Search Documents

By Peter Zhang

As simpler and easier reserves are exploited to depletion, underground coal mining continues in more complex geologic settings and conditions. Roof bolting for long-term entry stability is always a challenge. For long-term entries of a mine such as belt, track entries and return airways, roof could be under-supported if one uses the same bolting plan as in short-term entries or even seem over-supported with increasing use of longer, stronger bolts along with mesh, pans, straps or trusses. In most underground coal mines, roof deteriorates over time with cutters developing deeper in entry corners; roof sagging and popping out between bolts; entry width increasing due to rib sloughage; and separations developing deeper up into the roof. In deteriorated long-term entries, roof falls could occur at certain locations and re-bolting could be costly or sometimes unfeasible. Therefore, it is important to choose an appropriate bolting plan during development based on mining and geologic conditions if the entries will be used for the long-term. In both short-term and long-term entries, primary roof bolting is critical in building a strong and effective beam. But in long-term entries, supplementary bolting is necessary in certain cases to ensure roof stability can still be maintained even if a roof failure develops beyond the primary bolted horizon. This paper deals with roof bolting for long-term entries based on studying the progressive roof failure and roof falls under different geologic conditions. The paper presents bolting considerations in terms of bolt type, length, diameter and spacing as well as bolting strategy to minimize the risk of roof falls in long-term entries.

Jan 1, 2011

By Kot Unrug

The selection of tensioned versus non-tensioned roof bolts, for primary coal mine roof support, has been debated for well over 25 years. The wide spread use of fully grouted rebar marked the beginning of the discussion of the relative importance of bolt tension and the resultant immediate roof compression. The focus is on roof conditions encountered in coal mines however the discussion may be extended to hard rock mines as well. The theoretical bases for both systems are presented, with the analysis focused on the bolt's ability to actually fulfill the theoretical assumptions. Of specific concern are assumptions about bolt tension upon installation and the likelihood of maintaining useful bolt tension over time. The usefulness of tension roof bolting is limited by the mechanical interaction of roof bolt assembly and the rock, which may be valid at the time of installation only. Ambient mine atmosphere and fluctuation in moisture content changes the physical condition of rock, which may have a significant impact on the effectiveness and longevity of bolt tension. Changes in the material properties of the surface of the immediate roof can invalidate the assumptions, which were made in the design stage of roof support system. This means that the roof support in the long term excavations may be inadequate requiring costly rehabilitation. The presented arguments are illustrated with the examples of some relevant experimental data. Tension primary roof bolting is recommended where the immediate roof is: ? Stiff with a high modulus of elasticity ? Surface layers are continuous and fairly thin ? Not easily weathered

Jan 1, 2004

By S. P. Singh

Rockburst is a sudden manifestation of the release of elastic strain energy stored in the rock. Therefore the tendency of the rock to burst should depend upon its ability to accumulate elastic strain energy. The use of Burst Proneness Index as a simple and reliable indicator to recognize the burst prone situations in the mines has been advocated in this study. The Burst Proneness Index exhibited a strong dependence upon the brittleness, compressional and shear wave velocities and the point load strength index. It was also observed that the Burst Proneness Index increases with the increase in the strain energy per unit volume, c ressive strength and the modulus of rigidity of the rocks. In addition, important causes and the measures to alleviate rockbursts have been outlined in this paper.

Jan 1, 1986

By Andrew Starkey

Ground anchorage systems in the form of rock bolts are used extensively throughout the world as supporting devices for civil engineering structures such as bridges and tunnels. It is widely recognized that non-destructive testing methods for ground anchorages need to be developed as a high priority [1 ], with only a small proportion of anchorages currently being monitored in service [2]. The condition monitoring of anchorages is a relatively new area of research, with the objective being a wholly automated or semi-automated condition monitoring system capable of repeatable and accurate diagnosis of faults and anchorage post tension levels. The GRANIT tm system developed by the University of Aberdeen is capable of measuring the integrity of ground anchorages and has won two major UK Awards - the National John Logic Baird Award for Innovation in 1997 and a Design Council Millennium Product Award in 1999. The normal procedure with the GRANIT tm system [3,4] is to train a neural network on dynamic response data that has been sampled from an anchorage over a range of post-tension levels. Further data is needed in order to test the neural network. The paper presents neural networks that have been trained on data taken from a number of anchorages on different sites, and the results of the diagnosis of these neural networks on new test data is presented. The results show that the cross-diagnosis of anchorages is possible, where a neural network trained on one anchorage is capable of successfully diagnosing the post-tension level for an anchorage of similar construction, and the high level of accuracy that can be attained with the GRANIT tm technique is shown. The paper also introduces a mathematical model of the GRANIT tm process.

Jan 1, 2001

By Saeed Zhandi, Behdeen Oraee

"Rock bursts remain an important problem in longwall coal mining. These bursts are due to a sudden and severe failure of rocks from a high stress concentration in deep underground excavations that occur with the instantaneous release of strain energy stored in the rocks. They can potentially cause irrecoverable damage to equipment and personnel, thus accurate rock burst prediction and control is expected to be carried out by the mine design engineer. As a result, this can constitute major challenges for said engineer. In this paper, forensic engineering has been used to evaluate the possibility and extent of rock bursts in deep coal mining. For this purpose, established mining engineering principles, including factors influencing the severity of rock bursts, have been incorporated in the forensic engineering technique. The analyses took place in five steps:• Assessment of regional and local conditions prior to the event• Assessment of conditions after the event• Hypothesize plausible ways in which pre-event conditions can become post-event ones• Search for evidence that either denies or supports various hypotheses• Apply engineering knowledge to relate the various facts and evidence into a cohesive scenario of how the event may have occurred.The paper concludes by demonstrating a method for predicting rock bursts and preventing their reoccurrence. The methodology used in this paper, together with the results obtained, can serve as useful tools for the coal mine design engineer in the primary evaluation of rock burst potential in underground coal mines.INTRODUCTIONA rock burst is defined as the sudden and great failure of rocks due to a high stress concentration in deep underground mines. This phenomenon occurs with the instantaneous release of strain energy that is stored in the rocks, and can cause irrecoverable damages to both equipment and personnel (see Figure 1). Rock bursts pose a hazard owing to their lake of predictability; in fact, research into understanding rock burst mechanics and implementing techniques for their monitoring and prediction has been taking place for over 50 years."

Jan 1, 2015

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 Revendra Awasthi

The application of a horizontal inclinometer and a tiltmeter to measure tilt, curvature, and deformations of shallow footings during mine subsidence is described. The measurements were taken during mining of a longwall panel in Southern Illinois and were supplemented with precision level survey data. Monitoring was conducted under a cooperative agreement between the Institute for Geotechnology at The University of Tennessee, Knoxville, the U.S. Bureau of Mines, the National Science Foundation, the Illinois Mine Subsidence Insurance Fund, and Old Ben Coal Company. Six different footings were constructed and instrumented at two sites; one above the panel centerline, and the second inside the edge of the panel in the anticipated zone of maximum tension. A brief description of the inclinometer casing and tiltplate installation is described, and the results obtained from the two instruments on a plain concrete footing compared. Observations of footing deformations and curvatures calculated from the inclinometer and tiltmeter data are also presented, and the results compared with observed structural damage.

Jan 1, 1990

By Syd S. Peng

Full-face shield leg pressures in a longwall panel for a three-month period were collected and analyzed. The pressure data were presented in the following forms: full- panel full-face shield pressure, individual shield pressure, full-face shield pressure, full-panel time-weighted-average pressure, and relationships among setting pressure, final pressure and pressure increment. Although shield pressure varies from shield to shield and from cycle to cycle, the setting pressure and face advance rate do control the 6nal pressure and time-weighted average pressure.

Jan 1, 1997

By David B. Plumeau

The Cayuga Mine, a deep rock salt mining, operation, changed from a standard room and pillar mine design to a yielding pillar/pressure arch design five years ago. In general the roof conditions improved. However, entries adjacent to the abutment pillars continued to fail. It was noticed that where abutment pillars were accidentally "notched" (by driving cross- cuts one cut into the pillars), the abutment entries had better roof conditions than where there were no "notches". Experiments with intentionally notched abutment pillars showed that notching did improve abutment entry conditions. During the past four years notch configuration has been varied to suit local ground conditions. Recently, "pre-cutting" of notches has proven itself as a means of protecting abutment entries under severe ground conditions. As an added bonus, the notches improve productivity by providing more places to work and reduce costs because it is not necessary to bolt the notches.

Jan 1, 1981

By Steve Signer

Researchers at the National Institute for Occupational Safety and Health (NIOSH) have developed several new tools for evaluating roof support performance. A miniature data acquisition system (MIDAS) was developed that can collect readings from up to 16 strain gauges at regular time intervals and store the readings for later retrieval. Three LED lights change from green to yellow, then red, based on the reading levels and/or rates of change of rock movement. This feature can be used to warn miners of rock instabilities. The system can be used with any strain-gauged support (resin bolt, friction bolt, cable bolt) or with a newly developed rock strain strip (ROSS) developed by NIOSH to measure rock strain. A ROSS is grouted in a hole drilled adjacent to the rock support to be evaluated; the deformed shape of the ROSS provides anchorage on either side of the strain gauges. Data from both laboratory tests and field evaluations are presented that describe the capabilities of both the MIDAS and ROSS instruments.

Jan 1, 2003

By Ben L. Seegmiller

Specially truss systems? including the Intersection Truss®, Continuous Entry Truss®, Cutter Roof Truss® and Roktrus® have been used in many coal mines in the mid- west and western United States during the past four years. These truss systems offer greater strength and support than other standard trusses and many other support devices. Mining operations have found that speciality trusses can give effective long-term support in areas that must remain stable for many years. At some mines, the Intersection Truss® has become the most-widely accepted method of providing secondary support in potential problem areas. Other applications for specialty trusses have included longwall start- up and recovery rooms, arched roofs, access rock tunnels and tailgate entries.

Jan 1, 1990

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 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 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 K. Goshtasbi Goharizi

This paper discusses experimental results of physical scale models employed to study the stability of a pillar between rectangular roadways. A programme of laboratory investigations utilising a bi-axial test rig has been undertaken to examine the influence of various stressfields on the stability of pillars between mine roadways. The effect of pillar height/width ratio on the overall stability is studied. Observations of failure characteristics in and around pillars are discussed especially in relation to the various stressfields applied to the models.

Jan 1, 1992

By Axel Preusse

A high advance rate of the longwall face increases inevitably the mining dynamics within the rock and on the surface. Measures to limit the mining dynamics are the adjustment of the face advance rate to stretch the influences of mining over time and continuous winning to maintain a uniform influence of mining. This paper deals with the dynamics of mining subsidence within the range of a transmission overhead line for transportation of gasoline, diesel, kerosene, heating oil, mineral oil and other hydrocarbons. In a certain segment of the 24"-pipeline it was possible to predict, due to technical, operational and laying conditions, that undermining the pipeline would cause local stress maxima within the pipeline string. To limit these expected local stress maxima, the face advance rate of panel 497 was adjusted to the sensitivity of the pipeline towards the influences of mining. Comparison of the predicted mining dynamics, using the theory of stochastic media, with the measured ground movements led to the conclusion that the effects of dynamical influences which occurred within the range of the transmission overhead line and acted on it were low. This confirmed that the decision to adjust the face advance rate to the sensitivity of the transmission overhead line was correct (Kateloe et a)., 2002). A later relaxation cut in range of the pipeline siphon scarcely produced relaxation movements of the pipeline. The appointed sensitivity data of the undermined pipeline and the according maximum admissible face advance rate were thus correctly measured.

Jan 1, 2003

By André Vervoort

To learn more about the fundamental roof behaviour in South African coal mines, accurate underground measurements of roof deflection were conducted in bord and pillar panels during development and pillar extraction. To conduct these measurements a survey levelling technique was adapted. The overall system reading accuracy was established as being within 0,4 mm on a 15 m sighting distance. The underground sites were modelled and, for a roadway development, similar symmetrical curvatures were calculated by a linear elastic model, as were measured underground. In a pillar extraction panel, a significant amount of bed separation was measured and the roof underwent an a-symmetric movement due to the close presence of the mined out area at one side of the roadway.

Jan 1, 1992

By Liu Changyou

This paper analyzes experimentally the stress-strain relationship and the characteristics and variation trends of acoustic emission signals of the medium- and high-burst-prone coal/rock fracture process. The precursor and sensitive characteristics of the acoustic emissions when the burst-prone coal/rock fracture occurs were obtained. The characteristic index and the feasibility for predicting rock bursts using acoustic emissions technology were proposed. Keywords: burst-prone coal/rock, acoustic emissions, energy, prediction index

Jan 1, 2003

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

The applicability of four empirical strength criteria for rock masses, namely, Hoek and Brown, Bieniawski - Yudhbir et al., Rarnamurthy - Arora and Johnston - Sheorey et al. has been tested against the triaxial test data for jointed plaster of Paris of Arora. The following modified Bieniawski criterion shows best agreement with the triaxial test data for jointed plaster of Paris:- where st = axial compressive stress at failure, MPa; s3 = confining pressure, MPa; scm = uniaxial compressive strength of jointed plaster of Paris, MPa; and B = - 3.988 + 9.674 (scm )-0? 185 The modified Bieniawski strength criterion can be used to estimate the strength of a coal pillar, based on a knowledge of stress distribution in it.

Jan 1, 1992