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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 Shu Cheng

Abstract: Based on the consolidation theory of soil dewatering and land subsidence prediction methods in soil mechanics, preliminary analysis of surface subsidence and movement due to groundwater withdrawal was performed using the stochastic theory and the principles of effective stress and the method for subsidence prediction was derived, Surface subsidence due to coal mining was then superimposed to obtain the total subsidence. Keywords: land subsidence, rock and soil consolidation, stochastic theory, effective stress

Jan 1, 2002

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 Shugang Li

The layout and efficiency of the degasification system is heavily dependent on the distribution of and the methane migrating mechanism in the mining induced fractures in the overburden strata In this paper, physical modeling and numerical simulation are employed to study the mining-induced fracture zones and the stress distribution in these zones. The physical models show that both the fracture and compaction zones form a parabolic shape in the over- burden strata along the panel transverse direction. When this is generalized to the panel longitudinal direction, the shape of the mining induced fracture zone over the longwall gob can be represented by a 3-D elliptic paraboloid function. Through the observation of bed separation, fracture density and distribution pattern in the physical models, it is believed that the development of the parabolic fracture zone can be divided into three distinguishable phases. The expansion characteristics and sizes of the fracture zones can be estimated. The numerical stimulation studies can be used to estimate the extent of the stress release zones. The stress release zones can be also divided into a high compressive stress - fracture closure zone, stress relieved - fracture expansion zone, and intact zone. The methane desorption model is then used to analyze the methane migrating pattern in the fracture zone. The developed model has been verified by the results of a longwall gob degasification tests at a coal mine in Yangquan, China with a fair agreement. Such studies show that the model can provide a reasonable accurate tool for designing the gob-well degasification system for longwall mining operations.

Jan 1, 2005

By Dion Pastars

Resin based grouts are the main form of rock bolt anchorage in the underground coal industry in Australia and New Zealand. To be effective, the system requires the mixing of the catalyst and mastic components of the resin, as well as shredding of the laminate cartridge that contains the resin. An unknown measure is the load transfer characteristics of a bolt where the resin is well mixed but remains encased in the cartridge (gloved). Laboratory and in situ field investigations have been undertaken to quantify the performance loss due to mixed gloved bolts. This work showed repeatable results, indicating serious performance loss of the gloved and mixed system, with load transfer approximately 10- 15% of a non-gloved system (MacGregor, 2004). The in situ testing has demonstrated the relationship between the adhesion qualities of the resin and the mechanical interlock generated by radial confinement with progressively increasing tensile load. Effective load transfer is defined by the ability of the system to sustain shear stress on the bolt hole wall.

Jan 1, 2005

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 Kot F. v. Unrug

In spite of the extensive research and accumulated experience from mining operations, an appropriate pillar design remains an open issue. Due to the shrinking reserve base, present mining operations move to more challenging conditions. There are usually thinner seams and geologic conditions are often unfavorable. For instance, mine roof and floor are weak or deteriorate rapidly when exposed to moisture fluctuations in mine atmosphere. It is common that pillars of current operations are more- squat than several decades ago when the bulk of pillar research have been carried out in thicker seams. The methods of pillars calculations were developed regionally (in particular countries) to suit design needs for the most important seams, from which the majority of coal production was coming (e.g. Pittsburg or Pond Creek seams in the eastern of US). Most of the pillar formulae use similar approach by assigning weights (using constants) to the influence of coal (seam) strength and pillar widthheight ratio on its final strength. That mechanistic approach has sound theoretical base and good data supporting its validity. However, what seems to be missing is a qualification system of the mine conditions, which are outside of the data base or do not comply with the theoretical model applied in pillar strength formulae. This paper attempts to address that issue in two major categories, first when only a drill core data are available, and second during the mine development observations can be made directly on a coal seam. The composition of a coal seam is also addressed, (which often changes rapidly across the reserve), and discussed its influence on pillar's structural performance. In summary a set of practical design rules is presented.

Jan 1, 2005

By E. Bane Kroeger

In underground coal mining, there is often a need for supplemental support for mine openings. In the past, one of the most common types of supplemental support was wooden posts that were installed against the roof using opposing wooden wedges. Underground, wood has the tendency to shrink from moisture loss and dislodge if not under load. Another drawback to using wood is the variability in strength between posts. Defects in the wood such as knots can cause weaknesses that often lead to premature failure of supplemental support. Many mines that install large amounts of supplemental support in certain areas of the mine such as bleeder entries in longwall mining have switched to using steel props rather than wood posts or cribbing. The reactions of steel props under loading are well- defined and there is little variability in strength between props. Steel props and can be fabricated so they can be used in a variety of roof heights without modification. Wooden posts often have to be cut to a usable length underground before installation. This adds to the installation time of the wooden posts. Steel props are more compact and can be lighter than their wooden counterparts, depending on the length and carrying capacity. Often times it is desirable to prestress the supplemental support to improve roof stability. This is often problematic and time consuming with some types if supplemental support. With these factors in mind, research was undertaken at Southern Illinois University Carbondale (SIUC) to design a new steel roof prop. Several new clamping designs were developed and prototypes were fabricated and tested in the labs at SIUC in the spring of 2003. The designs were modified and refined and the first full-size prototype roof prop was tested in January 2004. With the success of the full-size prototypes, a cooperative research program was established between SIUC and Excel Mining Systems, Inc. to further develop the new prop. Development went smoothly and the prop has performed as expected. In February 2005, SIUC and Excel entered into an exclusive licensing agreement. The prop design under license, the K-Prop, will be installed using a lightweight portable hydraulic jacking system under design that can be used to prestress the props, applying load to the roof and floor during installation. By varying the installation load, the prestress load can be varied from about 1,000 to 10,000 pounds (4.448 to 44.48 kN) and at the same time, the carrying capacity of the K-Prop can be varied from 10,000 to 100,000 pounds (44.48 to 444.8 kN). The K- Prop is composed of only 3 pieces and laboratory testing has suggested that it can be installed by a pair of workers in less than two minutes. The speed of installation of the K-Prop should significantly reduce the total installed cost for supplemental support and should provide a safer working environment by prestressing the prop, which will add support load to the roof and floor during installation.

Jan 1, 2005

By Shuangsuo Yang

Coal ribs can roughly be divided into three types: (1) roof and floor rocks are similar to ribs, (2) roof and floor rocks are stronger than ribs, and (3) roof and floor rocks are weaker than ribs. Different types of roof/floor and ribs will exhibit different types of ground pressure around an entry. Most coal mines belong to the 2" type with ribs fractured or even broken. In this case the mine ground pressure depends on the occurrence and development of deformation and fracture of the ribs. If it's not properly controlled, the following deteriorating process will follow: the roof bent downward > the ribs squeezed and broken > rib sloughing and collapse > rib support to the roof reduced > the roof bent downward further > the ribs broken further. In this paper rib instability is divided into four types: compression/shear sliding rib fall, gravity sliding rib fall, cross arch rib fall, and cleavage rib fall. Since failure in thin walls is largely of tension type whereas that of thick walls by compression or shear, a concept is proposed here to utilize a thick bolted rib to stabilize entry ribs, i.e. the thickness to height ratio of the thick bolted ribs

Jan 1, 2005

By Naj Aziz

n Australia, the common method of laboratory testing of bolt for load transfer capacity determination is by short encapsulation push testing. Some concerns are raised about the validity of the test methodology, as the method does not reflect on the actual load transfer characteristics of bolt in real field situation. Thus, laboratory testes were carried out to examine the load transfer mechanisms of bolts in both the push and pull conditions. Tests were conducted by shearing a short resin encapsulated bolt out of a cylindrical steel sleeve. Three types of bolts with different surface profile configurations were tested. The study was complemented with numerical simulation of the test methods. Irrespective of bolt type the average shear load and shear stress values were found to be greater in push test, and the displacement at peak shear load was greater in pull test. The average shear stifhess values were greater in push test. The numerical simulation of the bolts provided a clear understanding of the stresses and strains generated by different bolt profiles during both the pull and pull testing process, thus allowing a better appreciation of the load transfer mechanism process.

Jan 1, 2005

By Luis Giraldo

Rock bolt installation characteristics near roof falls have been identified as contributing to failure. One documented and regularly occurring failure mechanism is loss of anchorage between the grout and the rock wall of the bolt hole. Key contributors to the integrity of the grout interlocking with the rock mass are the diameter of the hole relative to the diameter of the bolt, resin versus cement type grouts, rock type, and condition of the hole. Smooth bolt holes consistently exhibit less rock bolt load bearing capacity than rough walled holes. The goal of this study was to quantify the benefits of borehole conditioning and develop a systematic approach that would allow improvement of roof bolt performance. A new rock drilling technology, the Helical Drag Bit (HDB), was used in the early stages of this project to condition the borehole. The HDB cuts a prescribed helical groove in the borehole wall. When a bolt is grouted into a hole that has been conditioned using the HDB, the grout fills the helical groove and provides a stronger, more reliable mechanical lock between the rock and the grout. This paper presents results of a series of tests conducted in the laboratory and in the field. Short encapsulation pull out tests have demonstrated considerable increase in pull out strength and more consistent bolt performance when hole geometry is modified using the HDB. The results have also shown that greater load bearing capacity improvements are obtained in mines with weak roof rock, where it is most needed. Additionally, bolts recovered for examination suggest that hole conditioning during roof bolting reduces the effects of finger gloving. Therefore, mines with stronger roof rock could also benefit from hole conditioning. A new roof bolt that uses the HDB principle has been designed and tested. The new system works with existing bolting equipment requiring minimal modifications to the traditional procedure used for installation of fully grouted rebar bolts. The use of these higher anchorage capacity bolts will allow mines to reduce severe injuries and fatalities resulting from ground failures while maintaining or increasing productivity.

Jan 1, 2005

By Jisheng Han

Interaction effect exists when two or more neighboring seams are mined. The mining activity in one seam causes stress redistribution and strata movement in both the roof and floor. When the thickness of interburden between two neighboring seams is small, some hazardous situations like roof cutters, roof falls, floor heave and rib spalling in one seam may be encountered due to the effect of the mining activity in the other seam. An investigation about roof cutters, roof falls and floor heave was conducted in a West Virginia mine to study the interaction of multi-seam mining. The mine is mining the Sewickley Seam, which is about 90 ft above the Pittsburgh Seam which was mined in the 1960's using room and pillar method. The roof falls and other stability anomalies of entries observed underground in the Sewickley Seam were found closely related to the location of barrier pillars and gas well pillars left in the Pittsburgh Seam. Then numerical modeling was carried out to check the interaction between these two seams of mining. It was found through the modeling results that in the areas where pillaring operation was conducted in the Pittsburgh Seam, the stress concentrated less in the Sewickley Seam while near the edge of the gas well pillars and barrier pillars, high stress concentration was observed. It was concluded that under the geological conditions of the study mine, the entries stability would be affected by the lower seam mining activity, especially above or around the edges of pillars.

Jan 1, 2005

By Dr. Daniel W. H. Su

t is both an honor and privilege for me to nominate Dr. Daniel W. H. Su for the inaugural Syd S. Peng Ground Control in Mining Award. I can think of no better candidate than Daniel that is deserving of this award ? an award to recognize technical excellence in advancing ground control technologies [and] approaches. Daniel?s education and career spans over three decades. Having graduated from National Cheng Kung University in Taiwan in 1972 with a B.S. degree in Mining and Petroleum Engineering, he spent the next four years in the Taiwanese military and mining industry, before moving to the United States, where he enrolled in West Virginia University?s Mining Engineering Department. He received his M.S degree in Mining Engineering from WVU in 1978; then in 1982 became the first person to ever receive a Ph.D. in Mining from WVU. His thesis was titled, ?Development of Ultrasonic Techniques for the Measurement of In Situ Stresses?.

Jan 1, 2006

By J. F. T. Agapito

Overcore measurements at the North Fork Valley (NFV) coal mines in western Colorado have shown that horizontal stresses are highly anisotropic. Measurements have been made in four mines at various depths. In many measurements, the maximum horizontal stress is three to four times higher than the minor horizontal stress. At the West Elk Mine, maximum and minimum horizontal stresses of 24 MPa and 6 MPa, respectively, have been measured at depths of 640 meters (m). Under highly anisotropic stress conditions, ground control problems associated with both high and low horizontal stresses can develop. While high horizontal stresses can produce cutter failures and floor heave, low horizontal stresses can allow block fallouts. This paper summarizes the horizontal stress measurements in NFV mines and, in particular, in the West Elk Mine where their role in roof fall and floor heave failures is discussed. This experience has led to an improved use of ground support and safer mining operations at depths of 640 to 700 m.

Jan 1, 2005

By S. Mehrdad Heidari

The concentrator plant is located near a very high (105m), large (- 500m) and steep (a, > 45' ) wall. Geologically, the wall was composed of very complicated formations and broad range of soils and rocks type in which a variety of local typical failures models have been occurred on different soil and rocks formations. The construction of concentrator plant structure and other activities have been overshadowed by these local failures. Safety and risk of huge failures were serious concern. For distinguishing the wall conditions a broad campaign of slope stability investigation with detailed geotechnical drilling on berms of wall have been done and the structure of slope was fully identified and further Lab tests and slope simulation models done. Slope stability of the wall was analyzed and the effects of instability factors were described in short term (for construction phase) and long term (for production phase).The results indicated, if a well-appointed drainage system is implemented, the overall stability of the wall in short term will not cause any particular problem but about long term stability, the wall must be supported by rock bolting, consolidation grouting, shotcrete reinforced with fibers. The results have been confirmed by passing one year after this investigation.

Jan 1, 2005

By S. S. Peng

To acquire knowledge on the general trend of primary and supplemental supports used in tailgates of US longwall mines a survey of 76 mines with 86 longwall panels was carried out in the summer of 1993. This paper attempts to provide some insight on most commonly used support types as primary and supplemental supports in tailgates. Analysis of the surveyed data show that fully grouted resin bolts and cribs are the most commonly used primary and supplemental supports in tailgates in longwall mines of USA.

Jan 1, 1994

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 Kresho Galovich

Quinsam Coal Mine located in Vancouver Island, BC, Canada is mining the No. 1 coal seam at the 2 North/3 North mine and the No. 3 coal seam at 4 South Mine. This paper describes its geology and structure, mining and processing, development and depillaring practices as well as methods of roof supports under various geological conditions.

Jan 1, 2005

By Gift Makusha

Total extraction mining of coal is being undertaken on a small scale by Anglo Coal South Africa. Longwall mining has been done in the group at New Denmark colliery since the 80s and recently the company started experimenting with pillar mining at Goedehoop colliery. Total extraction mining methods result in caving of the overburden strata that is often accompanied by displacement of air causing wind blasts. The windblasts can cause injuries to underground mining personnel, damage to mining equipment, and ventilation disruptions. Some incidents of windblast damage and injuries have been recorded at both New Denmark and Goedehoop Colliery that prompted mine management to experiment with the goaf warning device, an instrument developed by CSIR mining technology to give early warning of impending god. The initial version of the Goafwarn was originally developed to operate as a stand-alone unit, with LED lights mounted on the device that would flash if the unit was triggered'. That set up would be effective in small working areas where the flashing lights are visible to all workers. This requirement is more difficult to satisfy in longwall application of the unit where mining activities are spread over large linear distances of up to 250 m and the flashing lights would not be visible to all workers in the longwall face. It was also found that the single unit had limitations in distinguishing between man made vibrations and cracking in rockmass leading to an unacceptably high incidence of false alarms. The case studies describe the experiments that were carried out to ameliorate the two issues mentioned above through networking more than one unit. This was achieved through the development of radio communication from the warning devices to a central control (Trollex Data Acquisition Unit) from which the alarms are issued once the prescribed triggers are satisfied. The operating system was made up of 4 Goafwarn units installed 30 m apart with 2 in the tailgate and other 2 in the main gate. The alarm state was defined where 3 or more units were triggered simultaneously. The concept was applied successfully in real time during longwall mining, and management procedures have been developed to achieve safety through the system including switching off the shearer, and flicking the lights to warn people.

Jan 1, 2005

By Quanxi Wang

This paper presents a new stability mapping system which tightly integrates structural and geologic mapping with geo- mechanical stress analysis for use in support design and mine plan- ning. The structural and geologic input can include such local characteristics as: rock strength, layer thickness, faults, stream val- ley effects, etc. The geo-mechanical stress inputs can include overburden stress, multiple-seam stresses, abutment stresses, etc. The mapping system utilizes the popular AutoCAD/SurvCADD platform as a foundation for inputting the geologic characteristics, and then integrates the boundary-element program LaModel for determining the geo-mechanical stress influences. Additional func- tionality in the system allows the user to transform each individual geologic or stress factor into an individual rating and then the user can apply various weighting functions to the individual factors to generate an overall stability index map for the mine property. A sample case study is included in the paper to demonstrate the basic concepts, procedures and utility of the stability mapping system.

Jan 1, 2005