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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 Brian Clifford

A wide range of different cablebolt systems are available and in use in hard rock and coal mines around the world. The birdcaged cablebolt was initially used in UK coal mines in conjunction with rockbolts in the late 1980's. More recently other cablebolt configurations have been used. Innovations in cablebolt design in the UK have been driven by a demand to find a system as effective as the birdcaged cable, but which can be installed more rapidly and offer immediate support. A laboratory short encapsulation pull test (LSEPT) has been developed for both rockbolt and tendon systems which measures bond strength and stiffness in rock and assesses the full range of potential failure modes. This test is now used along with conventional 'gun barrel' and shear tests to provide full assessment of bolt and tendon performance. This test may be incorporated into a new performance based British Standard for rockbolts and long tendon systems. This paper describes the performance of some of these systems as being applied to UK and South African coal mines.

Jan 1, 2001

By K. Biswas

So far, all pillar design methods or approaches consider the coal seam as a uniform structure From some mine visits in northern Appalachian Region. it is found that some coal seams have binder or parting material within the coal scan with varying thicknesses and numbers- And it has also been found that these kinds of hinder materials tire prone to weathering when exposed to ambient mine moisture In this paper an attempt has been made to study the weathering susceptibility of parting materials and coal when exposed to moisture and a two dimensional finite element analysis has been carried out to study the effect of the presence of partings on the structural integrity of the coal pillar in terms of short-term and long-term stability.

Jan 1, 1995

By Syd S. Peng

A total of 209 cases of subsidence data over longwall panels in 16 US coal seems have been collected and built into a subsidence database. The database is developed under MS Windows environment. It uses a very user-friendly menu driven system. The empirical formulae for a number of commonly used subsidence parameters have been derived from those collected subsidence data.

Jan 1, 1995

By Christopher Mark

The Bureau of Mines is conducting research to assess the effectiveness of different chain pillar designs in maintaining gate entry stability. The study described in this paper was performed in a 1200 ft long, experimental headgate section which contained three pillar designs: - a three-entry, yield-abutment pillar' system, - a four-entry, yield-yield-abutment pillar system, and - a five-entry, all-yield pillar system. As the longwall mined by the experimental section, Bureau engineers monitored entry convergence, roof sag, and changes in roof quality. The results of the study indicate that the all-yield system performed at least as well as, and probably better than, the two abutment pillar systems.

Jan 1, 1988

By Nicholas P. Kripakov

The unpredictable massive collapse of roof in openings developed under apparently safe and stable roof conditions in coal mines of the United States has been of much concern for many years. One type of massive caving phenomenon is often attributed to cutter roof, a failure that initiates at one and/or both upper corners of an entry and propagates nearly vertically resulting in overall failure with little or no warning. This paper reviews current methods being used to control cutter roof and suggests new alternatives. The impact of basic parameters that include mine geometry, overburden geology, rock properties, and in situ stress conditions is discussed in the context of their relationship to this problem. The effects of various stress control measures are investigated with model analysis to gain insight into means of reducing the high level of stress known to exist at the entry corners of a West Virginia coal mine prone to cutter roof. The results are presented in the paper and the feasibility of their practical implementation is discussed.

Jan 1, 1982

By S. J. Jung

The goal of this research is to demonstrate how optimization methodology can be coupled with the finite element method for greater stability of underground mine openings. As a result of increasing mining depth and intersection of geologic discontinuities, mining environments are becoming more complex. Current design procedures are rapidly becoming inadequate. In this research, the finite element method has been coupled with optimization criteria to design more stable and safe underground openings

Jan 1, 1993

By Khaled Morsy

Longwall mining is one of the most widely practiced underground coal mining method. The behavior of the longwall gob is very critical in the understanding of the complex ground response to longwall mining. In the limited data available, the values of gob moduli used in numerical modeling ranged from 1,000 psi to over 300,000 psi. Such a wide variation in moduli would greatly affect the results of the numerical analysis. A better understanding of the behavior of the gob will allow numerical models to be more accurate for simulating longwall mining conditions. In this study, a gob model based on the Terzaghi's model was incorporated into the comprehensive finite element package, ABAQUS, to simulate the loading behavior of longwali gob material. A case study was used to verify the proposed gob model.

Jan 1, 2002

By Richard G. Burdick

The Bureau of Mines', Denver Research Center has been conducting research for the past few years on the use of resistivity methods to locate abandoned mine workings. As this work has progressed, it is apparent that in some cases, in addition to detecting the old workings; stoping, precursive to surface subsidence has also been detected. This paper describes the Automated Resistivity method and discusses sites in Colorado. Wyoming. and Illinois where the subsidence phenomenon has been observed.

Jan 1, 1982

By P. Forrest

Based on the results of mathematical analysis, photoelastic modelling, and case studies, the mechanisms of longwall under- mining interaction has been thoroughly investigated. The effects of using yield pillars in combination with rigid pillars in lower seam mine design have been found to provide significant improvement in ground conditions during undermining. Off- setting of solemnized longwall panel entries relative to upper-seam remnant structures has proven to be effective in alleviating adverse stress conditions.

Jan 1, 1988

By Gregory Hendon

Jim Walter Resources, Inc. (JWR), has successfully longwall mined for many years at depths ranging from 1200'-2500'. However, full pillar extraction has proven difficult and generally economically unfeasible. Overburden and redistributed stresses at these depths require relatively large pillars, which become unmanageable with most pillar extraction practices. The economic benefits of taking gateroad pillars as part of a longwall face have been recognized for some time, including increased life of mine recovery, elimination of belt moves, reduced overcast requirements, and improved longwall to continuous miner ratios. Until recently, the associated risks of pulling pillars prevented any extensive use of this concept at JWR. Due to geological, economic, and longwall repeatability problems, JWR accepted the risks, and undertook pillar extractions with longwall faces in several different applications. The applications included taking a stable pillar (250' wide) as a longwall panel, taking yield pillars on the headgate and tailgate of panels, taking yield pillars and stable pillars in the middle of a panel, mining through chain pillars in the middle of a panel, and taking a support pillar on the tailgate end of a panel. This paper describes the background and experience of pillar extraction with longwalls at JWR.

Jan 1, 1998

By Andrew J. Hyett

The ISRM commission on testing methods has recently presented Suggested Methods for rock stress determination (Kim & Franklin, 1987). This proposes that even if the magnitude of in situ stress is not sufficient to cause significant ground problems, the optimum shape, orientation, and layout of underground structures can be significantly affected by it. In the design of underground excavations, the engineer often requires a knowledge of the average in situ stress tensor in the rock mass. However, attempts to measure a stress state applicable to design have proved frustrating, and a high degree of uncertainty is introduced by the variability in results. In the belief that deduced fluctuations are random in nature, many separate measurements are often made, so that a more reliable stress tensor average, and some indication of the associated variance (see Dyke et al., 1986) can be calculated. Unfortunately, this approach is limited by economic considerations and, as with any sampling procedure, it is imperative to understand the sources of measured variability, in order to design an optimum measurement programme. Cundall(1987) itemised the main causes of stress variability as a. Instrument and measurement errors, and b. Natural fluctuations of stress from point to point, induced by the action of a boundary stress on the internal rock structure. At least four categories exist: i. inhomogeneous rock, bedding, intrusions etc.; ii. differential contraction, creep, or changes in rock properties with time; iii. proximity to faults or discontinuities; iv. cycles of tectonic activity that cause movement on joints. The latter was successfully investigated using the distinct element programme UDEC. In this paper, the contribution of items (iii) and (iv) (i.e. the items related to discontinuities) to the total stress variability will be discussed, for the kind of rock mass shown in Figure 1. Although many authors have concluded that rock joints have an influence on the stress in their vicinity, a more analytical approach is required if the stress distribution associated with different jointed rock masses is to be (i) compared, and (ii) related to routinely measured joint parameters. If this can be achieved then, prior to any attempt to measure stress in a jointed rock mass, a consideration of joint geometry, joint deformability, and the geological kinematics of joints (e.g. the magnitude and direction of previous movements), can place valuable `common sense' constraints on the expected stress system for minimal extra cost.

Jan 1, 1989

By Stanislaw Prusek

The present state of Czech and Polish coal mining industry is presented at the beginning of the article. This paper will inform the reader of the general data concerning hard coal reserves? number of mines, number of miners, output, number of longwalls, productivity, depth of extraction, and extraction methods?in both countries. The main part of the paper describes the methods of gateroad development as well as gateroad support characteristics (support types, gateroad shape and dimensions, distance between support sets, types of reinforcements). The development prospects of gateroad support in the Czech and Polish coal mining industry are also presented.

Jan 1, 2011

By Fangting Dong

The broken rock zone (BRZ) and the tunnel support theory are investigated. The relationship between the BRZ and the related factors, the interaction between the BRZ and supports, and the function of supports are analyzed in detail based on the authors' underground observations and model taste. A summary of the application of the BRZ theory in the past is also presented.

Jan 1, 1988

By G. Skybey

The Directional Drill Monitor (DDM) is an advanced instrument for real time in hole surveying. It was developed by Du Pont (Australia) to suit Australian gassy coal mining conditions. The instrument consists of three units: i. Down hole transmitter ii. Acoustic transducer iii. Data receiver processor unit (i.e. up hole unit). This paper describes the function of each unit in detail and highlights the importance of the DDM in long hole drilling efficiency. Surveying is carried out independent of hole depth. Each survey takes 2-3 minutes to complete, the frequency of surveying to control borehole trajectory is determined by the accuracy required for a particular hole. The DDM is currently being used successfully for long hole drilling in a number of collieries. A high degree of directional control has been achieved. Borehole trajectories were controlled to within +l¬2m of the planned path with intersurvey distances varying between 6 to 18 metres over distances up to 700 metres.

Jan 1, 1992

By Bruce W. King, David A. Newman

"Underground mines have always been an integral part of the limestone industry. However, beginning in the 1990s, the number of underground limestone mines has increased for many reasons including• Stripping ratios at surface quarries have become uneconomic.• Existing surface quarries have reached the mineral boundaries, and the remaining stone reserves are underground.• Surface access to the entirety of the limestone reserve is limited by development or infrastructure.• The once-rural setting of the surface quarry is now surrounded by subdivisions and development. An underground mine is less intrusive and takes the mining operation out of the public eye.• For larger markets, an underground mine enables year-round production without the sequence of overburden stripping in the winter to expose mineable stone for summer production. This is common at many surface quarries.Multiple-level operations are a natural extension of the decision to mine underground. Most limestone reserves, for example, the Tyrone-Oregon-Camp Nelson, Hermitage-CartersLebanon- Ridley, and Salem Limestones, are sufficiently thick to accommodate multiple levels. Multiple-level mines should be planned by considering each level separately and then integrating the individual levels into a larger mine plan. Failure to plan all the levels of a multiple-level mine in concert can result in ground control (back, pillar, floor, and inter-level sill pillar) problems attributable to stress transfer and interaction between the levels or the necessity to leave a thicker sill pillar between adjacent levels.Multiple-level mine planning begins with geology. Once the upper level back horizon is identified, the geology, rock strength, and physical properties in each level are used to• identify the optimal back horizon• quantify whether back support is required and, if so, the type, spacing, and length of the back support• determine the pillar centers• determine the total mining height, including floor shots"

Jan 1, 2016

By Baisheng Zhang

The definition of Ultra-close multiple seams as defined by the floor disturbance depth, h, due to mining in the upper seam is proposed: it refers to those coal seams that are spaced very closely, i.e. h,, the distance between seams, is small or equal to h, , and mining of any one of the seams will significantly affect the others. Those seams are called the ultra-close multiple seam group. Theoretical formula for defining it and practical examples are given in this paper. Underground measurements and numerical analysis show that in ultra-close multiple seam mining, the characteristics of ground pressure due to lower seam mining is different from that of single seam mining. The major differences is that the roof, as a result of damaged caused by the upper seam mining, is full of factures; the roof broke up in the belt entry and fell down easily; support loading at the face was small without clear periodic weighting phenomenon. Based on these results the roof structure model and roof control theory for ultra-close multiple-seam are established.

Jan 1, 2005

By Chao-jiong Hou

The necessity of using tensible rockbolt in the roadway with soft rock, high in-situ stresses, mining influence and other complex conditions was discussed, the structures of three kinds of tonsibte rockbolt and design principle were introduced in this paper. The tests of analogue material simulation under plane strain condition and underground industrial tests indicated that the effect of controlling deformation surrounding roadway by use of tensible rockbolt was good.

Jan 1, 1989

By Clifford McCartney

The PropsetterTM System has been designed and developed to cost effectively solve problems experienced with conventional underground supports used in tailgates, headgates, bleeder entries and as supplemental supports. The PropsetterTM System utilizes the strength and orthotropic properties of timber to achieve initial stiffness and controlled yield as the roof and floor converge. Utilizing a prestressing system developed in South Africa by our parent company, the PropsetterTM prestresses/preloads the roof and floor on installation. The PropsetterTM System consists of five integrated elements which are easily assembled to achieve the objectives of ? Improved support. ? Improved safety, ? Reduced cost, ? Reduced material handling, ? Improved installation productivity, and ? Improved ventilation.

Jan 1, 1995

By Thomas M. Barczak

Wood cribs are used extensively to stabilize mine openings by providing resistance to deflections of the immediate roof and floor and by supporting the weight of unstable rock masses. While the unit costs of these supports are relatively low, their extensive use results in significant costs to coal mining. The Bureau of Mines has developed a model that computes the load capacity of wood cribs as a function of the displacement of the crib structure induced by mine roof and floor convergence. This permits comparison of the loading characteristics of various crib constructions and enables systems to be designed with consideration of the load conditions imposed by the mine environment. The design method matches the stiffness, strength, and stability of the crib structure with expected rock mass behavior to determine a crib design and employment spacing that will provide the lowest cost support.

Jan 1, 1993