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By Jay Hilary Kelley

This paper proposes a modification of longwall roof support to meet new difficult mining conditions that are anticipated in the future. A gob canopy is a movable appendage attached on the rear of a longwall shield to counterbalance the troublesome forward downward force couple (FDFC). The gob canopy can be swung in both vertical and horizontal directions, powered by hydraulic cylinders. Several functions and applications are de- scribed

Jan 1, 1997

By Jan A. Nemcik

Steel mesh has been used successfully for many years to control friable roof conditions and prevent loose roof and rib material from caving into the roadway. Despite its extensive use, the installation of steel mesh is difficult to automate and many other products have been evaluated as an alternative for skin control. Ideally, the properties of these new products should be similar or better than those of the steel mesh. In particular, development of a strong and resistant shell that minimizes movement along the fractured rock and coal surfaces between the bolt anchors is recommended. A strong surface adhesion and the strength of a reinforced polymer skin can provide the necessary toughening mechanism required to enhance roadway surface support by forming a reinforced polymer/rock surface layer. Most of the Thin Spray-on Liners (TSL) evaluated in the mines are weak with slow curing times, and the plastic mesh currently used to support the coal ribs is relatively weak. Therefore, neither material can seriously compete with steel mesh. Currently, a new polymer product that cures in seconds and forms an instantaneous strata binder that surpasses the properties of steel mesh is under development at the University of Wollongong.

Jan 1, 2009

By John Bullock

Limestone and dolomite quarries were mapped to determine specific failure mechanisms for the various geologic and structural conditions. Algorithms for analyzing these mechanisms were incorporated into a software package together with ranges of physical properties such as joint shear strength, angle of internal friction, and cohesion for specified lithologic units. Reliability options using Monte Carlo sampling and sensitivity studies are included to evaluate alternative highwall designs. The package is designed to facilitate progressive stability evaluation of quarry highwalls as mining continues. This package includes an optimum design protocol that can be followed to avoid the potential of massive failure. The program was developed in conjunction with the quarry industry and is demonstrated using case studies.

Jan 1, 1993

One of the problems associated with the maintenance of roadways in underground coal mines is that of dealing with roadway closure. Sidewall weakening technique is based on the transference of the high stress zone further away from the vicinity of roadway. This transferral is carried out by weakening parts of sidewalls of roadway by cut- ting slot, drilling a series of parallel holes or blasting during development stage. In the paper, the effect of sidewall weakening on the stability of roadway is studied by finite element analysis and field experiment. The finite element analysis shows that sidewall weakening can control the development of yield zone around a roadway, improving the stability of roadway. It is also shown that if in- situ horizontal stress is lower or higher than the in-situ vertical stress, it is difficult to obtain roadway stability by sidewall weakening. The field experiment shows that roadway closure can be controlled by sidewall weakening with blasting.

Jan 1, 1990

By Robert Anderson

Advance knowledge of roof conditions can lead to improvement in roof control strategies that will reduce roof falls and ultimately will reduce injuries to mine workers. J.H. Fletcher and Company, in cooperation with West Virginia University, has been developing a system to meet these needs by collecting torque, thrust, and rotation speed data during drilling operations. Recent improvements in analysis algorithms to detect anomalies in the roof structure as well as various display enhancements for real time mapping and evaluation of roof structures have been incorporated into current Fletcher Information Display system. The unit has also been housed in an explosion-proof container to allow use in coal mine applications. Recently, the system has undergone extensive field trials in several limestone mines and one coal mine. During these trials, the National Institute for Occupational Safety and Health assisted Fletcher in verifying the accuracy of the roof mapping capability of the system by comparing the display information to bore hole scoping observations and drill corings. It was concluded from this assessment that the intelligent drilling system can complement scoping and core analysis to provide a more complete picture of roof strata and is capable of seeing features or conditions that are not always obvious in these more traditional methods.

Jan 1, 2007

By M. P. Nesterov

The method of predicting the time-dependent surface subsidence for gently-inclined potash deposits where total extraction using various sizes of yielding bamier pillars was discussed.

Jan 1, 1998

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 Adam Mackenzie, Dan Payne, Yass-Marie Rutty

"Historically, the Crinum mine has experienced significant falls of ground when longwall production slowed to prepare for recovery in weak roof areas. These conditions continue through the recovery process and result in both safety concerns and delays. When mine plans and exploration revealed that most of the mine's future longwall recoveries were located in weak roof areas, a decision was made to try pre-driven recovery roads as a solution to the problem. After completing eight pre-driven recovery roads with varying degrees of success and numerous lessons, the Crinum North mine now utilizes a modified Pre-driven Recovery Roadway (PDRR) to improve the longwall take-off process in weak roof areas. During mine development, a standard roadway is driven where the final recovery location of each longwall is planned. After the installation of secondary support, the PDRR is backfilled with a cement-flyash mix to provide support to the roof, and confinement to the ribs and floor of the roadway. The method has been refined over the last four years to provide greater strata stability and improved operational and safety performance compared with conventional take-offs at Crinum, and has resulted in a site record of longwall relocation in 11.5 days (pull mesh to picks in coal). This paper describes the evolution of the PDRRs from Crinum East to Crinum North, including lessons from initial attempts and changes to the secondary support regime, the operational approach during the final stages of retreat, the backfill strategy and plans for the future.INTRODUCTIONThe Crinum mine is the underground component of BHP Billiton Mitsubishi Alliance's (BMA) Gregory Crinum mine, located northeast of Emerald, Queensland (Figure 1).The Crinum mine has a history of roof control problems coming into and during longwall recoveries. Weak roof, combined with the slow mining, which is a consequence of preparing for equipment recovery, pulling mesh (which also creates tip-to-face issues), and the lengthy bolt-up process, often results in loss of immediate roof and subsequent roof falls. In fact, approximately 50% of the first 13 longwall recoveries (which were at depths of 180 to 200m) experienced roof fall delays, the longest of which took four months to mine the last pillar and recover the longwall. This recurring problem prompted the mine to investigate options to reduce or eliminate the problem."

Jan 1, 2016

By Luis Giraldo

Rock bolt installation characteristics near roof falls have been identified as contributing to failure. One documented and regularly occurring failure mechanism is loss of anchorage between the grout and the rock wall of the bolt hole. Key contributors to the integrity of the grout interlocking with the rock mass are the diameter of the hole relative to the diameter of the bolt, resin versus cement type grouts, rock type, and condition of the hole. Smooth bolt holes consistently exhibit less rock bolt load bearing capacity than rough walled holes. The goal of this study was to quantify the benefits of borehole conditioning and develop a systematic approach that would allow improvement of roof bolt performance. A new rock drilling technology, the Helical Drag Bit (HDB), was used in the early stages of this project to condition the borehole. The HDB cuts a prescribed helical groove in the borehole wall. When a bolt is grouted into a hole that has been conditioned using the HDB, the grout fills the helical groove and provides a stronger, more reliable mechanical lock between the rock and the grout. This paper presents results of a series of tests conducted in the laboratory and in the field. Short encapsulation pull out tests have demonstrated considerable increase in pull out strength and more consistent bolt performance when hole geometry is modified using the HDB. The results have also shown that greater load bearing capacity improvements are obtained in mines with weak roof rock, where it is most needed. Additionally, bolts recovered for examination suggest that hole conditioning during roof bolting reduces the effects of finger gloving. Therefore, mines with stronger roof rock could also benefit from hole conditioning. A new roof bolt that uses the HDB principle has been designed and tested. The new system works with existing bolting equipment requiring minimal modifications to the traditional procedure used for installation of fully grouted rebar bolts. The use of these higher anchorage capacity bolts will allow mines to reduce severe injuries and fatalities resulting from ground failures while maintaining or increasing productivity.

Jan 1, 2005

By Andrzej Walentek

In polish coal mining due to specificity of seams lying the multiple extraction is used. In this kind of mining, it is essential to take into account all geological and mining conditions which have an influence on stability and functionality of workings during their using. In view of above, for one of polish coalmines, by mean of Phase2 program, the numerical calculation were made for assessment of the influence of advancing excavation front on convergence of roadway located 30 meters above longwall was analysed. In order to verify developed prognosis, in analyzed roadway the underground investigations in scope of convergence measurement and changes of load exerted on working support were carried. As a result of these investigations it was found that real convergence on certain part of roadway was significantly larger then determined in prognosis. Additional calculations and analysis showed that the main cause of differences was underestimation of influence of dynamic load, which was an effect of numerous low energetic rockmass tremors. These tremors, related to excavated longwall, were located in close proximity of roadway in its part where the biggest convergence occurred. In current practice, in this type of convergence prognosis, the maximal value of dynamic load exerted on roadway resulted from single rockmass tremor is taken into consideration. Quantitative consideration of all tremors including their location is impossible at the stage of convergence prognosis because of lack of this type of data. This paper presents results of roadway convergence prognosis compared to results of underground investigation. Conclusions formulated on that base allow in future similar designing works for obtaining more accurate results.

Jan 1, 2013

By Mark F. Sindelar, Yi Luo, Hua Jiang, Michael T. Shook

"Convergence of roof and floor in underground mine openings is a common occurrence. This convergence not only adversely affects the ability of workers, equipment and supplies to travel through the mine, it also reduces the effectiveness of the mine ventilation system, which is essential for the dilution of methane gas and airborne respirable dust. While installing secondary standing supports to control floor and roof convergence, such supports, by nature, partially obstruct a portion of the airway. These added obstructions inhibit the ability of the ventilation system to operate as efficiently as it could by increasing the resistance in and reducing the cross-sectional area of the airway.This study introduces and demonstrates the benefits of The Eye CAN™ standing roof support, which controls floor and roof convergence and is less obstructive to air flow than conventional wooden cribs. Laboratory findings show that the normal resistance of a supported lined airway is reduced by using this new product from Burrell Mining Products, Inc., while providing the same roof support characteristics of an established product-The CAN®. Load vs. displacement curves generated from laboratory tests demonstrated that this new product behaves with the same roof support characteristics as others in The CAN product family. Ventilation data gathered from a simulated mine entry was then used for computational fluid dynamics (CFD) modeling. The CFD analysis showed an improvement with The Eye CAN versus other accepted forms of standing roof support.This proof-of-concept study suggests that, when using this new product made by Burrell Mining Products, Inc., not only will the convergence from the roof and floor be controlled, but airway resistance will also be reduced.INTRODUCTIONOne of the basic elements of underground mining is the necessity to support the mine roof. It is well-known that easier reserves have been mined so that today's underground mines present challenging conditions. Factors influencing roof support decisions include mine depth, mining method, overburden composition, and agency regulations. Mining engineers, therefore, have to balance three objectives: safety, engineering, and cost. While safety is obviously paramount, and engineering often complements safety, the cost effectiveness of any product, method, or device warrants additional consideration in today's harsh business climate. Avenues for adaptation and improvement are found at the intersection of the three objectives."

Jan 1, 2016

By Anastasia M. Suchowerska

The design of effective ground support requires a sound understanding of the pre-mining state of stress in the stratum to be mined. Irregularities in the stress field encountered during multiseam mining are governed primarily by the previous mining in the surrounding strata. The aim of this study is to quantify the effects of rock mass transverse isotropy on the vertical stress redistribution in the rock strata beneath supercritical longwall panels. Wilson?s equations for determining the stress distribution induced in and beyond a single longwall panel after it has been mined have been adopted, and a plane strain finite element analysis of an elastic medium, together with the stress changes predicted by Wilson, was used to calculate the final pre-mining stresses in the underlying strata. Values of the independent shear modulus (G¢) of the rock strata, ranging from 0.1 to 1 times the isotropic shear modulus (Giso) in the plane of the strata, and a Poisson?s ratio (?) of 0.25 have been investigated. The key finding of this study is that the predicted magnitude of vertical stress in the transversely isotropic strata beneath a mined supercritical longwall panel is larger and changes more quickly with horizontal distance than is the case for an isotropic rock mass. This finding is of significance for the design of ground support measures in multi-seam mining operations.

Jan 1, 2012

By Eric Westman

Ground control problems caused by multiple-seam interaction frequently sterilize coal reserves. Interaction mechanisms differ for overmining and undermining and are controlled by mining, spatial. and geologic conditions at each site. A multiple-seam interaction analysis software program, UGLY (Uppersearn Gateroad Longwall Stability), has been used to evaluate sterilization of coal reserves due to overmining at three locations. Four case histories were reviewed to determine the effectiveness of improved ground control in mitigating interaction effects.

Jan 1, 1997

By Thomas M. Barczak

The performance characteristics of Mobile Roof Supports (MRS's) were evaluated through full-scale testing in the Strategic Structures Testing Laboratory at the National Institute for Occupational Safety and Health, Pittsburgh Research Center. Safety issues pertaining to the operation and design of these supports for pillar extraction during retreat mining are examined. A unique load frame, called the mine roof simulator, provided a realistic simulation of mining conditions by inducing vertical, horizontal, and lateral loading on the support. The purpose of these experiments was to determine the capabilities and limitations of the supports and to investigate factors that influence the measurement of loading and loading rate. An evaluation of the support design and load conditions that can cause support failure or loss of support capacity are discussed relative to the laboratory tests. In general, lateral loading perpendicular to the longitudinal axis of the canopy is most severe, although horizontal loading in the direction of the long axis of the canopy can also produce critical loading in some cases. The experiments indicate that both setting force and leg pressure measurements are influenced by the staging of the leg cylinders. The implications of these factors on load rate measurement are evaluated. A comparison of MRS capacity and stiffness characteristics is made to conventional timber supports.

Jan 1, 1997

By Asmaa Yassien

Finite element models were developed to study the effect of tensioned and fully-grouted bolts on the stability of the mine opening in a three-entry development system using a typical geological column for the Pittsburgh Coal Seam. The models were used to study the effect of each bolt type on the stress distribution in the immediate roof, yield zone development and the vertical deformations. Comparison of all of those features show that the effect of a tensioned bolt on the stress distribution in the immediate roof is restricted to limited areas, whereas the fully-grouted bolt has a much wider effect on the stress distribution. Both types of bolts can reduce the yielding and deformation in the immediate roof to different degrees. The fully-grouted bolt reduces the sliding along the bedding plane more than the tensioned bolt. Both types of bolts were able to maintain the bedding planes without separation. The induced forces in both types of bolts are analyzed. A rapid increase in the shear force and bending moment occurred in the fully¬grouted bolt due to the relative motion of bedding planes.

Jan 1, 2002

By E. Bane Kroeger

Rib failure is a hazard that has resulted in many injuries and fatalities in underground coal mines, especially in soft coal seams. Conventional rib control methods including rib boarding, shotcreting. steel meshing and geogridding, are fairly expensive and time consuming to install. Some methods, such as rib boarding, have met with marginal success at controlling rib failure. Under this research, the Rib Strap System (RSS) introduced by Dr. Chugh in 1999, was modified and new components were added. Both bench scale and full size prototypes were tested in the labs at SIUC to determine the best materials for fabrication and easier methods for assembly and installation. This laboratory research has developed new methods of securing the strap ends to smaller rib hoards, a new device to post-tension two straps together, and a technique for replacing straps without having to drill new rib bolts or for retrofitting straps to rib hoards that are already in place. The method used to secure the strap end to rib boards developed during bench-scale testing was dubbed the wrapped staple technique. It is very simple, requiring only 10-15 staples, with the end of a strap secured to a rib board in about 45 seconds. Several different prototype designs for a new device to join the strap ends together and tension the strap were fabricated and tested in the labs at SIUC. The device, dubbed the rib strap buckle, was very easy to install and held the straps firmly. One specific buckle design worked extremely well during testing and exceeded all expectations. The laboratory research also developed a method of replacing a damaged rib strap that can also be used to add rib straps to existing rib boards. The technique uses nails, spacer blocks, and band clamps to secure the new board and strap. This technique was developed to allow one person to replace a strap using only a hammer and screwdriver, negating the need to drill and install a new rib bolt using mine equipment such as a roof bolter or scoop. In addition to the technical feasibility, the cost of materials for the rib strap system was compared to the rib boarding method currently being used at a mine in the Illinois Basin. The mine is using 20 rib boards on each pillar at cost of about $223.60. The costs for the rib strap system depend on the materials chosen for fabrication but for the base case, were about $97 per pillar.

Jan 1, 2004

By John L. Edwards

The underground coal mines in the Lake Macquarie area of the Newcastle Coalfield, New South Wales, Australia are constrained by surface subsidence restrictions as low as 150 mm. The mining environment is shallow, generally between 100 m and 200 m, often multi-seam and commonly overlain by residential development or tidal waters. Most mines use partial or panel and pillar extraction techniques to meet the subsidence restrictions and allow maximum resource recovery. This paper gives an overview of the results of a recent two year government funded research project aimed at developing a mechanistic, geomechanically based mine design criteria for subsidence control. Three extraction layouts were monitored in detail to assess the relationship between mining geometry, pillar behaviour and surface subsidence. Pillars at each site were instrumented with stress cells, convergence monitors and extensometers. Field data was used to initially calibrate, and subsequently, verify stress and displacement predictions made by the three-dimensional displacement discontinuity code, SUBSOL. Back-analysis of the modelling results indicate SUBSOL's capability as a tool to satisfactorily predict surface subsidence magnitudes and pillar loads for alternative pillar and panel layouts.

Jan 1, 1993

By Wolfgang Schott

The propagation velocity of in-seam seismic (ISS ) waves depends on frequency which is also known as dispersion. This dispersion is not only determined by the thickness of the coal seam and its dirt hand content (i.e. thickness and position), but also by the rock type and its condition (soft or compact) above and below the coal. A coal panel completely surrounded by roadways was investigated using the in-seam seismic transmission survey technique. Along the roadways the immediate roof was either sandstone and shale- The seam thickness varied between 1.20 m (sandstone in the roof) and 1.40 m (shale in the roof) and a fault system (normal faults) crossed the panel with a total throw of about the seam thickness. Below this coal seam another coal scam with a thickness of about 80 cm exists. Its interval shown by exploratory drilling changes from about 60 cm to more than 6 m (scam splitting). Tomographic inversion was applied to the dispersion of the measured ISS waves and velocity distributions were calculated at frequencies found to he most sensitive to changes in the seam thickness and the distance to the coal seam below. By correlating the velocity distributions with known values along the roadways approximate distributions of the coal seam thickness and the seam splitting within the panel were produced. The mining of the coal panel had already been finished and comparison was made between the survey results and the actual situation as far as it was documented. Prediction by the seismic survey was partly in good agreement with the actual held conditions or showed at least the same trends.

Jan 1, 2003

By Vincent Scovazzo

This paper compares the two most common pillar design equations used worldwide by John T. Boyd Company; the Mark-Bieniawski Equation, an empirical approach, and the Wilson Equation, an analytical approach. The equations are compared in two ways; by their stress profiles and by the Mark-Bieniawski Equation stability factor. Contrasting the two equations based on the visual comparison of their stress profiles is straight forward but evaluating their resulting pillar designs is problematic. Complications arise due to the differences between the use and development of stability factors for empirical equations and safety factors for analytical equations. To work around these differences, comparisons are made between the two equations by first calculating pillar sizes using the Wilson Equation and then determining the Mark-Bieniawski Equation stability factor for these pillars. Pillars designed using the Wilson Equation were based on site-specific data from 23 mines located in the Americas and Australia. Comparing sizes of pillars designed by the Wilson Equation to the Mark-Bieniawski Equation; the Wilson Equation pillar sizes were smaller in 77% of the cases, similar in 3%, and larger in 20% of the cases.

Jan 1, 2008

By Taghi Sherizadeh, Pinnaduwa H. S. W. Kulatilake

"This paper examines the effect of different geological and mining factors on roof stability in underground coal mines by combining the field observations, laboratory testing, and numerical modeling. Observations at an underground coal mine that uses the room-and-pillar mining method showed several failures of the immediate roof, and is used as a case study. Three-dimensional distinct element analyses were performed to study the intrinsic roof failure mechanisms and to evaluate specifically the effect of in-situ stress magnitudes and directions, discontinuity mechanical properties, and variation of pillar to room size on roof stability.INTRODUCTIONThe lithology of the immediate roof with respect to the underground openings plays an important role in stability of the roof in underground excavations. Laminated, or thinly bedded shale, is one of the most frequently seen overlaying strata above the coal deposits. The bedding planes and interfaces have very low or close to zero tensile strength in the direction perpendicular to the bedding planes, and their shear strength is much lower than of the rock layers. As a result, the bedding planes are much weaker than the rock layers. Therefore, due to the stress concentrations caused by excavations, slippage and separation can easily happen along the bedding planes before initiation of failure within the rock layers.In the past, several attempts have been made to understand the mechanism behind the roof failure in underground coal mines. Most of these earlier attempts were based on empirical techniques and continuum based numerical methods that have limited capability in simulating the response of discontinuum rock mass behavior to excavation. Different factors such as the magnitude and direction of the horizontal in-situ stresses, type and mechanical properties of roof rock, surface topography, geological anomalies, gas pressure, slippage and separation of bedding planes at the roof and floor, entry width and excavation sequences are identified as key factors that could significantly impact the stability of the roof. Among all the aforementioned factors, in spite of its importance, the impact of slip and separation of bedding planes on roof stability has not received enough attention from researchers. In a few available numerical modeling studies, the bedding planes are modeled using the continuum based numerical codes such as ABAQUS (Chen, 1999) and FLAC3D (Ray, 2009). However, since the bedding planes among the roof layers become discontinuous during the numerical analysis, this behavior is most likely not be accurately captured by the continuum based numerical models. Chen (1999) conducted finite element analysis to study the impacts of the roof and coal interface slip and the bedding planes slip in the roof on the stress distribution in the roof strata by using the finite element analysis software named ABAQUS. To avoid the complications associated with the finite sliding between two deformable bodies, he limited his study to the two-dimensional finite element analysis (plane strain). He concluded that the slip and separation of interfaces and bedding planes have significant impact on the behavior of the roof strata and without considering the interfaces, the roof bolt functions cannot be simulated properly by using the numerical analysis methods. Gadde and Peng (2005) performed three-dimensional finite difference analysis using FLAC3D software with strain softening material behavior to simulate the cutter roof in an underground coal mine. Their numerical model was fully continuum and no bedding plane was included in the model. They suggested that the strain softening material model can help to simulate the cutter roof failure up to a satisfactory level (Gadde and Peng, 2005))."

Jan 1, 2015