to see their predictability on our present and future mining conditions. The face cleat runs N 55° E and butt cleat runs S. 25° E. On a regional basis, there are several anticlinal belts that trend in a northerly direction. The one major restraint is that Inland does not own surface subsidence rights. Therefore, a partial extraction mining plan is being followed. The Holland-Gaddy formula for pillar strength design is used to calculate theortical pillar sides. When production began in March of 1979, it was noted that ground control problems occurred as we drove north entries. East-west cross cuts tended to have better working conditions. This phenomia occurred from a couple hundred feet, but seemed to dispate as we got away from the men and material shafts. Once the production shaft was connected and enough entries extended for installation of belts, ground control problems were again experienced in north-south headings. It was also observed that when mining north-south headings with east-west crosscuts, if the crosscut was driven through ahead and supported, the intersection was stable upon entry connection. This held true even where fairly extreme ground control, problems were encountered. The reverse held true. when driving east-west headings with north-south crosscuts. At this time, we started searching in earnest for a working theory to explain this roof phenomia. It also became the first step of what eventually was a four step sequence to derive a workable solu¬tion to the problem. The combination of the many geological changes and the fact that north-south condition was at times intermitent, helped to disguise the stress field. The mine also being in a fairly virgin area with no mines in approximately 25 milt, radius, meant we didn't have a reliable case history to examine. Various theories were researched before it was concluded that our primary ground control problem was a directional lateral stress Field, with the major stress axis in an east-west direction. The second stop of the procedure was then to test. mining methods and determine if these stresses could be modified to a

Jan 1, 1982