Search Documents

The effects of corrosion upon the load bearing capacity and ultimate tensile strength of galvanised friction bolts were determined. Corrosion rates were measured by overcoring of the reinforcement elements in situ. Overcoring was achieved at several sites using a specifically designed overcoring drill rig. This rig has the capacity to drill up to 3.0m long 140- diameter core in any orientation and up to a collar height of 7.0m. The recovered core and friction bolt elements were pull tested in the rock mechanics laboratory at the WA School of Mines (WASM) in order to determine their load-displacement characteristics for various embedment lengths. The friction bolt elements were then analysed for rates of corrosion and tested for their ultimate tensile strength. Relationships were formed between these data and the environmental conditions to which the friction bolts were exposed, in order to determine the major factors affecting the corrosion process and how they affect bolt performance.

Jan 1, 2005

By Jian Yong Chen

The results of a laboratory investigation of pullout tests conducted on grouted, plain cable bolts are reported. The main purpose of the study is to determine experimentally the shear bond stiffiess of cement grouted and resin grouted cable bolt systems, which is an important input parameter for numerical modeling of cable bolt support. Three nominal embedment lengths of 152mm (6 inch), 203mm (8 inch) and 254mm (10 inch) were used and two types of grout namely cement and Anchortite resin were used. Two water- to-cement ratios of 0.3 and 0.4 were employed for the cement grout. The concepts of effective shear stress and effective shear stiffiess are explained and adopted to calculate the shear bond strength and the elastic shear bond stiffiess per unit length. A comparison of results with previous research is presented to reveal the effect of water-to-cement ratio and embedment length on the shear bond characteristics of grouted cable bolts.

Jan 1, 2005

By R. Karl Zipf

Stress analysis programs such as MULSIM/NL, LAMODEL, MinSim 2000, and EXAMINE TAB are used in the mining industry to analyze stresses and displacements in coal mines, platinum mines, gold reefs, and tabular-type deposits. These relatively simple numerical models can efficiently simulate yielding and failure of a rock mass near a mine opening and subsequent stress transfer. The main input parameters for these models are a family of nonlinear stress-strain curves for the in-seam material. For many applications of these models, such as in coal mining, extensive field measurements and observations exist from which to estimate these stress-strain curves and calibrate the numerical models; however, such measurements are lacking for other types of mines. A three-step method is presented to determine nonlinear stress¬strain curves for boundary-element (BE) programs used in many mining applications. The method requires a suite of laboratory¬scale strength tests at various confining pressures. Note that this analysis uses laboratory-measured strength and modulus properties of alluvium. The dependency of the mechanical properties of alluvium on scale has not been established, although more tests are being planned. However, it is believed that the relative comparisons performed are valuable regardless of scale effects. First, the FLAC2D computer program is used to model the laboratory tests and determine cohesion (c) and friction angle (0) for a Mohr-Coulomb material model. Second, another FLAC2D model uses the c and 0 values to calculate vertical stress and strain around a single opening in the rock mass. This model calculates the stress-strain path of points at various distances from the opening boundary. Finally, based on these stress-strain paths, the stress-strain curves needed for a BE analysis are derived. This three-step method was demonstrated in a large, flat-lying underground test facility in very weak rock. The BE analyses agreed well with observations of failure at the test facility and provided a basis for evaluating the behavior of alternative layouts.

Jan 1, 2003

By Simon H. Prassetyo

Violent failures of coal pillars, known in practice as coal mine bumps, have long been a subject of investigation. Many field investigations have considered geological conditions that create high stress in the pillar to be the main causative factor leading to bumps. In recent years, constraint at interfaces has been observed to be associated with the occurrence of coal bursts. This paper shows how interface friction and width-to-height (W/H) ratio affect the degree of violence of coal specimen failure. A series of UCS tests on 178 coal samples from bump-prone (Utah, called UT coal) and non-bump-prone (West Virginia, called WV coal) mines were conducted. Three violent failure parameters? peak sound pressure level, core zone failure, and ultimate stress? were used to assess the violence of failure. The degree of violence were investigated at varying interface frictions (high: µ = 0.40, medium: µ = 0.22, and low: µ = 0.13) and W/H ratios (W/H = 2, 3, 4, 5, 6, 7, 8, 10, and 12). A simulation series of modeled coal specimen under uniaxial loading was also conducted using the finite element method to observe the equivalent plastic strain distribution experienced by the modeled coal specimens. It was found that the violence of coal specimen failure depends on W/H ratio and interface friction. Interface friction has more significant influence on the violence of failure at high W/H ratios than it does at low W/H ratios.

Jan 1, 2011

By Raymond M. Stateham

The Bureau of Mines has developed a portable, lightweight device for determining the integrity of resin-grouted roof bolts. The instrument functions by sending a known pulse of ulstrasonic energy into the bolt and comparing the amount of energy reflected hack with the original measured pulse. Where the bolt is properly bonded or coupled with the rock, the energy passes through the bolt-resin-rock interface and dissipates in the rock. Where the bolt is not properly coupled, the energy is reflected back along the bolt to the instrument. The ratio of reflected energy to the pulse approximates the fraction of the bolt's surface that is improperly bonded. The instrument is easy to use. The operator holds the sensor head of the device firmly over the head of the bolt to be tested, causing the pulse of ultrasonic energy to be imparted to the bolt. The device then listens for reflected energy. This cycle is repeated about 200 times per second until an evaluation of the bolt installation is made and indicated by colored lights in the display window. Bolts having 50% or less of their surface bonded are considered had and cause a red light to he illuminated. Bolts with 50% to 75% of their surface bonded cause a yellow light to be lit and are considered marginal. Bolts having more than 75% of their surface bonded are considered good and cause a green light to be lit. The indicator lamp remains on for about 4 seconds and then goes off, indicating that the instrument is ready for another test.

Jan 1, 1982

By James H. Fletcher

Much data and experience has been accumulated, especially in the last 5 or 6 years, to show that modern roof trusses, both of the Birmingham type and the bolt-and-channel type, perform well to support certain kinds of coal mine roof. For some time now, roof trusses of the Birmingham type have been effective in controlling thick shale roof, which tends to sag and fall over a period of about eight weeks. These falls leave dame-shaped cavities which are reminiscent of the pressure arch theory of roof failure. Lately there has also been significant interest in supporting some longwall panel entries with channel and-bolt systems. These systems utilize two vertical center bolts and two 45 degree end bolts through a heavy gage channel. Both types of truss systems have long bolts, which require 8 to 9 Foot holes on a 45 degree angle, and point anchor resin bolts, which utilize a 2 or 3 foot rebar grouted in a thin annulus of polyester resin. Consequently, many coal mining people have been impressed with the very good anchorage characteristics and tensionability features of these systems. However, the same people have been proportionately dismayed to find that very often the most difficult part of introducing a trussing program to a mine is deciding on the machine specifications which determine the truss installation rate. The purpose of this paper is to indicate aspects of truss bolter design and use which are not shared by roofbolter design and its vertical pattern of bolts. Of the two types of drill units conventionally accepted, arm feed units and mast units, only one 15 judged suitable. While the arm feed unit offers low collapsed height with a long drill feed length, a great attribute for low seam use, it has not lent itself to being rolled over to a 45 degree angle. One reason for this inability is that many items which are conventionally mounted on the arm feed unit cannot be allowed to rotate with the unit. These items include drilling controls, tool Cray, lights, canopy or safety post, panic bar, and stab jack(s). If these items were independent of the arm feed drilling unit, they would have to be out of the way of the boom arms, links, and feed cylinder as the unit moves. To ease this problem, the arm feed unit can be made very short and even mounted across the machine. However, the resulting short feed length is undesirable. Several short drill steel extensions are required to drill a relatively long hole, making this procedure time-consuming as well as increasing the likelihood of drilling a crooked hole. Another reason for abandonment of the arm feed unit is that a very stiff drill guide (or "centralizer") is required to start the hole. Using the mast tilt control, the roof bolter operator will force the bit against the roof and will rotate the bit without feeding to cut sideways into the roof to form a hole face. Omission of this step often results in the skidding of the bit toward the rib. To date, no such sturdy drill guide has been designed for an arm feed drill unit. The mast style unit has been the most successful design for angle drilling. It is easily rolled over to a 45 degree angle or to 90 degrees and, on single head models, often has been made to roll through 180 degrees. The mast style is compact, in plain view and is easily fitted with heavy duty manual or hydraulically operated drill guides. It can be mounted directly on a machine chassis, or onto a swinging boom, or onto a slider arrangement in a broad variety of configurations which will accommodate controls, tool trays, stab and roof support jacks, lights, canopies, and panic bars. The only significant difficulty with mast drilling units occurs in lower mining heights when desirable low mast height is accompanied by undesirable short feed length. While the lead extension in the drill string can be as large as the mining height will allow, a short feed length will permit only the use of shorter middle extensions and drivers, and perhaps will. require the use of two different length wrenches. The associated problem is the relatively short distance from the drillhead to the drillguide. This distance is further shortened when raising the drillhead to get the bit to the roof. One might keep longer lead extensions on the machine; drill a vertical pocket for the bit, starting the angle hole at this pocket; or modify the mast. If the mast is made so that the distance from the drillhead to the drillguide remains constant while the mast extends, after which the drillhead travels up the mast, the shorter mast can be made to start the hole more easily. However, short teed length will still result in longer drilling time. It is obvious from these points that a proper choice of mast is simply the highest one which will be moved easily in the given mining height. This, of course, goes against the grain of most experienced coal mining people who realize that a relatively small decrease in mining height can create a problem correctable only with the purchase and installation of shorter masts. If the mining

Jan 1, 1982

By Michael Karmis

During the last 25 years, technological advancement and subsidence research have resulted in more accurate and diverse prediction capabilities. The work presented in this paper focuses on the development of integrated prediction capabilities for dynamic deformation indices and for strains over sloping terrain. In order to assist subsidence engineers with accurate prediction methodologies, the research also addresses the development and validation of techniques that can enable model calibration using alternative measured subsidence parameters such as horizontal or ground strain instead of the traditional vertical subsidence values. This paper presents the basic concepts and validation of the above mentioned enhanced prediction and control methodologies, which are necessary for effective assessment and control of mining-induced subsidence, using examples and case studies. In addition, a risk assessment approach for evaluating landscape stability over mined areas is presented. The enhanced prediction methodologies have been incorporated into the Surface Deformation Prediction Software (SDPS) package.

Jan 1, 2008

By Herryal Z. Anwar

In recent years, many both mining and civil engineering projects are developed in the area which may be categorized unstable, such as fractured and jointed rocks or "water sensitive rocks". For example, in underground coal mining, in which the floor strata normally consist of weak shale, the problem that may arise in roadway is floor heaving. In this case cement injection in borehole appears to be an economically effective method in order to make roadway more stable. This method may be also applied in the projects in which the constructions are laid on fractured and jointed rock. Nevertheless, the successful cement injection is much dependent on the water-cement ratio and the injection technique. This paper discusses the application of cement grouting to improve the ground stability in longwall mining, the laboratory experimentation of cement grouting and the method of floor injection.

Jan 1, 2000

By Erdal Unal

In order to define the support parameters, by use of empirical approaches, it is particularly important to analyze the rock-load height that should be controlled by supports. The controlling mechanism however, depends on the type of support used there- fore, the rock loads should be treated differently. In this paper, an empirical equation is introduced which relates the rock-load height (ht) to a quantitative rock quality index (REIR), and roof span (B) as fallows:

Jan 1, 1986

By Hamid R. Nikraz

This paper reports the finding of a comprehensive investigation carried out to evaluate the compaction characteristics of saturated sandstone aquifers in the Collie Coal Basin under conditions anticipated during dewatering operations. A triaxial technique has been adapted to evaluate the compaction characteristics of the sandstone aquifer in the Collie Basin. This technique, which allows the strata stress regime to be reproduced by triaxial loading with zero lateral strain, also provides a precise evaluation of lateral stresses and consequently Poisson's ratio under in situ conditions. The paper contains a description of the equipment commissioned, test techniques, results, analysis and interpretation of the data obtained. The testing evaluation techniques are general in nature and can be applied to field situations in locations where similar weak sandstones occur.

Jan 1, 1993

By Shengli Yang

Solid backfill mining, as a green mining technology, has the advantages of a higher resource recovery, less damage to the environment and the ability to dispose of waste underground. It has been applied successfully in the Xingtai and Xinwen mining areas in China. Strata behavior and the movement of overburden strata in this mining method are very different from fully mechanized longwall mining, which results in a natural caving, because the gob is filled with waste rock, fly ash and other backfill material. The results of field measurements showed that the periodic weighting interval of the main roof is longer; the magnitude of periodic weighting is smaller; the peak and zone of influence of the front abutment pressure are smaller than those in fully mechanized mining; that major deformation of mine roadways occurs within the influence zones of the abutment pressures; that the falling space of immediate roof is smaller, with the lower portion falling more sufficiently than the upper; that the main roof subsides slowly; and that the falling height of the overlying strata is greatly reduced.

Jan 1, 2011

By Tetsuro Esaki

This paper describes the present condition of mining-induced damages in Japan and the characteristics of them. The change of mining operation and environmental condition has caused various significant impacts on the ground environment: subsidence, cave-in, springing out of ground water etc. The characteristics and the influences of these phenomena are discussed by investigation of the actual conditions and by fundamental studies. Some counter measures are also introduced.

Jan 1, 1989

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

Retreat longwall method with re-use of gateroad has been traditionally employed in Japan, mainly because of the advantage of higher recovery. But the Miike Colliery has newly introduced the interpanel-pillar system to maximize the production efficiency. The Department of Mining, Kyushu University, in cooperation with the Miike Colliery and the Coal Mining Research Center (CMRC), has conducted research into the optimum design of interpanel-pillar, because this system may be adopted in other coal mines as well. Field measurements of the deformation of the gateroads and of stress distribution in the pillar show that the designed layout of the panel and pillar and the supporting system are proper from the viewpoints of gateroad maintenance and spontaneous combustion in the pillar.

Jan 1, 1990

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 Megan N. Witkowski

The purpose of this study is to investigate the susceptibility characteristics of domestic and agricultural water supplies that may be vulnerable to diminution or total loss of supply when active longwall mining is in the vicinity of the potable aquifer systems of southwestern Pennsylvania. The study focuses on water supplies in Greene and Washington Counties with reported losses or reductions in supply that were attributed to the seven active longwall mines in operation between 2003 and 2008. The undermined water supplies represent a variety of hydrogeological settings and mining conditions prevalent in the southwestern Pennsylvania longwall coalfield. The susceptibility characteristics investigated include hydrogeological setting, proximal location to mining, and climatic conditions at the time of impact. The investigation considers the relationship between geological and mining characteristics. Geological characteristics include overburden composition and geomorphology of the surface; mining characteristics include mining method, depth, and extraction thickness. It was determined from the study that the average overburden thickness at each mine site is indicative of hydrological changes to potable aquifer systems, with the overall impact frequency at each site decreasing as depth to mining increases. However, depth to mining was not the governing factor when hydrological responses were determined on a case-by-case basis. To fully analyze and hopefully predict potential impacts by longwall mine subsidence, it is necessary to consider additional factors including topography, distance to mining, mining method, and climatic conditions at time of undermining. The investigation serves the purpose of enhancing our understanding of hydrogeological responses to longwall mining and will aid in predicting potential impacts to southwestern Pennsylvania?s wells and springs in proximity to future mining.

Jan 1, 2011

By Linsheng Xu

When coal seam is under the integral thick sandstone (or conglomerate), the structure of thick sandstone is integral and its strength is rather high. After coal is extracted, the roof is hanging and caving is difficult. When the method of full caving is adopted in the longwall working face to control this kind of hard and difficult caving roof, the effect of roof-controlling affects directly the production and safety. In China Datong Coal Mines the extracting coal deposit is Datong Jurassic period. Most of the roof is sandstone and conglomerate. It is hard and difficult to cave. Having used special hydraulic power supports and other special ways to deal with the roof, the hard and difficult caving roof has been controlled.

Jan 1, 1986

By André Zingano

Pillar collapses have been studied for several years and can be classified into two types: nonviolent squeeze or violent pillar collapse, i.e., controlled or uncontrolled pillar collapse. Underground coal mines in Brazil are considered shallow mines, because the overburden thickness varies between 20 and 400 meters. For this reason, the coal pillars should be designed as permanent pillars to avoid subsidence and groundwater problems. In the last two years, various pillar collapses occurred, one being a violent collapse and the others being squeeze collapses. All these collapses were considered massive pillar collapses because many pillars were involved. In the violent pillar collapse case, the width¬to-height (w/h) ratio was less than three; although, the pillar safety factor (SF) was more than 1.3. There was a collapse of about 100 pillars in less than three hours. The objective of this study is to determine the mechanism of pillar collapse for this violent pillar collapse. Convergence monitoring and numerical modeling were applied to combine field data and theory to simulate the pillar collapse. Convergence field measurements introduce the arc-effect behavior for the roof. The results showed that other factors besides pillar geometry caused the pillar failure. Other parameters, mainly dip of the coal seam and presence of fractures and faults may affect the strength and failure mechanism of a pillar.

Jan 1, 2004

By Gregory J. Chekan

Developing a coal seam that has been influenced by previous mining in seams either above or below can result in severe ground control problems. In many instances, interactions between operations are inevitable. but improvements in ground stability can usually be achieved by-predicting potential problem areas in advance and adjusting the mining plans accordingly. This report investigates four case studies involving interactions between operations that used both room-and-pillar and longwall mining methods. In each case, geologic and mine design parameters are addressed, and those parameters found critical to the interaction are further defined in relation to observed ground problems.

Jan 1, 1990

By Collin L. Stewart

A summary of the results of a limited subsidence monitoring program at Cyprus Amax Coal Company's Shoshone Mine No. 1 in the Hanna Basin of south central Wyoming is presented. The monitoring program was designed to meet regulatory requirements for confirmation of subsidence predictions made in the mining permit. The longwall panels were 380 ft to 600-ft wide. Mining heights varied from 11 ft to 13.5 ft. Gate road development included three-entry and two-entry head and tail gates. Overburden depth varied from 150 to 945 ft. The coal seam and overburden dip at approximately 11 degrees. The overburden is very low to low strength mudstone and carbonaceous shale with low to moderate strength, silty sandstone interbeds. Panel W/h ratios varied from subcritical to supercritical. Smax was 0.9 of the mining height. The subsidence ratio was about 0.6 for subcritical width. The draw angle on the up-dip side averaged 20 deg. The draw angle on the down-dip side averaged 12 deg.

Jan 1, 1995