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By C J. Carr

The Ernest Henry Mine (EHM) is owned and operated by Xstrata Copper and is located in North-West Queensland, Australia. The mine is currently undergoing a transition from an 11 Mtpa open pit mine to a large sublevel cave underground mine producing 6 Mtpa. Due to a number of factors, including geometry of the pit and crusher depth, the decision has been taken to develop a hoisting shaft rather than inclined conveyors.During the feasibility study the strip and line shaft sinking methodology was preferred over blind sinking due to scheduling opportunities and interactions with level development. Other methods of sinking considered included large-diameter raise boring, which was rejected due to concerns relating to centreline accuracy. The strip and line decision was reviewed during the tender process where better than anticipated blind sink advance rates and costs were presented. This in turn enabled the higher cost of the blind sinking method to be justified by a combination of reduced risks including schedule and ground water management. This paper discusses the pros and cons of both strip and line and blind sinking methodologies in the Ernest Henry context including sinking rates and total schedule impacts of the two methods, the key risks associated with each method, cost implications of blind sinking and safety implications of the work methods in general. The paper then concludes with the reasoning behind the decision to change to blind sinking for the EHM shaft.

Mar 21, 2011

By W. C. Whittaker

Introduction When one thinks of a coal stripping deposit, the picture that usually comes to mind is that of a more or less flat seam from two to twenty feet in thickness overlain by ten to fifty feet of overburden, in suitable ratio for economic operation. This is the usual type of stripping deposit and is quite a common occurrence in various parts of the prairie provinces. Rather unusual types of coal stripping deposits are being presently worked as open-cut mines by Sterling Collieries, Limited, and the Co al Valley Mining Company, Limited, at Miles 47 and 48 on the Alberta Coal Branch, some 150 miles south west of Edmonton. These deposits are of interest chiefly because they represent turned-up and greatly thickened sections of the Mynheer seam, and as such they present their own problems, peculiar to the commercial exploitation of a seam of this type. The area in which these deposits occur was originally owned by the Pacific Pass Collieries, whose operations, however, were confined to under-ground mining at Lovett, some eight miles to the southeast. It was not until l918 that the value of these deposits was realized, when the Company now known as Sterling Collieries, Limited, opened their pit at Mile 47. At the inception of the work, coal was loaded directly into the railway cars by men using wheelbarrows. In 1920, a tipple was built, after which the coal was loaded by steam shovel into mine cars, hauled by steam dinkeys, and loaded over the tipple into railway cars. In 1922, the Coal Valley Mining Company entered the field at Mile 48, with an entirely mechanical equipment, and at the present rime these two are the only stripping properties in Alberta using mechanical equipment both for the removal of overburden and the loading of the coal. These Companies have operated continuously since that time, and have to date mined and shipped 4,000,000 tons of coal, and moved some 5,255,000 cubic yards of stripping. The present annual coal output for the two mines is about 240,000 tons.

Jan 1, 1935

The direct removal of lead ions from D2EHPA-kerosene using metallic zinc as the reducing agent was evaluated. The electrochemical process, called galvanic stripping, is a potential alternative stripping technique when standard chemical methods are not adequate. Four parameters found to be important in the rate of lead removal were studied in a factorially designed experiment. The variable evaluated included D2EHPA concentration, temperature, zinc surface area and solution agitation. Temperature and surface area were found to be the most significant, while agitation and D2EHPA concentration had less influence on the reaction. An activation energy of 22.5 Kcal/mole was calculated indicating a chemically-controlled process. The reaction was also sensitive to the concentration of oxygen in the system. The zinc required was considerably in excess of stoichiometry, possibly due to the dissolution and redeposition of lead: In general, the results were encouraging and demonstrated that lead impurities could be removed from D2EHPA using cementation type reactions.

Jan 1, 1995

Stripping of Fe (III) from D2EHPA Using Different Reagents The common way to extract Fe(III) from an organic solvent is by using concentrated acidic solutions. The stripping of Fe(III) from organic solvent Di-2-ethylhexyl phosphoric acid (D2EHPA) by hydrochloric acid (HCl) solutions has been found to increase with increasing the acid concentration. However, the use of a concentrated acidic solution such as 6 M HCl can degrade the organic molecules. In this research, to increase the efficiency of Fe(III) stripping from D2EHPA, effect of some acids, salts and buffer solutions was studied. Influence of Tri-butyl phosphate (TBP) as modifier on the Fe(III) stripping operation was also investigated. According to the results, the best Fe stripping recovery was achieved in the presence of low acid concentration and the salts which was 72.4 per cent oxalic acid, 70 per cent ammonium oxalate and 75 per cent buffer of oxalic acid and ammonium oxalate. The results also showed a better recovery in the presence of the buffer rather than the salt. Finally, increasing the concentration of TBP, HCl and H2SO4 resulted in a higher stripping efficiency of Fe(III). However, in the presence of oxalic acid and its salts increasing the reagent concentration did not increase the stripping efficiency and at this condition, the recovery of iron remained constant.

Sep 13, 2010

By Paul L. Russell

Of the potential applications of nuclear explosives to mining, excavation is perhaps the most obvious and the best understood, and probably the most practical for use in the near future. Large quantities of rock and earth can be moved by proper emplacement of nuclear explosives, but economic and practical relationship must be determined between the known capabilities of nuclear explosives and the required excavating to remove overburden. On an energy yield basis, even the cheapest of the present commercial explosives cost more than the most expensive nuclear explosive. Comparing the energy yield of a megaton thermonuclear device with the yield of TNT shows that one energy yield per dollar is 400 times greater for the thermonuclear device. However, small-yield nuclear devices (a few 10's of kilotons yield) are less efficient blasting materials than chemical explosives, with the result that the heat of explosion is utilized less effectively. While only 1 lb of conventional explosive is required to excavate 1 cu yd of rock, or about 2 million cu yd per kiloton, nuclear devices excavate from 100,000 to 300,000 cu yd per kiloton. This changes the economic situation, and it is thus apparent that nuclear explosives are cheaper only for the large explosions.

Jan 6, 1964

By O. W. Shimer, D. C. Helms, C. E. Brown

THE early history and progress of anthracite stripping, from the first known operation at Summit Hill in 1821 through 1917, was covered in 1917 in a paper by J. B. Warriner,1 then chief engineer, now president of the Lehigh Navigation Coal Co., and stripping methods between 1917 and 1931 were the subject of a paper in 1931 by H. H. Otto,2 of the Hudson Coal Company. The present paper is confined to methods used today on pitching beds in the western middle and southern anthracite fields. The anthracite industry, not unlike other large industries, had been affected for more than a decade by the adverse economic conditions of the country. High development and operating costs and the competition of other fuels caused some collieries to be closed temporarily, while others were permanently abandoned. In order to obtain a larger tonnage at a lower development cost, a larger stripping program was put into effect by many companies. In 1932, stripping operations accounted for 334 million tons, or 7 per cent of the total anthracite production of the year, and in the next 10 years this increased until more than double that amount, or over 14 per cent of the total production, was from stripping operations. These figures represent the total production of the entire anthracite region; unfortunately, the figures have not been separated from the different fields. There are some operations in the southern and middle western fields where the tonnage obtained from stripping amounts to more than 30 per cent of the total colliery production. Stripping operations on pitching beds present difficult problems. Owing to the various geological formations, a different situation is encountered in nearly every stripping pit; and because of the proximity of an overlying bed, it is very often necessary to load the entire overburden into trucks and haul it out of the pit to the dump area, although back filling and casting is done whenever possible. Beds in the areas described in this paper pitch from 20° to 90° and some are on the invert in, many sharp folds and faults. PLANNING The more successful strippings have had adequate engineering analysis. Some work is being done without much preliminary planning, but many strippings would have failed completely if close supervision by the engineering department had not been maintained. Locations of stripping areas are determined as follows: 1. By constructing cross sections over the areas and drilling sufficient diamond-drill or well drill holes to study the strata and to determine the location, thickness and pitch of the beds.

Jan 1, 1944

By Tim O’Neil

Upbeat, surprisingly so, might describe the mood at the2002 SME Annual Meeting and Exhibit in Phoenix, AZ. More than 4,000 mineral professionals gathered for three days of education, networking and product perusal. The Annual Meeting featured the theme, “Minerals for the Future@ SME” at the Feb. 25-27 gathering. Meeting attendance reached 4,039along with 479 exhibit booths show-casing 325 companies. Exhibitors found a good mix of industry types:37% from mining, 19% from processing, 20% from manufacturing and 28% from environment, education and government.

Jan 1, 2002

By Peter Ay, Vincenz Mehlstaubl, Satyanarayana Narra

"This study gives an insight into non-destructive testing methods suitable for porosity analysis at a small scale, firstly summarizing relevant state-of-the-art technology and pending patents, secondly realizing and documenting the start-up of the VHX-2000 digital microscope by the manufacturer Keyence and thirdly demonstrating practical application with the extended analysis of three fundamentally different agglomerates for which important application-related parameters are determined. Various analysis procedures on the different agglomerates the efficient and timesaving operational features of the digital microscope are outlined. Firstly an analysis of flax fibre agglomerates determines the process conditions best suited to produce agglomerates with specific properties by determining sphericity, cubicity, density and porosity as well as fibre damage and alteration. Secondly the production of iron-hydroxide - hemp-fibre agglomerates serves to obtain information on porosity values as a result of a cauterization of the fibres. The third analysis provides estimations of porosity values for acid-treated lignin agglomerates. Measurements were carried out with compact dry agglomerates and with soaked, expanded agglomerates, determining factors for total water carrying capacity. Practicability of non-destructive testing by digital microscopy is demonstrated and explained concerning the applications in this study by obtaining definite structural findings about the analysed agglomerates. Information on its operation is made available by a practice-oriented manual for the VHX-2000 digital microscope. Further suggestions for more in-detail analysis results with different instruments are given."

Jan 1, 2014

By Giuseppe Bonifazi

Minerographic and petrochemical research was carried out on crude ore samples from the complex polymetallic Zn-Cu-Pb sulfide ores of , Sotiel in Huelva province west of Seville, Spain. A computerized image analysis system was used to define the structural and textural characteristics of the ore to obtain information on the degree of liberation of the different minerals (i.e. pyrite, sphalerite, galena, chalcopyrite). Techniques of digital multispectral image processing of reflected light microscopy images were used. Particular structures were identified by means of mineral associations and numerically quantified to obtain useful information for designing beneficiation processes.

Jan 1, 1991

By Stanley Davidson

Introduction The ore deposit mined by Falconbridge Nickel Mines, Limited, is one of the numerous nickel-copper sulphide bodies marginal to the large oyster-shaped mass of differentiated norite in the Sudbury district, famous as the world's most important nickel producing field. The unique character of the ore deposits of the district has resulted in its intensive geological study over a period of more than fifty years. The results of these studies have given rise to a voluminous geological literature and to a great deal of controversy as to the genesis of the ores. This description of the structural geology at the Falconbridge mine is presented in the hope that it may form a small part of the chain of evidence leading to a reasonably satisfactory explanation of the ore genesis. The mine, named after Falconbridge township in which the deposit occurs, lies along the southeast margin of the norite mass which, however, is here concealed beneath a deep mantle of glacial sands and gravels averaging over 100 feet in thickness. As a consequence, the deposit was not discovered by the early prospectors, and it was not until 1916-17 that E. J. Longyear, of Minneapolis, located the ore by drilling. The discovery was foreshadowed in 1899 by the work of Thomas A. Edison, who found indications of the deposit by what is believed to have been a combination of magnetic and electrical geophysical surveys. He sank a prospect shaft over what has to date proven to be the largest and richest ore section in the mine. A wet quicksand layer at the rock surface proved impenetrable, however, and the "Edison' shaft never reached bed-rock.

Jan 1, 1946

By H. Q. Peng

The study of synergistic reagents combination of mineral floatation is more and more important with the decreasing of bonanzas now. In order to determine the chemical reaction mechanism, here we present the synergistic reagents combination: amyl-xanthate, butyl-dithiophosphate and N,N-diethyldithiocarbamate ester, which dissolved by different organic solvents. The reagents were applied by an Anhui mine to float low-grade covellite as a substitute for ethyl-xanthate. One reaction products Na2HPSO3?5H2O (disodium hydrogen-thiophosphate pentahydrate) which was re-crystalized and measured by X-ray diffraction analysis. In its crystal structure, the two crystallo-graphically different Na ions are six-coordinated and are bridged by two covalent bonds with the O atoms of water molecules to give a basket unit. However, we can?t get Na2HPSO3?5 H2O by combining butyl-dithiophosphate and N,N-diethyldithiocarbamate ester under the same condition. This discovery may provides a intuitionistic evidence that in the process of reaction, the three com-pounds would cooperate with each other, but the decisive reaction may be the hydrolyzation of amyl-xanthate.

Jan 1, 2014

By E. M. Williams

The U.S. Bureau of Mines recently conducted a limited-scope study on the use of lineament analysis and electromagnetic geophysics to better identify the surface expressions of faults at the Cimmaron Mine near Raton, NM, an operating longwall mine in the York Canyon mine complex. Two normal faults were encountered by the mine, one during gate road development and drilling and another only through drilling. The potential ground control problems and offsets associated with these faults will severely impact long-range development plans, so it is essential to know the lateral extent of these faults within the coal reserves. The surface topography at the site is fairly rugged, yet outcrop exposure is poor. There is no obvious visible geologic evidence to indicate that the faults identified at depth extend to the surface. Both the geophysical and remote sensing techniques provided information on the presence and extent of important geologic structures which could not be gained just from drilling and surface outcrops. The geophysical surveys also were invaluable in confirming the presence of subsurface geologic structures in the vicinity of the lineaments assumed to be associated with the faulting.

Jan 1, 1995

By H. L. Cameron

Introduction Since the beginning of gold mining in Nova Scotia in the year 1860, the biggest problem o the mine operators has been to find and follow the ore zones in the narrow quartz veins. It was early recognized that all veins were not gold bearing and that, in those that were, the commercial ore was confined to limited zones. A large number of geologists and mining engineers have been interested enough to give time and thought to the problem, but, until the last four or five years preceding World War II, they were handicapped by lack of exact data from mining operations. The necessary data could only be obtained by daily checking of underground operations. Such daily checks were instituted at a number of mines during the period 1935-40 and, in every case, were successful in delimiting the ore zone or zones for the particular mine. The present paper deals with the results of this work as applied to practical ore finding. It is an attempt at extrapolation, from observations in the four districts with which the writer is personally familiar, co other districts in which similar conditions or structures are reported.

Jan 1, 1947

By D Howe, J Conn, J G. McLellan, K Gates

The Red October Gold Mine, owned by Saracen Gold Mines Pty Ltd, is located approximately 80 km south of Laverton, Western Australia. It is considered to be of a similar nature to many of the Archaean granite-greenstone deposits in the Laverton Tectonic Zone, and presents as an intriguing and relatively young deposit in terms of developed structural and genetic models. Regional deformation events have been proposed for the area, and are relatively well accepted. The most important of these deformation events in the region were the D4b (WNW–ESE sinistral transpression) and D5 (ENE–WSW dextral transpression) which have been attributed to the main Au mineralising phases. At the local scale there appears to have been a rotation of the regional stress field to provide the optimum orientation of maximum principal stress for sinistral movement on the main shear zone.Fully coupled 3D finite element analysis (FEA) was used to simulate the geomechanical response of the system based on structural observations. The geomechanical modelling resulted in highlighting anomalous patterns of stress and strain and subsequent fluid flow as a result of the two main deformation events. The D4 simulation provided a good correlation with the known distribution of subsidiary structures on the main shear zone, and furthermore provided several geomechanical targets that suggested locations of additional subsidiary structures. The D5 simulation provided a great insight into the response of the main shear zone and known subsidiary structures also highlighting additional targets indicating potential for further mineralisation. These targets have since been followed up with a 732 m deep surface reverse circulation (RC) drill hole, resulting in the discovery of two high-grade shoots, with follow up testing currently planned for mid-2014.CITATION:McLellan, J G, Conn, J, Howe, D and Gates, K, 2014. Structural controls and strain partitioning in the Red October Gold Mine, Western Australia, in Proceedings Ninth International Mining Geology Conference 2014 , pp 315–322 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Aug 18, 2014

Structure is the most important control on the location of hematite deposits in the Hamersley Province in the Pilbara region of Western Australia. Unravelling the structural history of mineralised areas, however, is complex because the Province underwent three orogenic events, and three regional extension events followed the main episodes of compression. Steeply E to ENE dipping normal faults immediately predated deposition of the Beasley Quartzite of the lower Wyloo Group after the Ophthalmian Orogeny. Steeply NE and SW-S dipping normal faults predated deposition of the McGrath formation of the middle Wyloo Group, following the Panhandle Orogeny. The resulting interpreted rift was subsequently reactivated and tilted during the Capricorn Orogeny. Late NE trending normal faults are confined to the eastern half of the Province. High-grade hematite deposits occur in rocks that range from intensely folded (Mt Whaleback, Giles Mini) to rocks with less, or only little deformation (Paraburdoo, Channar), suggesting that folding had at best limited influence on ore genesis. In contrast, extensional normal faults are always associated with high-grade hematite deposits, and in particular faults that link the underlying dolomites of the Wittenoom Formation with the overlying iron formation, thus providing a pathway for mineralising alkaline fluids.

Jan 1, 2005

By T X. Le, H T. Tran, K Zaw, T Manaka

"An EXTENDED ABSTRACT is available for download. A full-length paper was not prepared for this presentation. The Truong Son fold-thrust belt (TSFTB) is a prominent tectonic component of mainland Indochina that occurs as a north-west–south-east-trending tectonic entity that extends from north-eastern Laos, straddles the Laos-Vietnam border area and is submerged under the sea basin in east central Vietnam. The TSFTB hosts numerous regional-scale gold and polymetallic base metal deposits, and the mineralisation occurs along the strike of the belt; however, the potential of many of these deposits are inadequately evaluated. The ore mineralisation is hosted by different rock assemblages, including highly metamorphosed Late Proterozoic metasedimentary rocks and Palaeozoic sedimentary and/or volcanic packages, which were intruded by several phases of magma and underwent a number of tectonic events. Several ore-forming styles – including porphyry, Carlin-type and sediment-hosted orogenic Au – were formed during at least two phases of regional thermotectonism that have been identified in the TSFTB. However, all of the major gold deposits/mineralisations are closely related to region-wide structural breaks and many of them are controlled by large-scale, high-strain zones. The fold-thrust structures in the TSFTB therefore play important roles for regional gold mineralisation and metallogeny.CITATION:Tran, H T, Zaw, K, Le, T X and Manaka, T, 2015. Structural controls on gold mineralisation in the south-eastern Truong Son fold-thrust belt and its significance in regional metallogeny, in Proceedings PACRIM 2015 Congress, pp 521–530 (The Australasian Institute of Mining and Metallurgy: Melbourne)."

Mar 18, 2015

By A. H. Mumin, A. K. Somarin, K. S. Oran

"Synvolcanic structures played a fundamental role in the genesis, morphology, and siting of volcanogenic massive sulfide ores and associated hydrothermal alteration in the Archean South Sturgeon Lake caldera complex. The most voluminous and persistent hydrothermal venting and massive sulfide deposition occurred along synvolcanic rifts and grabens associated with faults and tectonic fissures that created permeable fracture zones deep enough to access the underlying hydrothermal reservoir. The type of fracturing is highly variable and changes with the composition, competency, degree of consolidation, and alteration of host rocks. Synvolcanic structures and fracture styles also vary according to the amount and type of tectonic movement, including extension-related collapse, shearing and faulting perpendicular to the principal direction of extension, and orthogonal faulting and shearing. Permeable conduits were created by tension fracturing along fault zones, brittle deformation at the intersections of orthogonal faults, and by extensional fractures in stockworks. In texturally uniform footwall rocks, the distribution of alteration zones was controlled by the morphology of the structural conduit. In rocks with vertical and/or lateral facies, permeability, and competency changes (e.g., Lyon Lake graben), there was an additional stratigraphic control over fluid migration. Some crosscutting synvolcanic structures, alteration zones, and intrusions appear as stratiform units at the present erosion surface due to regional deformation and the present attitude of the volcanic stratigraphy.Hydrothermal mineral assemblages (e.g., quartz, carbonates, chlorite, pyrite, chalcopyrite) infilling structurally induced fractures provide good evidence of fluid migration pathways. However, mineralogy can vary significantly according to the fluid characteristics, host rock geochemistry, and subsequent metamorphic history of the area.Clearly, one of the best methods for locating volcanogenic massive sulfide deposits is to delineate the attitudes of synvolcanic structures, and explore those that show evidence of associated high-temperature hydrothermal mineral assemblages. Excellent exploration targets occur where synvolcanic structures with hydrothermal alteration intersect paleo-seafloor horizons."

Jan 1, 2007

By J Widjaja, E Hermawan, M Muharam, L Damanik

"An EXTENDED ABSTRACT is available for download. A full-length paper was not prepared for this presentation. The island of Java lies within the Sunda-Banda magmatic arc, which is characterised by multiple phases of volcanic activity and extensive compressional deformation. Along this zone, significant gold and copper mineralisation occurs, including some of the world’s largest deposits. In West Java, epithermal gold mineralisation is typically located in a series of extensional and strike-slip faults that cut Miocene to Pliocene volcanic rocks. These are all low-sulfidation, epithermal, vein-stockwork-style deposits developed in Miocene to Pliocene calc-alkaline volcanic rocks, which have been intruded by shallow-level plutons.This paper discusses an example of epithermal mineralisation at Mt Subang, West Java. Gold mineralisation was originally identified from stream sediment, panned concentrate and rock geochemistry sampling in the early 1990s. Geophysical surveys (magnetics and induced polarisation resistivity/chargeability), trenching, pitting and diamond hole drilling has subsequently revealed silicified-quartz vein epithermal gold mineralisation hosted within fault structures. It is clear that these structures acted as hydrothermal fluid pathways leading to the development of mineralised offshoots along north–south extensional and en-echelon arrays of north-east–south-west and transtensional faults. A number of vertical offshoots also occur in flexures and fault jogs. Post-mineral faulting has subsequently dissected and offset a number of the offshoots. Therefore, a detailed knowledge of structure and fault kinematics and displacement magnitudes is required in order to understand the geometry and continuity of mineralisation across major structures and maximise the resource potential. A range of exploration targets were modelled, indicating a minimum and maximum of 0.2 and 1.8 Moz of gold respectively.CITATION:Cunningham, M J M, Muharam, M, Damanik, L, Hermawan, E and Widjaja, J, 2015. Structural controls on the localisation of low-sulfidation epithermal mineralisation in West Java, Indonesia , in Proceedings PACRIM 2015 Congress, pp 227–234 (The Australasian Institute of Mining and Metallurgy: Melbourne)."

Mar 18, 2015

By C. W. Dunham

DESIGN of the structures for the Morenci Reduction Works involved many interesting problems. Naturally, the chief purpose of these structures is to house and support the equipment and other things necessary for or incidental to the conduct of production. hence most of the features of the design were developed to suit these particular conditions; however, several are so generally applicable as to warrant description.

Jan 1, 1942

By Johannes Jäger

"A composite shell lining typically consists of an outer permanent sprayed concrete shell, a water proofing layer and an inner sprayed or cast concrete shell. Composite shell linings become more and more popular and have been built recently on large scale infrastructure projects. Surprisingly there is very little published literature on the actual structural design of composite linings. As a consequence there is a lack of common understanding on how to design a composite lining structure. Composite action of the outer shell (primary lining) initially carrying short term ground loads and the “unloaded” inner shell (secondary lining) require nonlinear concrete modelling to be able to assess the bearing capacity of the composite lining structure adequately. The composite action of the waterproofing membrane and the inner and outer shells can be considered either by simplified closed form solutions or by numerical models. Strength and stiffness parameters of membrane and concrete govern interaction of inner and outer shell. This paper should contribute to understanding composite lining behaviour with focus on the bearing capacity at limit state by taking into account the actual loading history. Possible ways and concepts on how to deal with these topics are presented, however a broader discussion on limit states of the composite lining materials – also in the long term will be required to come to an agreement on composite lining design methodology. INTRODUCTION Composite Lining Technology Traditionally the construction of shotcrete lined tunnels incorporates primary and secondary linings, separated by a sheet or a sprayable waterproofing membrane. The primary lining shotcrete is considered to be structurally temporary and the secondary lining to be permanent, leading to designs which ignore the bearing capacity of the primary lining in permanent condition. There was general perception that the shotcrete lining installed immediately at the tunnel face, experiencing strains up to 1% and even cracking in the early age, combined with mesh reinforcement additional support measures like lattice girder, spiles and bolts cannot be considered as a permanent structure. But also former shotcrete mixes showed significant weakness with respect to durability suggesting a more temporary nature of primary lining shotcrete. On the other hand it was clear that primary lining shotcrete does not disappear, it may lose probably a certain percentage of its bearing capacity, but discounting any contribution from the primary lining at all was considered inherently conservative. There were different strategies to account for the risk of reduced durability and bearing capacity of the primary lining, either by selecting moderate ground loads for the secondary lining design, by assuming that some residual load will be carried by the primary lining. This approach is very much based on experience. Another more rational possibility is to assume that the primary lining degrades into a rocklike material, which is weaker than the primary lining shotcrete but still imparts some strength to the ground around the secondary lining, but does not need to comply with any concrete design codes any more."

Jan 1, 2016