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By Kikro Matsui

Longwall mining 1s more productive in comparison to other methods but requires particular conditions for its effective use This paper relates to longwall roof instability at Ikeshima Colliery In order to make clear the cause of the longwall roof instability, scientific investigation in terms of geomechanical factors and longwall support system was planned and carried out

Jan 1, 1997

By Brian E. Travis

The recent development of polymer grids for the underground coal mining industry has created new alternatives for supplemental ground control practices. The grids utilize strong, lightweight polymers - usually special grades of polypropylene. High tensile strengths and resulting load support characteristics are achieved by molecular orientation of these polymers in the manufacturing process. One of the most important applications of polymer grids as supplemental ground control is in longwall shield recovery. When shields are moved from one face to another, the determining factor in the success of the recovery is the ground control provided by roof support structures along the old face. Whereas primary support is usually provided by roof bolts and cables which run the full width of the panel, supplemental support is often provided by metallic meshes of welded wire or chain-link fence. lightweight, high-strength polymer grids replace these heavy, cumbersome metallic meshes, giving the operation increased productivity by decreasing installation time and reducing injury downtime.

Jan 1, 1990

By John C. Stankus

From research started six years ago, a new design criteria has been developed for mine roof bolt systems This design criteria precisely and quantitatively determines parameters such as bolt length, installed tension, and spacing that will produce the Optimum Beaming Effect (OBE) OBE, a new roof control term, is defined as the roof beam that has no separation above and within the bolted range with the shortest bolt passible Utilization of this design criteria has shown that in many cases shorter roof bolts with higher installed tension can produce a much stronger beaming effect (Stankus, 1995) This paper will first present a brief deception of how the design criteria was formulated From there, actual case studies where the design criteria was utilized will be discussed In conclusion, current and future research efforts will be presented

Jan 1, 1997

By Ryan Garvey

A numerical investigation is made into unstable failures of coal pillars (i.e. coal bumps) using the finite difference software FLAC3D. A static energy balance is first derived to calculate the excess energy released as a consequence of unstable failure in rock. Two- and three-dimensional mining layouts are then assessed on their bump-potential based on the magnitudes of excess energy released during simulated pillar failures. A three dimensional pillar model is developed to represent tributary area loading of an infinite room-and-pillar layout. Pillar strength and brittleness characteristics are calibrated using a Mohr-Coulomb strain-softening constitutive law to simulate width-to-height ratio coal pillars ranging from 1:1 to 5:1. Width-to-height ratio pillars 1:1 to 3:1 release large magnitudes of excess energy as the pillars fail, representing massive collapse of room-and-pillar layouts. The larger pillars maintain their residual strength during failure; however significant excess energy is still released during temporary reductions in pillar strength. A two-dimensional model is then constructed of a gate road pillar failing due to elastic rebound of the surrounding rock mass. Soft loading conditions initiate unstable failure for all pillar widths, while the total magnitudes of excess energy increase with the size of pillar being modeled. The results from these tests indicate that excess energy may be used as a direct assessment of the bump potential of simulated mine layouts.

Jan 1, 2013

By Song Guo

This paper will present the roof bolting finite element model developed at Jennmat Corporation used to study the control mechanism of the tensioned bolt system in the laminated roof with a large horizontal stress. In addition, it will discuss general support concepts pertaining to the tensioned bolt system which are derived 6om a series of studies in conjunction with the authors' working experience and extensive research efforts over the years.

Jan 1, 1997

By Erik Westman

Tomographic imaging, commonly known as a CAT-scan or CT-scan, is used in the medical field to generate a picture of the interior of a body. Current research is applying this method to bodies of rock so that rock failure can be better understood and predicted. This paper discusses the basic principles of tomography and describes recent developments in the use of seismic tomography in longwall coal mines, including results from a bump-prone longwall coal mine and continuing research.

Jan 1, 2003

By Mohummad Akram

Statistical analysis of multi-seam mining case studies demonstrated that geologic structure and lithological type and sequencing were predominant factors in entry failures. Finite element analysis was used to evaluate the effects of stress field changes on entry instability under differing geologic conditions. Analysis showed the roof to be particularly vulnerable to shear with the location of the failure site changing to meet loading and geologic conditions. Effects of external support on roof stability were examined. Results were correlated with field data to determine stress levels at which failure could be expected to occur. Based on this research design, criteria were developed for roof stability evaluation under specified interactive loading conditions.

Jan 1, 1993

At any given lime, the tidal force subjects massive regions to a spatially uniform load. The deformation of any large structure due to these forces is proportional to the elastic properties of the structure. In this aspect, the tidal force applies a load in a manner similar to that of a large-scale testing machine. Any anomalies in the measured tidal response must reflect alterations in the average elastic response of the massive structure. In the present work, tidal tilt measurements obtained in the wall of a large dam are used to infer changes in the response of the dam to applied loads. In particular, it is found that the dam response has changed by approximately 13 percent, on average, for two monitoring periods separated by 5 years. It is possible that these changes were produced by the different volumes of water in the dam for the monitoring periods. The tidal measurement technique can also be applied to monitor changes in the average structural response of large underground and surface masses relevant to coal and metal mines.

Jan 1, 1992

By E. M. Williams

The U.S. Bureau of Mines is investigating the use of in-seam seismic methods to monitor stress-induced changes in coal near the working face of longwall mines. Seismic surveys performed through a longwall panel and pillars near the face can provide valuable information on geologic anomalies, high stress regions, tailgate support performance and other conditions that could endanger miners or interrupt production. This paper describes the details of the equipment and data analysis techniques used, and the significant results obtained and their implications for ground control. Using seismic tomography, it is possible to conduct measurements on the pillar and reconstruct an image representing the internal velocity distribution. Successive measurements over several days identify changes in velocity attributable to changes in stress. Several field measurements were conducted at a western U.S. longwall mine to observe stress induced changes within the panel and pillars during mining. A permissible data acquisition system was designed to meet mine safety requirements for operation in gassy air. Certification from the Mine Safety and Health Administration (MSHA) was obtained, and this approved system was used to monitor stress changes in several pillars as the longwall face passed during first panel mining. Tomographic surveys were completed across the longwall panel to map the apparent abutment stresses ahead of mining. Velocity distributions show an increase in vertical stress across the panel from the headgate to the tailgate. Additionally. a yield zone along the face near the tailgate and the forward abutment approximately 100 feet outby the face in the tailgate side of the panel were detected. These results agree well with data obtained from other instrumentation emplaced in the panel near the tailgate. Using velocities for both compressional and shear waves, the Poisson's ratio was calculated for the panel.

Jan 1, 1994

By Michael Alber

Pullout tests on resin grouted rock bolts have been executed in coal mines and surface test sites. The goal of this study was to provide some qualitative information about the rock mass quality from mandatory tests with a bit little more effort. Movements of rock bolt head and steel plate at different load levels were analyzed and were found to yield information about the rock mass deformability. The tests were interpreted similar to plate bearing tests and a rock mass compressibility index C was assigned. This index reflects the complex interactions between the rock mass, resin grout and bolt under load. Although further research is necessary it may be concluded that those tests may aid in recognizing geological conditions.

Jan 1, 2003

By T. N. Singh

With a view to producing increased target output every year, Indian coal mining industry is fast moving towards mechanisation but the present approach of mechanisation suffers from lack of standardisation, mainly due to nonavailability of the cutting data for various machine/rock combinations. The coal seams of the country are relatively hard in nature and the coaling machines are reported to be affecting production and productivity of the coal mine and the very trend of mechanisation. This paper presents the result of extensive study performed to visualise the cutting characteristics of coal seams of compressive strength ranging from 20 MPa to 65 MPa. In a specially designed drag bit type coal plough rig, continuous recording of cutting forces was done with the help of a microprocessor based data logger and the specific energy of cuttability was evaluated at different depth of cut and speed of cutting. The geomechanical properties such as compressive strength, orientation and frequency of cleats etc. and different indices of coal seams were measured in the laboratory and field and relationship was developed for easy determination of specific energy.

Jan 1, 1992

By David I. Hoyer

It is shown that by monitoring longwall leg pressures in real time, warning can be given for significant weighting events and the formation of roof instabilities, such as roof cavities, several hours in advance. Longwall Visual Analysis (LVA) is a software package that continuously monitors shield pressures and shearer position in longwall mines. LVA has been running on 19 Australian longwalls for up to four years, and as a result a very substantial database of shield pressure trends in a wide range of longwall situations has been collected. This database has been analyzed to develop indicators that will give operators and geotechnical engineers advance warnings of developing conditions such as weighting events and difficult roof conditions. LVA displays live charts of data, such as shield leg pressures, loading rates, and yield frequencies. Data analysis techniques were applied to historical data records from multiple longwall sites in order to develop a ?Cavity Risk Index? (CRI). The CRI indicates the risk of roof cavities developing, and it is based on pressure trends that indicate significant yielding and loading rates spanning a region of relatively low support. Case studies from different longwalls are given, showing how the CRI can give real-time advance warning of the formation of roof cavities with their anticipated location on the face. The limited number of case studies done so far indicate that the predictions have a high degree of reliability, but more work is required to quantify this.

Jan 1, 2011

By S. P. Singh

Destress blasting is commonly used in the mines to minimize the occurrence of rockbursts. The fracturing created by destressing in the vicinity of the face reduces the stress concentration by pushing the zone of high stress deeper into the rock mass. But the destressing practice by blasting may be devastating near the faults or other sensitive high stress zones. To avoid severe rockbursts in such situations, the application of chemical destressing has been proposed in this paper. A laboratory investigation was conducted on the rocks in the Sudbury basin to examine the change in the material properties with an aim of assessing the feasibility of destressing by chemical means. The favourable change observed in the rock properties led to the suggestion that the injection of surfactants in boreholes would enhance fracture propagation similar to what is currently done by blasting but in a safer, inexpensive and non-violent manner. The current destressing practices in Ontario mines have also been reviewed in this paper.

Jan 1, 1987

By Edward S. Custer

Recent investigations of mine roof falls have demonstrated that roof failures are generally associated with geologic conditions. The roof falls can be predicted by using the techniques of depositional modeling for the coal bed and roof rocks, and by lineament analysis. The ability to predict potential roof failure areas will allow the engineer and mine planner to develop a mine plan to avoid the roof failure areas and thus reduce overall mining costs. The ability to avoid roof failure areas also will permit maximum recovery of the coal resources.

Jan 1, 1981

By Peter Griesenbrock

In German underground coal mining mostly arch and some rectangular roadways are used in the development work. Due to its positive results from numerical and physical modeling techniques as well as good underground experiences, the numbers of rectangular roadways has increased significantly. Today the bolt support - independent of its cross-sectional shape - represents an undeniable support element. During the introduction of new bolt techniques a detailed investigation of the rock and support mechanical effectiveness of bolt support in roadways at all operation stages is of basic importance. This effectiveness can be predicted and verified using different simulation techniques. For the planning of rockbolted roadways two supplementary simulation techniques are presented in this paper. By combination of all available planning tools the best result for planning assurance can be achieved. Keywords: numerical modeling, physical modeling, arched roadway, rectangular roadway, roofbolting, advancing longwall.

Jan 1, 2002

By Rossen A. Halatchev

This study introduces a new probabilistic approach to seismic stability analysis of embankments and slopes. The approach is based on Sarma's solution, which is modified through presentation of the seismic force by its horizontal and vertical components; i.e. the force may have an arbitrary inclination. The probabilistic modelling includes Monte Carlo simulation of the shear strength parameters of the soil mass, which are assumed to have a normal distribution. The probability of failure is determined based on the seismic co-efficient, which is treated as a random quantity. The probabilistic approach is illustrated through a solution of a numerical example from an opencast coal mine.

Jan 1, 1992

By Stephen C. Tadolini

A new innovation in coal mine roof support has been developed by the Bureau of Mines at the Denver Research Center. The technique, called "Thrust Bolting,"1 converts a traditional passive roof support system, a full or partial column rebar bolt, into an active support system by following a specific installation process. The thrust bolting system provides two advantages from a ground control standpoint. First, because the system is active, no roof movement is necessary to put the bolt into tension. This eliminates the possibility of strata separations in sedimentary roots, minimizing progressive-type roof failures. Secondly, the tension placed on the system is independent of mechanical devices or torques. This eliminates the highly variable frictional losses that occur between the bearing plate and the bolt head and in the threaded portions of traditional active support systems. These two improvements create a safer working environment for mining personnel. An additional advantage of this improved support system is the reduced cost of installation and maintenance. The initial support cost is reduced by a minimum of 30 percent per bolt when compared to similar conventional support systems. The labor intensive and potentially hazardous torque¬tension test, required for most tensionable active support systems, is not required for thrust bolts. The potential for improved support performance and cost reduction of thrust bolting has generated a considerable amount of interest from both industry and MSHA. This paper will describe, in detail, the theory and application of thrust bolting and present the results of two underground coal mine case studies.

Jan 1, 1990

By Hamid Maleki

During the last decade, a significant amount of research has been conducted by Energy West Mining Company, government agencies, academia, and consulting companies to develop predictive tools for assessing the stability of the Castlegate Sandstone, which is found approximately 250 m above multiple-scam coal reserves, Energy West Mining uses longwall mining techniques in its operations near Huntington, Utah, and these studies were initiated to satisfy requirements for maintaining the stability of the Castlegate Sandstone and resource recovery. In this study, the authors have used multiple-regression analysis techniques and a wealth of data collected over many years on geology, mining, and escarpment stability. The volume of failed rocks is used as the response variable after several other factors were considered, including measurement of surface deformation and frequency of mining-induced surface fractures. Geologic and geometric variables were obtained along 3.7 km of escarpment exposure at 130 study locations. Regression analysis of data for the first 29 study location, which had been fully' undermined, showed that surface topography played a critical rule in influencing escarpment stability. Preliminary regression analysis results from 70 study loca¬tions identified several other important geologic arid mining factors that influence be stability of the sandstone escarpment These are canyon slope, sandstone thickness, and mining influence ante.

Jan 1, 2000

By Graham Daws

State of the Art Room and Pillar mining techniques have been employed at two coal mines in the UK in the last two years. This paper describes the design of pillars and rockbolted support systems for Room and Pillar mining operations in these collieres. Within the last year, rock mass classification systems have been used in the design process of a room and pillar mining layout at one of these collieries. In particular, the roofbolt support system was detailed using these techniques and the support system was linked with operational aspects such as depth of cut. The depth of cut was initially estimated using rock mass classification systems and then verified by a stand up trial which was monitored remotely. The stand up time has a direct influence on the operational aspects of the mine and this is described. The design was verified by performance monitoring and the results are discussed. The importance of understanding the role of rockbolting is emphasised along with some examples of accurately determining in-situ bolt performance. The paper also recommends some modifications to rock mass classification systems when applied to underground coal mining operations.

Jan 1, 1998

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