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By Edwin M. Chance

During the past 10 years, the safe and efficient lighting of the coal mines of this country has received an ever-increasing amount of attention. Several States have passed laws attempting to regulate the type of lamp to be used and the nature of the fuel to be burned, and the mining departments of coal-mining States have generally shown a keen and intelligent interest in this subject. The passage of the recent Employers' Liability Act in Pennsylvania has made it necessary for many coal-mining companies to take out liability insurance, and the companies underwriting such insurance have made it desirable for the insured to permit the use of none but illuminants of established worth. While these conditions have not obtained for a sufficient length of time to permit the statement that the illumination of coal mines by portable lamps has been standardized, still, considerable progress has been made and the direction of future practice in this branch of coal-mining technology is very evident. Under these conditions it seemed that a review of the methods now used in coalmine illumination, together with a brief consideration of the principles underlying these methods, might be of some interest. Miners' lamps may be divided into three classes: The open light, the electric cap lamp, and the flame safety lamp. It will be desirable to consider each of these classes separately, as each has properties peculiar to itself and one class is hardly comparable with another. In this country, without question, the open light is the most generally used of all miners' lamps. This fact is explained by the relative freedom of a large proportion of our mines from gaseous conditions, and by the admirable systems of ventilation installed in those that show a tendency toward such conditions. In the metal mines, the miner's candle has had and still has considerable vogue. Because of the small amount of ventilation usually supplied in metal mines, the freedom of the candle from any tendency to produce noxious gases or offensive odors, and the small amount of air it consumes, are valuable assets, while, because of the light color of the rocks in which workings are driven, the meager light is not a serious handicap.

Jan 1, 1918

ALASKA Established November 19, 1947 Meets fourth Monday monthly except June through August Term of office ends October Peter O Sandvik, Chairman Denny G Breaid, Vice-Chairman Douglas W Huber, Secretary-Treasurer School of Mines University of Alaska P O Box 337 College, Alaska

Jan 1, 1958

By D. K. Atwood

Experiments resulted in a satisfactory laboratory method for restoring permeability to clay-containing cores damaged by fresh water. Clay contents of a number of field cores were measured, and permeabilities of plugs from these same cores were then deliberately reduced with fresh water. This damage is attributed to swollen and dispersed clays occupying the pore space. After damaging, a number of experiments were performed to meaJure the amount of damage and to establish some means by which permeability could be restored. The experiments included flooding the damaged cores with water-miscible fluids such as salt water, acetone, isopropyl alcohol and ethanol. Permeability was not successfully restored in these experiments. However, part of the damage was repaired by flooding with oil; when water was removed by distillation in the presence of immiscible fluids such as air or toluene, permeability was completely restored. This evidence suggested that swollen and dispersed clays could be collapsed to their original volume by strong interfacial and capillary forces. It was further postulated that the required forces could be generated by flooding the damaged cores with a solvent partially miscible with water. The flooding experiments were repeated using n-hex-an01 as the partially miscible solvent. Permeability was restored to five of six damaged cores and substantially increased in the sixth. A large fraction of the restored permeability was retained even after water saturation was raised to its original value with 12 per cent salt water. INTRODUCTION Sharp reductions in permeability often occur when relatively fresh water contacts clay-containing formations during drilling and workover operations. These permeability losses are caused by removing inorganic ions from the environment surrounding the clay, and consequent swelling and/or dispersion of clay minerals into the available pore space.' This phenomenon is generally termed clay damage, fresh-water damage, or simply formation damage; it causes large losses in current revenue by preventing oil wells from making their allowable production. Attempts to repair the damage and restore permeability by flowing salt water solutions or brines through clay-damaged cores containing montmorillonite have been unsuccessful.' This irreversibility is thought to result from formation of brush-heap, or edge-to-face, structures when the dispersed clay is flocculated. The brush-heap structures occupy much more space than the close packed domains present before damage.' One solution of the problem is to destroy the clay-water brush-heap and thus restore permeability. Because no satisfactory method existed for restoring permeability to clay-containing formations damaged by fresh water, the work described in this paper was under taken. The laboratory experiments generally consisted of deliberately damaging fresh cores containing clay and then attempting to repair this damage by various means. Results indicate that generating strong interfacial forces within the pore space of damaged cores collapses the clay brush-heap and restores permeability. These forces are most conveniently generated by flowing partially water-miscible solvents, such as n-hexanol, through a core. THEORY OF THE DAMAGE PROCESS The most common clay mineral groups known to cause permeability damage to formations are the mont-morillonites, kaolins, chlorites and illites. These clays are constructed of particles which can adsorb water on their surfaces and edges and, in the case of montmorillonite, between layers of the basic particle itself. This adsorption increases as water salinity decreases. At low salinities the particles disperse into the aqueous phase. When the clays present in the formation are kaolin, chlorite and illite, dispersion accounts completely for permeability damage to porous media. However, unlike the other clays, montmorillonite particles can imbibe water and adsorb ions between layers of sub-particles, or platelets. These platelets have net negative charges on their faces and are held together by exchangeable (or removable) cations such as Na and Ca decrease in ion concentration (salinity) in the fluid surrounding a particle causes migration of water into the clay layers and disperses the basic particle, while diffusion removes the original exchangeable ions from between the platelets. Once these ions are removed, the facing negative platelets repel each other, causing the montmorillonite to swell until, for all practical purposes, the individual platelets are dispersed. For this reason, fresh-water* damage is much more severe in sands containing montmorillonite than it is in sands containing other clays. Many investigators have shown that even trace amounts of montmorillonite can be responsible for marked reduction in the permeability of reservoir sands in the presence of fresh water." ." Monaghan and others have shown that fresh-water damage in montmorillonite-containing cores cannot be

Jan 1, 1965

By R. E. Reed-Hill

A simple accurate graphical method for orienting fcc crystals using (111) slip traces on a single surface is described. Solutions placing the pole of the surface in a unit stereographic triangle are obtained using a Wulffnet, a (111) standard projection showing the four (111) planes, and a (111) standard projection ruled into unit stereogmphic triangles. The multiplicity of solutions is described in relation to this method. AN accurate, rapid single-surface technique for orienting fcc crystals, using (111) slip or twin traces, has been a matter of continuing interest, as evidenced by the periodic appearance of papers on this subject.'-4 This note presents a new method possessing some advantages over those previously reported. It is an analog to the technique for hcp metals5 where the solution is based upon a stereographic rotation of the specimen surface on a (0002) standard projection that contains the planes of the possible features that might account for the observed traces. In the hexagonal solution, the axis of rotation is the basal-plane trace in the surface. In the fcc case, the axis of rotation is one of the observed ( 111) slip or twin traces. It has been observed that the solution is greatly facilitated if the indicated movements of the (111) traces during the rotation of the surface are plotted directly on a paper copy of a Wulff net as shown schematically in Fig. 1. The trace selected to represent the axis of rotation is assumed to fall at the north-south poles of the Wulff net. The directions of the other traces are then plotted as points at the proper angles around one-half of the basic circle of the net. The loci of points corresponding to the movements of these traces (directions) during a 90-deg rotation of the surface, about the polar axis of the net, are then easily and accurately drawn by following the appropriate small circles on the Wulff net. The solutions to the problem are obtained with the aid of a (111) standard projection of the (111) planes, see Fig. 2. This is normally a Xerox copy of a carefully drawn master made on high-quality tracing paper. The (111) standard projection is superimposed over the Wulff net and rotated about its center until the points of intersection of the loci of surface traces, drawn on the Wulff net, and the great circles representing the (111) traces, all lie along a common meridian of the Wulff net as demonstrated in Fig. 3. This latter great circle represents a possible position of the surface. The pole of this plane is then plotted on the (111) standard projection. Its location in a unit stereographic triangle may be readily obtained by superimposing (over the standard projection) another tracing paper (111) standard projection subdivided into unit stereographic triangles. The result of this last operation is shown in Fig. 4. MULTIPLICITY OF THE SOLUTIONS The maximum possible number of (111) traces on a single grain surface is four. Any one may be selected as the axis about which the surface is rotated. Corresponding to each such choice are three possible surface-pole orientations, related to each other by a simple 120-deg rotation about the pole of the (111) basic circle. This is a direct result of the threefold symmetry of the (111) standard projection. Since each of the four rotations will normally give a different set of three-pole positions, there are, accordingly, twelve possible solutions. Each of these twelve solutions places the surface pole in the same relative position in a stereographic triangle so that all are crystallo-graphically equivalent. Accordingly, only a single solution is required to obtain a basic orientation of the crystal. It should be noted, however, that the orientations obtained by reflecting* any of these twelve orien- *The reflected orientations may be obtained by a 90-deg rotation of the opposite sense to that shown in Fig. 1. This rotation places the correspond-ing trace loci in the lower hemisphere of the Wulff net rather than in the upper hemisphere. tations through the surface are also possible orientations that could account for the surface traces. Thus, a given set of four traces always corresponds to a pair of basic orientations related by reflection through the specimen surface. This ambiguity can only be re-

Jan 1, 1967

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Jan 1, 1918

By Frank A. Hill

A visitor to the Northern anthracite coal-field in 1844 wrote: " History and song have hallowed the valley of Wyoming, and everything pertaining to it seems wrapped in an atmosphere of romance." Song and history have since been almost forgotten in the rush of enterprise and business energy, while the invested capital which has built 160 colliery plants, producing last year 17,031,826 tons of coal and giving employment to 50,000 men and boys, lends an atmosphere the inspiration of which is the very sustenance of prosperity and progress. The fundamental cause of this great valley's financial strength, however, rests on the fulness of its mineral wealth. Its mountains surround and its cities are built upon anthracite coal. Its foundation, physically, financially and socially, is coal. Its geology has governed its growth and developed its greatness. In 1768, ten years, prior to the celebrated Wyoming massacre, two Connecticut Yankees, named Gore, settled near Wilkesbarre, Luzerne County, and, burning Pennsylvania anthracite in their blacksmith's forges, were the first to utilize it as a fuel. Its first use in the household was also at Wilkesbarre, where, in 1808, Judge Fell burned it in an open grate. Carbondale, Wilkesbarre, and Scranton were early recognized as the coal centers of the field.

Jan 1, 1887

By Frank T. Wheby

The cost of tunneling is highly sensitive to the rock characteristics through which the tunnel is to be driven. These characteristics affect in a major way the rates at which tunnels and shafts can be driven and the degree to which the openings must be supported to maintain them in a permanent stable condition. Tunnel and shaft design and cost criteria have been computerized, permitting estimates of tunnel-shaft systems to be made directly from inputs of rock mechanics characteristics and other data. Curves of quantities of steel, rock bolts, and other rock support items as related to tunnel size and rock quality designation are presented. An illustrative example of the use of the program in calculating the cost of a hypothetical tunnel system is included.

Jan 1, 1971

By G. L. Gates

The comments by George L. Gates are appropriate to the characteristics that we sought to feature in our study and they are appreciated by the authors. In our laboratory deposition tests the smoothness of the surface seemed to be more important than the type of surface. However, we also noted that even where very smooth glass, plastic or polished metal coupons were used, scale deposition still occurred though the rate varied. Much more information is needed, however, and new work to solve the mineral scale deposition problem by mechanical and chemical means is expected to continue.

By R. C. Ferguson, B. Bernstrom

R.C. Ferguson (District Manager, Hardinge Company Inc., Hibbing, Minn.) — In the opening paragraph of Mr. Bernstrom's paper he stated that the Cascade mill performance information is preliminary. With the completion of the 1962 operating season, more reliable performance information became available. The main bearings, which had been damaged, were reconditioned at the factory. New Gatke bearing bushings with a 120° are of contact were supplied. Provision was made to introduce high pressure lubricant at two points in each bearing. New spherical bearing balls that operate in spherical seats were aligned at the factory and held in place by several setscrews to aid in installing the bearings in the field. The screws were not loosened prior to, or during, operation; therefore, the bearings operated as rigid bearings. The lubricant cooling system was changed to the cooling tower type to provide lubricating water of a suitable temperature. Another refinement consisted of installing a device to add a set percentage of soluble oil to the make-up water.

Jan 1, 1963

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Jan 1, 1938

By Arthur F. Taggart

ENGINEERING education today is like a crazy quilt of somber wools and gaudy shoddy, chain-stitched on an academic assembly line, and sold at ever mounting prices to inexperienced youths for lifetime use. Each engineering school offers a half-dozen or so patterns with essentially the same skimpy wool content. Sales appeal is in the color of the shoddy, denoted by the hopeful adjectives chemical, civil and the like. This article is argument for an all wool product made by slow, careful handwork. Bluntly it proposes a thorough four-year undergraduate drill in fundamental science and general engineering empirics, leavened by continuing study of the third of the engineer's troubles: Man. It is not contended that existing methods of engineering education have failed to start men on the path to successful engineering practice (too many of you have started that way); rather, social conditions and the bounds of knowledge have changed tremendously, and corresponding alterations in the pattern of engineering education are overdue. We'll Teach the Specifics The first part of this contention is illustrated by excerpts from a letter received recently from an officer of one of the largest corporations: "We are interested in employing two or three outstanding young engineers to be placed on a special training program to develop them as design and construction engineers.... "We propose to put these men on a four-month indoctrination and orientation course after which they would be given job-training assignments for a period that will vary from about six months to about a year and a half depending upon the aptitudes, abilities and intelligence of the candidates.... "I am not too fussy about what kind of engineers the men might be insofar as academic courses are concerned. They could be.. . in almost any engineering classification.... We can furnish the additional training beyond the academic fundamentals. "The starting rate for these jobs would be $326 per month...." This letter is not unique; rather it is representative of a practice that has become common. It is industry's enforced response to the general refusal by young graduates to submit to union domination in order to gain operating experience at the labor level. The message to engineering schoolteachers is plain: You teach the fundamentals; we'll teach our practice and pay the graduate to learn it. The letter means to the student that time spent in school studying the specific practices of particular industries is waste compounded. The school can't give the equivalent of what industry teaches, wherefore industry will still demand apprenticeship. The student must pay for the specific instruction at school instead of being paid to take it later. And time-consuming specifics crowd out drill in basic scientific and technical subjects as well as liberal arts.

Jan 1, 1952

For his achievement in building the Panama Canal, Major-Gen. George W. Goethals was presented, on May 22, the John Fritz medal, the highest mark of distinction in the engineering profession. In the absence, in Europe, of Clemens Herschel, chairman of the Board of Award, Charles F. Rand presided over the meeting. Col. John J. Carty, who was chairman of the Board of Award at the time the medal was awarded to Gen. Goethals, and who ordinarily would have presented the medal, was detained in Paris by Government work, so the presentation was made by Ambrose Swasey. When calling the assembly to order, the Chairman said: This medal has been awarded by a Board representing four of the National Engineering Societies: American Institute of Mining and Metallurgical Engineers, American Society of Civil Engineers, American Society of Mechanical Engineers, and American Institute of Electrical Engineers. The medal was established in 1902, in honor of that great American engineer, John Fritz, and has never been awarded except to very distinguished men. In. 1902, the first award was made to John Fritz for scientific and industrial achievement. In 1905, to Lord Kelvin for his work in cable telegraphy and other scientific attainments. In 1906, to George Westinghouse for the invention and development of the air brake. In 1907, to Alexander Graham Bell for the invention and introduction of the telephone. In 1908, to Thomas A. Edison for the invention of the duplex and quadruplex telegraph, the phonograph, the development of a commercially practical incandescent lamp, the development of a complete system of electric lighting, including dynamos, regulating devices, underground system protective devices and meters. In 1909, to Charles T. Porter for his work in advancing the knowledge of steam engineering and in improvements in engine construction. In 1910, to Alfred Noble for notable achievements as a civil engineer. In 1911, to Sir William H. White for notable achievements in naval architecture. In 1912, to Robert W. Hunt for his contributions to the early development of the Bessemer process. In 1914, to Professor John E. Sweet for his achievements in machine design. In 1915, to Dr. James Douglas for notable achievement in mining, metallurgy, education, and industrial welfare. In 1916, to Dr. Elihu Thomson for achievement in electrical invention, in electrical engineering and industrial development, and in scientific research. In. 1917, to Dr. Henry M. Howe for his investigations in metallurgy, especially in the metallography of iron and steel. In 1918, to J. Waldo Smith for achievement as engineer in providing the City of New York with a supply of water. And now to General Goethals for achievement as the builder of the Panama Canal_

Jan 7, 1919

Jan 1, 1928

By J. A. Corbyn

An analysts of mining investment from the point of view of financial optimization is presented. The method uses differential equations to describe financial performance rather than the more usual method of modeling mine accounts from year to year. The analysis shows there is a optimum mine life that can be easily determined if certain simplifications are made. Tables are given to facilitate computation of the optimum life.

Jan 1, 1985

The plan recently started for the organization of a Division of Re-search and Statistics in the American Petroleum Institute will probably shortly be consummated. It is proposed to expend $500,000 annually for this research work. The organization's work, as now outlined, pro-vides for a Technical Director, who is to be in general charge of all the work. He is to be assisted by Consulting Technical Experts, who are to be distinguished specialists in the industry and are to serve without pay. In addition, there is to be an advisory committee made up of one representative from the Bureau of Standards, Bureau of Mines, Geo-logical Survey, American Institute of Mining and Metallurgical Engi-neers, Society of Automotive Engineers, National Automobile Chamber, of Commerce, American Society for Testing Materials, American Chem-ical Society, and the National Research Council. This committee is to meet regularly to discuss the problems of the petroleum industry. An engineer and chief economist are to assist the Director.- The econ-omist is to be in charge of statistics, economic phases of the industry, and publicity. It is the intention to centralize in the Institute all infor-mation that may be required by the States or Federal Government, which would be of value to members of the Institute. This includes prices of crude oil, refined products, and natural gas. The engineer of the division is to coordinate the research work to be undertaken by, the subdivisions of production, chemical engineering, utilization, and special research. Systematic study is to be made of the

Jan 12, 1919

By Jan M. Mutmansky

The primary purpose of this presentation is to analyze the strategies available to an operator using either stripping or stripping and subsequent punch mining on a given coal property. The initial concern is the formulation of a mathematical model, which allows an operator to determine and utilize the particular highwall height, which will maximize : (1) Total profit or (2) Profit per unit of operating time. The analysis centers on the relative costs and recoveries involved in a typical coal seam amenable to stripping, augering, and punch mining. The model defines the point to which the high-recovery stripping operation with increasing overburden removal and reclamation costs should proceed before the low-recovery augering and punch mining methods are employed so that the desired objective is achieved. Examples are presented, which exhibit the use of the model on typical operations. Present mining methods as practiced in the Eastern United States are examined in the light of the results. Recommendations for improving present strategies are made and the resulting economic ramifications are analyzed. Particular attention is given to the stripping ratios presently used and recommendations for improving present mining practice are presented. Implications of these recommendations to the design of strip mining equipment are discussed.

Jan 1, 1977

By A. B. Cook

Gas cycling is generally considered a much less efficient oil recovery mechanism than water flooding. HOWever, recoveries from some fields have been exceptionally high as a result of gas cycling. Recovery from the Pick-ton field, for example, was calculated to be 73.5 perceni of the stock-tank oil originally in place. In evaluating pressure maintenance projects, determining how much of the recovery is due to displacement by gas and determining how much is due to vaporization of the imrnohile oil in the flow path of the cycled gas is very difficrilt. Even though most of the oil is recovered by displacetr~ent, the success of a project may depend on the amount of oil vaporized. A limited number of experiments have heen performed with a rotating model oil reservoir that simulates gas cycling operations and allows a separation of the oil from, tile free gas flowing into the laboratory wellbore at reservoir conditions, thus revealing which is displaced oil and which is vaporized oil. It Iras been determined that the amount of varporizatio'n is .significant if proper conditions exist These experiments show that oil vaporization depends on pressure, temperature, volatility of the oil and amount of gas cycled. Increases in each of these conditions increase the volume of oil vaporized. Data from six experiments affecting vaporization are presented to illustrate reservoir condition that range from favorable to unfavorable. 111 these eaperitnenis recovery by vaporization ranged from 73.6 to 15.3 percent of /he immobile oil (oil not produced by gas displacerrlt). INTRODUCTION Between 1930 and 1950, gas cycling was a popular. oil recovery practice. especially for the deeper reservoirs. Later, with many case history-type studies published for both gas cycling and waterflooding, it was generally believed that waterflooding was far superior to gas cycling, even when gas cycling was conducted as a primary production procedure by complete pressure maintenance. A good example illustrating the advantage of water-flooding over gas cycling is given in a paper by Matthews' on the South Burbank unit where gas injection was followed by waterflooding. The author concluded in part that "Early application of water injection, without the intervening period of gas injection, would have recovered as much total oil as ultimately will be recovered by waterflooding following the gas injection, and total operating life would have been shortened". This appears to be a logical conclusion. However, it should not be applied to all fields. Pressure maintenance with gas in the Pickton field, as reported by McGraw and Lohec;' will result in a much larger percentage of oil recovery than was obtained in the South Burbank unit. The great success in the Pickton field resulted partly from vaporization of the immobile oil in the flow path of the cycled gas. The amount of vaporization is related to the following conditions: volatility of the oil as reflected by the APT gravity of the stock-tank oil; reservoir temperature; reservoir pressure during gas cycling; and the amount of gas cycled. Therefore, the U. S. Bureau of Mines is investigating these effects on vaporization in a research project using a model oil reservoir. Three different stock-tank oils having 22, 35 and 45" API gravities are being used as base stock to synthesize reservoir oils. Experiments are being performcd to determine vaporization at 100, 175 and 250F and at 1,100, 2,600 and 4,100 psia. This is a progress report showing the results from six experiments. Other Bureau of Mines reports"- concerning vaporization are listed. LABORATORY EQUIPMENT AND PROCEDURES The equipment ' consists of an internally chromium-plated steel tube packed with finely sifted Wilcox sand. The tube is approximately 44 in. long and has an ID of 13/4 in. The sand section contains approximately 570 ml of voids, has a porosity of 32 percent, and a permeability to air of 4.3 darcies. A unique feature of the laboratory reservoir (Fig. 1) permits the tube part to rotate at 1 rpm while the outlet and inlet heads are held stationary. The outlet end contains diametrically opposed windows to permit observatlon of the flowing fluids, and two valves, one on the top and the other at the bottom. Oil and free gas. when being produced simultaneously, can be separated by manipulating the two valves to keep a gas-oil interface in view through the windows. Thus, only gas is produced through the top valve and only oil flows through the bottom valve. The laboratory equipment was designed to study vaporization. Therefore, a uniform reservoir was made using dry sifted sand as opposcd to using a consolidated sand core with interstitial water. Furthermore. the reservoir was tilted to minimize fingering of gas. This tilting also in-

R. B. BRinsmade, Ixmiquilpan, Hgo., Mexico (written discussion").—how far ahead of the United States are the self-governing British colonies in their application of economic democracy is well illustrated by Mr. Godson's paper. To prove this, I need quote only two of his assertions, the italics being mine: "lt is a stern fundamental principle that a11 natural resources are the inalienable property of the crown.'' "The right to a claim should only exist as long as it is worked or taxed to insure work. Once depart from that sound maxim, there results improper control of the natural resources of the country, which primarily belong to the people." These assertions formed the basis of my criticism1 of Dr. Raymond's paper in 1913, and they should be inscribed in letters of gold above the entrance to the office of every county recorder in the United States. I venture to assert that the existing United States mineral land laws are essentially unfair to the nation and a hindrance to the legitimate miner, because they fail to embody the above quoted principlca; and this applies both to state laws east of the Mississippi and to federal laws in the West. That these laws favor the forestaller and the monopolist, defraud the government of its fair share of the profits of mineral production, and tend to industrial feudalism must be evident to any competent unbiassed observer. The fact that no serious political movement arose to change them, until the conservation agitation of the last decade, may be largely explained by two salient conditions. First, as stated by President Winchell, "for a long time there was such a vast surplus, or excessive amount of unwanted, undesirable land in the West that the land offices did not scrutinize the claim sought to be located;" second, that the millions of individual farm owners throughout the country could easily be aroused, by the special pleaders hired by the great land speculators and monopolists, to use their vast political power for the suppression of any statesmen striving to incorporate in our semi-feudal land laws any such principles as those quoted from Mr. Godson. While agreeing with Mr. Godson that a freehold or fee-simple title, if coupled with obligatory continuous work, may have the same practical effect as a lease, it seems to be the experience of the most active and

Jan 1, 1920

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Jan 1, 1942

By M. L. Jeremic

Mining practice and underground studies show that, besides mining stresses, lateral tectonic stress also influences roadway stability. Roadways perpendicular to the direction of major lateral stress experience the majority of roof falls, accelerated floor heave, and extrusion of coal ribs into openings. They have the most severe deformations and their maintenance is difficult. Roadways parallel to the major lateral stress exhibit roof sagging and bed separation, moderate though sometimes uneven floor heave, and slabbing of coal ribs into openings. Roadways oblique to lateral stress have, to a certain degree, self-supporting roofs, uniform floor heave, and limited sloughing of coal ribs into openings. The optimal orientation of roadways is, therefore, oblique to the lateral stress direction with fewer crosscuts between them.

Jan 1, 1982