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By M. Terry Hoch

This paper presents information on low-coal ATRS systems that are now in use. A study was conducted consisting of an in-mine evaluation of ATRS- equipped roof bolters capable of bolting in seams below 42 inches. Machine manufacturers, MSHA personnel, and bolter operators were asked to assist in reviewing the operation of various low- coal ATRS systems. Features of each system are outlined and operational problems and limitations of each are analyzed. Specific applications, such as ATRS-equipped bolters designed for use with auger-type miners, are also examined.

Jan 1, 1988

By Ross W. Seedsman

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

Jan 1, 1992

By Y. P. Chugh

The paper discusses laboratory and field studies to analyze time-dependent deformation behavior of immediate weak floor strata in an Illinois coal mine. In the laboratory, core samples of the immediate floor strata were subjected to incremental creep tests under unconfined compressive stress. Both axial and lateral creep deformations were recorded. Time-dependent behavior was reasonably characterized by a Burger model with one Kelvin-Voigt unit. The elastic and viscous parameter8 of the model were found to be stress-dependent, but in the range of 40% to 80% of the failure stress, the effect of stress was relatively small. Therefore, a linear visco-elastic model was developed to represent the laboratory time-dependent behavior of immediate floor strata. Such a model may be acceptable for the design of partial extraction mining systems having extraction ratios of 40% to 60%. Field observations of roof-floor convergence (floor heave) at the mine also indicated a time-dependent behavior similar to that observed in the laboratory. Field time-dependent behavior was also characterized in terms of a Burger model with one Kelvin-Voigt unit to obtain scaling factors between the field and laboratory parameters. This approach should help in predicting likely floor heave in the mine based on laboratory creep tests. It was concluded that additional studies in the field are required to arrive at more reliable correlations between the two sets of parameters.

Jan 1, 1987

By Maynard O. Serbousek

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

Jan 1, 1984

By 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 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 William Bookshar

The development of increasingly more sophisticated and powerful retreat longwall equipment has led to ever wider and longer panels. To maximize gateroad development efficiency, provide for effective face recovery, andlor reduce ventilation pressures, entries are cut across longwall panels in advance of longwall retreat mining. These entries must be supported to prevent falls of immediate roof in advance of the face. Recently, cut through entries and predriven recovery room entries have been supported with various combinations of roof bolts and passive support. The cost and relative success of these support systems has varied considerably. Eighty-Four Mining Company, Fosroc, and Sub-Technical have developed and implemented an efficient, yielding, multiple component, cement crib to support longwall panel cut through entries and pre-driven recovery rooms. The cutable crib consists of an inexpensive flyash cement pillar poured in a bag, within a temporary metal form. The form is moved to a subsequent cutable crib location after the initial hardening of the flyash pillar. The remaining entry height between the mine roof and the top of the flyash cement pillar is filled with a specially designed bag filled with Fosroc's Tekset cement formulation. The diameter of the pillar and thickness of the Tekset bag are designed to accommodate the anticipated front abutment loading and entry convergence.

Jan 1, 1998

By N. P. Kripakov

This paper presents a brief overview of current bump mechanics theories and pillar design methodologies, and relates these concepts to experiences at two mines located in a north-central Utah coalfield where different pillar designs were used to control mountain bumps. Experiences gained at the first mine demonstrated the successful implementation of a two-entry, 9.8 m (32 ft)-wide, yield-pillar design. A U.S. Bureau of Mines field study quantified the timing of chain pillar yielding and resulting load transfer from the gateroad. In-mine pillar response, although apparently sensitive to site-specific conditions, compared favorably to estimates derived using two yield-pillar design methods. A second study conducted at another mine located in the same district, but subjected to different geologic conditions, documents the unsuccessful attempts to employ progressively narrower three-entry pillar designs based on successes achieved at the first mine site. This second mine never achieved a true yield-pillar design. Use of pillar widths ranging between 9.1 m (30 ft) and 27.4 m (90 ft) always resulted in violent bumps in the tailgate pillars. The pillars were either too large to yield, or too small to support peak operational loads. Hypothetical gateroad systems were evaluated using both analytical and empirical approaches for configurations comprised of two rows of conventional pillars, and systems incorporating both yield and abutment pillars. Analysis concluded that yield pillars less than 6.1 m (20 ft) wide and abutment pillars ranging between 30.5 m (100 ft) and 39.6 m (130 ft) square would be required to achieve a stable gateroad design. However, results of a field study conducted on a two-entry, 36.6 m (120 ft)-wide abutment pillar concluded that abutment pressures from the first panel overrode the pillar, and that a still larger pillar may be required to preclude bumps in the tailgate during second panel mining. Without the benefit of a demonstrated in-mine success, it is not clear which design would ensure elimination of gateroad bumps.

Jan 1, 1992

By Hani S. Mitri

The behaviour of grouted cable bolts used for rock reinforcement as a means of ground control in hard rock mining is investigated. The study utilizes an analytical model which is based on the variational principle to calculate the shear bond stress distribution along the embedded length of cable bolt. In this paper, a comprehensive parametric study which covers both geometric and mechanical design parameters involved is presented. Special emphasis is placed on studying the characteristics of the grout model represented by Winkler springs. This involved varying the maximum shear strength of the grout, its residual strength and the slip occurring between the cable and the borehole wall.

Jan 1, 1990

By Zhanjing Yu

In this paper, an equation is developed to characterize the relationship between the UBC and plate size using a boundary integral approach. The equation was validated by the results from mo& than seventy (70) plate loading tests of different sizes conducted in various underground coal mines throughout the Illinois Coal Basin. The equation indicates that the UBC decreases as the plate size increases. The fundamental solution for this boundary integral problems is the Boussinesq equation. The load exerted by the plate on the floor surface is assumed to be uniformly distributed. Both normal and tangential loads are considered. It is also assumed that the floor fails in shear in a plate loading test. The equation may provide the mine operators with a useful tool to obtain from a plate load test of any plate size the UBC value required by the regulations.

Jan 1, 1993

By Kot Unrug

Fully grouted tensioned bolts provide a desirable compressive of laminated roof and bondage along the entire bolt length, at the same time the entire surface of the hole is protected against penetration of moisture. This is particularly important in shale strata, where, when left unprotected by a partial grout column, moisture penetrates to the annular space, and weathers rock around the bolt hole, having an easy access to the bedding planes which are strongly hydrophilic. The most desirable are thrust tensioned rebar bolts that can be installed with one procedure step, and can secure a desired pre-stressing. Such bolts have a forged conventional head and offer no hazard to miners in low clearance mines, whose number increase constantly due to the depletion of reserves in thicker seams. The presented field-testing program has been based on instrumented bolts allowing for monitoring of tension at installation and at a chosen time interval after. The tested system is based on rebar bolts installed with two speed Fosroc resin (fast at the top and stow at the bottom), and mechanical compression by the bolting machine at installation. Combination of several different washer -- plate arrangements have been tested. The results are reported with their interpretation, and recommendations directly towards strata control requirements, as well as the operational considerations for efficiency of installation.

Jan 1, 2002

3-D finite element modeling was performed to study the effect of bedding plane sliding and separation and tensioned bolt in layered root It was found that the major influence zones of a tensioned bolt are restricted to the neighborhood of the anchor points; that bedding plane sliding and separation releases the vertical stress in the roof above entry but greatly increases the horizontal stress within the first layer of the roof, and that there is a triangular zone between the two adjacent bolts at and near the roofline level that is beyond the influence zone of the bolts which is liable to fail if conditions exist. Based on the results, three possible modes of roof failure are proposed for the design of tensioned bolting.

Jan 1, 2001

By Bob Thomas

The roof drill operator productivity can be enhanced by the introduction of control technology The operators control station can be improved by considering human factors such as handle forces and control location. The smaller electrical controls can be located much easier than the traditional hands on valves. The operators control can he improved by using processors with programmable subroutines that assist the operator in the decision making process. For example to increase feed rate - to slow rotation - to avoid clogged steels - etc. The overall affect is to improve drilling and bolting consistency. The control system uses a dedicated processor packaged as an intrinsically safe control station. The roof drill operator is assisted in the drilling; arid bolting by a closed loop feedback system. The processor monitors the drilling parameters such as feed force, feed rate, rotation torque, RPM, drill position, and vacuum condition Based on the drilling parameters the processor executes pre-determined condition statements intended to enhance drilling The drill operator has an active display to alert him to the real time machine status. For example if the feed were to slow or delay due to a high vacuum level the display would show "high vacuum". The dedicated processor is available on the arm feed style machines, The prototype machines used programmable logic controllers to develop the feedback relations. The ability to record drilling parameters was extremely helpful in this development project. The extended use of the recording feature will aid operators in mapping the mine roofs Interpreting the recorded drilling parameters can reveal a void in the top or hard competent drilling The prototype machines also showed the importance of vacuum monitoring This feature is extremely valuable in softer materials, The benefits of the control system range from efficient use of the consumable bolting materials to uniform operator performance.

Jan 1, 1999

By James G. Schotsch

Unmapped abandoned mines are hazards for under¬ground mines, surface mines and surface structures. The Radio Imaging Method (RIM) provides a cross borehole technique to remotely detect abandoned mine works. RIM was used at the Buck Creek Mine, an underground coal mine, to determine the extent of unmapped mine works. The information provided by the RIM survey assisted the mine planners in developing their mine.

Jan 1, 1990

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

By Russell Frith

Probably the most expensive but least well understood roof falls in underground coal mining, are those that occur ahead of the powered supports in the centre of longwall face. Even small roof cavities result in the dilution of the ROM product and large cavities can stop the face for days if not weeks and the incurred cash flow losses can be catastrophic. Expensive and time-consuming remedial measures (e.g. PUR, cavity filling, removal of stone from the AFC by hand) are required and equipment wear and damage can be significant when the need to blast and convey stone is also considered. Through four major research projects within the Australian coal industry and numerous consulting assignments, the author has spent the best part of fifteen years attempting to identify and defme the various geotechnical and operational factors that determine whether a major roof cavity occurs ahead of the powered supports or a normal goaf fall occurs just behind them, the difference in operational outcome being enormous. The paper presents a condensed summary of the overall technical findings covering such issues as roof fall mechanisms, the measured influence of cover depth, periodic weighting of massive strata units and its quantification, the role of cut-throughs in chain pillars, face width and height, the role of the powered support and the influence (both positive and negative) of mine site operations. The big picture outcome is the view that high production longwall extraction is well-suited to certain geotechnical environments, is marginal in others and under certain conditions is fatally flawed and beyond the control of mine operators, although fortunately the latter category is a relatively rare situation. Various risk-based research findings in this regard will be given as a basis for future discussion.

Jan 1, 2005

By Syd Peng

In designing a proper roof support system, one must know the features of roof geology in advance of mining. The drilling parameters obtained during normal roof bolt installation cycle can provide a large amount of information on the roof geology when properly interpreted. It is also an economic way for identifying the geological features of the roof. This research develops a method for predicting the roof geology based on the drilling parameters obtained during roof bolting operation. From the results of a series of laboratory and underground tests, feed pressure is found to be a good indicator for identifying the voids/fractures and estimating the roof rock strength. This paper describes the method for determining quantitatively the location and the size of void/fracture and estimating the roof rock strength from the drilling parameters of roof bolter.

Jan 1, 2005

By Garth Kuhnhein

The Black River and Maysville underground limestone mines operated by Carmeuse Lime and Stone, Inc. in northeastem Kentucky are two of the largest underground limestone operations in the United States. The combined output of these two options has consistently averaged over 5.5- million tons per year for the last decade. The mines operated beneath 650-ft and 1,200-ft of overburden and are affected by high horizontal stress. In 1982 changes to the mine design were made at the Maysville mine to improve ground conditions. A series of design changes were initiated in 2002 at Black River to improve ground conditions. This paper reviews the results of the changes made at Maysville, recent horizontal stress measurements conducted at the Black River mine, and the recent modifications to the Black River mine design. Roof fall statistics at both operations are used to compare the distinctly different mine layouts used at the two mines.

Jan 1, 2004

By John C. Stankus

From research started six years ago, a new design criteria has been developed for mine roof bolt systems This design criteria precisely and quantitatively determines parameters such as bolt length, installed tension, and spacing that will produce the Optimum Beaming Effect (OBE) OBE, a new roof control term, is defined as the roof beam that has no separation above and within the bolted range with the shortest bolt passible Utilization of this design criteria has shown that in many cases shorter roof bolts with higher installed tension can produce a much stronger beaming effect (Stankus, 1995) This paper will first present a brief deception of how the design criteria was formulated From there, actual case studies where the design criteria was utilized will be discussed In conclusion, current and future research efforts will be presented

Jan 1, 1997

By P. Tsang

In order to deal with ground control problems such as roof falls, floor heaves and pillar failures in underground coal mines, a new pillar design method based on the "yield pillar" concept is proposed. Using the finite element model simulation, the functions and mechanisms of the "yield pillar" are studied. The important variables that affect the stability of longwall entry-pillar system are identified. To simplify the geological and mining conditions, a new pillar design method classifies the in-situ roof and floor conditions into four categories: 1) strong roof and strong floor, 2) strong roof and weak floor, 3) weak roof and strong floor, and 4) weak roof and weak floor conditions. The design formulae are derived based on the finite element model simulation, stability study and nonlinear regression analysis. The finite element models used consider the plastic failure properties and time-dependent behaviors of rock. To better organize the finite element model simulation, the orthogonal experiment design is used to arrange the finite element models and proved to be very effective. An application example is used to illustrate the basic design procedures involved in the new pillar design method.

Jan 1, 1993