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By David P. Conover

The effects of stress distribution on pillar stability were evaluated through rock mechanics studies conducted in three underground room-and-pillar coal mines. Results of the three field instrumentation programs are presented to illustrate the use of borehole pressure cells to determine vertical stress distribution in coal pillars at various stages of mine development. Vertical stresses and stress changes are determined using methods developed at the Bureau of Mines. Measured stress distributions are compared against theoretical distributions and possible explanations for observed behavior are discussed. Results indicate that stresses and stress changes calculated from pressure cell data are inconsistent with theoretical values; however, the data are useful for qualitative evaluation of trends over time. Calculated values are found to be sensitive to coal properties and initial setting pressures of the cells.

Jan 1, 1989

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 David Newman

The latest version of the Surface Deformation Prediction Software (SDPS) package is presented through its application to a number of case studies. The package now exploits all the benefits of the Microsoft Windows platform and has a direct interface to CAD software, thus facilitating input and output. Also, the package features advanced calibration routines to handle predictions over specific regions. The case studies presented highlight the features and capabilities of the new package.

Jan 1, 2001

By Guang Ping Cui

In this paper the imminent relationship between rock burst and its phenomenon with normal rock pressure and its appearance, as well as the factors of one changing into another have been analyzed. Based on this, the necessary and sufficient appearing factors of rock bursts have been put forward, and the term "burst pressure" has been defined. According to the statistics data of tremendous rock bursts which appear in collieries in Hunan Province, the futures of high frequency, great strength and shallow depth of the beginning burst have been found. There ere some special structures in the coal seams, which are closely related with the burst phenomenon. A hypothesis of "star structure" has been suggested for the appearing mechanism of bursts. Inside and around this star structure the distribution rule of burst pressure and rock strength has been researched depending upon the theory of the limited stress field. A new description of the appearing causes of bursts has been raised.

Jan 1, 1984

By S. Kramadibrata

It has been well accepted that water content significantly changes the physical and mechanical properties of rocks. Experimentation data of Uniaxial Compressive Strength (UCS), Modulus of Elasticity (E) and Brazilian Tensile Strength (UTS) of sandstone showed that increasing water content from oven dried to saturated condition will decrease their values. Ultrasonic tests carried out together with the UCS tests also indicated that the sonic wave traveled faster as rock specimen became saturated. During triaxial tests, the sonic velocity changed with the applied load. It was found that constitutive behavior of dried rock specimen was dependent on the confining pressure, whereas saturated rock specimen did not seem to be dependent on the confining pressure. Dependency of failure criteria such as Mohr-Coulomb. Bieniawski and Hoek & Brown on the water content was conformed as well. In order to show the influence of water content on the stability of an underground opening, a numerical modeling, based on finite element method, was carried out. The model was run using the mentioned failure criteria.

Jan 1, 2000

By Cao Shenggen

Experiences in the developed countries have shown that continuous increase of panel width (or face length) is one of the major technical trends for high production high efficiency mines. Increasing the panel width will increase not only the panel output, simplify the gateroad layout, facilitate high concentration of production, but also increase resources recovery and facilitate protection of surface structures. This paper uses the No.4326 super- wide panel, Wangzhuang Coal Mine as an example. This mine employs the fully-mechanized top-coal caving longwall mining method. Through surface subsidence monitoring, the dynamic characteristics of surface movement were obtained. The dynamic surface movement during mining and the final surface movement after mining were predicted using the Mining Subsidence Prediction System. The results indicate that after mining, the surface above the super-wide panel reached a state of full subsidence, So it can ensure the No.309 National Highway above is located in the flat bottom of the subsidence basin, and there is little impact on the national highways when panels on both sides of 4326 Panel are mined.

Jan 1, 2003

By E. Bauer

During 1987 and 1988, the Eighty Four Complex Mine of BethEnergy Mines, Inc. used the open entry longwall recovery method to recover one partial and three complete longwall faces. Various cement- concrete supports were utilized to support the open recovery entry until the approaching longwall face cut into this entry. This recovery method and the supports used were studied by the mine personnel and the Bureau of Mines. This paper discusses the findings, results and conclusions of that study.

Jan 1, 1988

By Stephen P. Signer

This paper presents the results of a case study conducted in a two-entry gateroad in a coal mine where excessive roof deformation and bolt loading resulted in failure of many roof supports. The instruments consisted of 16 fully grouted, strain gaged resin bolts, load cells on both full and partially grouted cable bolts, and vertical deflection multipoint extensometers. These instruments were installed on-cycle to measure loading on bolts in an existing support pattern and to determine if cable bolts could be used to improve roof stability. Results showed significant amounts of movement within the bolted zone. In sonic cases, these movements were enough to load the supports past their ultimate strength. Geology appeared to be a significant factor in localized roof degradation; that is, a very weak rock layer in the immediate roof overlain by a very strong rock layer contributed to the development of shear planes. Only one instrumented bolt had a maximum strain of less than 2,000 microstrain, which was the yield point of the steel. The average maximum strain on all bolts was 20.000 microstrain. However, electrical continuity to many gages was lost, and a pattern was noted that would indicate possible bolt failure. Wire mesh and concrete cans installed as secondary support performed very effectively.

Jan 1, 1998

By Z. T. Bieniawski

This paper presents the results of a survey of room and pillar dimensions and design practice in U.S. coal mines aimed at improving the design procedures in room and pillar mining. A review is given of the current pillar strength formulas and suggestions are made for better utilization of research results in engineering practice.

Jan 1, 1981

A high strength steel strand bolt has been developed by Barrett, Fuller & Partners as a replacement for conventional rigid bars for primary roof support in coal mines. It is based on a 23mm diameter multiwire steel strand with a tensile strength of 55 to 60 tonnes, The paper summarizes the development of FLEXIBOLT flexible roof bolts, their reinforcement performance and their potential to provide operators, currently reliant on medium to high density roof bolting systems, with a viable, low density roof bolting system offering high roof reinforcement capacity along with the option of installing bolts longer than headroom, in a single pass at the face.

Jan 1, 1993

By Vincent Scovazzo

Cost-effective underground mine configurations have been designed in highly fractured rock masses where the stone possesses a high material strength. A limestone mine in the eastern United States is used to demonstrate procedures for the geotechnical design of one such operation. The mine discussed is located in both limbs and the axis of a recumbent fold that fractured the rock mass. In a highly fractured rock mass, where the host rock has high strength, a design can be developed to alleviate intact rock failure. This is possible because in most cases as the stress levels needed for intact failures are approached, the stresses are quickly dissipated due to movement along discontinuities, which represent planes of weakness. As a result most underground failures in highly fractured rock are due to sliding or toppling. Even when intact rock fails in a highly fractured rock mass, it is the movement along an existing joint or other weakness plane that often initiates or causes this failure. It is therefore important to obtain the characteristics of weakness planes that are the discontinuities within the mass. The required information on weakness planes, geology, and strength of materials is discussed and recommendations for obtaining this information are presented. A discussion is provided on failure modes and the use of developed information for their evaluation. The impact on geotechnical design is demonstrated through pillar sizing procedures; however, this approach can also be applied to mine roof and pillar foundations.

Jan 1, 2002

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 Karl Jr. Zipf

The famous Homestake gold mine in Lead, SD. closed recently alter 125 years of operation. However, the mine may receive a new lease on life as a National Underground Science Laboratory (NUSL) supported by the National Science Foundation (NSF) and dedicated to high-energy physics experiments involving neutrinos. These experiments require the construction of many large underground chambers tells of meters wide at depths of over 21 18 m. Construction of such large openings at such extreme depths has few precedents m civil-engineering-type construction. The Spokane Research Laboratory has a long history of ground control research at the Homestake Mine and has supported NSF and the physics community in their efforts to develop a NUSL at I Homestake or other suitable location. This paper will summarize underground science and describe proposed underground science laboratories ill the United States, in particular tile proposed NUSL at Homestake. Next, the paper describes an empirical design approach using the Q system of rock mass classification for these large Openings. .M initial site investigation that produced three good case histories is described. The actual support system used at the three sites agrees well with support recommendations from current support design charts, giving credibility to estimates of the classification parameters and Q. Feasibility of, and support requirements for, 25-, 50-, and 100-m spans arc estimated in the anticipated rock mass. This analysis indicates that 25- and 50-m spans are probably feasible, but a stable 100-nn span may be unachievable. The empirical predictions are then compared to detailed numerical analyses. The paper concludes with preliminary support recommendations and necessary site investigations needed to make a proposed NUSL a successful reality. These studies have been important input to NSF for their decisions on the technical feasibility of developing the proposed NUSL. If NUSL is developed, it could enable scientific experiments in areas beyond high-energy physics. 'the laboratory could also enable research in rock mechanics. ground water hydrology. mining engineering, nine safety and health, and ground control.

Jan 1, 2002

By John L. Hill

Over 50 mines of the Appalachian and Illinois Basins are presently experiencing poor ground conditions believed to be caused by overlying stream valleys. The Bureau of Mines is conducting research into the causes of this problem with the aim of developing a predictive method for determining the safe limits of mining beneath stream valleys. In addition, the factors that control instability in these regions will be analyzed to provide design guidelines for adjusting support requirements. This paper presents the theorized causes of poor ground conditions beneath stream valleys and compares the theories with the results of a Bureau study of the Birchfield Mine No. 1 and a preliminary investigation of the Davidson Mine, both of southern West Virginia. The Birchfield Mine excavated four main entries beneath a 50-ft wide stream with only 50 ft of overburden. The valley flood plain is over 500 ft wide, flanked by valley wall slopes of 25 deg and relief of over 950 ft. In-mine monitoring of strata movement and detailed geologic mapping revealed that roof rock strength was greatest under high cover and weakest along outcrops and beneath the valley, with localized areas of decreases in rock mass quality. To improve the rock mass quality beneath the stream and reduce the threat of inundation, an extensive drilling program was undertaken to inject grout into the overburden from the surface. As a result, mining beneath the stream was practically routine from a ground control perspective, and water was not encountered until mining had advanced to a point beneath the floodplain that had not been grouted. The only ground control problems which were encountered were entirely a result of localized poor rock mass quality. In contrast to the Birchfield Mine, the Davidson Mine experienced failure of intact roof rock at a depth of 300 ft beneath a V-notch shaped valley with no change in rock mass quality. Finite element analysis was conducted to study the premining in situ stress at the two sites. The results are discussed within this paper and provide insight into the causes for the types of failure seen beneath valleys of varying shapes and depths.

Jan 1, 1988

By Robert C. Garson

The Mine Roof Simulator at the DOE'S Mining Equipment Test Facility is described. Its use in the research and development on the behavior and response of longwall roof supports to simulated mine environment is investigated. Advanced technology analysis methods such as finite element modeling, strain gage instrumentation, and photoelastic techniques are investigated for application in roof support design two case studies are presented of test data and results.

Jan 1, 1982

By K. Hanna

This manuscript describes an extensive ground control study at the Inland Steel No. 2 Mine near McLeansboro, IL, conducted by the U.S. Department of the Interior's Bureau of Mines (USEM) in cooperation with Inland Steel Coal Co. The mine experiences severe ground control problem primarily due to high horizontal stress, particularly at entry intersections. Mine experience has shown that reorienting mine entries, modifying the bolting pattern, and altering the mining sequence are effective in reducing the severity of roof falls and out-of-seam dilution. However, the frequency of roof falls has not been significantly reduced. This research project was designed to provide a thorough explanation of the intersection failure mechanism through combined analyses of company in-mine experience and Bureau field measurements. Stresses and deformations were monitored in a four-way intersection during all stages of development and were related to site observations and theoretical analyses. The results of the combined analyses indicate that high horizontal stress is the predominant factor contributing to intersection roof falls in the Inland Steel No. 2 Mine. The horizontal stress induces shear zones to form along the outby 1 ribs, causing the immediate roof layer to detach 1 and behave as a cantilever beam oriented diagonally across the intersection and parallel to i the maximum horizontal principal stress. As the shear zone progresses upward, roof layers successively detach and fail, forming a dome-type roof fall.

Jan 1, 1986

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 Gift Makusha

Pre-feasibility work at Goedehoop Colliery indicated that stooping of the undersized pillars could be undertaken safely and economically. In light of the findings a decision was taken in may 2003 to undertake the project in the 600 area where three panels (628, 630 and 632) were identified for pilot test sites. The findings of the pre-feasibility studies were presented in a paper to the 22nd International Conference on Ground Control in Mining held at Morgantown in 2003 (1). This paper is a progress report for the project 12 months down the line. Work commenced to prepare the sites for mining including, acquisition of equipment, equipping the panels, selecting and training people to undertake the project. Three monitoring sites were set up distributed evenly along panel 628. Each site comprised of a surface extensometer station in the overburden strata adjacent to a pillar underground containing hard inclusion vibrating wire extensometer cells. This combination would be used to record pillar loading ahead of stooping as well as the depth of caving and strata deformation in the goal zone behind the stooping face. In July 2003, support work began to secure the roof and rib sides. The support was required to be at least three-pillar center distances ahead of the mining front to ensure that the effective stooping zone is secured at all times. The NEVID method of pillar extraction developed at Sasol and described in detail by Ismet and Moolman (2) in a Coaltech 20-20 report was adopted for the pilot project. As part of training the selected stooping team was send for a week to Sasol's Bosmanskrans Colliery to familiarize with the NEVID method of stooping. Further training sessions were carried out on site especially on the use of Mobile Roof supports. The first cut was taken on 20 August 2003 by implementing a form of chequerbord mining taking every second row of pillars. Three rows were mined this way which period was considered training to get workers familiar with the new equipment and mining method. On the fourth row the full design span equivalent to mining 4 pillars in a row was implemented taking out all the pillars in successive rows. Production figures gradually increased from 300 to more than 1,000 tons run of mine coal per shift. Panel 628 was mined out at the end of December 2003 and the second of three panels (630) is being mined. Substantial data from the monitoring set up has been recorded since the start of the project. Monitoring will continue to the end of the pilot project and the data will be analysed and fed back into the design tools to improve the design in future undertakings. The pilot project has been a success so far and other old workings are being evaluated for stooping.

Jan 1, 2004

By Archie Richardson

Periodic weighting of longwall supports occurs during retreat ofthe face under certain geologic conditions. Strong stratain the immediate and main roof tend to cantilever over the gob, weighting the supports periodically. The distance between weighting peaks, as well as the intensity ofperiodic weighting, is determined by the suength and thickness of roof members, their location relative to the seam, the frequency of joinling, and characteristics of the gob. In this paper the effects of varying several of these variables ate investigated using block modeling. The models are calibrated against a published case history of periodic weighting in the Pittsburgh Seam. It is concluded that block modeling is a useful tool for correlating measured support pressure histories with expected modes of strata behavior in the roof, with potential for predicting shield behavior in new geologic enviromnents.

Jan 1, 1998

By S. S. Peng

The hydraulic powered supports have been employed in U. S. longwall mining for the past two decades. Yet the sciences of roof control at the powered supported faces remain largely unknown. In order to improve longwall roof control and increase production, a series of questions in longwall roof control should be studied and answered: what is the basic rules of roof behavior in longwall coal faces equipped with the hydraulic powered supports? What is the interaction between the roof, supports, and the floor What is the contact condition of the canopy with the roof, and consequently the load distribution on the canopy? Des the periodic roof weighting really exist in the powered supported face and if so, how does it affect the face area? This paper presents the preliminary analysis of underground observations and instrumentations in a longwall panel with 4-leg shield supports designed to address the questions mentioned above.

Jan 1, 1982