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By P. J. D. Lloyd

The widely employed IPCC models for the release of methane from underground coalmining operations are reviewed, and put in the context of the South African National Communication on greenhouse gas emissions. Then attempts to calibrate the model via measurements on South African coalmines are described. These attempts failed because it rapidly became clear that the IPCC model, involving methane adsorbed by the coal and released (a) by the depressurization of the strata caused by mining, (b) from the broken coal and (c) from coal remaining in roof, floor, pillars or goaf after mining, was not valid. Instead, measurements showed that the methane release was very variable, sporadically falling to zero. Carbon dioxide concentrations were monitored at the same time as methane, and followed methane concentrations. An alternative model was developed, in which methane was largely displaced from the coal and held in fissures, cleats and pores, which, when intersected by mining, released methane into the ventilation stream. In a key test of this hypothesis, methane concentrations around a continuous miner were monitored both during operation and after shutdown. Even during operation, methane became undetectable. It is hypothesized that heating of the South African coal-seams during the emplacement of the igneous dykes that cut so many of the seams caused displacement of the methane from the coal into the pores. This fissure-held methane, or free methane, is released rapidly once a pathway through the coal into the mine atmosphere is established. In contrast, methane adsorbed in the coal, and measured as the seam gas, desorbs relatively slowly. Measurements suggest that on average about 50%of the adsorbed methane is lost one day after mining. By that time, the coal will have left the mine, so that the remaining seam gas will be released to the atmosphere outside the mine. Attempts to measure methane emissions from surface coal-mining operations showed very little detectable methane. Coal from surface operations, which was milled and exposed to atmosphere, adsorbed gas from the air. It is concluded that methane release from surface operations is probably less than 3Gg/a. It is estimated that the total methane release from the coalmining industry amounts to about 72 Gg/a, as opposed to the National Communication estimates of 323 Gg/a in 1990 and 317 Gg/a in 1994. However, there are large errors associated with these estimates, and it is recommended that the methodology outlined in this report be extended to as many mines as possible to reduce the errors that are inherent in the physical processes underlying the release of methane from South African coalmines.

Jan 1, 2005

By P. F. Kerr

This is not a conventional professional paper. It deals mostly with personal experiences. These concern a background in applied mineralogy. Your committee asked me to do this. I accepted with a California enthusiasm which has since diminished with the approach of a Chicago winter. Nevertheless, I wish to thank your committee and also to apologize in advance for omissions and inadequate statements, as such a paper must possess. My introduction to applied mineralogy took place in 1922 at Stanford University. Physicists had recently demonstrated that crystalline powders would reflect x-rays in a systematic manner. Professor A.F. Rogers sug¬gested that significant x-ray reflections might be obtained from powdered opaque minerals. With the aid of Professor D.L. Webster in the Physics Department, I constructed an x-ray diffraction outfit. It was a crude affair, but it worked. Starting with 1924, we secured recognizable patterns of a goodly number of ore minerals (1,2,3). Today x-ray diffraction outfits are common, and a card system of mineral identification is constantly used.

Jan 1, 1981

By S. Managi, M. C. Williams, N. H. Behling

Particle processing is a key to using solid particles in fuel cell applications. Selectivity with regard to reactivity, impurities, etc. is important feature and the treatment of particle surfaces could greatly impact the performance of direct carbon fuel cells. Solid fuel particles will become increasingly important in the future. Present energy conversion systems for solid fuels are too inefficient. New energy conversion systems for solid fuels with higher energy conversion efficiencies are possible. Fuel cell technology is a key-technology in these new conversion systems. The direct carbon fuel cell (DCFC) operates on carbon particles obtained from a variety of solid fuel feedstocks. The DCFC is the only fuel cell designed to directly oxidize carbon particles in a special anode chamber. The particles are generally graphite structure with high purity. The electrolyte used is the high temperature solid oxide, molten carbonate or hydroxide electrolyte. Since a pure stream of CO2 is produced the stream can easily be sequestered and disposed. Pure carbon dioxide produced as a by-product would also have a market in many industries. A well-defined technology roadmap identifying key research and development (R&D) issues is necessary to provide a framework for the development of these systems and to prevent entrenchment in inherently inefficient technologies. This review paper describes the direct carbon fuel cell and its system, how it works, the developmental status, the characteristics of the carbon particles needed, and the research and development issues for the technology. INTRODUCTION The DCFC has been under development for many years. It was researched as early as 18961), but an intense research and development is necessary to realize its potential. William W Jacques, a US electrical engineer and chemist, described a DCFC in 1896 in US Patent 555511 for a “Method of converting potential energy of carbon into electrical energy”. Jacques used coke electrodes in a molten NaOH electrolyte. Molten carbonate fuel cell (MCFC), solid oxide (SOFC) or hydroxide electrolyte (AFC) technologies can be used as the basis for the DCFC technology. A schematic of the direct carbon fuel cell in the MCFC mode is shown in Figure 1. The molten carbonate fuel cell operates at approximately 650°C (1200°F). The high operating temperature is needed to achievesufficient conductivity of the carbonate electrolyte, yet allow the use of low-cost metal cell components. A benefit associated with this high temperature is that noble metal catalysts are not required for the cell electrochemical oxidation and reduction processes. For the hydrogen reaction the half-cell electrochemical reactions are

Jan 1, 2019

By D. Grabowski, Z. Pokryszka

ABSTRACT: Some residential areas of the iron basin of Lorraine (in the North-East of France) are affected by noxious gas emissions (under oxygenated, carbon dioxide loaded and radon loaded gas mixtures). Site investigations showed that the gas flow was mainly due to natural thermal air circulation between old mines and the external atmosphere. The main hazards associated with this flow of gas and its potential accumulation in confined volumes within buildings are asphyxiation and intoxication. A further study is in progress in order to better understand the origin of the gas generation within the old iron mine workings. Following the first hypothesis, the carbon dioxide emission is mainly due to the specific hydro-geo-chemical process associating the pyrite oxidation with the acid dissolution of carbonates. KEY-WORDS : gas, emission, mine, flow, pyrite. ZUSAMMENFASSUNG: Einige Wohngebiete der Eisenmulde Lothringen (im Nordöst Frankreich) werden von Ausgasung gesundsheitsschädlichen sauerstoffarmen Mischungen, die mit Kohlendioxid und Radon beladen sind, betroffen. Untersuchungen am Ort haben gezeigt, wie diese Gasstroemung von natürlichem thermischen Luftumlauf zwischen den alten Bergwerken und der externen Atmosphäre ausgelöst wird. Erstickung und Vergiftung sind die wichtigste Risiken, die mit dieser Gasströmung und ihrer möglichen Speicherung in begrenzten Lokalen zusammenhängen. Eine weitere Untersuchung wird durchgefuehrt so dass man die Herkunft der Gaserzeugung in den alten Grubenräume besser versteht. Der ersten Hypothese folgend, Ausgasung von Kohlensäure liegt vor allem an dem hydro-geo-chemischen Prozess, der Pyritrösten mit der säuren Auflösung von Karbonate verbindet SCHLÜSSELWÖRTER: Gas, Ausgasung, Bergwerk, Strömung, Pyrit RESUME: Certaines agglomérations du bassin ferrifère lorrain (au Nord-Est de la France) sont affectées par des émanations de mélanges gazeux sous-oxygénés et chargés en certains gaz nocifs (dioxyde de carbone, radon…). Les investigations effectuées in situ ont montré que les écoulements gazeux à l’origine de ces émanations sont liées au phénomène de tirage naturel entre les anciens travaux miniers et la surface. Les principaux risques engendrés par ces émanations de gaz et son accumulation possible dans des endroits confinés sont l’asphyxie et l’intoxication. Une étude est en cours afin d’expliquer l’origine de la modification de l’air au sein des vieux travaux miniers. Les émissions de dioxyde de carbone seraient dues à des processus hydro-geo-chimiques associant l’oxydation de la pyrite et la dissolution de carbonates. MOTS CLES: gaz, émanation, mine, écoulement, pyrite.

Jan 1, 2003

By P. G. Killeen

Proper calibration of portable gammaray spectrometers and spectrometric logging equipment is essential if they are to be used for accurate quantitative determinations of radioelement concentrations. In the case of both types of equipment, it is necessary to determine the system calibration constants (stripping factors and sensitivities). In the case of borehole logging equipment, it is further necessary to determine the system response characteristics if the data are to be computer processed to improve the accuracy and resolution of the logs. Facilities are now available near Ottawa for the complete calibration of portable gamma-ray spectrometers and spectrometric logging equipment, in addition to already existing facilities for calibrating airborne gamma-ray spectrometers. Limited additional facilities are also available or planned in several other areas of Canada.

Jan 1, 1978

By Bhaskar B. Thapa

The design of the Caldecott 4th Bore, located along State Route 24 in Oakland, California, is based on the principles of the New Austrian Tunneling Method (NATM). Typical NATM initial support design practices used in Europe were adapted for this project to account for U.S. conditions and requirements, such as degree of experience with NATM construction, the prevailing contractual environment, and preferences for contractual simplicity. Key design features are: (1) support selection criteria based on ground behaviors and ground conditions; (2) a prescriptive design with allowance for support adjustments based on observations during construction; and (3) organization of support requirements into only four major support categories, while permitting some adjustment for variations in ground behaviors and conditions using a few subtypes and additional support measures.

By L. C. IlsLey

The theory and practice followed by the Bureau of Mines in investigating electrical equipment4/ made for service in gassy mines are of particular interest not only to manufacturers but also to those organizations that deal with safety anal inspection at the mines. This publication purposes to discuss the theory upon which tests of equipment in explosive atmospheres are based and to consider inspection and other matters pertaining to investigations made by the Bureau in ascertaining whether a machine merits approval. The material in this report is taken from an earlier bulletin,5/ which is row out of print, and has been revised and brought up to date to make the information again available to manufacturers, inspectors, and others interested in the development of safe equipment.

Jan 1, 1954

By Groom P.

Discussion of the paper presented at the Midland Institute of Mining, Civil, and Mechanical Engineers' general meeting held in Leeds, March 21st, 1916, with T. Beach, vice-president, in the chair included contributions by Beach T., Fearnsides W.G., Lloyd W.D., Gill J. and Nadin R.

Dec 1, 1916

By Paul Worsey, Dave McCool, Ryan Freeman, Chris Wolters, Rick Hines, John Bowles

Carroll Cave, located in Camden County, in south central Missouri, is one of the most significant caves in Missouri with over 12 miles of mapped passages and over 100 known but unmapped side passages. In the early 1990’s there was a breakdown of the originally excellent landowner/caver relations as the ownership of the only known entrance moved to a new generation and grottos squabbled for control. Subsequently the current owner of the natural cave entrance refused to grant anyone permission to enter.

Jan 1, 2003

By Maurice J. Peterson

The applications of optical spectrochemical methods of analysis for determining trace impurities in metals are described, and the need for developing analytical procedures capable of accurately determining fractional parts per million of impurity elements in metals is stressed. Factors which determine sensitivity of detection and means to extend sensitivity levels in spectrochemical methods are discussed. Examples from the current literature illustrate the diversity of spectrochemical techniques that have been applied to the analyses of varied sample types, Many of the methods use preconcentration procedures to increase sensitivity limits. A bibliography containing abstracts of recent methods in included. Optical spectrochemical analysis is based upon the measurement of radiant energy emitted by atoms of the elements to be determined when a sample is subjected to a suitable source of excitation. The behavior of the valence electrons during excitation, usually by an arc, spark, or flame, produces radiant energy of specific wavelengths. This is recorded as spectral line images on a photographic film or plate by the spectrograph; for direct-reading instruments, the spectral lines are detected with photomultiplier tubes. The spectral lines result from the excitation of atoms in the sample. Each variety of atom produces its own characteristic set of lines, thus serving to identify the elements in the sample. Increased concentrations of elements in the excited sample results in increased blackening of the spectral line images.

Jan 1, 1961

By D. Harrington

Metal-mine fires in the United States are not of frequent occurrence and relatively for of those that do occur result in loss of life, although many cause heavy property loss or are costly to extinguish. Over a span of years, however, more lives are lost in metal Trines from fires than from any other of the untoward occurrences which afflict metal mining, such as electrical storms, floods, cave-ins, magazine explosions, or strikes. In perhaps more than 90 per cent of the fires. in metal mines' there is no loss of life; hence, the general public, even the general mining public, seldom hears of these fires, though the property loss frequently reaches hundreds of thousands of dollars; on the other end, in some instances the property damage from an underground metal-mine, fire has been negligible and the loss of life comparatively high. In any comprehensive study of metal-mine fires, ventilation is certain to be found a vital factor. The influence of ventilation on fires and the influence of fires on ventilation are of vital importance. In metal-mine fires very few persons meet death from actual contact Pith heat or flame; nearly all fatalities are duo to asphyxiation or suffocation. A well-planned, well-installed, and well-operated mechanical ventilation system is unquestionably one of the best available present-day protections from loss of life and property in case of fire in a metal mine; that this is true is attested by the usual helplessness of mining officials in trying to handled underground fires in a mine that has not had the foresight to install mechanical equipment to control underground air flow. On the other hand, the establishment of high-velocity air currents without sufficient day by day attention to such details as the method of coursing, or controlling, or confining them most certainly tends to increase dangers from fires rather than to aid in the avoidance or elimination of these dangers; the truth of this latter statement is shown by the fact that the greatest, loss of life in metal-mine fires during the past 20 years has been in relatively well-ventilated workings, and in every case the life loss has been due to some more or less trivial or easily remedied defect in the Ventilating system - usually a defect that has had some reference to doors used or supposed to be used to deflect or control air currents.

Jan 1, 1933

By Helmut Hass, Marco Ziller

"Abstract: 1 Sophia Spoortunnel Marco Ziller, Trevi S.p.A.:For the construction of 14 cross passages connecting the twin tunnels of the Sophiaspoortunnel the Artificial Ground Freezing technique was applied as temporary soil improvement. AGF by Brine and Liquid Nitrogen was implemented. Automatic data acquisition and recording systems were installed to monitor the variation of the temperatures in the soil during the freezing intervention in order to control the behaviour and the efficacy of the treatment.IntroductionThe Sophia rail tunnel is a bored tunnel with a length of 4.24 km, comprising two separate parallel tubes with an outside diameter of 9.45 m. The twin tunnels were excavated by a TBM equipped with hydroshield. Seven service shafts, approximately 30 m deep, were constructed in between the tunnels. From each shaft two cross passages were constructed connecting the shaft with both tubes, for a total of 14 cross passages.The Artificial Ground Freezing technique utilizing both the Brine and Liquid Nitrogen methods was used for temporary waterproofing and soil support to allow the excavation of the cross passages. The natural soil to be frozen was consisting in loose sand and clay under a hydrostatic pressure up to 250 kPa.For the measurement of the ground temperature. Automatic data acquisition and recording systems were installed to monitor the variation of the temperatures in the soil and at the contact with the tunnel lining. Furthermore, a comprehensive computerized system of sensors and cryogenic valves was designed and implemented in order to manage the whole Nitrogen process.Soil Main CharacteristicsThe soil stratigraphy is generally consisting in Clay (Holocene) from ground level to -15 m depth, followed by Pleistocenic Sand, from -15 to -25 m, and Clay below -25 m."

Jan 1, 2014

Mining One (M1) is often challenged with the task of evaluating deposits considering open cut and/or underground (UG) mining methods. In many near surface deposits a combination of both methods results in the optimum economic means of exploitation. As common as it is to have open pit mines eventually develop into underground mines, theory associated with determining the optimum underground/open pit interface location continues to be subject to refinement. With recent developments in underground software packages, the generation of rudimentary underground mining stopes from basic economic inputs is not only fast, it provides a practical underground mining solution. The determination of the open pit/underground interface is not new to Gemcom's Whittle 4X (Whittle). However, its solution has only ever claimed to be indicative, mainly due the rudimentary nature of how Whittle represents any potential underground mining. The efficient generation of the aforementioned underground mining solution has provided mining engineers with a unique opportunity. Underground mining can now be represented within the Whittle environment to allow the open pit/underground mining scenario to be modelled with increased accuracy. Providing Whittle with the ability to now consider mineable underground stopes combined with the consideration of time value of money (discounting), has delivered a solution with higher credibility than ever before.This paper will elaborate on the potential value gains associated with improved accuracies in the modelling of any potential underground mining in the Whittle environment. Furthermore, it will explain the importance of incorporating time value of money in the optimisation process to provide a consistent solution through to pit selection and final analysis.

Dec 6, 2010

By J Heyes, R Getty, M Sanapala

Closure is an inevitable part of the life cycle of every mine. Successful closure outcomes critically depend upon early and ongoing integration of closure planning and mine planning. Early and holistic integration minimises residual risk and maximises opportunity to leave a cost-effective, positive legacy for the company, investors and future land users. Both disciplines are inherently interconnected and aim to maximise ‘value’ via trade-offs. Historically, integration of closure into mine plans has been poorly implemented across the industry. There is still a perception that closure planning only starts as the end of a mine life approaches. This inevitably leads to tension when making business decisions, with progressive closure often being deferred due to short-term drivers and incentives. To create a positive legacy after production has ceased, this tension can be managed by transparently and fully informing business decision-makers, such that the impacts of short-term decisions on long-term closure objectives are understood. BHP recently updated a business-wide standard ‘Our Requirements for Closure and Legacy Management’ designed to strengthen collaboration, education and integration of closure across the entire life cycle of the asset. Concurrently, the ‘Life of Asset’ definition was refined to include the postproduction phase, until closure objectives have been met. This paper discusses the criticality of mine planning integration with closure and how updates to Our Requirements sharpen the focus from transactional to transformational planning. These concepts are applicable to any mining operation aiming to achieve successful closure across the full asset life cycle, from exploration to post-production.

Sep 18, 2023

By J. Diaz

"Antioquia region, in Colombia, is considered one of the areas with the highest mercury contamination in the world due to its use in gold recovery through artisanal mining, emissions to the environment between 100 and 150 tonnes of mercury are estimated by year. Therefore it became necessary to develop a project in which it was proposed aim to empower traditional artisanal mining community, or in different areas, such as gold mining in the region of Antioquia, in the implementation of safe mining practices and environmentally sustainable, from the introduction of clean technologies, in order to prevent or mitigate the impacts generated by traditional mining on the environment and the communities involved. In order to achieve this training to traditional mining community developed and technical visits to artisanal mining were performed in order to optimize operating processes and promote safe mining practices. In the project more than 1400 artisanal miners were trained.INTRODUCTION In the developing world, a combined 30 million individuals extract more than 30 different minerals using rudimentary techniques. Gold is the preferred mineral extracted by artisanal operators due to its high unit value and its market price, which has increased almost sevenfold over the last decade (Veiga et al, 2014). The artisanal gold mining sector in Colombia has 200,000 miners officially producing 30 tonnes Au/a. In the Northeast of the Department of Antioquia, there are 17 mining towns and between 15,000 and 30,000 artisanal gold miners (M. Veiga, 2010) (Upme, 2014). This production, however, has caused environmental problems, including high levels of river siltation and mercury pollution (Veiga et al, 2014) (UPME, 2007) (Cordy et al., 2013). The mercury contamination in Colombia by artisanal gold mining has been recognized by a number of researchers and local authorities for over a decade estimated that the annual mercury emissions from artisanal gold miners in Colombia in 2007 were between 50 and 100 tonnes, however given recent gold value increases and gold rush activity, current mercury emissions are likely much higher. Colombia is likely the world's 3rd largest source of mercury emissions from AGM after China (240 to 650 tonnes of Hg/a) and Indonesia (130 to 160 tonnes of Hg/a) (Telmer and Veiga, 2008) and is the world's highest per capita mercury polluter (M. Veiga, 2010)."

Jan 1, 2016

By M. Springett

Organization of teams for rapid and effective evaluation of international mining properties requires special considerations beyond those needed for domestic activities. Experience, technical ability, a sense of perspective, humor and self-discipline are vital characteristics to look for in the selection of individuals for forming an efficient and productive team.

Jan 1, 1994

HISTORIC SIGNIFICANCE OF EXPLOSIVES TO MINERAL PRODUCTION The ancient method of breaking rock was to build a wood fire against the face, heat the rock through and then pour water on the heated ore-bearing rock to cause it to break by tension. It was too slow and costly as the forests around the mines were cleared and transport of wood became difficult. Black powder had been used for 300 years as a gun propel- lant. ('Black' after Berthold Schwartz, a German monk, Figure 1, and the inventor of the gun who was given the epitaph `Here lies Berthold the Black, the most abominable of inhumans who by his invention has brought misery to the rest of humanity'). Only at the beginning of the 17th Century, black powder was introduced for loosening rock in European mines. In 1802 Dupont started black powder manufacture on the Brandywine Creek. The ignition train of powder was enclosed in goose quills, straws, rushes, paper or wooden tubes. The time interval between igni- tion and blast was pure speculation.

Jan 1, 1988

By T. K. Akopyan, E. A. Naumova, V. V. Doroshenko, N. A. Belov

"A qualitative and quantitative analysis of the new Al-Ca-Mn-Fe phase diagram in the region of the aluminium corner is carried out using both thermodynamic calculations and experimental methods. Using the obtained data, the principal possibility of creating both cast and wrought sparingly alloyed aluminium alloys with calcium, manganese, scandium and zirconium at allowable iron contents of up to 0.4% that do not require solution treatment and quenching is justified. At the selected concentrations, the entire amount of manganese (1%) is practically included in the aluminium matrix, while calcium (~2%) and iron are in the multiphase eutectic, which has a fine structure. The joint introduction of 0.25% Zr and 0.1% Sc into the alloy leads to precipitation hardening due to the formation of the L12-Al3(Zr,Sc) phase during annealing. Based on the experimental data for the model multicomponent Al2CaMnFeZrSc alloy, it is shown that such alloys can combine high mechanical properties and good manufacturability during both casting and metal forming. This is due to the combination of a narrow temperature range of solidification with a fine eutectic structure.INTRODUCTION Hardening in branded aluminium alloys (both cast and wrought) is generally achieved by heat treatment of the T6 type, including quenching and artificial aging (Zolotorevskiy, 2007; Kaufman, 2004; Mondolfo, 1976). Heat treatment significantly increases the cost of the final products. Therefore, industrial companies are interested in developing alloys that can be hardened to the desired level without quenching. To solve this problem, alloys with scandium are the most suitable (Toropova, 1998; RØyset, 2005). This element (usually in an amount of ~0.3%) is one of the most effective hardeners in aluminium alloys, due to the formation of Al3Sc (L12) phase secondary precipitates, the size of which is less than 10 nm (Knipling, 2010; Fuller, 2005). These nanoparticles are formed during the annealing via decomposition of the supersaturated aluminium solid solution (hereinafter referred to as (Al)). This feature allows us to significantly increase the strength of aluminium alloys by simple annealing (at ~300°C), i.e., without heat treatment, including quenching. However, scandium is a very expensive metal, so its use as an additive is severely limited. The most effective method is the introduction of scandium together with zirconium (Lefebvre, 2009; Booth-Morrison, 2012). The optimal combination is the introduction of ~0.1% Sc together with 0.20–0.25% Zr, which, according to (Belov, 2006), gives approximately the same strengthening effect as the introduction of 0.3% Sc."

Jan 1, 2018

By Curtis Wilson

FROM the results of his investigation of the hardenable copper-nickel-silicon and copper-cobalt-silicon alloys, M. G. Corson1 explained the dispersion-hardening effect by assuming the precipitation of the inter-metallic compounds nickel silicide and cobalt silicide, and on this basis he constructed diagrams for the pseudobinary systems copper-nickel silicide and copper-cobalt silicide. A consideration of these diagrams and of the factors that exert such an important influence upon the rate of precipitation, such as degree of supersaturation, nucleus number and rate of diffusion, leads to the expectation that the rate of precipitation of the silicides on reheating the quenched solid solutions at temperatures below the alpha-phase boundary should increase continuously as the reheating temperature is raised. The present investigation shows that while this rate does increase with the reheating temperature in a general way, it does not do so continuously. Supplements to Corson's diagrams are offered, showing the discontinuities, and slight modifications in the solubility curves are suggested.

Jan 1, 1933

By D. A. Greenwood

Dr David Greenwood opened the first technical session of the conference by highlighting the diverse nature of the papers in this session but drawing attention to the common theme that each solved a particular problem. PAPER 1.1 Tony Elliott introduced his paper on deep mix-in-place soil/cement stabilisation by stressing its simplicity. The engineering solution adopted at Elland was detailed in his paper and had formed the key to success of the project. The process itself is not widely used or known in the UK, although it has the potential to achieve significant benefits, subject to proper engineering control being exercised. It was explained that although the concept was not new, development of the technique for soil modification was relatively recent. The alternatives to mechanical mixing are injection, which is very dependent on soil type, site geometry, etc, and jet grouting; both processes are soil-responsive, and not easily monitored or programmed, difficulties which can largely be overcome by mixing. To achieve this at depths to say 30m requires high-powered, high torque machines and has resulted in significant equipment development.

Jan 1, 1991