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By Francis Kendorski

One of the most dangerous events in underground coal mining is unexpectedly encountering water inrushes from undetected abandoned mines in the same seam. The surest and most confident method is probe drilling either from the mine or from the surface. However, drilling is expensive and may even miss the suspected mine voids entirely by drilling through pillars. Many operators rely upon one or more of several remote sensing techniques for detecting mine voids. However, mine "voids" often are not air- or water-filled open cavities, but are collapsed, rubble-filled chimneyed columns in the strata. Geophysical techniques, such as seismic reflection and refraction, electrical resistivity, magnetics, ground-penetrating radar, and others, often assume a continuous or fractured rock mass that has varying properties which provide the signatures that allow discrimination of one strata from another, or of strata from voids. However, a rubble-filled cavity has rock block-to-rock block contact throughout its volume and can still respond as a continuous rock mass with the rock blocks allowing signal transmission or mass detection, rather than a void space. Hydraulically, a rubble-filled cavity is essentially as transmissive to water as an open mine void. Thus, the problem of detecting a mine void with confidence is significantly compounded.

Jan 1, 2004

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 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 Andrew P. Schissler

Cyprus Shoshone is an underground coal mine in southern Wyoming owned by Cyprus Coal Company, a subsidiary of Cyprus Minerals Company. To date, seven longwall panels have been extracted from the pitching seam under increasing depth of cover from 150 ft to 1200 ft. The purpose of this paper is to describe the geology, rock mechanics needs identification, design, testing and successful application of the current headgate/tailgate cribbing system. The immediate roof of the Shoshone is unique in that the first 50 ft of lithology is interbedded mudstones. The mudstone is slickensided, highly friable and has characteristics of unconsolidated fill relative to self-spanning and beaming abilities. Finite element modelling analysis of the overall geomechanical environment showed the mudstone to yield under applied longwall abutment stress. Roof bolt system design methodology also pinpointed the relative weakness of the mudstone roof. In-mine entry vertical convergence and weight response from the mudstone identified the need to design a crib which had superior weight capacity over a wide range of convergence. Design of a structure ensued to achieve the superior strength properties of wood parallel to the grain and the displacement properties of wood perpendicular to the grain. Testing of the prototype structure along with other alternative wooden cribs was done at the U.S. Bureau of Reclamation facility in the Denver Federal Center, Colorado. The test results verified the design. Crib placement pattern was chosen to maximize support with a confined core crib under the high convergence area. The design has been successfully installed in over 5,000 ft of headgate entry in the 11 left panel.

Jan 1, 1993

By James Pile

Since C.C. White introduced coalmine roof trusses in 1967. many different systems have evolved. Beginning in the mid 1990's, several types of bar/cable truss systems have been introduced. A new and innovative bar/cable truss recently introduced offers several advantages. By eliminating many small parts, and simplifying the installation, this new truss increases operator safely. In order to fully benefit from the new truss system, a closer look at the individual components and their interaction was undertaken to understand critical implications on preload and overall support capabilities.

Jan 1, 2004

By Hamid Maleki

A new failure criteria is proposed for assessment of roof stability in underground sedimentary rocks. Unlike other stability assessment techniques, this method is simple and practical, having improved mine safety and productivity in several mines. It is shown that roof movements accelerate to a critical rate prior to actual roof collapse. Critical rates vary between 0.025 inches per day to 0.2 inches per minute depending on rock mass - pillar mechanical properties. Specific critical rates are determined for three rock mass - pillar categories which are common to mining in sedimentary rocks. Time-to-caving varies from one minute to ten days on rock structural controls and type of primary support utilized.

Jan 1, 1990

By David L. Kuck

This paper describes a system for providing structural, support in underground mines, employing water ice for pillars. packwalls and brattices. It consists of a refrigeration brine chiller, brine circulation system, and disposable plastic containers. The system provides for the timely freezing of uniform ice pillars, and for ice maintainance and/or melting on demand. Unique, useful ice properties include: expansion upon freezing, resilience to dynamic loads, and plasticity. Ice will not dilute or contaminate coal or ore. Economic advantages include: 1) lower labor and material costs (water can be piped to the ice pillar site through mine-water, spray-water, or drill-water lines); 2) replacement or supplementation of timber. concrete or other roof support equipment; and 3) potential use of ice brattices as ventillation control; and 4) conversion of ice bratices to ice pillars upon retreat. See Figure . A safety advantage for personnel and equipment is that ice pillars can be placed in hazardous sites, by remote control, if neccessary, without human entry. All filling, freezing and melting operations can be controlled from a safe distance.

Jan 1, 1987

By Ian W. Farmer

A description of the factors causing outbursts and rockbursts in coal mines is given. The mechanics of both outbursts and rockbursts can be described in terms of conversion of stored strain energy in the coal and surrounding rocks into kinetic energy through failure. The major difference between the two phenomena is that gases adsorbed onto the coal structure and desorbed during deformation contribute to the energy available during outbursts and may also affect the loading and unloading characteristics associated with failure. The amount of energy involved and the type of phenomenon are related principally to gas content depth and rock behavior.

Jan 1, 1988

By Leigh J. Wardle

Significant coal reserves in New South Wales lie under areas with steep topography such as valley slopes and cliffs. Subsidence predictions are difficult to make for these locations as existing empirical databases cannot cover all permutations of topography and mining geometry. Two dimensional numerical model alternatives are of limited applicability. Consequently a novel three-dimensional numerical model is developed including the detailed panel layout, realistic rock mass properties including the full stratigraphy, goat behaviour and actual surface topography. Model results show the same trends as subsidence measurements from Baal Bone Colliery in the Western Coalfield of New South Wales

Jan 1, 1992

By Yunxin Qiu

A new technology has been introduced - the use of the High-Water Solidifying Material for providing pumped roadside supports to maintain a roadway for re-use in longwall retreating. As a gob edge support, its initial use at Hebi No. I Colliery was successful, providing acceptable performances in support and control of convergences and lateral closures of the gob-edge roadways. This acceptability applied to both first use and re-use when the roadway became a gateroad for the adjoining longwall, remaining stable throughout the associated loadings and convergences. The technology promises economic and social benefits.

Jan 1, 1992

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 Victor V. Nazimko

Longwall mining technology without interpanel pillars is feasible from a geomechanical point of view. A set of pillarless longwall layouts are proposed including tailentry pillar recovery, headentry pillar recovery and drivage of a new tailentry, partial headentry pillar recovery and combination mining in which advancing panel entries are used for retreating panels. The following mitigative measures are proposed in order to enhance the stability of entries near the panel: driving crosscuts obliquely to the longwall face line, advance support of entries and crosscuts, localized destressing of the chain pillars by horizontal slotting, and partial stowing of the gobs behind the pillars being recovered. A new technology of recovering barrier pillars on the setup entry side is also proposed. This technology uses the same regular longwall face for interpanel pillars extraction.

Jan 1, 1994

By Craig Compton

The The National Institute for Occupational Safety ane Health (NIOSH) Pittsburgh Research Laboratory (PRL) is continuing to investigate the behavior of fully grouted roof bolts in the weak roof rock of the Safety Research Coal Mine (SRCM). This paper describes the results of three studies: A series of 24 pull tests of bolts installed fully grouted and overcored to leave 12 inches of grouted bolt. The tests compared the pull-out performance of offset-head roof bolts with that of standard 518-inch bolts; A second series of 24 tests that compared the pull-out loads obtained by bolts installed with 1 R of resin (a Short Encapsulation Pull Test or SEPT) to that of fully grouted bolts overcored to leave only 12 inches of grouted bolt, and An investigation into the pressures generated during the installation of fully grouted resin bolts. All of the bolts installed and pulled were overcored and removed from the roof after the pull tests to allow inspection of the resin. The first study found that there was no significant difference in pullout load between the offset-head bolts and the standard bolts. The second study, comparing the SEPT bolts and the partially- overcored fully grouted bolts, indicated that SEPT can significantly underestimate bolt anchorage grip factor, suggesting that the SEPT is a conservative measure of actual bolt performance. Significant loss of resin to cracks in the roof was observed in the bolts overcored in the SRCM. The third study explored one possible explanation for the resin loss, high pressures generated during bolt installation. These tests employed bolts installed in strain gauged steel tubes. The results confirmed that significant pressures (greater than 4,000 psi) can be generated during bolt installation. The results of the pressure tests, combined with the observations of resin loss in bolts installed in the SRCM, suggest that under some circumstances resin loss and under-encapsulation could take place and lead to degraded bolt performance. Glove fingering was observed in all recovered bolts to various degrees. The limited comparative data suggested that the effect of glove fingering ranged from slight to moderate. Complete mixing of the resin was observed on all of the 40 bolts recovered (28 fully grouted and 12 SEPT). Australian and U.S. resins were compared and the Australian resins were observed to have much smaller catalyst compartment areas and lower resin viscosities. These differences are suggested as potential explanations for occasional resin mixing problems noted in Australian resin bolt installations.

Jan 1, 2005

By J. A. Nemcik

Although floor failure at longwall faces has been traditionally associated with a soft floor, in many csses floor buckling has been observed just ahead of the longwall supports, despite the floor being a competent rock The floor failure is associated with the presence of weak bedding planes in the floor and the movement of strata towards the goaf opening. This paper presents the principles of the floor failure based on numerical modelling and field observations and outlines the practical approach to estimate the risks involved with floor failure.

Jan 1, 1995

By John C. Stankus

The success of cable bolts for mine ground control have been well documented in papers presented at this conference too numerous to reference. Cable bolts have proven effective over the years mainly utilized in the form of supplemental support. Some limited work has been conducted on the use of cable bolts as primary support This work has been mainly confined to the West and the bolts utilized were of a non- tensioned type MSHA requirements for primary support have been that the bolt system he tensioned or if non-tensioned, then fully encapsulated with resin. Many operators are now interested in using cable bolts as primary support Further, despite the enormous success of cable bolts, there have been questions raised about the longevity and corrosion characteristics of the cable itself particularly for long tern applications. To improve the corrosion characteristics of cable bolts, galvanized cable has been used for some applications However. although improving the corrosion resistance of cable, galvanized cable for mine ground control does not provide complete protection under certain types of situations and mine environments. Realizing a need for a cable bolt that can be used for primary support and provides consistent installed tension, complete corrosion protection, and a fast installation, Keystone Mining Services. LLC (KMS), an affiliate of Jennmar Corporation, has developed the INSTáL CableOx system. This paper will detail the development and specifics of this system including two underground case studies

Jan 1, 2000

By Kikuo Matsui

In a difficult economic environment, the coal mines in Japan have been making extensive efforts in order to survive by rationalizing the mining operations. Introduction of a new longwall mining system is one of these efforts to establish the rational production structure. In the high production longwall mining system, gateroads should be developed rapidly and maintained effectively during their lifetime. Strata bolting support system has been employed in the gateroads to raise their stability levels and reduce their maintenance cost. However, excessive gatemad closure caused by longwall mining is still occurring, impeding the passage of equipment, personnel transportation, and ventilation air in the longwall panels. This paper describes the development system of gateroads and proposes a new mining method to control the excessive roadway closure problem.

Jan 1, 1995

By Kewal K. Kohli

An investigation was performed to determine the sequence of events which caused the 1987 surface subsidence and related damage to several homes in Walker County, Alabama, USA (see Figure 1). Surface affects compared to mine maps indicated the subsidence to be mine related. However, two coal seams had been worked under this area. The upper seam, the American seam ranged from 250 to 280 feet beneath the surface in the area in question. It was mined-out before 1955 by room-and-pillar method leaving in place narrow-long pillars to support the overburden strata, and abandoned in 1955. The lower seam, the Mary Lee seam, ranged from 650 to 700 feet beneath the surface. The Mary Lee seam had been abandoned in 1966 and subsequently became flooded. The dewatering of the Mary Lee seam workings in 1986 caused the submerged pillars to be exposed to the atmosphere. Due to multiple seam mining and the fact that workings had been inundated then dewatered, a subsurface investigation ensued to determine the sequence and ultimate cause of surface subsidence. Core sample tests with fracture analysis in conjunction with "down-the-hole" TV camera inspections provided necessary information to determine that the subsidence started in the lower seam and progressed through the upper coal seam to the surface. Evidence from the investigation program established that dewatering of the lower seam workings caused the marginally stable support pillars and the roof to collapse. This failure triggered additional subsidence in the upper seam which broadened the area of the influence at the surface.

Jan 1, 1992

By Qiang Fu

ABSTRACT This paper aims to identify the main issues concerning top-coal loss and the optimum drawing interval of LTCC (Longwall Top-coal Caving) mining by underground tests and DEM (Distinct Element Method) modeling. By extracting a new roadway in the goaf of the mined LTCC Panel No.4318 in Wang Zhuang coal mine of Lu-an CMA (Coal Mining Administration), and underground observation of the distribution of loss coal and broken rock in different sections of the roadway, this paper analyses the composition and presents a 3-D distribution map of the loss coal in the exploration roadway. This study indicates that the DEM modeling of broken top-coal is reasonable and scientific as it can simulate the movement of broken coal and waste rocks and determine the optimum drawing interval of top coal.

Jan 1, 2002

By Steve Signer

The San Juan Mine and the National Institute for Occupational Safety and Health conducted a study to measure how development mining effected bolt loads. Twelve fully grouted, instrumented roof bolts were installed at two three-way intersections as part of the standard bolting pattern. Newly developed miniature data acquisition systems (MIDAS) were used to measure bolt load changes during initial face advance and cross-cut breakthrough. The effects of cut placement and depth on roof bolt loads were studied This test showed how bolt loads increased at five positions along the bolt length during initial mining. Both entry advance and cross-cut breakthrough produced a similar percentage of increase in bolt loads. Geologic differences between the test sites were probably responsible for the differences in amounts of bolt loading. The test site with more top coal and a higher rock quality designation (RQD) had lower bolt loads.

Jan 1, 2004

By Winton J. Gale

Coal mine pillar design formulae available to mine planners are typically based on relationships determined for specific site conditions. Application of these methods is difficult when other factors, relating to mining or geological conditions, not specifically addressed in these relationships are encountered. A review of pillar design methods, coal strength properties and relevant field monitoring of pillar behaviour has shown pillar strength and deformational characteristics are extremely sensitive to in situ behaviour of the rock and other material components that make up the coal pillar system. A method has been developed that has been found to provide excellent correlation between existing field data using the strength and deformational characteristics of materials determined in selected laboratory tests. This method is based on definition of the leading conditions, accurate modelling of the strength, confining conditions and deformation occuring in the roof, floor and coal seam and definition of the in situ strength of coal based on underground monitoring of appropriately sized coal pillar. The design method is considered to provide an accurate and rigorous pillar design method that will give mine planners a more rational mine design capability which is based on the mine geometry and geological setting of the operation. It is envisaged that availability of such a method will allow significant optimisation of pillar layouts and more closely define the geological and mining issues that relate to pillar design optimisation.

Jan 1, 1992