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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 D. Bigby

The current British Standard on rock reinforcement components used in coal mining (BS7861:1996) employs the axial double embedment tensile PET) test to determine the main system load transfer performance criteria, in terms of bond strength and stiffbess. This test involves measurement of load/displacement characteristics of a tendon fully grouted into internally threaded thick-walled steel tubes. Part 1 of the Standard covers rock bolting systems and Part 2 covers birdcaged cable bolting systems. Both parts are highly prescriptive, defining a specific design of bar (AT) and cable bolt (birdcaged). In 2000, it was agreed by the UK Rockbolt Research Liaison Committee that Part 1 of the Standard should be revised to allow further industry innovation of bolts and resins. Amongst other aspects, it was recognised that the double embedment test was limited by its inability to examine load transfer between the bolt and the host rock and the UK Health and Safety Executive commissioned an RMT research project to develop a new test to replace the DET test. The new test, the Laboratory Short Encapsulation Pull (LSEP) test has now been incorporated into a draft revision of BS7861 Part1 which has been issued for industry wide consultation. In 2003, it was agreed to begin revision of Part 2 of the Standard to encompass the many types of long tendon reinforcement which were, by then, being used in the UK coal mining industry. These included flexible bolts, deformed strands and tensionable systems encapsulated with resin, cementitious grout and combinations of the two. A second RMT research project was funded by the HSE to examine adapting the Laboratory Short Encapsulation Pull Test for the revised long tendon Standard. This project is nearing completion and a revised Part 2 of the Standard is being drafted. This paper presents some of the results of the two research projects, describes the laboratory tests and acceptance criteria developed and discusses the implications of the research on improving the understanding of reinforcement performance and its characterisation. In particular it shows how the new test can reveal very different load transfer characteristics to the double embedment test, as it takes the load transfer at the holefrock wall into account.

Jan 1, 2005

By Frank Schmidt

Due to changing geology, rib bolting is an ever-changing problem in ground control. With the economic status of the coal market, the need for safely increasing productivity in rib bolting is at an all-time high. The current solutions used in the market often require a long installation time (e.g. if collapsed holes have to be redrilled) or result in poor quality installations that can lead to insufficient ground control. A new bolting product based on the simple screw principle has been developed to bolt ribs faster and with a more repeatable, higher level of installation quality. The paper covers the product invention and development, the fastening principle and its advantages over current systems and first experiences in the market. The first underground trials have shown that the screw anchors can be installed safely and efficiently, often faster than traditional methods. The screw anchor does not suffer from conventional bolting issues such as collapsed holes due to bad ribs. The screw anchors are designed to obtain steel failure and load testing underground shows consistent fastening in all types of conditions.

Jan 1, 2013

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 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 P. Palmer

Using 3D numerical modeling as a tool to predict rock behavior during mining extraction is still an uncommon practice at many underground operations. This paper will discuss how 3D numerical models were used to identify potential underground instability problems in the later stages of mining. Detailed modeling was then used to make modifications to the mine design in order to minimize the impact of the potential instability problems. This paper will also discusses how the back analysis of a previous underground collapse event at the same mine in a room and pillar area was used to identify the instability problems for a new orebody being developed deeper in the mine. A 3D numerical model was first created in the MAP3D software to determine the conditions of the pillars and the magnitude of the vertical displacement above the mining zone prior to the underground collapse. The results from the ground collapse model were then used as a threshold limit for predicting potential instabilities and developing a model for the new mining zone. sing the results from this analysis, mine management has been able to implement a much higher percentage recovery of the reserve in a new, high-grade area of the orebody while minimizing potential instabilities in the future.

Jan 1, 2005

By A. Wilkinson

An investigation into the performance of support systems that are currently used in South African coal mines was conducted. Five most critical components of a support system have been identified. These components are resin, bolt, hole, machinery/equipment and the rock type. All five of these components are equally important as failure in any of these components will result in an inadequate support system. The performance of resin used in South Africa was investigated through in situ short encapsulated pull tests. The results indicated that in the majority of pull tests, failure took place at the rock-resin interface, indicating that the rock failed before the resin shear strength had been reached. It is therefore concluded that the strength of resin currently used in South Africa is adequate. It is found that although the maximum bond strengths achieved form both resin suppliers were similar, there was a considerable difference in the stiffhess of the resins. Roof bolt diameters and rib heights of bolts from three major manufacturers were evaluated (approximately 80 roof bolts from each manufacturer). The results showed there is a significant variation in bolt diameters and rib heights in the same batch in roof bolts supplied by certain manufacturers. The results also showed that on average there are insignificant differences between the parameters that determine the bolt profile (rib angle, spacing between the ribs and rib thickness). The influence of these parameters on bolt performance was very small and could not be established in situ. Roof bolts from four major suppliers were also evaluated through in situ short encapsulated pull tests. The results showed that the reinforcing system using roof bolts from all four manufacturers performed almost identically in sandstone, but somewhat differently in other rock types. The effect of bit types was evaluated. Both the stresses and the maximum loads obtained from the two-prong bits were greater than those obtained from a spade bit. Borehole annulus was found to be another important parameter that determined the support performance, and was investigated. The results from these tests showed that an annulus between 2.8 mm 4.5 mm resulted in the highest bond strengths. The results also showed that as the annulus drops below 2 mm, it appears to have a negative effect on the bond strength. The effect of wet and dry drilling on support performance was investigated. The results showed that wet drilling can-provide relatively greater system stiffnesses and slightly greater bond strengths. Short encapsulated pull test results showed very distinct differences between bolt system performance in different rock types and that sandstone produces significantly better results than shale and coal. From the results it is concluded that rock type is probably the primary factor influencing support system performance. A detailed investigation into the specifications of roofbolters that are currently being used indicated that the quality of installation of a support system is directly related to the performance of the equipment that is used to install the bolts. The important parameters, torque, thrust and speed of bolting equipment were investigated. The results showed that there are significant variations in these parameters in South African roofbolters.

Jan 1, 2005

By Vince Caggiano

The newly developed Hilti OneStep roof bolt combines the standard 5-step anchoring process (i.e. drilling, transfer of drill steel, insertion of resin, insertion of rod and tightening) into one easy operation. It also replaces 4 items (drill head and steel, adhesive cartridge and anchor) for simplified material management. This paper describes its history of development, and bolt, bolt drilling and bolt resin specifications, experimental test results in Germany where it achieved a reduction of 36-42% in roof bolt cycle time and 25-30% improvement in gateroad development rate. In the U.S. coal mines, a time study showed that OneStep can save approximately 50 seconds in installation time per bolt.

Jan 1, 2005

By Timothy Batchler

"Standing roof supports help to provide a stable working environment in coal mine gateroads. NIOSH participates in the development of these support systems by conducting full-scale tests in their Mine Roof Simulator. These tests have established the performance characteristics of every coal mine support system currently in use in the United States. Through these tests, mine operators and regulatory officials have acquired a fundamental understanding of the various types of support performance that guides the assessment and use of these support systems. However, some little known factors that influence support behavior can lead to misconceptions and, perhaps, a poor assessment of support behavior at times. This paper highlights several of these factors and discusses their significance. The paper also examines a current longwall tailgate support and discusses the inefficiency with this approach. Finally, the NIOSH Mine Roof Simulator (MRS) is the world’s most powerful load frame built specifically to test full-scale roof supports, including longwall shields. are addressed in this paper to provide a more in-depth assessment of the limitations of the MRS and what the MRS testing protocol is designed to do. INTRODUCTION Over the years, considerable research has been conducted to develop standing support technologies and to evaluate their performance characteristics. The load and displacement characteristics of the standing supports have been determined from full-scale testing conducted using the National Institute for Occupational Safety and Health (NIOSH) Mine Roof Simulator (MRS) (Barczak and Tadolini, 2008) located in Bruceton, PA. Performance traits, installation patterns, and ground responses are observed in various geological and mining conditions and are evaluated to determine the support performance in the field (Dolinar, 2010; Campoli, 2015; Zhang et al., 2012). Despite widespread use of standing supports, not every aspect of their performance and resulting ground support capability are always well understood. This paper discusses 10 factors, several of which are surprising, that impact standing support performance."

Jan 1, 2017

By Phillip E. Gramlich

Through extensive practical experience and multiple studies, tensioned roof control systems have proven beneficial in difficult mining conditions. Due to increased mining of less desirable seams, tensioned roof control systems have claimed a progressively larger share of the total number of roof bolts installed in the US coal industry. Each tensioned roof control system has its benefits, but also has problems concerning cost, cycle time, stress concentration, load retention, and protrusion into the mine opening. For example, resin-assisted mechanically anchored bolts (also known as resin point anchor bolts) have a fast cycle time and the potential for high tension loading, but the stress from the mechanical shell is concentrated in a small area and can create concerns with certain roof structures (Zhang and Peng, 2001). Resin point anchor bolts can be difficult to fully grout. The cost of resin point anchor bolts can be an issue; a two-piece resin point anchor bolt has a large number of parts (10) and manufacturing steps (9), adding to its higher cost. Tension rebar bolts can achieve high loads and are easier to fully grout but have longer cycle times. In addition, a high percentage of the installed load can ?bleed off? if the resin mixing time or the ?hold time? before tensioning is short-circuited. Tension rebar bolts can be notorious for protruding into the mine opening, creating an obstruction for man and machine (Figure 1), in particular, with reduced mining heights. A new tensioned roof control system has been developed that doesn?t have moving parts, is cost effective, provides fast cycle times, has a low profile, and is simple t install.

Jan 1, 2014

By Peter Zhang

Room and pillar mining under shallow depth is usually conducted with a high extraction ratio, which leaves small pillars with a low safety factor in some old workings in abandoned mines. Pillar sloughage, squeezing or even collapse may have occurred in those old workings with small pillars, either at the time of mining or later. The conditions of the pillars in abandoned mines can affect mining above them. Subsidence fractures and abutment pressure are indications of multi-seam interaction caused by squeezing or collapse of small pillars. To mine over areas with potentially unstable small pillars, the panel layout in the upper seam should be such that multi-seam interaction can be minimized and proper roof and rib support should be installed in the influence zones. This case study presents an example of multi-seam mining above small pillars under a shallow depth of cover. Though the interburden is relatively thick and competent, interaction is still present over the influence zones. Pillar stability and multi-seam interaction analysis, numerical modeling and in-situ monitoring are presented to demonstrate how the upper seam can be mined safely over the small pillars.

Jan 1, 2014

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 Thomas Barczak

Longwall mines typically use some form of standing support for secondary roof support in longwall tailgate entries. Although there have been several new support products developed for this application, there remains no universal design criteria to optimize the application of these support technologies. The requirement for optimization and proper support selection is to understand the degree of control that the support has on the ground behavior. The ground reaction curve and numerical modeling was used to evaluate the impact of standing support on ground behavior. LaModel was used to evaluate the impact of standing support on main roof and floor behavior and pillar yielding. The conclusion drawn from this study was that standing supports do not have sufficient capacity to control main roof or floor loading or prevent the resulting convergence of the tailgate entry. However, it is imperative that this "uncontrollable convergence" be considered in the support design to prevent premature failure of the support. A FLAC model was used to evaluate the near-seam roof and floor behavior in conjunction with the global vertical and horizontal stresses. The model suggests that standing roof supports can have some impact on the ground behavior as the elastic response of the rock is exceeded and rock structure deteriorates from the stress concentrations that develop around the tailgate opening. During this phase, the capacity and stiffhess of the standing support can be critical to the stability of the opening, as eventually the rock mass will be transformed into a partially detached structure whose weight must be supported by the standing support. This work is part of a ground control program of research at the National Institute for Occupational Safety and Health (NIOSH) aimed at improving mine safety by reducing roof fall injuries and fatalities.

Jan 1, 2005

By Tom Vandergritft

With advances in system design driving higher productivity, safety, and coal recovery, highwall mining is becoming an attractive option for extending reserve life at surface mines. Typically, highwall mining is performed by contract, with the system owner charging the mining company on a per ton basis, plus a mobilization fee. For this arrangement to benefit both parties, geotechnical planning is required to minimize risk to highwall mining personnel and equipment, while maximizing coal recovery. This paper discusses how highwall mining has been successfully implemented at a large surface coal mine in Colorado, including design procedures, operational factors, and productivity. The operation includes an area where four seams are mined in succession from the lowermost upward, requiring careful attention to seam interaction issues.

Jan 1, 2005

By Murali Gadde

Numerical simulation of cutter roof failure is a daunting coal mine excavation design issue. The complex progressive failure mechanisms associated with cutter failures are extremely difficult to replicate in computer models. However, this paper shows that with a proper selection of constitute behavior and implementation of cutting sequence, it is possible to realistically reproduce cutter failure using continuum models. Strain-softening material behavior and a systematic step-by-step model solving for a four-entry-and- three-crosscut model produced results that are similar to cutter patterns noticed in field. These results show that traditional modeling with geometries created in one model-step has limited applicability and may even provide misleading conclusions under some circumstances. Most importantly, the proposed modeling methodology provided some insights into the reasons for the inconsistent spatial distribution of cutters often noticed in coal mines. The modeling also revealed the usefulness of cutter mapping to determine the directions of in situ horizontal stresses.

Jan 1, 2005

By Xianxin Shi

In surface electrical exploration the high resolution earth resistivity method (HRRM) is a very effective method for detecting abandoned drift mines workings. When the abandoned mines are more than 500 ft (150 m) deep, its detection capability reduces greatly and requires more effort to implement. In Yan Quan coal mine, Shanxi province, we tried to adapt this method for underground application. Two survey lines were designed with the spacing of current and potential poles 20 m and 10 m, respectively and measuring points at 2m. The measurement radius of I line and I1 line were 140 m and 60 m, respectively, and the infinitely far pole was 1200~2000 m from the survey line. (Note the I and II lines are located on the north and south ribs of main tunnel, respectively). The survey results showed that abandoned workings were located at 25-70m, Survey Line I and at 80-1 10m, Survey line 11. Based on this finding, the longwall panel gateroads and set up entry were properly located thereby providing safe mining of the No. 15 Coal seam. Key Words: Abandoned coal mine, High resolution earth resistivity, Underground electrical exploration

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 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