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By Babcock. C. O.

This report presents equations derived from experimental testing to convert borehole pressure cell data from vertical and horizontal cells to the biaxial ground stress changes. While hydraulic pressure cells in boreholes have been in extensive use since 1957, methods of data reduction have been slow to develop. The methods of Sellers (1970) and Bauer et al. (1985), respectively, indicate a linear relationship between cell pressure change and the rock stress change. Lu presented a theory for creep behavior (Babcock et al., 1982). In the present work, in laboratory testing, the logarithm of the cell pressure was related to the stress change in seven elastic materials for a range of Young's moduli of 3.45 to 85.5 GPa (0.5 to 12.4 x 106 psi). The gage response depends upon the setting pressure of the gage, the gage pressure at any time, the rock properties, and the cross-sensitivity of the gage to transverse loading. The setting pressure of the cell should be proportional to the Young's modulus of the rock, the absolute stress can be determined by overcoring in strong rock, and the in situ Poisson's ratio can be estimated in some cases from the cell responses.

Jan 1, 1986

By Peter Kolitsch, Galyn Rippentrop

INTRODUCTION On April 14, 1994, the joint venture of Kiewit Construction Company and Kenny Construction Company was awarded a 76 million dollar contract for the construction of the New Hampshire Avenue Tunnels in Washington D.C. The Notice to Proceed was given on May16, 1994 and scheduled completion is September 1997. This project is one of five contracts for the Mid-City E Line as part of the Washington Metropolitan Area Transit Authority (WMATA)Greenbelt Route. Once completed, the Mid-City E Line subway tunnels will connect service between the U Street-Cardozo and Fort Totten stations (See Figure 1). The New Hampshire Avenue Tunnels project consists of approximately 6800 feet of twin single track subway tunnels, three vertical shafts, associated utility work and comprehensive restoration work. Section E4b involved two methods of tunneling along the project alignment. The first method consisted of the well known soft ground open face shield driven tunnel and the second method consisted of tunnels driven using the New Austrian Tunneling Method (NATM) . The NATM section of twin tunnels begins at the Buchanan Vent Shaft and proceeds approximately 2630 feet under Rock Creek Cemetery, including the Farragut Emergency Access Shaft and connects approximately 390 feet beyond to the existing Fort Totten running tunnels (See Figure 2). Special considerations had to be given to the portion of the tunnel alignment which passed beneath Rock Creek Cemetery. The analyses and subsequent selection of the tunnel support system were based on anticipated geological conditions and the sensitive tunnel alignment. The New Hampshire Avenue Tunnels project was the second soft ground NATM project in Washington D.C. The first, by no coincidence, was completed at the adjacent Fort Totten Station and Running Tunnels Project by Mergentime/Hochtief, AJV. Currently, on a nearby WMATA jobsite, the J.F. Shea Construction Company has started a third NATM soft ground tunnel. Tunnel support systems were studied with respect to three tunneling methods: •A one-pass system consisting of a bolted precast concrete segmental lining •A two-pass system consisting of an unbolted precast concrete segmen

Jan 1, 1997

By C. McLean, S. G. Kolb, A. J. Erickson, S. C. Moore

The Grossschloppen vein-uranium deposit, Bavaria, West Germany, was examined with underground workings during 1980-82 by Esso Erz GmbH, an affiliate of Exxon Minerals Company (EMC). Geologic evaluation entailed dense drilling of a portion of the deposit from workings constructed specifically for the program. Discovered in 1977, the deposit was initially explored by surface diamond drillholes which allowed definition of a 30-60 m wide vein system discontinuously mineralized along a 1000 m strike length and to at least a 450 m depth. The underground program was conceived as a cost-effective procedure to answer questions on vein correlation, grade continuity and variability. A 1200 m decline allowed access for detailed sampling of approximately 10% of the known area of mineralization. Fanned drillholes, logged by gamma probe, were spaced to provide intersections of veins at 10 to 20 m intervals. Six cross-cuts also penetrate the pitchblende and uranophane mineralization which occurs in 0.1 to 2.5 m thick quartz veins. Detailed cross-sections and level plans were constructed for resource estimates of the intensively studied portion of the vein system. The program resulted in the discovery of local, high grade areas and an average grade in the evaluated area nearly double that expected from surface drilling.

Jan 1, 1992

By Tom H. W. Loomis

The department of the Interior is a federation or holding company of several fiercely independent bureaus which have either jurisdiction or expertise - or both - in a wide range of environmental fields. Each bureau is an advocate for its own concerns and an enforcer of its own mandates. This can be confusing to the company operating in the field. I will try to explain our concerns, help you see where we are "coming from," and give you some insight as to how we can help you. Other speakers have stressed the environmental concerns and site selection factors for uranium wastes so I will focus on the concerns of the Department of the Interior and some of the assistance we can give you in this area People are more aware and more concerned about environmental factors now than they used to be. Thus the past decade has brought about an evolution in both siting and design of uranium mill tailings ponds. Site selection criteria used to be simple - find a tract of land as near to the ore supply as possible with enough space and suitable topography for the expected volume of wastes. Site selections in the past have left a legacy of problems to be corrected - radioactive wastes on stream banks and in floodplains, mill wastes in and near established communities, and unprotected wastes subject to constant erosion. This is no longer acceptable environmentally or socially to a newly aware and concerned public. (Series of 30-35 slides showing uranium mine and mill wastes in Western United States. These illustrate environmental problems, reclamation and environmentally acceptable sites. The first group of slides are aerial shots taken in Wyoming and shows a surface mine and mill area. In the various views the pit, overburden piles, mill, and tailings pond can be seen. The next series were taken in Wyoming and New Mexico. These show various views

Jan 1, 1980

By Thomas J. Rudolphi

Efficient transport of bulk materials by belt conveyor systems is an important engineering design issue, especially as conveyors become longer and power requirements increase. It is well known within the industry that the parasitic energy loss of a belt conveyor, due to the indentation of the idlers into the belt backing material can be upwards of half the total energy required to drive it on a horizontal flight. Other loss sources includes the rolling resistance of the idlers, belt/roller misalignments, material trampling, frictional and acceleration losses on loading, etc., but energy absorbed by indentation of the belt backing as it passes over each successive idler is the dominant power consuming factor. INDENTATION LOSS MODELS Models to predict the rolling resistance of a cylinder (idler) on a viscoelastic foundation (backing material) have been available for some time. As a contact problem, it is nonlinear (contact length or indentation depth is a dependent on the load and vice versa) in load/deformation response. Rubber backing material is generally assumed to behave as a linear viscoelastic solid, even though it is known that most rubber compounds retain linearity only at very low strains. Because completely general solutions are impossible, practical solutions require simplifying assumptions. Those assumptions relate naturally to two separate pans of any analytical model: I. The deformation model-aspects of the problem relating to the geometry, boundary conditions and kinematics of how the material deforms. 2. The material model-aspects of the problem relating to the material properties, or constitutive relationships, i.e., the stress/strain relationships.

Jan 1, 2008

By Ted D. Johnson

INTRODUCTION The report of the workshop on Alternate Energy Strategies estimated that the world's reserve of uranium oxide is about 700,000 tonnes. In order to extract these reserves, it is estimated that between 100 and 200 million tonnes of solid waste and of the order of 1 billion cubic meters of liquid waste will be generated. In addition, radium within tailing will continue to generate radon gas as it decays for many years after closure. Hence, the handling, storage and possible use of uranium mine waste is a matter which deserves serious consideration and planning. Since the properties of the waste will largely dictate the method of handling and storage or use of the waste, the properties of the waste should be determined, to the degree possible, prior to the design and subsequent operation of the mine. Uranium mine waste has both solid and fluid constituents. The solids include soil and rock removed during site preparation, similar material removed during development of the mine and uranium tailing generated by the mill. Soils can be stockpiled for reuse during reclamation or used as earth materials for construction of fills, roads, dams and soil linings. Rock can be stockpiled, used as mine backfill or can be used for earth construction in fills, roads and dams. The tailing is usually placed in a tailing storage area. The fluid constituents are both liquid and gaseous. The liquid waste is generally composed of the mill water; however, additional waste water can be generated by dewatering processes and diversion of surface runoff. The gaseous constituent of the uranium mine waste is radon generated by the radioactive decay of radium in the ore tailing. DEFINITION OF PROPERTIES The following presents a brief definition of some of the more important properties discussed later. The definitions are not intended to be comprehensive, but are useful as a basis for understanding.

Jan 1, 1980

By Pieter Berendsen

Lamproite occurs in two locations in southeastern Kansas that are in close proximity to each other. In both locations, the lamproite is restricted to north- east-elongated, ellipsoidal depressions measuring about 3 mi by 1.5 mi. At the surface and in the shallow subsurface, lamproite occurs as sills that were intruded into Middle and Upper Pennsylvanian rocks consisting of shale, limestone, and minor sandstone. Isotopic age determinations by Zartman and others (1967) indicate that the lamproite was intruded during the Late Cretaceous (88-91 my). Drillholes for hydrocarbon exploration down to a depth of about 1,400 ft (top of the Mississippian) and numerous shallow drillholes (mostly 200-300 ft) by the Kansas Geological Survey in the northern portion of Silver City dome helped to understand the structural complexity and extent of the near-surface sills. Numerous radial faults showing small apparent vertical offsets limit the lateral continuation of individual sills. Concentric faults are believed to mark the edge of the depression. Wagner (1954) mapped the geology of Silver City dome in considerable detail and recognized a northward dipping, 120-ft-thick, lamproite sill exposed at the surface along the northeastern edge of the dome. The upper 40-50 ft of the sill weathers olive-brown and has a soft, clayey consistency in which only the phlogopite mica is recognizable. The weathered material has been mined by Micro-Lite LLC since 1982 from the exposed sill in an open-pit that measures about half a mile extending in an east- west direction. The bulk of the material, about 98%, is used as an additive in mineral supplements for cattle feed.

Jan 1, 1999

By Donald G. Wieland, James H. Meinholz

INTRODUCTION The Milwaukee, Wisconsin Metropolitan Area provides an ideal opportunity to review the advancement of the art and science of mixed face tunneling in the past 65 years. Although there were earlier tunnel projects in the Milwaukee area, by other agencies, the majority of those driven in the Twentieth Century have been done under contract with the Milwaukee Metropolitan Sewerage District through the agency of the Sewerage Commission of the City of Milwaukee and the Metropolitan Sewerage Commission of the County of Milwaukee. Legal Description of District The Milwaukee Metropolitan Sewerage District was created by the Wisconsin Legislature in 1921 together with the Metropolitan Sewerage Commission of the County of Milwaukee. This concept of subregional wastewater control was an outgrowth of an investigation and report from a commission appointed by the Common Council of the City of Milwaukee in 1909 to study final sewage disposal for the City. In 1911 that Commission, comprised of J. W. Alvord, George C. Whipple and Harrison P. Eddy, recommended that the City construct a water filtration plant, construct a Menomonee River flushing works, expand the intercepting sewer system and, after a thorough study of the then existing and developing methods of sewage treatment, construct a sewage treatment plant. In response to those recommendations, the Wisconsin Legislature, in Chapter 608 of the Laws of 1913, created the Sewerage Commission of the City of Milwaukee. Recognizing that the improvement of surface water quality would require action on a watershed basis, the Legislature, in Chapters 554 and 590. of the Laws of 1921, created the

Jan 1, 1979

By G. S. Grainger, R. A. Cummings, F. S. Kendorski, R. L. Butts

Georgia Power Company's Rocky Mountain Project, located approximately ten miles northwest of Rome, Georgia, in the Valley and Ridge Physiographic Province, is a pumped-storage facility rated at approximately 800 megawatts of peaking energy. Geological field explorations and borings enabled the division of the rocks along the tunnel and shaft alignment into distinct lithologic units and defined the ground--water regime. Field explorations included a detailed analysis of the rock mass discontinuities at available rock unit exposures. Testing of rock strengths was performed in several phases in both the field and the laboratory. The field testing consisted of point-load testing, geophysical testing, in situ load testing, and hydro-fracture testing. Laboratory testing determined the Brazilian split-tensile strength, the unconfined compressive strength, the triaxial compressive strength, the joint-direct shear strength, and the pulse velocity. The rock mass along the tunnel and shaft was also characterized by empirical classification systems. The classification systems included the Rock Mass Rating (B1IR) and the Rock Mass Strength Determination (RHSD). The results of the field explorations, field testing, laboratory testing, and rock classifications were used to divide the rocks into two structural groups. These rock groups were defined as volumes of rock wherein the rock mass behaves similarly in response to the excavation. Each group, therefore, required separate rock engineering consideration.

Jan 1, 1986

By T. P. Lim, A. J. Vivyurka, N. K. Davé

Chemical and radioisotope stratification studies were carried out in an abandoned uranium tailings pile at Elliot Lake, Ontario, Canada. The tailings, with an average thickness of 7 m over an area of 16 hectares, were deposited approximately 20 years ago on a layer of black peaty material which overlays deposits of permeable glaciofluvial sands. Geohydro-chemical investigations of this tailings area have been ongoing since 1978. The radioisotope profile study was conducted on three adjacent sites. Solid core samples, collected with a split spoon sampler at various depths, were analyzed for various chemical and radioisotope constituents. The results indicated by 2 distinct zones of stratification; an upper zone, where the constituents were uniformly distributed, and a lower zone, near the peat-tailing interface, which showed increasing concentration with depth. Within the peat layer, the concentration decreased rapidly with no significant levels of contaminant penetration below the peat layers observed, though the samples were limited. The pH profile showed that the top 5 m of the tailings have been oxidized, the average soil pH in this zone was 3.4 compared to 5 .8 underneath it. The groundwater hydrology of the system was observed to be complex showing downward and lateral flow in the dry season and lateral and upward flow during the spring and periods of heavy rainfall. The results are discussed in terms of a model based on the combined segregation and settling process of the tailings fine fraction, and the geohydro-chemical interaction of the solid and liquid phases.

Jan 1, 1981

Throughout the 1970s, the US Bureau of Mines rode an ebb tide in Washington political circles. In rapid fire succession, the agency lost its mine health and safety enforcement responsibility (1973), a large portion of its energy research responsibility (1975), and then its fuel statistics functions plus a sizable portion of its coal R&D (1977). By 1978, johnny-come-latelies in the Energy, Labor, and Interior Departments had siphoned away from the Bureau some $125 million in program responsibilities as well as 900 of its employees. The bruising decade came to a close with a Department of Natural Resources proposal that, in its original form, would have scattered the Bureau’s responsibilities to the winds. There is no doubt in my wind,” said Lindsay Norman, acting director of the Bureau of Mines (see box), “that were the Bureau to continue down the path it was on, the next step was closing the doors and going out of business.” The doors are still open, but erosion of responsibilities had a devastating effect on the resources available to fulfill the Bureau’s mission. Devastating,too, in terms of internal morale, considering the Bureau had six directors in a 10-year span. “When the troops are in the trenches and are being shot at,” Norman said, “they need a leader—hopefully, a permanent one— that can get them advancing. This is something the Bureau did not consistently have in the 1970s.” In September 1978, Roger Markle climbed aboard as director of the Bureau to rejuvenate the 70-year-old agency. He resigned only six months later* but the program Markle set in motion is being carried through by Lindsay Norman. Since Norman’s appointment as acting director, he has spent considerable time on the road explaining his agency’s redefined goals. At one such stop in Duluth, Norman and his two deputy directors, Charles Kenahan and Mike Kaas, answered the following questions.

Jan 6, 1980

By S. R. Wardwell, M. J. Robison

INTRODUCTION The first phase of the Los Angeles Metro Rail Project consists of 7.1 km of subway and stations running through downtown Los Angeles from Union Station to MacArthur Park. Contract A146, Tunnels from 5th/HILL Station to 7th/Flower Station, involves tunnel construction through the heart of downtown Los Angeles. The alignment of the tunnels passes beneath twelve buildings, ranging from a 3-story below grade parking structure to numerous 12 to 14 story buildings. The soil along the alignment is alluvium, consisting of silts and clays, clean sands and gravels located above the ground water table. Because of unstable ground conditions, which led to excessive ground losses in several instances, and because of the potential for excessive building settlements, it became necessary to design and perform a program of chemical grouting. The chemical grouting program was designed such that tunneling could proceed on a one shift per day basis, with chemical grounting being performed on the remaining two shifts. Each day 21,000 to 31,000 liters of sodium silicate grout were injected through sleeveport grout pipes installed 18 m deep through the tunnel face. Tunnel advance rates of 12 m per day, 60 m per week, under full chemical grout cover were achieved. The chemical grouting program virtually eliminated both ground loss and settlement and permitted successful excavation of the tunnels. The purpose of this paper is to describe the chemical grouting program used to control ground losses and settlement on Los Angeles Metro Rail Contract A146. The paper begins with a description of

Jan 1, 1991

By R. V. Ramani, S. Skankar

Available literature on the effect of re-entrainment on overall airborne dust concentration in mine airways or the fundamental mechanisms of coal dust re-entrainment in mine atmosphere is scant (Shankar and Ramani, 1991). Therefore, as part of the Respirable Dust Center's ongoing research, the following three-part study was undertaken: •a study of the re-entrainment phenomena of anthracite and bituminous coal dust particles in a wind tunnel under the influence of varying air velocities; •a study of the same in the Lake Lynn Laboratory Mine of the USBM with both air flow and walking by personnel causing re-entrainment; and •the mathematical modeling and computer simulation of the re-entrainment phenomenon. In this paper, results from the re-entrainment studies in the wind tunnel and the Lake Lynn Laboratory mine are presented. Comparing the results from the two studies, it was observed that increasing the air velocity increased the rate of re-entrainment (Shankar and Ramani, 1994a) and the rate of re-entrainment decreased with the time of exposure of the source dust to the flowing air. In addition, when the source dust was composed of particles of different sizes, the airborne dust concentrations due to re-entrainment were higher than when the source dust was composed of particles of the same size. The contribution of airborne dust concentration due to airflow was negligible compared to the contribution due to walking.

Jan 1, 1996

By L. M. Petrie, D. C. Marozas, S. E. Paulson

The US Bureau of Mines (Bureau) is conducting research to evaluate the potential for developing an in situ leach mining process that will selectively extract manganese from oxide ores, leaving gangue relatively undisturbed. To accomplish this, a geochemical approach based on deposit-specific information, such as mineralogy and geology, is used to constrain the operating parameters of the system. Among the studies being conducted are: • leaching experiments to define reaction mechanisms and kinetics, • detailed petrologic characterization at the microscopic scale to assess manganese distribution and leaching efficiencies, • deposit characterization and evaluation, and • development of potential manganese recovery schemes. Data from these studies are being used to develop engineering design information for an in situ mining process. Specific details of the Bureau's research project are discussed, including results obtained to date.

Jan 1, 1993

By Edward J. Zeigler, John T. Miller

INTRODUCTION Construction of Phase I, Section A, of the Baltimore Region Rapid Transit System required that six underground stations be built using a cut and cover excavation technique. For five of these stations, soldier piles, used as support of excavation, were installed with tip elevations well below the bottom of the excavation. However, in the case of the sixth subway station, the Mondawmin Station, soldier piles were installed only through soil and decomposed rock, with the tips of the piles being stopped well above the bottom of the excavation in partially decomposed rock. Below the tips of the soldier piles, and below a cut slope where piles were not installed, the sides of the excavation consisted of partially weathered and fresh rock and were supported by a combination of rock bolts, wire mesh and shotcrete. An extensive installation of geotechnical instrumentation permitted detailed monitoring of horizontal and vertical movements of the soldier piles and the walls of the excavation. Preliminary studies indicated that the excavation support system selected for the Mondawmin Station would have several advantages over a conventional system consisting of soldier piles extending to the bottom of the excavation, braced with prestressed struts. These advantages included lower cost of installation, a decreased time required for installation, and a more open excavation for the contractor during construction of the station. The Mondawmin Station is situated in the northwestern quadrant of Baltimore City, approximately 6.4 km (4 miles) from the center of the City. The station lies midway in a project 12.9 km (8 miles) long identified as Phase I, Section A, of the Baltimore Region Rapid Transit

Jan 1, 1981

By George M. Potter

During the late 1960s the mining industry became interested in USBM's efforts to develop the gold-silver heap leaching process. In the last 10 years, successful commercial heap leaching applications increased. The process provides a relatively low-capital means of processing treatable ores with reasonable operating costs. Present gold and silver prices compared to capital and operating costs favor the continued growth of the heap leaching method. The heap leaching process is defined as the percolation leaching of relatively coarse gold-silver ore piled on an impervious surface. To date, dilute alkaline cyanide solution is the lixiviant of choice. Other leaching reagents such as thiourea and hypo have been suggested, although they are not currently used in heap leaching practice. Ores may he run-of-mine in size or crushed as fine as about 10 mm. Gold and silver-bearing so¬lutions draining from the heaps may be processed in activated carbon columns to recover the metals, followed by stripping and metal recovery, or treated by the Merrill-Crowe zinc dust precipitation method. All solutions are generally recycled to the leaching step, and the heap leaching system is relatively easy to control from an environmental viewpoint. One advantage of the heap leaching process is its flexibility. Plants may be very simple, comprising leaching of uncrushed ore on a locally-obtained clay-lined pad and shipment of gold and silver recovered in the form of loaded carbon or zinc precipitates; such a plant would have a relatively low capital investment. More elaborate plants for larger ore bodies may be planned to include crushing, cyclic pad use, and done gold-silver recovery. General planning is similar in each case. This article outlines some key factors in designing heap leaching operations and discusses how these factors are applied to develop a working operation. Since individual design requirements and local costs vary so much, only a few general comments on costs are included.

Jan 3, 1981

By A. C. Lyons

Foreword Sir William Halcrow and Partners are responsible for the design of the Ahmed Hamdi Tunnel as Sub-contractor to the Contractor, Osmac, a joint venture formed by the Arab Contractors, Osman Ahmed Osman and Company of Cairo, and Tarmac International of London. Throughout the paper the term "Client" refers to the Employer, the Ministry of Housing and Reconstruction, which latterly became the Ministry of Development and New Communities of the Arab Republic of Egypt. At the time of writing of this paper only a few hundred metres of the tunnel have been driven, but the design is substantially complete. Introduction The Ahmed Hamdi Tunnel, which is at present under construction, will be a two-lane tunnel crossing beneath the Suez Canal approximately 15 km north of the town of Suez at El Shallufa. It will become the first permanent connection across the Canal since 1967 and is visualised by the Egyptian Government as an early step in the development of the Sinai Desert along the eastern bank. The tunnel will carry pipelines for treated water and electric power cables to assist in this development. The tunnel, which will be 1640 m in length, is being driven through hard clay and mudstone using a tunnel shield fitted with hydraulic excavators. The primary lining is being constructed with precast, bolted reinforced concrete segments with an internal diameter of 10.4 m. The road deck structure and the walls of the air supply ducts beneath it are formed with precast, reinforced concrete units erected immediately behind the shield.

Jan 1, 1979

By T. Shungu, N. Vermaut

The various copper-cobalt ores mined by Gecamines, together with typical plant applications and results, are briefly described. Intensive laboratory investigations have been undertaken in recent years to improve the metallurgical performances of the various concentrators. As a result of these tests, it was shown that the flotation of the siliceous oxide ores could be improved by increasing the efficiency of the palm oil collector (energetic conditioning, chemical emulsifier, petroleum sulfonate addition) and by varying the froth depth in the cells. A new flowsheet incorporating these modifications and a regrinding stage has been approved for the rehabilitation of the Kolwezi concentrator. For what affects dolomitic oxide ores, hydroxamates were tested as secondary collectors and, based on successful laboratory results, plant trials were run but failed due to excessive frothing. In other plant tests, xanthates were replaced by a xanthate- fatty acid mixture; plant trials over more than one year proved successful for copper, but not for cobalt. Mercaptans were tested as secondary collectors for the sulfides flotation from sulfide or mixed oxide-sulfide ores; laboratory results proved successful but, to date, no plant test has been run.

Jan 1, 1987

By J. L. González-Chávez, A. Ballester, M. L. Blázquez, F. González

In recent years, the bioleaching of sulfidic raw materials has been of great interest. However, the application of this biotechnology is now very limited for many different complex reasons. The mechanism of the chemical and electrochemical reactions taking part in the process is not well understood. The application of electrochemical techniques is useful in its elucidation. Among the common sulfides, galena has been studied very little with respect to its behavior during bioleaching. Therefore, the electrochemical evolution of massive electrodes of lead sulfide in the presence and absence of' a mixed culture of mesophilic microorganisms was investi¬gated by the application of potentiometry, anodic and ca¬thodic polarization and cyclic voltammetry. The agitation rate, the use of pH control and the composition of the nutrient mediums did not have any significant affects on the oxidation of galena. However, the temperature (25 'to 45 °C), the scan rate (4 to 100 mV/s) and the acid used to adjust the pH were significant variables that influenced sulfide oxidation. There¬fore, the peak (600 to 1,000 mV) corresponding to the oxida¬tion of sulfide ions from the mineral was only detected in the presence of a complexing anion of lead. In the presence of bacteria, the peaks corresponding to galena oxidation de¬creased and, at the same time, new peaks appeared both during the oxidation at -300 mV and during the reduction of the mineral surface at 150 and -300 mV. These signals were more important when the attack time increased. The rest potential of bioleached mineral increased with an increase in leaching time, which was related to the electrochemical transformation of the galena surface.

Jan 1, 2000

By Ian McFeat-Smith, Robert J. Fowell

Roadheader machines have a specific place in modern tunnelling, not simply as a compromise between drill and blast techniques and full face machines, but as an economic and versatile form of mechanised tunnelling where the method of excavation is easily integrated with tunnelling operations. In the past, roadheaders have been subjected to misuses by engineers who failed to realise that, as in all mechanised tunnelling, a planned and systematic approach is essential if the full potential of these machines is to be gained. An increasing awareness of this, together with the introduction of the more powerful roadheader carrying their cutting potential into the hard range of rock is changing this situation and now makes the prospect of using roadheaders particularly attractive (1). The advantages gained by employing these machines can be considerable and Table 1 lists some of them. This table illustrates that from both cost, and installation points of view, the roadheader has considerable benefits over the full face machine. However, the principal advantage gained is in broken ground where heavy supports can be installed immediately at the tunnel face. In addition, access for planned maintenance and repairs to most working parts of the machine is at a premium and average machine utilisations of 40 - 60% can be expected. In comparison to drill and blast methods, up to 40% better progress can be regularly achieved, and the subsequent savings in overbreak costs alone would more than pay for the cost of a roadheader used to drive a 3km long tunnel. As with full face machines, ground disturbance is minimal and for short tunnels in urban areas a road-

Jan 1, 1979