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By U. F. Beichmann

INTRODUCTION The City of Oslo together with the neighbor municipalities, Baerum and Asker, are presently building a jointly owned sewage treatment plant located on the west side of the inner Oslofjord. The project is scheduled for completion late 1981. Because the location of the treatment plant is at a distance of about 20 miles (30 km) from the city center, a discharge of more than 13 yds 3/sec (10 m3/sec.) of sewage and storm water will have to be transported over considerable distances. The topography of the area and the geological conditions early indicated that the only realistic transport method had to be through tunnels. As planning proceeded, full face tunnel boring emerged more and more as a favorable alternative to conventional drill and blast medhods. Today the scheme comprises nearly 25 miles (40 km) of tunnels being excavated by full face tunnel borers. In addition a number of access tunnels and cross cuts are to be excavated by conventional methods.

Jan 1, 1979

By E. D. Attanasi

Exploration for non-fuel minerals has declined since 1988 as recessionary pressures have slowed growth in the industrialized economies. By most accounts, however, 1990 expenditures were still significantly greater than the low levels experienced in the mid-1980s. The Persian Gulf crisis introduced instability in energy prices and greater uncertainty into an already cloudy economic picture. As 1990 unfolded, the US savings and loan problems spread to the commercial banking system, leading to the failure of several major US commercial banks. The Japanese stock market also suffered significant losses as Japan's Central Bank raised interest rates. The Soviet Union experienced shortages of food and other basic items. Its government faces resistance to changes leading to a market economy. Individual Soviet republics also are seeking more control of their own destiny. The cost of German reunification, in terms of the capital required to bring the east's infrastructure and industrial base to western standards, also appears to have been underestimated.

Jan 1, 1991

By Saied S. Navid, Michael A. Piepenburg, Richard F. Escandon, Robert C. Lindsay

INTRODUCTION Phase I of the Los Angeles Metro Rail Project, known as MOS-1 (Minimum Operable Segment-1), is the first phase of the 27 km± (17 mi) subway starter line. MOS-1 is currently under construction and involves approximately 6.4 km (4.4 mi) of subway, five stations, yards and shops routed through the downtown area of Los Angeles. The project will form the initial heavy rail backbone of a more extensive light and heavy rail transit system planned for the greater Los Angeles area in the future. Construction Contract A-171 is one of 4 tunnel contracts within MOS-1 and involves construction of twin tunnels running from the downtown metropolitan area west to MacArthur Park. The heavily populated and developed downtown area, potentially gassy ground conditions, and the possibility of undocumented abandoned oil wells in seismically active southern California made the design and construction of Contract A-171 a uniquely interesting project. This paper summarizes the construction history to date (December, 1988) including design geotechnical considerations, tunnel support design, as-built geologic conditions, shield tunneling performance through soft ground, and ground response. The findings presented herein may aid in the design and construction

Jan 1, 1989

By Lawrence K. Nordell, Vinit Sethi

Published standards and specifications do not adequately cover engineered class pulleys for modern high strength steel cord belt conveyors. This paper discusses the Conveyor Dynamics, Inc. (CDI) design criteria and stress analysis techniques emphasizing the finite element method (FEM), fatigue failure criteria, design limits and manufacturing requirements to ensure successful and safe pulley installations.

Jan 1, 1993

By A. P. Grippo, L. B. Phelps

Many coal mining companies have potential surface mine reserves that either are known or suspected to have had prior drift mining activity on them. In many cases, there is no documentation on these mines or, at best, very fragmented records. These undocumented abandoned underground mines are creating engineering, financial, and environmental difficulties and concerns for both surface coal mine operators and regulatory personnel. This paper examines the use of resistivity techniques in detecting entries of abandoned drift coal mines. The detection of such a drift mine entry would confirm the presence and orientation of an abandoned underground coal mine.

Jan 1, 1987

In 1992, United States mine production of barite decreased 8.5%. Barite consumption, sold or used by grinding plants, decreased by 36.9% and imports decreased by 61.6% vs. 1991. World mine production decreased 1.5%, according to the US Bureau of Mines. The value of domestically produced barite, f.o.b. mine, decreased 11.9% according to the Bureau. The declared value, c.i.f. United States port, of all imported crude barite during 1992 decreased from $42.35/t ($38.42/st) in 1991 to $40.09/t ($36.37/st). Nevada continued to be the leading producer of barite with 88% of the total, followed by Georgia and Missouri. It is estimated that 80% of the domestically mined barite was used as a weighting agent in drilling fluids. The remaining 20% was used in barium chemicals, glass or as a filler. Most of the production from Missouri, Georgia and Ten¬nessee was used in the non-oilfield sector. Of the total barite used by grinding plants and chemical manufacturers, it is estimated that 90% was used in drilling fluids and 10% in the non-oilfield sector. This included barium chemicals and fillers in paper, plastics and glass. The tonnage consumed in the drilling fluids market decreased due to the lower average rig count during 1992. The average number of rotary rigs operating (717) was 16.6% less than the 1991 average of 860 in the United States. Crude barite imports were 518 kt (571,000 st), 61.6% less than in 1991. Imports accounted for 44.7% of the barite consumed (apparent) in the United States. During the year, China continued to be the leading exporter of crude barite ore to the United States with 67.2% of total crude ore imports. India followed with 31 %, and others with 1.8%. About 90% of the imported barite ore was used in the oilfield sector. Ground barite imports decreased from 46.3 kt (51,000 st) to 30.9 kt (34,100

Jan 1, 1993

By N. K. Davé

Water quality impacts of shallow water submersion of partially weathered and oxidized waste rock containing reactive sulphide minerals were investigated in laboratory column lysimeter studies. The waste rock investigated was highly reactive, contained oxidation reaction products from previous exposure and weathering, and had a low sulphur content of approximately 2 % S, mostly as pyrite. The submerged waste rock, though previously weathered, had high oxygen consumption, resulting in rapid depletion of dissolved oxygen in its pore water, within a shallow depth of ~2?5 cm from the surface. The surface and pore water qualities of the submerged waste rock were significantly impacted by the dissolution and release of previous-oxidation-reaction products, which contributed to their rapid acidification and having increased concentrations of dissolved SO4, Fe total and other metals: Al, Mn, Ca, Cu, Ni, Zn and Pb. Despite the high porosity and permeability of the waste rock, the diffusion controlled mass transfer from the submerged waste rock to the overlying water cover was relatively low for most ionic species. Additional surface oxidation of the sub-merged, partially weathered waste rock was on-going, but its impact on the overall surface water quality was low and masked by the dissolution and release of previous oxidation reaction products to both surface and pore water regimes.

Jan 1, 2006

By J. A. Herbst, Z. Rogovin

In order to develop more accurate procedures for scale-up and design of semi-autogenous grinding mills through detailed mathematical models, a better understanding of the charge motion and rocklball segregation is needed. To achieve this a 0.61 m (2.0 ft) diam by 0.305m (1 .0ft) length plexiglas mill was constructed as a scale model of a commercial SAG mill. Rock motion in the rotating mill was simulated using various sizes of ceramic balls. Quantitative measurements of dynamic angle of repose, water and slurry motion, holdup and size segregation of the charge were taken. In addition, a video presentation (available to the public) was prepared to elucidate charge and water motion at various mill speeds, charge volumes, rocksize distribution, and flow rates.

Jan 1, 1990

By Bernard Scheiner

A number of wells were drilled at two phosphogypsum stacks, one at P-21 owned by IMC and the other at Agrico's South Pierce Stack. Each well was sampled, and composites representing 5-ft increments were leached with synthetic rainwater to determine the extent of metal and nonmetal migration. To simulate what was occurring in the stack, composite samples were placed in a column, so that the column represented the stack in miniature form. Leaching experiments showed that both metals and nonmetals moved down the column when water was added to the top of the column. Data obtained from P-21 showed that with time the concentration of metals and nonmetals reached equilibrium values. The concentration of Ca, SO4, F, Si, and PO4 were -600 ppm,~1,500 ppm, 32 ppm, 14 ppm, and 22 ppm, respectively. Column leaching of South Pierce material, representing 0 to 55 ft of stack, showed that after 355 in. of water, the stack was near equilibrium values down to the 33-ft level. With continued addition of water, the portion of the stack at equilibrium would continue to increase and eventually reach the values observed for P-21. The test showed that the sulfate concentration of the effluent from the stack would never reach the USEPA standard value of 250 ppm. During the leaching test, it was determined that the permeability of the phosphogypsum in the columns was 0.000103 cm/sec. This number was used in the modeling. The data obtained from the leaching experiments cast some doubt on the practice of entombment now being practiced for closure of phosphogypsum stacks. The data clearly shows that with time, the amount of material leaching from the stack is low in concentration, with only the calcium and sulfate at higher levels. When a stack is closed at an existing operating plant, allowing rainwater to fall on the stack and capturing the solution in the existing pond system would eliminate the need for covering the stack, since with time the closed stack would continue to leach and at some point reach the equilibrium values of the P-21 stack. Entombment means that the leachable material in the stack will always be there and will require monitoring forever. Allowing rainwater to decontaminate the stack may be the best environmental solution to the problem.

Jan 1, 1999

By Mark W. Springett

The examination of relatively well behaved distributions subject to truncation by a cutoff grade in the mine clearly demonstrates that a bias is inevitably introduced when the selection is based on the results of samples which are subject to even the most idealised error distribution. The idealised error used in this paper is assumed to be normally distributed with a mean of zero and is thus unbiased. The bias introduced can be minimised but it cannot be eradicated. A better understanding of the bias can lead to improved procedures for mine sampling, sample preparation, assaying and estimation of selection units which will not only reduce the discrepancy but more significantly will increase the efficiency of mining selection and hence profitability.

Jan 1, 1989

By David B. Campbell

INTRODUCTION This submission was made to the Royal Commission of Inquiry into uranium mining in British Columbia. It discusses the stability of waste rock piles that might be developed in conjunction with uranium mining and the control of leachates that might be generated within these waste rock piles over the short and the long term. Short term refers to the interval during which a waste rock pile undergoes either continuous, or alternatively intermittent development, and in general can be considered to extend over the operating life of the mine. Long term refers to the postmining period, i.e. after the ore-body has been exhausted, and mining operations have ceased. Two major classes of waste material are generated in the course of mining and milling operations. Tails, which are the solid waste materials produced by the milling operations commonly exit the mill in slurry form and are stored in tailing retention ponds. Management and control of these slurried wastes are the subject of other papers to the Commission and this conference. In the course of the mining operations, it may be necessary to remove rock materials that are below cut-off grade. That is, ore concentrations within some of the mined rock may be lower than the concentrations which on the basis of economic considerations justifies milling the material. Rock which is removed in the course of the mining operations, but which is below cut-off grade is referred to as waste rock. This waste rock is commonly consigned to a specific area for disposal where it is placed in a mound or pile, i.e. a waste rock pile. Ore concentrations within the waste rock may vary from zero to just below cut-off grade. Since an increase in the market value of uranium could at some time in future result in reduction of the cut-off grade, it follows that waste rock containing ore concentrations slightly below today's cut-off grade could become ore grade rock at some time in future. For this reason, the mine operators may deem it advisable to separate waste rock according to grade, and to consign the waste rock to two or more individual waste rock piles. For purposes of clarity and simplicity, this submission treats the stability of waste rock piles and leachate control in a manner which assumes that these potential problems are isolated and separate from other aspects of the mining and milling operations. For many site specific cases, the problem of potential leachate control might be integrated with the tailings management aspect of the project, thereby realizing economic benefits while at the same time reducing the potential for adverse environmental impact.

Jan 1, 1980

By R. K. Komarek, F. E. Jr. Briber, G. M. Hanson, Fred D. DeVaney, J. H. Blake, C. L. Matson, T. E. Ban, N. E. Shields, R. D. Frans, R. B. Greenwalt

Metallurgically, the term agglomeration is used to describe processes whose goal is to form balls, briquettes, nodules, flakes, or other sized shaped particles from loose, usually fine, incoherent particles. In practice it includes such varying processes as the briquetting of coal dust and other substances, the balling and induration (heating) of such nonmetallics as cement, fluorspar, lightweight aggregates, etc. It includes such widely used processes as the sintering of nonferrous concentrates, such as those copper, lead, zinc, etc. The agglomeration of specific nonferrous metal concentrates is discussed in detail in the section of this handbook devoted to that particular metal. Agglomeration processes are of particular value to the iron and steel industry where the nature of the primary reduction process, the blast furnace, requires for best results a burden essentially free of fines. This is a change that has taken place since about 1960. Prior to that time many fine unsized ores were fed to the furnace. The development of the pelletizing process gave to the industry a chemically controlled closely sized product that increased the produc¬tion rate and reduced the costs so much that unsized fines are seldom used in modem furnaces. Steel plants produce many fine sized waste products such as fine dust (fine ore coke and gangue) and mill scale. These products, which have a valuable iron content, are usually sin¬tered directly at the plant for reuse. Fine sized ore concentrates are usually pelletized at the mine and shipped as pellets to the steel plant. Common practice in some areas producing natural high grade ores is to screen the ore at the mine to about 3/8 in. The oversize is shipped "as is" and constitutes blast furnace feed. The / in. may be shipped to the steel plant where it may be sintered or otherwise agglomerated. At some mines the fines may be pelletizied prior to shipment. With the development of large ocean ore carriers, and with the transportation of slurried concentrates or fines, a trend has developed of constructing large pelletizing plants or other types of agglomeration plants near large consumption centers (i.e., in the Netherlands or Japan) where ores of many types are brought together, ground if necessary, blended, and agglomerated. As an indication of tonnages of agglomerated materials produced, for iron materials alone (the largest share of the total) production in 1970 in the U.S. was Million gross ton Sinter 40.7 Pellets 43.1 Briquettes, nodules, & others 0.1 Total 83.9 During the 1960s, U.S. production of sinter rose from 40.3 million gross tons in 1961 to a high of 48.7 million in 1964, then settled back to almost the 1961 figure in 1970. Pellet production rose steadily from 14.4 million gross tons in 1961 to the 59.9 figure for 1975. Worldwide, sinter production in 1966 was estimated at 340 million mt (mainly in Europe, Russia, Japan, and the U.S.) and pellet produc¬tion the same year at 64 mt (primarily Canada and the U.S.). World pellet production capacity rose to 200 million tons in 1976.

Jan 1, 1985

By Robert L. (Original by) Llewellyn

Many of the problems that occur in the preparation plant originate from practices in the mine. Impurities in raw coal can be in the seam itself or from extraneous material taken in mining from the roof or bottom. In the case of underground mines, the choice of mining equipment will affect the size consist and the amount of extraneous material included with the coal. Also, governmental regulations concerning the degree of rock dusting, gobbing underground, and cleanup have increased the dirt in raw coal. Sulfur dioxide emission standards, on the other hand, are pressuring the coal industry to clean coal more thoroughly. Coal supplied for coking purposes is nearly all upgraded by washing, while for the largest market, electric power generation, a high proportion is shipped as a raw product with processing, in whole or in part, only necessary when excess impurities are mined with the coal. This chapter will deal primarily with plants that process or wash coal to some extent (Fig. 1) and only incidentally with mines shipping raw run-of-mine coal crushed to a top size of, usually, 51 to 102 mm (2 to 4 in.), with little, if any, other treatment.

Jan 1, 1981

By L. Diels

About 45% of the contaminated sites are dealing with heavy metal problems. Metals are spread in the environment by mining activities, surface treatment and non ferrous processing. As heavy metals can not be degraded, removal or immobilization (leading to bioavailability reduction) are the only risk reducing measures that exist. The currently best available technique for remediation of heavy metal contaminated groundwater is to pump the contaminated water and to treat it in a wastewater treatment plant installed on the surface. Next to this often used, but expensive, pump and treat technologies, heavy metals can be immobilized by inducing sulfate reducing bacteria (SRB) to transform the sulfates, that are very often present in the same groundwater (due to the metal mining or processing activities), into sulfides. These sulfides will precipitate the metals as insoluble metal sulfides. At the moment several studies have demonstrated the feasibility of this In Situ Bioprecipitation Process (ISBP) for the removal of heavy metals from ground-water as well at lab scale (batch and column tests) as at field scale. However, some questions arise concerning the continuation of the process, the efficiency and the sustainability of the precipitates. The presented study will try to answer these questions. The study is based on more than 10 different studies, all done by the same authors, on different groundwaters and aquifer samples. The presentation will give an overview of the guidelines necessary for a correct and successful bioprecipitation process with stable metal sulfide precipitates. An important question in the ISBP is related to the sustainability of the heavy metal precipitates. It could be observed that the interruption in the carbon source delivery also stopped the in situ bioprecipitation process. Depending on the metal also some release from the precipitate could be observed. It was shown that release can occur with Co and Ni. Aquifers, on which the metals precipitated during the ISBP, were treated in a sequential extraction process under an-aerobic and aerobic conditions. It could be shown that especially under the anaerobic conditions the Zn and Cd precipitates changed from the leachable and the exchangeable fraction to the organic or Fe-Mn fraction (i.e. from the unstable to the stable fraction). The same could be observed for Co and Ni but still some fraction was present under a more available form. On the other hand it could be shown that large differences were observed concerning the heavy metal speciation on the aquifer in function of the used carbon source during the ISBP. The molasses treated systems showed a much more available heavy metal fraction on the aquifer compared to the lactate or HRC® treated groundwater. The different speciation forms based on the use of different carbon sources will be discussed in detail. It will pay attention to the influence of the carbon source, the efficiency of the induction of the bioprecipitation process, the influence on the competence between sulfate reducers and methanogenic bacteria and the influence of low pH on the ISBP. It seems that acetate, which is also a degradation product of fermentation of some carbon sources, can inhibit some sulfate reducers. Also the relation between slow release compounds and immediately available compounds will be explained. Finally a pilot study will be presented showing the problems that can be encountered during this process. Focus will be put on the induction of the process, the keeping on of the process and the follow up of the process by the installation of in situ mesocosm socks. These results will allow the correct implementation of the ISBP with an eye on longevity and sustainability of the process and present the ISBP as a much more sustainable alternative to the pump and treat technology as measure for heavy metals contaminated groundwaters.

Jan 1, 2006

By A. W. Khair, S. S. Peng

Discussion by J. Parker Gentlemen; Rainy day in Upper Michigan, a good time to catch up on reading. Am doing that, and have a couple of questions concerning the K and P paper on roof problems and supports. I wonder if the authors can enlighten me. The compressive strength of the coal was reported as 4.4 MPa (633 psi) and compressive stresses measured in the coal seam were reported at around 27.6 and 48.2 MPa(4000 and 7000psi ). Can such a soft coal actually contain such a high level of stress? Can it be measured as if the material were elastic, or does the coal behave plastically? Can that stress level be confirmed with hydraulic load cells? And if the coal is so soft, and if it is plastic, why is the stress field not hydrostatic'? The intact roofrock appears to be unusually strong, with compressive strength reported at around 145 MPa (21,000 psi), and some failures were noted as soon as the rock was exposed. Were tests run to find out how that rock responded to the addition of moisture, from sprays or from humid air? Did moisture weaken the rock? Did it cause the rock to expand quickly, thus to induce internal compressive stress? How can the near-vertical cutters, as shown in the photographs, be attributed to primarily horizontal stresses? Did the cutters follow any particular pattern in the development panels, which might be related to a particular stress pattern? For example, did they always appear alongside the stiffest pillar? Or always along the northwest rib? Was there any evidence of the pillars and entries "rotating" under the influence of differential horizontal stresses? How much did the entries and pillars rotate as the longwall face approached and passed? After rooffalls, did the openings assume shapes which would indicate a preferred shape in that stressfield and geology? Did crosscuts always survive without damage, as shown in the sketch?

Jan 1, 1992

Little faith can be placed in a coal resource estimate without a clear understanding of the estimate's accuracy. Different types of coal resource estimates require different levels of accuracy. Prospecting surveys typically are done with widely spaced boreholes, while a much higher level of accuracy is required for mine design. Either way, a measure of the accuracy clearly indicates the level of confidence that can be placed in the results of the estimate. Geographic information systems (GIs) are becoming a commonly used tool for resource estimation because they not only provide a database management platform but also the graphical capabilities to visualize resource characteristics. interpolation methods used commonly in these resource studies do not typically provide a quantitative answer to the question: "How accurate is the estimate?" By comparing localized measurements at multiple locations to the interpolated thickness distribution used in a resource estimate, a quantitative analysis of accuracy can be obtained. The technique is demonstrated with a case study from Central Appalachia.

Jan 1, 2000

The choice between EPB - and Slurry Shields is influenced by the following factors: grain size distribution, cohesiveness, occurence of boulders, occurence of gas, feasibility of soil separation and muck disposal, and settlement properties of the soil. The simple equation cohesive soils = EPB Shields and non cohesive soils = Slurry Shield is very often superimposed by other facts. For example, the occurence of a large amount of boulders in weak cohesive soils, where the application of roller discs is not recommendable, may lead to the use of a Slurry Shield with a rock crusher, thus accepting the disadvantage of a sophisticated separation plant; whereas the occurence of contaminated groundwater or gas may exclude the use of a Slurry Shield. In the following examples the choice of a Slurry Shield or EPBS will be explained and justified with regard of the above factors.

Jan 1, 1995

By Paul R. Thomas

This paper addresses two issues affecting the international ferrochromium market as they pertain to the changing pattern of U. S. chromium import dependency. These issues are 1) the long-term ferrochromium production cost differential between the United States and the Republic of South Africa and 2) the ability of Zimbabwe to expand ferrochromium production and exportation. This paper draws upon the data and analyses of a recent Bureau of Mines Minerals availability Program appraisal that evaluated the international availability of chromium in the form of chromite and ferrochromium from 10 market economy countries. South African ferrochromium producers are shown to have a long-term production cost advantage over U.S. producers of $0.03 to $0.04 per pound of contained chromium, in 1981 dollars. Zimbabwean ferrochromium production and exportation are expected to increase in the future. Major expansions of production capacity will be dependent upon alleviating potentially serious constraints in electric power, rail, and port facilities. In the long term, ferrochromium production costs in Zimbabwe are expected to increase faster than the rate of inflation, because as chromite mining from the podiform resources in Zimbabwe decreases, production from the Great Dyke resources will increase.

Jan 1, 1984

By Victor L. Steven

10.1-SHAFT SINKING AND RAISING BY CONVENTIONAL METHODS VICTOR L. STEVEN 10.1.1-SHAFT SELECTION A great deal of thought and planning should go into the selection of a mine shaft to provide the most effective use over the greatest, period of time. It sometimes is better to make an additional capital expenditure in the initial shaft program to achieve the greatest overall shaft usage. It is possible to sink a shaft which could be used for mine development, and initial production and later convert it to a ventilation, service or escape shaft. It is important to consider a second shaft in the original planning to provide ventilation, an escapeway and services for overall efficient production and safety. A second shaft usually is required to comply with state and federal mining laws. The main items to consider in shaft selection are: 1. Installation from surface or underground. 2. Vertical or inclined (sec Sec. 10.1.4L for discussion of factors involved in choice). 3. Tonnage to be hoisted. 4. Size of mining equipment to be lowered through the shaft to the underground workings. 5. Amount of water to be handled in the mining operation. 6. Ventilation requirements. 7. Type of ground the shaft is to be sunk through. 8. Position of shaft with relation to ore body. After the main features of a shaft arc determined, such as vertical or inclined, shape, size, type of lining and support, compartments, and shaft. facilities (air, water, ventilation, pump lines, power cahles, etc.), then :I sinking plant and procedures should be planned. At this point it should be decided whether the company will sink the shaft with its own crews or award the job to a competent shaft contractor. Company sinking generally will cost less if competent shaft men are available. Money also will be saved in plant costs, such as for hoist, headframe, compressed air, buildings, etc., as these will be part of the permanent plant and can be used in the sinking operation. Labor costs generally will be lower than contractor construction rates. The advantage of having a contract, or is the speed of completion of the project. Contractors usually have headframes and hoists ready to move on the job immediately and have trained crews ready to complete the project in the shortest possible time. Next, the type of surface sinking plant (hoist, headframe, etc.) should be selected and a determination made whether it is to be temporary or permanent. In most cases it is less expensive to install the permanent, headframe and hoist, and use them for sinking. The type of actual sinking equipment to be used should be evaluated for the most efficient sinking cycle, e.g., mucking method, drilling method, and lining or support means. The proper cycling of drilling, mucking and lining is extremely important in obtaining the most favourable sinking costs. 10.1.2 - GROUND STABILIZATION AND WATER CONTROL Prior to the shaft-sinking operation an investigation should be made into ground-water conditions. If water is anticipated, then pregrouting of the ground

Jan 1, 1973

By Charles T. Holland

The tonnage produced on mechanized automated retreating longwall faces in the United States is growing rapidly. This time last year some 14 faces were operating, and in addition to that, three were known to be down looking towards revision or repairs before being placed back in service. These, it is believed, are now in service. It is estimated at the present time, therefore, that somewhere between 20 and 25 faces are being operated in the country. A survey made last year by the School of Mines, West Virginia University, indicated that longwall mining in general has been beset with a great many problems, some more serious than others. The problems that we propose to discuss here are those pertaining to roof and floor trouble in the tailgates. Longwalls are operating in this country at the present time under an overburden ranging from 200 feet to nearly 3000 feet. Main and tailgate entries are operating in panels of up to five each, with the width of the pillars between these entries varying from 20 to 50 feet. Face lengths are running from 270 feet to 670; actually a face 800 feet long should be possible. The most economical face is often spoken of as being around 450 feet.

Jan 1, 1969