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By Deno Pappas

During the past few years, the Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) has been evaluating developments in sealant support systems from a ground control perspective. The proper selection and use of a sealant material can significantly enhance ground control which may result in a decrease in worker related injuries due to ground falls. The purpose of this paper is threefold: to briefly review the current state¬of-the-art technology in shotcrete and membrane developments, to evaluate the preliminary findings of a long-terra underground study of various types of sealant materials, and to examine installation practices that are critical for an effective sealant material. The underground study utilizes NIOSH's Lake Lynn Laboratory Experimental Mine to evaluate the long-term performance of several types of shotcrete and membrane materials. Sealant performance to date have been evaluated on a regular basis over a two year period. Although the study is still ongoing, critical mining practices were identified that may seriously effect the bond of the sealant materials to the mine roof and rib; most notably, the importance of scaling and thorough cleaning of the rib prior to application. Also, results from an extensive series of Schmidt Hammer tests found that the shoterete increased in strength by 70% during the humid summer months.

Jan 1, 2002

By C. Babcock

This Bureau of Mines report considers some equations for mine pillar design from 1833 to the present. Most of the equations are of essentially the same kind, using pillar width to height relationships. The effects of end constraint and test volume size, that were implied in some of these early equations have received special emphasis in recent years. Experimental testing in the laboratory throughout the period given has been on prisms or cubes of coal or rock. Most of the present concepts have been known since about 1912. Wilson has recently emphasized the effect of end constraint in his confined core concept, but a number of other earlier relationships have indicated an awareness of the effects of constraint as well.

Jan 1, 1981

By John Stankus

Seam interaction in multiple seam mining has significant effect on entry stability. The remaining pillar in an old working usually creates a high stress concentration zone while the gob creates a stress release zone. A good panel layout should take advantage of the stress release effect created by old workings. This paper presents two case studies of multiple seam mining. The Jennmar Finite Element Analysis System (JFEA) is utilized to analyze the seam interaction effect and identify the mess concentration and relief zones Panel layouts are designed bad on the results of the finite element analysis Appropriate roof support system are also designed for different seam interaction areas.

Jan 1, 1999

By Thomas L. Vandergrift

The type and severity of coal mine entry failures are affected by the strength and stiffness properties of the roof, floor, end coal. Knowledge of the mine-wide trends of these properties Is valuable in developing effective ground control plans. Within the context of a larger effort to Improve entry stability In high horizontal stress fields, the U.S. Department of the Interior, Bureau of Mines, has evaluated three portable, on-site test devices to determine their suitability for identifying mine-wide physical property trends. The devices evaluated were the borehole shear tester, the Schmidt harmer, and the point load tester. Despite the lack of standardized data Interpretation procedures, each device provided useful on-site physical property data. The Schmidt homer was preferred for trend identification because of Its speed and simplicity. Schmidt hammer data were collected over a large area of an underground coal mine to compare calculated roof strength trends with a known geologic feature.

Jan 1, 1990

By Rocky Wu

Rock bursts are one of the greatest challenges to ground control in the mining industry. With increasing mining depth and mining scale, there are more and more industry requirements on yielding rock support. Since 2008, Jennmar has been conducting large scale research and development works to develop a technically reliable and cost-effective yielding rock bolt. This paper introduces a new yielding rock support: the Yield-Lok bolt. The bolt is characterized by the designed yielding ability to produce 150?250 mm (6?10 in) of deflection at 8?10 metric tons (MT) (7.9?9.8 t) loads for every 16.4kJ (12,096 ft lb) energy input. The bolt?s performance characteristics are consistent through multiple and varying amplitude of impacts. In this paper, the design criteria of the bolt and the principle of performance are described; the dynamic testing results are discussed; and the features and application of the bolt are presented.

Jan 1, 2011

By Larry Stolarczyk

A clear vision of the future coal mining industry was developed into a hierarchy of technology needs by the National Mining Association (NMA) and the Chief Executive Officers of the mining industry. Although improvements in safety and environmental factors were the main drivers in the technology roadmapping workshop sponsored by the Department of Energy, the agile mining machine of the future must efficiently extract coal from deeper, thinner, and more geologically complex coal beds. One of the challenges will be to develop sensors and software that enable clean coal cutting in a changing geologic environment. The roadmap identifies imaging ahead of raining and horizon control within the top ten needed technologies. For these technologies to be adopted by the mining industry, they must be proven to be effective while driving down mining cost and risk. This paper describes how three-dimensional imaging ahead of mining and horizon sensors enable improved ground control and safety in the mining of underground coal.

Jan 1, 2003

By Kanaan Hanna

Abandoned mines pose a serious threat to public health and safety, as well as the environment. When active workings approach old mine workings, miners could encounter significant hazards. Additionally, the presence of mined-out areas or voids underlying residential/commercial properties, transportation, and infrastructure systems can cause subsidence issues ranging from differential settlements and sinkholes to catastrophic collapse. All these features can result in adverse impacts in terms of cost and public safety. Traditionally, mine void detection and related engineering investigations for mine subsidence risk assessment and mitigation have been greatly dependent on systematic drilling and grouting backfill programs. The unknown location and condition of abandoned underground mines represent significant challenges to geologists and engineers in accurately evaluating subsidence hazards and developing appropriate mitigation measures. A comprehensive, multi-phase mine subsidence investigation was conducted at an abandoned western U.S. mine site. The mine operated in an approximately 8-ft (2.4-m) -thick coal seam using room-and-pillar extraction from 1919 to 1922. The mine lies at depths ranging from 50 to 100 ft (15 to 30 m), and it is overlain by critical pipeline and utility corridors. Any future subsidence/ sinkhole that occurs beneath or adjacent to the corridors could pose a significant threat, specifically to the structural integrity of the pipeline corridor. Therefore, the engineering geophysical investigation was initiated to determine the potential subsidence risk. This paper describes a recent success achieved by using integrated engineering geophysics in subsidence risk evaluation. A summary of significant results with emphasis on subsurface characterization, mine workings and voids delineation, modeling analysis, and high risk zones identification and mitigation strategy is presented.

Jan 1, 2011

By Shosei Serata

Serious floor heave of up to 2.4 m in a 2.4-m high mine entry was eliminated by applying the stress control method of mining, as a last resort, at the No. 5 coal mine of Jim Walter Resources, Inc., in the Black Warrior coal basin near Birmingham. Alabama. Underground observation of the first 3-room entry created using the stress control method is discussed here. The behavior of the test entry, which eliminated the heave problem, is analyzed in relation to similar studies conducted in a salt mine under similar ground conditions by utilizing finite element analysis.

Jan 1, 1984

In recent years, improvements made to the longwall system have accelerated at a faster pace than those made to the gate entry development system. As a consequence, unless planned years ahead of time, extending the panel length is nearly impossible without some innovative approach. At Cyprus Amax Coal's Emerald mine the G panel length was extended by 3,500 ft- by developing three entries across it to facilitate development of its headgate and tailgate entries simultaneously when the adjoining F panel was in operation. After evaluating alternative supporting methods and associated risks, determination was made to backfill these entries and crosscuts using a low strength flyash cement mixture prior to mining thru. Instrumentation was also used to monitor the stress change or front abutment stress in both the coal and the backfilled material ahead of the face. Longwall mining thru these three entries took about four eight-hour shifts and proved to be very successful. The designed flyash cement mixture provided adequate roof stability and caused no difficulty in the shearer's cutting while eliminating an in-panel longwall face move and minimizing the risk of roof or floor failure in the in-panel openings, or in the worst scenario loss of the face during the longwall cutting thru. This approach provided the intended result of extending G panel and recovered about one million tons of coal. This paper summarizes the engineering design of the cutting-thru project and flyash cement mixture, front abutment stress changes within the longwall coal block, backfilled material, and coal pillars ahead of the longwall face, and our underground observations

Jan 1, 1997

By Yi Luo

The severity of disturbances caused by longwall mining subsidence to various residential structures can be re¬duced. The success of such mitigation efforts depends on accurate subsidence prediction, correct assessments of the subsidence influences, and careful design and implementa¬tion of the proper mitigation measures. This paper presents the systematic approach involved in the subsidence mitiga¬tion efforts with a case demonstration.

Jan 1, 2003

By Hui Chen

The paper describes the use of a physical model technique to investigate the failure modes of mine tunnels with reference to the in situ stress field. The characteristics of stratification commonly encountered within UK coal mining conditions have been taken into consideration. The investigation has indicated that the weak floor strata in coal mining tunnels, under the action of high rock stresses, will be subject to the floor lift problem in both predominantly horizontal and vertical stress fields. However, the failure mode and mechanism varies between the two stress field patterns. In the predominantly horizontal stress field, the tunnel floor tends to fail by gradual bending of the laminations, whilst in the predominantly vertical stress field, the floor tends to fail by a shearing action with the failed material lifting in the form of a block. The failure mechanism is discussed with reference to mechanics theory.

Jan 1, 1993

By G. C. Xiao

A database, incorporating historical data from British mines, to study the factors affecting inflows from proximate aquifers into longwall workings, showed that a main casual factor was the immediate roof strata lithology. When this comprised principally stronger sandstone, there was a tendency for periodic inrushes associated with face weighting. This phenomenon is explained by the collection of water in bed sepa¬ration cavities, where free water connected to a high pressure head aquifer source, may be trapped.

Jan 1, 1990

By Zhou Ying

Based on the mining condition and roof lithology of the 2-3 coal seam at Gengcun Mine, the simulated materials modeling method was used to study the movement process and characteristics of overburden strata movement and to analyze the movement laws of overburden strata during top coal caving mining. Based on the modeling results, variation of the peak abutment pressure and its effect on the top coal's crushing and caving in the mining process was discussed.

Jan 1, 2001

By Jinsheng Chen

Longwall mining-induced abutment loads on the surrounding coal pillars can be grouped into two categories in terms of the relative position of the coal pillars and the longwall face. They are the front and side abutment loads. Even though a lot of research and efforts have been made to study the nature and behavior of these longwall mining-induced abutment loads, their influence zones and magnitude are still not well defined and fully understood due to the complexity of geological conditions and the longwall muting-induced overburden strata movement. The uncertainty about the mining-induced abutment loads often forces consultants and mining engineers to employ a conservative approach in designing coal pillars and entry roof control plans. The problems with this approach are that: (a) it increases CM development¬longwall retreat footage ratio, (b) it decreases coal recovery, and (c) it increases !Ill' operating costs. However, a more liberal approach may create safety issues and result in loss of production. Therefore, the importance of accurately defining the influence zones and magnitude of the front and side abutment loads induced by longwall mining can never be over emphasized. The authors in this paper attempt to define the influence zones and discuss the magnitude of the longwall mining-induced abutment loads by analyzing the field data measured at RAG American Coal Company's longwall mines. In addition, the relationship between entry stability and the balance among roof/floor conditions, pillar design, and roof control plan is also briefly discussed.

Jan 1, 2002

By R. D. Lama

The concept of rigid or yielding bolts is discussed based upon support requirements for excavations of equivalent geomechanical behaviour. The concept of equivalent geomechanical behaviour is introduced for comparison purpose where the rock mass properties are varied for a given depth (stress) so that the deformation of the roof without any support remains constant = 300mm. A simple experimental theoretical analysis for a uniform stress field acting on an axi-symmetric excavation is conducted to establish roof bolt requirements under different conditions such as width of a given excavation, time of setting of support etc. Results are presented in graphical forms.

Jan 1, 1992

By L. Z. Wojno

Tunnels in South African gold mines are developed at depths down to 3 600 m below surface where the virgin rock stress approaches 100 MPa and, on occasions, through rock where the field stresses exceed 150 MPa. In addition, many tunnels are subjected to increases in field stress of over 50 MPa during their useful lifetime. As a consequence, the walls of most tunnels are fractured causing the rock to dilate into the tunnel. Quantitative data on the extent of this fractured zone are presented. The function of support in these circumstances is to reinforce and maintain the integrity of the fractured rock. In 80 per cent of the 800 km of tunnels developed annually, the support methods used are capable of maintaining the stability of the tunnels. However, the remaining 20 per cent of tunnels is subjected to severer conditions where dilation in excess of 250 mm occurs and/or where the probability of strong seismic ground motion is high. In these tunnels, additional support is required. The type and pattern of support for both normal and severe conditions is described. In spite of the improved support used, damage to tunnels may occur. To minimize this damage, the concept of a rock reinforcing tendon that will yield and do work, when subjected to quasi-static or rapid deformation, has been put forward. Specifications for such a tendon are presented and the development of certain yielding tendons are described.

Jan 1, 1987

By Ed D. Doney

Based on the subsidence data collected through a comprehensive subsidence monitoring program con: ducted over two longwall panels in an Illinois coal mine, mathematical models have been proposed for predicting final and dynamic subsidence in the Illinois coal basin.

Jan 1, 1990

By Robert Trueman

This paper details the results of an assessment aimed at understanding the shield loading mechanisms associated with strata-related issues on a longwall face. Shield load cycle analysis theories developed by the authors were used to quantify the interaction between the shields and strata. Shield pressure data was back-analyzed using a version of the Longwall Visual Analysis (LVA) software that has been extended to enable efficient load cycle analysis to be undertaken. The software outputs enabled the shield strata interactions to be characterized. The results of the analyses indicated that the major cause of the roof falls was high-level periodic weighting, resulting in periodic shield overload. Exploration borehole data was analysed to identify and characterize the overburden unit that was the likely cause of the shield overloading.

Jan 1, 2011

By John M. Karhnak

The Bureau of Mines has a long history of research in Ground Control. For many years, this work was done at Bureau facilities by Bureau researchers. As the tiles changed, however, the area of research interest broadened, and additional methods of implementation, such as con- tracts, were added. 'Ibis paper outlines the organization, goals, and methods of implementation of the Bureau Ground Control Program, along with procedures for project selection. This program organization is followed by several specific examples of Bureau research that will be soon ready for use in the mines.

Jan 1, 1981

Coal mass strength properties must be estimated to obtain realistic estimates of coal pillar strength. The limitations of existing techniques for approximating these properties are discussed. The coal mass is anisotropic. The Hoek and Brown approach to rock mass property estimation is modified to allow for this. This approach requires the Rock Mass Rating (RMR) for the strata. In the normal case where bedding is sub-horizontal, these major planes of weakness have little effect on the strength of pillars. As a result of this the RMR must be measured parallel to bedding. This means that a significantly higher rating than that usually obtained for coal is observed. The use of mass strength property estimates in extant empirical relationships is outlined. At high width to height ratios, an acceleration in the rate of increase in pillar strength with increasing width to height ratio has been observed. The empirical relationship developed by Salamon (the so called squat pillar formula) takes account of this. This relationship was found to have significant limitations with respect to its universal application. Modifications arc made to Bieniawski's empirical formula to extend its use to large width to height ratio (squat) pillars. This new squat pillar formula has none of the disadvantages associated with Salamon's relationship. Strain softening was incorporated into non-linear finite difference numerical modelling to estimate pillar strengths. The technique outlined above is used to obtain peak coal mass properties. Modelled pillar strength was found to be sensitive to both peak and post peak mass strength properties. Post peak properties were found to be difficult to obtain and required some back analysis. Modelling results were used to estimate both the strength of squat pillars and parameters for use in a now squat pillar empirical strength relationship.

Jan 1, 1992