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By Christopher Phillips

In light of recent events in the mining industry, there has been an increasing demand for technology to detect underground voids and abandoned mine workings. This paper summarizes the use of geophysical methods for this application. Geophysical methods have been successfully employed in the detection of underground voids in several disciplines, including foundation analysis, road and infrastructure analysis, and darn analysis, but is not yet being widely employed by the mining industry. The principal reasons are that the combination of the complex geological setting of a mine, mine design, surface conditions, and improper choice of geophysical method often makes the task of locating abandoned mine workings with geophysics a frustrating, time consuming, and often unsuccessful venture. While there are several geophysical methods that have been demonstrated for the successful detection of abandoned mine workings, there is still no 'silver bullet' to solve the problem in every case. The choice of geophysical method to detect abandoned mine workings should be made on a site specific, and sometimes target specific, level. This paper reviews current methods, and introduces recently developed methods, for the detection of underground voids and abandoned mine workings.

Jan 1, 2003

By Xinchuan Xia

Based on the stepwise strain features and the strength reduction law of the surrounding rock in extremely softrock roadway, this paper proposes a dynamic supporting principle. A new kind of stepwise reinforced technology, which consists of immediate shotcreting, timely bolting and delay grouting, was optimized after thorough analysis and illustrated with a successful engineering example in extremely difficult softrock roadway.

Jan 1, 2000

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 Erik Westman

Tomographic imaging, commonly known as a CAT-scan or CT-scan, is used in the medical field to generate a picture of the interior of a body. Current research is applying this method to bodies of rock so that rock failure can be better understood and predicted. This paper discusses the basic principles of tomography and describes recent developments in the use of seismic tomography in longwall coal mines, including results from a bump-prone longwall coal mine and continuing research.

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

By Yingxin Zhou

Data from these field studies and previous case study data have been combined to define ultra-dose seam mining environments and relate them to observed failure conditions. A rating system for ultra-close multi- seam design has been developed and field studies have verified the applicability of the design criteria according to geologic structures, lithology and spatial data. This rating system has been combined with design and depth considerations to create a protocol for quantitatively predicting potential damage in ultra-dose seam mining conditions. A brief discussion of the characteristics of ultra-dose seam mining and its related failure mechanisms is also presented

Jan 1, 1990

By Mohummad Akram

Statistical analysis of multi-seam mining case studies demonstrated that geologic structure and lithological type and sequencing were predominant factors in entry failures. Finite element analysis was used to evaluate the effects of stress field changes on entry instability under differing geologic conditions. Analysis showed the roof to be particularly vulnerable to shear with the location of the failure site changing to meet loading and geologic conditions. Effects of external support on roof stability were examined. Results were correlated with field data to determine stress levels at which failure could be expected to occur. Based on this research design, criteria were developed for roof stability evaluation under specified interactive loading conditions.

Jan 1, 1993

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 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 G. Skybey

The Skybolt is a new bolt concept, point anchored with resin in large diameter holes of specific sizes 38 mm and 45 mm. The new design achieves the optimum annular spacing required for mixing the resin anchor components which is essential for optimum resin bonding of the bolt to the surrounding rock. This paper explains the load transfer mechanisms between bolt/resin/rock, unique to this bolt concept. It compares the pull-out test results to those of paddle and expansion shell type bolts, and highlights the versatility of Skybolt over existing, currently used, rock support systems and the benefits mine operators can achieve when using the Skybolt.

Jan 1, 1992

By Thomas W. McClain

This paper describes the importance of considering ground control measures when importing U.S. continuous mining technology in foreign countries. Important ground control factors are discussed. Each year the USA exports a significant amount of continuous miner equipment (i.e. continuous miners, roof bolters, coal haulers, shuttle cars, continuous haulages, scoops and feeder-breakers). International customers see the opportunity to significantly improve their productivity and increase their longwall gateroad development rates by implementing the room-and-pillar (place changing) mining systems used throughout the USA. However, a number of issues are often overlooked or not thoroughly evaluated when selecting the equipment, often resulting in lower than expected productivity. Ground control is one of the critical factors that must be addressed. Since 1993, the author has assisted numerous international customers and manufacturers with implementing or improving the performance of continuous miner and room-and-pillar systems in Australia, India, Mexico, South Africa, and the United Kingdom. An essential first step is to conduct a thorough ground control study, and then integrate its findings into mine layout, equipment selection and section operating plans. Based on actual case histories, the author will present a number of examples and "Do's and Don'ts." Another key to success is the involvement of manufacturers and suppliers with customers when implementing new systems and technology. The importance of allowing for cultural evolution by both management and workers for acceptance and successful implementation of any new system is also discussed. Over the past ten years, the global coal industry has gone through some very dramatic changes. Countries that previously experienced a freight advantage, such as Australia shipping coal to Japan and Korea versus the United States, are finding they are now in a world coal market similar to other commodities and products. Today, coal prices arc stated F.O.B. at the consumer, and the origin is often irrelevant. Also, previously state-run coal industries have been privatized, such as the United Kingdom, and others such as Poland and India are currently going through the same transition from state-run to the private sector. In all cases, the key to continued viability, or survival, depends largely on reducing costs in order to compete for a market share. Two ways exist to reduce costs. First, almost everyone tries to reduce expenditures. Depending on the circumstances at a given mine or in a specific country, this helps to some point. However, most find that secondly, production must be increased to achieve the necessary cost reductions to compete in today's global coal market. As a result, over the past ten years, foreign companies and countries have looked to the USA for new mining methods and systems to significantly increase productivity. Thus, a major portion of American equipment manufacturers and suppliers sales have been exports, which has played a major role in helping to "keep-the-doors-open" for a number of companies through the lean 1990's, when American coal companies were postponing capital expenditures and expansions. As foreign companies and countries have looked to the USA for ways to improve productivity, they have become particularly interested in continuous miner equipment systems and most often "room-and-pillar," "bord-and-pillar" or "place changing" mining systems for four reasons. First, in countries such as the United Kingdom and Germany, who have been longwall mining for generations, there remain numerous remnant small pockets of reserves in existing mines that are either too small or too difficult to longwall mine, and are conducive to the flexibility of continuous miner systems. Second, for countries such as Australia, who have experienced the constant problem of developing longwall gateroads in a timely manner to avoid longwall move delays, new or alternate methods are required. Third, companies and countries using roadheader machines on coal production are recognizing the increased productivity potential of continuous miner equipment. Fourth, countries such as South Africa and India, where a substantial amount of the production comes from conventional mining methods, "drill-and-blast", companies are recognizing the opportunity to improve by implementing continuous miner technology. Colliers & Associates, Inc. has found that over the past eight years too often foreign companies or countries purchase American continuous miner equipment without first conducting a thorough geologic, geotechnical and ground control study for the reserves to be mined. The problems are compounded by not developing mine plans, and submitting them to the regulatory authorities for approval prior to purchasing and even implementing the equipment.

Jan 1, 2001

By Robert R. Thompson

Retreat pillar mining is highly productive but dangerous. The primary danger during pillar removal is premature caving of the roof. Because the safety of the miners is dependent on successfully controlling the roof, the roof must cave in a predictable and dependable manner. Temporary support during retreat is usually obtained by setting posts. They are set manually by a miner working in a dangerous area. A solution to this problem would be a system which could set and retrieve roof supports remotely and safely. The Bureau of nines has developed a remotely operated Mobile Roof Support (PIPS) machine which will place and retrieve temporary roof support. Two PIPS machines were fabricated under a cost-sharing agreement with Southern Utah Fuel Company and mining equipment manufacturer. Each machine carries four 50-ton jacks and is remotely- Controlled by radio. The machines are presently being used by Southern Utah Fuel Company under a long-team cooperative agreement. Not only have there been material and manpower cost savings in not having to set posts, but also the time saved bas resulted in increased productivity. The projected costs of mass-produced machines with tethered remote control rather than radio remote control are estimated at $150,000 each. The radio remote control is more expensive and is considered an optional extra.

Jan 1, 1986

By Vasundhara

Underground coal mining in soft floor environments has often led to strata control problems ranging from small scale issues such as floor heave which disrupts traffic and ventilation, rib spalling and induced fractures in pillars, through to large scale instability and long term subsidence. Past investigations of the problems encountered in Newcastle Coalfields of New South Wales, Australia, have revealed that the soft floor unit comprises of claystone which is very sensitive to moisture. It is prone to swelling, reduction in stiffness and loss of strength in the presence of moisture. Long term stability of mine panels in such an environment is governed by the extent of deterioration of the floor strata. The paper reports the field investigations conducted at Cooranbong Colliery to assess the extent of softened floor zone in the roadway and under a pillar.

Jan 1, 2001

By Ali Benbia

The paper shows results from the physical modelling of longwall excavations concentrating on the fracture pattern development above longwall panels. The modelling technique is briefly described and the detailed analysis of the fracture pattern is discussed. The physical modelling is particularly useful in visualising the caving process above longwall panels. This advantage has been exploited using the image analysis technique. Black and white pictures have been taken while the models are tested in successive stages and then enhanced and stored as binary images by scanning using an image analyser coupled with a video camera. These images are then processed using a custom C program which allows the discretization and characterisation of the fractures. This provides a valuable data base for further numerical and statistical analysis which subsequently may allow the assessment of the water inflow hazard to the workings. In this paper a sequence of different extraction thickness have been analysed using the technique mentioned above to show the effect of the extraction thickness on the fracture distribution and intensity.

Jan 1, 1993

By P. R. Sheorey

Four case studies of special geomining conditions from Indian coalfields are discuss- ed. These cases relate to (a) pillar extraction in two seams simultaneously under a difficult roof (Satpura I and II). (b) depillaring in an inclined seam resulting in rib instability (Sudamdih), (c) depillaring resulting in floor lift and side spalling (Jarangdih) and (d) contiguous pillar extraction under hard incavable strata. Solutions to these problems are given based on rock mechanics principles involving stress analysis, classification techniques, past experience and engineering judgement.

Jan 1, 1988

By Rob Robinson

Following a massive rib wall failure along the reclaim belt of the large underground surge bunker at CONSOL?s Bailey Mine, a multifaceted, cleanup and rehabilitation effort was initiated to rapidly stabilize the bunker rib walls and place the bunker back into production. Rehabilitation of the bunker included infilling and re-mining of the failed excavation area, installing a system of horizontal tie-back cable bolts, constructing a large knee wall at the base of the rib, reducing water inflow, and providing drainage for rib water. As a result of the coordinated effort, the bunker was safely placed back into production within 38 days. The 7500-ton surge bunker, an engineering feat in itself, was constructed in 2003 to accommodate production surges from two longwalls. In 2005, 14.8 million raw tons passed through the bunker. The 22-foot wide by 75-foot high surge bunker excavation extends as much a 52 feet below the base of the Pittsburgh seam. The bunker excavation measures 325 feet in length, with feeder and reclaim belt excavations extending the bunker excavation by approximately 500 feet. The massive rib roll, which occurred near the bottom of the reclaim belt slope, measured as much as 45 feet high by 60 feet long. Failure of the rib was attributed to progressive failure of a thick claystone unit present near the slope bottom area. Saturation of the claystone by water inflow, high insitu horizontal stress, and pre-existing rock joints were considered contributing factors. Immediately after rib failure, mine production was temporarily rerouted to a parallel belt. Two bulkheads were then constructed 175 feet apart along the reclaim belt slope, and a customized fly ash-cement mixture was pumped from the surface into the unstable reclaim slope area. A tunnel was then mined through the backfill along the original reclaim slope contour; the new excavation supported by a system of customized steel sets. To assure stability of the adjacent bunker rib wall, a pattern of 70-foot long cable bolts were installed downward from the adjacent entries to effectively tie back the rib. Drainage holes and a 12-foot high knee wall were installed. Adjacent gas wells were also plugged to help reduce inflow. Periodic inspections and continuous monitoring are being used to help assure bunker stability.

Jan 1, 2007

By Axel Preusse

This paper shows basic considerations about the prognosis and prediction of surface subsidence and other ground movements in the German hardcoal industry. The following case studies describe experiences collected in projects between university and industry.

Jan 1, 2000

By Steve Signer

The San Juan Mine and the National Institute for Occupational Safety and Health conducted a study to measure how development mining effected bolt loads. Twelve fully grouted, instrumented roof bolts were installed at two three-way intersections as part of the standard bolting pattern. Newly developed miniature data acquisition systems (MIDAS) were used to measure bolt load changes during initial face advance and cross-cut breakthrough. The effects of cut placement and depth on roof bolt loads were studied This test showed how bolt loads increased at five positions along the bolt length during initial mining. Both entry advance and cross-cut breakthrough produced a similar percentage of increase in bolt loads. Geologic differences between the test sites were probably responsible for the differences in amounts of bolt loading. The test site with more top coal and a higher rock quality designation (RQD) had lower bolt loads.

Jan 1, 2004

By H. Gene Wilson

Mobile Roof Supports (MRS) look similar to a longwall shield on crawler tracks. The MRS units offer a safe, productive method of extracting the 40% to 60% product left normally unrecovered in room and pillar mining. The units feature radio control, self-contained power units, crawler track drive, and a 600 ton yield. Cribbing and setting posts in retreat mining has been a necessary burden to production. The MRS reduces this burden. Long blocks of coal can be developed to further reduce move and tram time, thus increasing production. The MRS units have successful safety records. The workers are removed from the immediate fall area, and we feel back injuries have been reduced by the workers not setting posts. While MRS units have been damaged in falls, the other capital equipment has been unharmed. The MRS product line has undergone normal product improvement and evolution. Product evolution is seen in our development of machines for particular mine requirements. Through the continuing development of the MRS, Fletcher presents the coal industry with a safe, productive method of retreat mining with potential to develop new mining methods.

Jan 1, 1990

By Hamid Maleki

During the last decade, a significant amount of research has been conducted by Energy West Mining Company, government agencies, academia, and consulting companies to develop predictive tools for assessing the stability of the Castlegate Sandstone, which is found approximately 250 m above multiple-scam coal reserves, Energy West Mining uses longwall mining techniques in its operations near Huntington, Utah, and these studies were initiated to satisfy requirements for maintaining the stability of the Castlegate Sandstone and resource recovery. In this study, the authors have used multiple-regression analysis techniques and a wealth of data collected over many years on geology, mining, and escarpment stability. The volume of failed rocks is used as the response variable after several other factors were considered, including measurement of surface deformation and frequency of mining-induced surface fractures. Geologic and geometric variables were obtained along 3.7 km of escarpment exposure at 130 study locations. Regression analysis of data for the first 29 study location, which had been fully' undermined, showed that surface topography played a critical rule in influencing escarpment stability. Preliminary regression analysis results from 70 study loca¬tions identified several other important geologic arid mining factors that influence be stability of the sandstone escarpment These are canyon slope, sandstone thickness, and mining influence ante.

Jan 1, 2000