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By D. A. Payne

Large excavations, such as Iongwall panels, result in extensive vertical stress redistribution in the surrounding strata. The large abutment stresses developed may produce damage to pre-existing or planned excavations in the same seam or in seams above and below the workings. Knowledge of the magnitude and location of these stresses is therefore important in the design of mine openings; pillar sizes for panel and pillar layouts, roof supports in longwall gateroads and workings over or above pre-existing or planned extraction in adjacent seams. In an attempt to reduce costs, the Cape Breton Development Corporation (CBDC), a Federal Crown Corporation responsible for operating two retreat longwall coal mines, examined the potential for either interpanel barrier pillar width reduction or entire pillar elimination by the adoption of dual life gateroads for the longwall panels. In order to assess the potential for reduced interpanel barrier widths or total elimination, an investigation of the redistribution of vertical stresses around longwall panels in the Sydney Coalfield was established. The study was conducted jointly by the Cape Breton Coal Research Laboratory (CBCRL), of CANMET (a division of Natural Resources Canada) and the Denver Research Center (DRC) of the United States Bureau of Mines. The program included monitoring of vertical stress changes around longwall panels and gateroad behaviour in two seams. USBM-style hydraulic borehole pressure cells connected to chart recorders for continuous monitoring were deployed at four sites, two at Lingan Colliery and two at Phalen Colliery. This report describes the investigations conducted at Phalen Colliery. Contoured plots of stress redistribution around two sites are presented.

Jan 1, 1995

By Wei Wu

A new bench blast design method and its realization with the assistance of the computer is introduced. The basic design principle is to define a rhomb cell, called fully-fractured zone, through a series of full scale bench crater tests and apply this geometric pattern in multi-hole, multi-row blast design. To illustrate the design method, the optimization process and assessment of blast performance are presented in the context of a case study.

Jan 1, 1992

By N. C. Saxena

Subsidence investigations in Indian coalfields have led to development of relationships between subsidence movement parameters and defining of safe limits of subsidence movements for surface properties. The maximum subsidence over caved workings was generally not more than 60% of extraction thickness underground while in case of hydraulically sand stowed workings it was generally not more than 5%. The relationships have been useful in extraction of more than 7-million tonne of coal from underneath and in the vicinity of surface properties.

Jan 1, 1988

By John J. Bowders

The stability of surface slopes undergoing subsidence due to longwall mining was investigated to examine the effect the subsidence had on the stability of the slopes. Stability analyses were performed on all slopes along both longitudinal and transeverse sections of the longwall panel before and after subsidence. Both finite- and infinite-slope analyses were performed with both drained and undrained cases. The results indicated that the changes in the stability of slopes due to subsidence were insignificant for the cases analyzed. Although significant lateral ground movements were recorded, these were not attributed to slope failure. Explanations for these movements are presented in the paper.

Jan 1, 1988

By Robert W. Bruhn

A 125 foot high steel lattice frame tower supporting a 500 kV EHV transmission line and located over the gateroads of a longwall mining operation was subjected to ground movements from the mining of adjacent longwall panels. Under an agreement with the owner of the powerline, the mine operator was required to minimize mining impacts on the transmission line. In view of this requirement, the mine operator prepared a forecast of the expected subsidence ground movements prior to the onset of mining. These forecasts, in conjunction with finite element analyses of the tower superstructure under the imposed ground movements and wind loadings. suggested that the structural capacity of the tower would probably be diminished little, if at all, by mining. Ground movements determined by survey during mining of the two longwall panels were found to be in close agreement with predictions. Strains measured in selected tower members during mining generally agreed with values calculated by the finite element analyses. The study confirmed a methodology to predict subsidence and to assess its effects on steel lattice transmission towers.

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 Hamid Maleki

Results of geologic investigations, in-mine instrumentation, and numerical modeling are presented for two longwall mines in which pillars with large width-to-height ratios (8 to 9) are used. These investigations revealed that wide pillars unload when subjected to sufficient stress and that pillar unloading is influenced by geologic conditions. Field measurements in mine 1 clearly indicated load transfer from the longwall area toward the gate roads, resulting in pillar failure near the margin of a fluvial channel. Loads were shown to transfer to a harrier pillar (width height ratio equal to 9), contributing to progressive failure of this pillar. Unloading of the barrier pillar was associated with significant load transfer to the two-seam submains to the north of the harrier, contributing to roof falls, floor heave, and rib spalling within the submains. Stress analysis results addressed pillar peak strength and unloading behavior that was in agreement with the measurements and underground observations. Underground observations and stress analyses were completed before and after longwall mining near a six-entry submain in mine 2. This submain was intended to he used as a temporary bleeder and tailgate for a small longwall block consisting of one to two panels, depending on the development schedule in other areas of the trine and observed ground conditions. Numerical modeling using both elastic and inelastic pillar behavior for submain pillars with a width-to-height ratio of 8 was completed before mining began. Results indicated that pillar unloading was likely. During retreat of the first longwall panel, pillars unloaded. as evidenced by progressive rib spalling along the cleats, This behavior was mostly confined to the immediate face area, an observation that was in agreement with the modeling results,

Jan 1, 2000

By Lorraine Kent

Comparison of in-situ stress overcore measurement results with those determined from laboratory evaluation of Acoustic Emission (AE) testing on oriented sub-core samples has been carried out for a variety of mining environments. These include UK and German Coal mines at moderate depth, shallow-level South African Coal mines and deep-level South African gold mines. Results show that the `Kaiser Effect' or 'Stress Memory-Damage is readily determinable throughout the brittle deformation process, although particular care is required in interpretation of results during the 'bedding-in' phase of testing and the region associated with unstable crack growth prior to failure. The results suggest that AE provides a useful complimentary method for comparison with existing methods of stress determination. The success of the method can, however, be dependent on both rock type and the likely magnitude of in-situ stress in comparison to the strength of the rock. Keywords: in-situ stress, acoustic emission, Kaiser Effect

Jan 1, 2002

By Michael A. Berdine

In early 1997, Twentymile Coal Company took a dramatic departure from traditional longwall recovery methods. Previously, Twentymile's standard longwall meshing procedure included the use of chain link fencing material. Beginning with the 9-Right to 8-Right longwall recovery operation, Twentymile incorporated a polyester geogrid mesh system supplied by DYWIDAG-Systems International. This paper discusses the subsequent utilization of the polyester geogrid mesh system at Twentymile.

Jan 1, 2001

By John Stankus

Regional high horizontal stress has long been recognized as an important factor affecting roof stability However, localized high horizontal stress, although causing many ground control problems, has not received due attention, Through many ground control studies and underground experiences, it has been round that localized horizontal stress can be created by abrupt changes of overburden and stream valleys. The magnitude of localized horizontal stress depends on the valley configuration and overburden depth. Utilizing the Jennmar Finite Element Analysis System (JFEA), this paper analyzes the mechanism of the localized horizontal stress concentration based on two cast studies.

Jan 1, 1999

By Axel Preusse

Stowing or waste fill is common in the German mining industry since the beginning of mining activities, With increasing mechanization, flat and gently inclined seams (0 -- 18) were mined. These mines were located within the densely populated areas close to important infrastructures such as industrial complexes, roads, highways, railroads, rivers and canals. These buildings and infra¬structures were rarely protected against subsidence influences and other ground movements and dump capacities close to the mines were exhausted. For subsidence reasons and because of the low dump capacities pneumatic stowing methods were continuously developed and used in German longwalls until 1997. Due to techni¬cal standards and because of some restrictions pneumatic stowing in Germany is only used at a minimum seam thickness of 2.00 m and average costs of this method are $20 - 30 per ton. A significant reduction of the disturbance potential and by that a decrease in expenses for subsidence regulations due to reduced subsidence and ground movements could not be achieved. In this paper the experiences with pneumatic stowing in the Ger¬man mining industry are described. Possible subsidence reduction and economic aspects of pneumatic stowing are taken into account.

Jan 1, 2002

By Swapan Bhattacharya

Section 516 of Public Law (PL) 95-87, commonly known as the Surface Mining Control and Reclamation Act of 1977 (SMCRA), requires that underground coal mine operators must 'adopt measures consistent with known technology in order to prevent subsidence causing material damage to the ex- tent technologically and economically feasible, maximize mine stability, and maintain the value and reasonable foreseeable use of such surface lands, except in those instances where the mining technology used requires planned subsidence in a predict- able and controlled manner..." Implied in the constraints of Section 516 (b) (1 of SMCRA is a determination of what constitutes "material damage" to surface structures and renewable resource lands, and the capacity for sequential land use following underground mining. This Study was undertaken in an attempt to provide quantitative guidelines for estimating material damage to a wide variety of surface structures and renewable resource lands. Surface structures studied included buildings, bridges, railroads, highways. pipe- lines, d~, embankments, and water reservoirs. Damage criteria for buildings developed in this study along with details of the adopted methodology have been presented elsewhere (Bhattacharya et al.) (1). and will not be repeated here. Similar guidelines for renewable resource lands have also been discussed (Singh and Bhattacharya) (2)

Jan 1, 1984

By S. Bensehamdi

An advanced elasto-plastic finite element model adopting a modified Von Mises yielding criterion was used to simulate progressive failure of the rock strata surrounding a room and pillar mine structure. The model conformed to the strain-softening behaviour of rock and accounted for post-failure residual strength. The simulation was achieved in such a way that material values were updated after each FE run, according to the stress criterion until the solution reached the final equilibrium steady state of the mine structure. The suggested structural stability analysis approach accounted for the main features of the structure degradations produced during loading through changes in rock material stiffness and subsequent evolution of stresses. Comprehensive parametric analysis was performed and the inevitable effect of interaction of the roof, pillar and floor on the overall mine stability limit was investigated. The numerical results clearly showed that linear models could not realistically represent the true behaviour of the mine structure. The results obtained are in good agreement with that predicted in the field.

Jan 1, 1992

By Andre Zingano

It is known that the backfilling confinement increases the pillar strength and also reduces the rib lateral displacement. With backfilling, it is possible to increase the room-and-pillar mining recovery. This paper aims to study the behavior of coal pillars confinement using rock fill as backfilling material. Laboratory tests were carried out to determine the behavior of backfilling materials. Constitutive models were developed for the tested backfilling materials. Numerical simulations for the effect of backfilling materials on the ground stability in room-and-pillar coal mine entry were performed. The backfill method is a good strategy to (i) improve the ground stability in room-and-pillar coal mines for long term, especially for weak coal seams, (ii) increase the coal mining recovery and (iii) reduce the quantity of waste material on waste piles at the surface.

Jan 1, 2011

By Dieter Wittenberg

The successful extension of the applications in which rockbolts can be used in the German coalmining industry is, because of the very great raining depth combined with a high degree of stress in the native rock, only possible on the basis of a comprehensive quality management system. The quality assurance system consists of material tests on the individual component parts of a rockbolt, of test rig trials of installed rockbolts, of suitability tests underground and of measures for quality assurance during roadway drivage. Only on the basis of exact knowledge of the way in which installed rockbolts function in the underground and possible sources of error and faults can quality assurance be guaranteed during roadway drivage.

Jan 1, 2000

By Klaus Opolony

The German coal industry operates a multi-seam extraction system, with average working depths currently around 1,000 meters. The gate roads serving the longwalls are often used for a second time - a practice which has the advantage of allowing several panels to be extracted with a minimum of roadway drivage work. This preference for re-using gate roads is based on a number of factors: such an arrangement a) prevents the strata pressures in the subsequent panels from being adversely affected by the presence of support pillars, b) helps achieve the target of full mineral extraction and c) allows the roadways to be used for air conditioning and ventilation purposes as part of an advancing longwall system. This paper explains the planning procedure required for a rectangular rockbolted roadway, using as an example a gate road in the 3 meter-thick D2/C seam at Lippe colliery, and describes the subsequent operating experience. Here special attention is focused on modem procedures and techniques for mine roadway planning. The investigation also examines the part played by the mine ventilation system and the available machine technology, which are particularly important parameters for the design of the T-junction areas and support system. Keywords: multiple seam extraction, longwall mining, roadway drivage, roof bolting, roadside pack system, hydraulic frame type support.

Jan 1, 2002

By Frank Kendorski

Several startup or longtime underground stone mines have experienced unanticipated roof control difficulties due to the variable geologic character of the roof strata. Careful mapping of roof conditions, and identification of controlling geologic and mine geometric factors using sedimentological descriptors, lead to an understanding of the mechanisms causing roof problems. Types of bedding plane features can be categorized into geometric, internal structures, bedding-plane markings, deformed bedding, solution structures, and accretionary structures. Factors such as encountering open water-filled master joints; roof strata delamination due to internal water pressures; local karst features with mud, sand, and water hazards; and sudden loss of limestone roof due to paleo-channel erosion are some recent problems described.

Jan 1, 2002

By Binchuan Zhang

The combined reinforcement system (CRS) combines the strengths of roof' bolting, resin-anchored cable bolt, and chemical grouting to effectively control the deformation of the strata surrounding an extremely poor entry. It is easy to implement and fast in installation. It is one of the systems that are suitable for the reinforcement and maintenance of extremely poor entries.

Jan 1, 2001

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 Deren Zhu

Based on the results of field observation and physical model analyses, a computer simulation method, FEAEBP, has been developed for simulating the breakage of the main roof by considering it as a Kirchhoff plate on Winkler elastic foundation. By this method various parameters of rock property, different boundary conditions of the working face and the initiation, development and results of the breaking process of the main roof with different geometric dimensions can be effectively simulated. Since the main and immediate roofs, and the coal seams are treated as an integrated mechanical system, the interaction among them can be studied in detail.

Jan 1, 1988