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SUMMARY: The paper refers to the stability analysis for underground cavities of large cross section, which are to be constructed either without or with minimum lining. Since such cavities may only be constructed in favorable engineering-geologically conditions, i.e. in hard rock masses, their stability is influenced mostly by the number, configuration and characteristics, as well as by latent discontinuities of fissures. The proposed procedure makes it possible to identify the safety factor for each monolith, which is a fundamental condition for establishing of the number, magnitude and direction of the anchors in the underground cavity. The procedure enables us to choose the best location, orientation, size and shape of the underground cavity. RESUME: Cet ouvrage se refère à l'analyse de la stabilite des cavites souterraines de grande section transversale que l'on construit soit sans soit avec le revêtement minime. Ces cavites ne pouvant être construites que dans les conditions ingenieurs-geologiques favorables, c'est-à-dire dans les roches en place solides, leur stabilite est la plupart du temps influencee par le nombre, la configuration et les caracteristiques des fissures, ainsi que par les discontinuites latentes. Ce procede nous permet de determiner le facteur de stabilite des monolythes; tout ceci sert de base pour la determination du nombre et de la position des ancres dans une cavite souterraine. Ce procede rend possible de determiner la meilleure localization, orientation, forme et grandeur de la cavite souterraine. ZUSAMMENFASSUNG: Dieses Werk bezieht sich auf die Stabilitatsanalyse der grossen unterirdischen Hallen, die konstruiert mit oder ohne leichtberg Gesicherung sind. Nachdem diese Raume nur unter den guenstigen ingenieur-geologischen Bedingungen konstruiert sein können, das heisst in den soliden Felsenmassen, ihre Stabilitat ist am meistens bei der Zahle, der Forme und bei den karakteristischen Fugen der der Diskontinuitaten beeinflusst worden. Diese Methode macht uns möglich den Sicherheitssgrad der vershiedenen Monolythen festzustellen, unci dies könnte auch als die Unterlage fuer die Bestimmung der Zahle und der Verteilung der Anker in einer unterirdische Halle dienen. Dieses Verfahren ermöglichst die Feststellung der besten Orientierung. Ort, Form und Grösse der unterirdische Halle. The engineering-geological structure of sites in Yugoslavia often calls for the construction of undergroung works in hard rock masses With lithologically or tectonically predisposed fissures. The paper presents an attempt to find a simple procedure to get the prior information on the degree of monolith's stability in the zone around the opening of an artificial underground cavity. The work analyzes the stability of monoliths around circular underground cavity's section (Fig.1). The starting points of the analysis are the following: - Sliding commences when at each point on the sliding surface the shear stress τ, inducted by external forces, reaches the shear strength at that surface. - Stresses in the rock mass around the tunnel opening of circular cross section are determined in the same way as for a homogeneous, isotropic and elastic medium - The rock mass is in the plane strain state. The basic steps in the procedure are the following: - The first step is to define the state of stress around the underground cavity, what enables us to determine the forces acting on monoliths. - The second step is to analyse sliding stability of monoliths alongside the surfaces of discontinuities. Normal forces and tangential forces acting on monoliths are the resultants of the corresponding stresses б n, б n and бe /equations (7).... (11)/. Knowing normal forces and tangential forces, we examine the stability of monoliths alongside the surfaces of discontinuities. This procedure will be shown on a monolith M (Fig.5). As the result of the shear strength of surfaces A3 and A4, it comes to the increasing of the partial safety factors, i.e. we are getting the global safety factors. We obtain the condition that the global safety factors for sliding of monolith M, alongside the sliding surfaces Al and A2 are equal one to another, i.e. This procedure may be the basis for the analysing the stability of monoliths when in the rock mass there is the three dimensional state and in the case when the underground cavity has not only the circular cross section, but has different cross sections. In that case the most difficult problem is to determine the stresses, i.e. forces acting on monoliths. In those cases we may use the finite element method. This procedure put us in the way of obtaining information about the stability of the monoliths around a planned underground cavity, provided that the structure of the fissured medium and the shear strength characteristics at the relevant contact surfaces are known. In such a way we may obtain an advantageous location of the underground cavity connected to that geotechnical aspect. We may determinate the number, magnitude and direction of anchors, if they are necessary, and the best directions for pressure groundings, too.
THE COMMITTEE The U.S. national group responsible for participation in activities of the International Society for Rock Mechanics is the U.S. National Committee for Rock Mechanics (USNC/RM). The Committee was formed within the National Research Council, the operating arm of the National Academy of Sciences and the National Academy of Engineering, in 1967 for three major purposes:To provide coordination and promote cooperation among the technical and professional societies and organizations involved in rock mechanics. To effect appropriate participation in all activities of the International Society for Rock Mechanics (ISRM) through the National Academy of Sciences - National Academy of Engineering - National Research Council (NAS-NAE-NRC), which adheres to the ISRM on behalf of United States scientists, engineers, and technologists interested in rock mechanics. The Committee is composed of eighteen members, including eight members-at-large and ten who are representatives of the following professional societies and organizations: Association of Engineering Geologists American Geophysical Union American Society of Civil Engineers American Society of Mechanical Engineers American Society for Testing and Materials Geological Society of America, Inc Society of Exploration Geophysicists Society of Mining Engineers Society of Petroleum Engineers Transportation Research Board National Research Council Boards with liaison members on the USNC/RM are: Building Research Advisory Board Transportation Research Board An approximate balance of representation from government, industry, and academic/research institutions or organizations is maintained among the members-at-large. The Committee chairmanship changes annually. The present chairman is Dr. Don C. Banks of the Waterways Experiment Station of the U.S. Army Corps of Engineers. The other chairmen since 1974 were:Dr. Charles Fairhurst, University of Minnesota, July 1, 1974-June 30, 1975 Mr. George B. Wallace, Bovay Engineers, July 1, 1975-June 30, 1977 Mr. Thomas C. Atchison, University of Minnesota, July 1, 1977-June 30, 1978 Mr. Sidney J. Green, Terra Tek, Incorporated, July 1, 1978-June 30, 1979 ROCK MECHANICS SYMPOSIA One of the Committee's major activities each year is to arrange for a U.S. symposium on rock mechanics. The Committee selects a university to act as host and works with that university in planning and organizing the symposium. These annual symposia are the major technical meetings devoted totally to rock mechanics in the United States at which researchers, educators, and practitioners in the field of rock mechanics can meet to discuss mutual problems and newly acquired knowledge. The symposia provide a forum for exchange of information by approximately 300 to 400 participants, representing both national and international viewpoints. The U.S. symposium was not held in 1974 because of the participation of potential hosts in activities associated with the 3rd Congress of the International Society for Rock Mechanics conducted in Denver, Colorado that year.. During the remainder of the reporting period, symposia were conducted as described below. To disseminate the information presented at the symposia, the proceedings are published and distributed to participants and other interested persons and organizations. For the reader's convenience, information on the availability of proceedings is furnished with the description of the symposia.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (0.56)
- Government > Regional Government > North America Government > United States Government (1.00)
- Energy > Oil & Gas > Upstream (1.00)
1. INTRODUCTION For the past few years the C.E.R.N. (European Center for Nuclear Research) at Geneva is operating its SPS (Super Proton Synchroton). This facility consists of an underground ring in form of a 5.6 m diameter tunnel of approximately 8 km in length where protons (positive loaded particles, which jointly with the neutrons build up the nucleus of any atom) are accelerated almost to the velocity of light by means of electric fields and steered around the tunnel by means of magnetic ones. The protons are introduced into the mentioned ring, that lays 20 to 60 m below ground surface, through an inclined edit. After some 100'000 revolutions they are diverted from the ring and brought to two experimental areas through two other inclined tunnels. The overall situation of the SPS/across the Swiss french border may be seen on figure 1. Recently CERN decided to develop this facility in order to accelerate in the same ring at the same time as the protons also antiprotons (which have the same mass as the protons and are electrically negative loaded) but, of course,in the reverse sense. With this arrangement the possibility arises of bringing protons and antiprotons to collide together at very high velocity, which also means at a very high level of energy. To achieve this experiments a 1'000 t heavy machinery is needed, which has to be placed across the ring but has, also to be removed from it from time to time. This makes necessary, among other works, the construction of a 30 m deep, 40 by 20 m wide shaft across the tunnel as may be seen in figure 2. The structure has to be as stable as possible in spite of the displacements of a mass of l' 000 tons and in spite of the swelling properties of the rock. The problem is once more complicated by the necessity of reducing as much as possible the shut-down time of the SPS for construction purposes and because the activity of the of the fact that during same some radioactivity appears. The workers must be protected from it by at least 5 m of rock. THE GEOLOGICAL SITUATION A wide strip of Swiss territory reaching from Geneva to Austria is build up by terciary aged marine and lacustrine deposits, the so called "Molasse" formations. They consist mainly of sandstones and marls with all the possible intermediary gradations. It is worthwhile to mention that as a general rule the swelling potential of that rock strata increases with depth and that the swelling pressures may reach the value of the rock overburden. The way the swelling develops in time is very difficult to predict since it depends on the possibility of the water to penetrate each element of the rock mass. On the contrary, the maximum possible volume increase of the rock is easy to measure on samples and the maximum deformations and pressures in the works may be computed with some accuracy.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.57)
- Geology > Sedimentary Geology > Depositional Environment > Continental Environment > Lacustrine Environment (0.55)
- Geology > Geological Subdiscipline > Geomechanics (0.51)
SUMMARY: During excavations some movements not following the pattern of theoretical models, often produce. These secondary movements are difficult to monitor and may have important effects on the bulk characteristics of a rock mass. Three examples of secondary movements are presented in basalt, marl and quartzite formations. RESUME: Les excavations produisent des mouvements qui ne suivent pas les previsions des modèles theoriques. Ces mouvements secondaires sont difficiles à detecter et peuvent porter à des effets remarquables dans les caracteristiques de masse du complexe rocheux. Trois exemples de mouvements secondaires dans des formations de basalte, marne et quartzite sont presentes. ZUSAMMENFASSUNG: In Felsaushueben treten oft Verformungen auf, welche in Widerspruch zu den auf Grund theoretischer Modelle vorausgesagten Deformationen stehen. Diese sekundaren Verformungen sind schwer zu identi fizieren und können einen bedeutenden Einfluss auf das Verhalten der Felsstruktur ausueben. Drei Beispiele sekundarer Verformungen sind im Folgenden gegeben fuer Basalt-, Mergel- und Quarzitformationen. In trying to bridge the distance between practice and theory, the engineer is permanently facing the problem of knowing the behaviour of the structure he has to deal with. Natural phenomena forming the subject of geotechnics are characterized by a complex nature and, in addition, each case invariably possesses one or more peculiar aspects. The geotechnical engineer is therefore confronted with an especially arduous task: that of predicting and interpreting the behaviour of rocky or earthen structures (man-made or natural) representing in large majority special cases. The approach to a geotechnical structure ideally develops along four basic steps:INVESTIGATION PRE-CONSTRUCTION INSTRUMENTATION DESIGN BEHAVIOURAL EVALUATION Investigations even if based on large-scale in situ tests provide only a part of those information needed to properly handle a geotechnical problem. They must be complemented by a careful monitoring of the rock mass started well before construction. Design (both conceptual and computational) should be based on this double wealth of information. It should not be considered achieved until checked against the structure's behaviour duly analysed and interpreted. Quite logically, computational design is mainly directed toward the paramount phenomena and the basic mechanisms governing the behaviour of rock masses. Instrumentation, accordingly, is planned and used to second this approach, particularly for excavations and related movements. During excavations however, movements may take place which, not considered in computational models (although not necessarily conflicting with it) are in most cases not monitored in advance. I called these movements "secondary movements". In spite of their localized extent and exceptional appearance, they may produce important effects. I would like to present three examples of secondary movements induced by excavations in rock masses together with some hypothesis regarding their origin and consideration about their consequences. The formation extending laterally for several hundreds of kilometers and to an indefinite depth, consists of several superimposed, nearly horizontal flows. An overall view of the basalt formation during excavations is given in Figure 1. The excavation was intended to create a diversion canal some 80 m deep and 100 m wide. In the process of excavation a mass, limited for the size of the project but fairly large by normal practice, suffered a lateral displacement in the order of hundreds of mm. The movement, shown in close-up by Figure 2 took place to all appearances along a joint. It came to a halt quite soon as no additional displacements could be measured after it was detected. The cause of the movement may be the release of horizontal locked-in stresses in the rock behind, or the transient uplift produced by blast gases in some particular joint, or the combined effect of both. A similar secondary movement may have negligible consequences on stability in view of the particular joint setting but would drastically alter the permeability of the rock mass. While instrumentation crossing this particular joint would have been severely damaged it is not reasonable to say that any appropriate instrumentation would have allowed to predict the movement. The second example refers to a layered marl formation interbedded with argillite, at some levels intensely fractured and weathered. The strata dip nearly at 30°. An overall view of the marl formation during excavation is given in Figure 3. The excavation was intended to found a double curvature concrete dam. In spite of the great care taken to define the excavation geometry so as to follow stratification, some undercutting was inevitable. Individual rock slabs suffered movements of the type shown in a close-up by Figure 4, consisting of displacements and rotation over levels corresponding to depositional alternations of weaker rock which in the extreme case could be mm thin clay films. The cause of these movements may be the vibrations produced by nearby blasts, or the hydrostatic forces applied by rain waters filling particular joints not able to drain, or the removal of overlaying materials with machinery.
- Geology > Rock Type > Igneous Rock > Basalt (0.79)
- Geology > Rock Type > Metamorphic Rock > Quartzite (0.49)
SUMMARY: This group report from Sweden tries to describe the general conditions and background for rock mechanics activities during the last five years. The reporters have found that the main interest has been directed towards the following areas: a) pre investigations - methods, scope and prognosis value b) rock mass permeability and hydrogeology c) rock excavation d) rock support and reinforcement e) calculation and measurement of rock mass deformation f) influence of temperature on rock masses. In every area the trends and the most important works have been shortly described and references are given. RESUME: Le present rapport de groupe etabli en Suède tente de decrire les circonstances generales et la toile de fond qui ont conditionne les activites poursuivies dans le domaine de la mecanique des roches au cours des cinq dernières annees. Les auteurs ont constate que l'interêt principal s'etait porte sur les secteurs suivants: a) etudes preparatoires - methodes, etendue et valeur des previsions b) permeabilite de la masse rocheuse et hydrogeologie c) creusement de la roche d) soutènement de la roche et renforcement e) calculs et mesures de la deformation de la masse rocheuse f) influence de la temperature sur les masses rocheuses. Dans chaque secteur, les tendances relevees et les travaux les plus importants font l'objet de brèves descriptions completees par des references. ZUSAMMENFASSUNG: Der vorliegende Gruppenbericht aus Schweden stellt den Versuch dar, die generellen Bedingungen und Beweggruende fuer die gebirgsmechanischen Tatigkeiten der letzten fuenf Jahre zu beschreiben. Die Berichterstatter gelangten zu dem Ergebnis, dass das Hauptinteresse auf folgende Teilgebiete gerichtet war: a) Voruntersuchungen - Methodik, Umfang und Prognosewert b) Gebirgsdurchlassigkeit und Hydrogeologie c) Vortriebsmethoden d) Gebirgssicherung, -ausbau, -verfestigung e) Einfluss höherer Temperaturen auf das Gebirge. Fuer jedes Teilgebiet wurden der Entwicklungstrend und die wichtigsten Arbeiten kurz beschrieben und Referenzen gegeben. 1. GENERAL CONDITIONS FOR SWEDISH ROCK ENGINEERING The great expansion in Swedish construction - including rock construction - took place during the fifties and sixties. During this period our society exploited the ever-growing knowledge about rock blasting and excavating in building waterpower stations and tunnels, subways, underground sewage treatment plants, utility tunnels, fortifications in rock etc. Although the total construction activities has been considerably reduced during the seventies, the underground construction has been slightly increasing as a consequence of a general and growing understanding of the possibilities and advantages offered by use of underground space. However, during the same period some disadvantages connected with this technique have also been recognized, for instance lowering of the ground water table causing subsidence and deterioration of wooden pile foundations and blast vibration damage. It has also been found that the administrative rules of the society for regulating underground work do not meet such an intense activity satisfactorily in congested urban areas [2:2]. The trend of the development in mining as well as in underground construction is directed towards greater depth, larger spans and more concentrated, mechanized and consequently less manned working places. All these factors emphasize the need for a full control of the rock mechanic situation as regards excavation, stability and personal safety. Especially in the last respect there is a rapidly growing demand for still better health and safety conditions, which has been supported by society. Sweden is one of the most oil-dependent countries in the world, which has led to our building big oil storage reserves underground. The oil crisis in 1973 showed us clearly how sensitive we are in this respect and has created a policy to try to steer away from this dependency. On the other hand, there is also a policy not to be bound to a too strong dependence on nuclear energy. Therefore, energy conservation, alternative energy sources (solar energy) and more rational energy use is our road for the future. Here rock mechanics will probably play an important role, for instance for nuclear reactors underground, nuclear waste storage, storage and transportation of heated gases or liquids etc. Some of these projects have already strained our capacity up to and above our present rock mechanics knowledge. This situation has been a challenge to the rock engineering science and has, in recent years, motivated large economical and personal resources for R&D in these fields. 2. PRE-INVESTIGATIONS - METHODS, SCOPE AND PROGNOSIS VALUE It is, the world over, a well-known fact that the conditions which the contractor meets in the tunnels or openings very often differ considerably from those prognosticated from engineering geology pre-investigations. This is an unsatisfactory situation, which causes much trouble during construction and often leads to filing claims for extra payment. In Sweden such claims are usually settled by negotiation or arbitration, rarely by litigation, but even so there has, especially among the contractors, been a general wish to obtain better engineering geology prognoses as an important part of tender documents for underground work[2:l].
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Geological Subdiscipline > Environmental Geology > Hydrogeology (0.86)
SUMMARY: The Paper describes a method, for execution of oriented borings made without previous consolidation grouting of the rock mass. Interpretation of the data obtained by this method with the aid of a computer - controlled plotter, permitted the identification of potentially unstable wedges. The analysis of positions of the wedges, and the preliminary calculation of its safety factor as to sliding, were of great value in obtaining the optimum final design for excavation of the Itaparica Dam foundations. RÉSUMÉ: Ce travail presente la description d'une methode pour l'execution de sondages orientes, realises sans l'injection prealable de consolidation du massif rocheux. L'interpretation des donnes obtenues par cette methode avec l'aide d'une "Table Traceuse" (plotter) contrôlee par ordinateur, a permis l'identification de cales potenciellement stables. L'analyse des positions des cales et le calcul preliminaire de son factour de securite quant au glissement, ont ete de grande valeur pour l'obtention du projet final optimal pour l'excavation des fondations du Barrage de Itaparica. ZUSAMMENFASSUNG: Die Arbeit beschreibt eine Metode zur Ausfuehrung von orientierten Bohrungen ohne vorhergehende Verfestigung der Felsenmasse durch Injektionen. Die Beurteilung der durch diese Metode erhaltenen Resultate, mit Hilfe eines Computers mit Zeichentisch, erlaubte die Identifizierung von potentiellen unstabilen Felskeilen. Die Analyse der Position dieser Felskeile und die vorlaufige Berechnung des Sicherheitsfaktors gegen gleiten, waren von grossem Wert fur den Erhalt des optimal en Endprojekts fur die Fundierugen des ltaparica Dammes. 1. INTRODUCTION The Itaparica Dam is located in the lower-middle section of the Sao Francisco River, on the border between the states of Pernambuco and Bahia, 450 km from the city of Recife and 520 km from the city of Salvador, in the Northeast Region of Brazil (Fig. 1). The Dam will be built as an earth - rockfill embankment on the left bank joining the concrete structures, which will house the Spillway and Power House in the right bank and in the river channel the dam will also be an earth-rockfill embankment. The crest length will be 4,700 meters, of which around 720 meters will be the concrete structures with maximum length of 105 meters (Fig. 2). When completed its ten - 240 MW generators will produce a total of 2,400 MW of energy. The spillway will be comprised of nine floodgates that will make possible a discharge of 26,500 m3/second. 2. GEOLOGY OF AREA The Sao Francisco River, in the dam area, is a superimposed river which started to excavate its bed in crystalline racks after completely cutting and eroding the unconsolidated tertiary deposits. It is not also subject to area macro-structural control, being conditioned locally just by secondary structures, such as faults and fractures, in the pre-cambrian areas, and mainly by the faulting and by differential strength of the sedimentary rocks. The differential erosion is shown by the different degrees of hardness of the exposed rocks. In extreme cases, elevations of sandstone and lowered ground of the marshlands are seen cut away in soft ground of the Mesozoic stratified shale. Erosion was very intense during the Tertiary levelling cycle, a great movement of solid load occurring then in the Sao Francisco River and its tributaries, which was responsible for the vast mantles of gravel occurring in the area. Dunes fixed by sparse vegetation occur near the alluvium belts of the banks of the Sao Francisco River, and are located over more extensive areas of Tertiary deposits. There are several different lithological groups that occur in the area, and are classified as follows: A - Undefined Pre-Cambrian rocks: represented by a crystalline complex consisting basically of migmatites, gneisses, leptynites, micaschists, quartz - schists and granites. B - The Paleozoics: represented mainly by feldspathic sandstones, with or without conglomeratic beds, and with parallel - planes and cross bedded stratifications. C - The Mesozoic: represented mainly by stratified shales claystine, siltstone, sandstones and feldspothic sandstones. D - The Cenozoic: represented mainly by sandy-clayey deposits with gravel beds, dunes consisting of silty sands, and alluvium consisting of silty-clayey sands. 3. SITE GEOLOGY The axis of Itoporica Dam was chosen in such a way that the dam would be settled on granites and migmatites instead of the sandstone area situated 5 km upstream, in order to avoid percolation problems in the banks end foundations. The weathering of granite or migmatite rocks gives a soil with a high percentage of sandy fraction, whose thickness varies from 0 to 8 meters. These saprolite are generally covered by colluvial and/or alluvium layers with thicknesses varying from 0 to 2 meters and composed of clayey and/or silty sands. The existence of granite boulders is verified throughout the dam site area. They occur both superficially over the exposed rock mass, and/or interspersed among the saprolite, colluvium and/or alluvium soils. The concentration and diameters of the boulders at the surface increases from the banks to the river channel.
- Phanerozoic > Cenozoic > Tertiary (0.74)
- Phanerozoic > Paleozoic > Cambrian (0.54)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.45)
SUMMARY: Over break and ground vibrations can be much reduced and pitwall stability increased through greater engineering input to blast design. Blast-induced damage is minimised by maximising the use of bottom-primed charges of bulk ammonium nitrate/fuel oil. Where it is necessary to decouple or deck back-row charges, their effective burdens should be reduced in proportion to the decrease in energy yield per blasthole. Decoupled charges are particularly beneficial in the upper sections of back-row blastholes. Inclined blastholes are usually superior to vertical blastholes. Most importantly, back-row charges must be able to fragment and displace their burdens in a forward horizontal direction with relative ease. Excessive effective burdens, inadequate inter-row delay intervals and/or the presence of buffer rock prevent the required progressive relief of burden and contribute very considerably to larger over break zones and higher vibration intensities. RESUME: Cassure de roche au dèla du chantier, et tremblements du terrain peut être reduite sensiblement, et la stabilite du parement exterieur amelioree par moyens d'une plus soignee modelage de la detente. Degats provenant de la detente sont au minimum quand les charges ANFO sont amorcees au fond. Partout ou les charges au rang des trous derrière doit être decouplees or rempliee d'un façon intermittent, l'ecartement entre trou de mine et paiement exterieur doit être reduite au fur et à mesure que le dèbit d'energie de chaque trou est reduite. Decouplement des charges est particulièrement avantageux pour les parties superieures des trous au rang derrière. Habituellement des trous inclines donnent des meillieurs resultats que des trous verticals. Plus important même, il faut que les charges du rang derrière peuvent donner un fragmentation et deplacement de leures propres tranches du roche horizontalement d'une façon relativement aise. Si l'epaisseur effectif des tranches est trop large, ou le delai entre tirage des rangs est insuffisant, et/ou du roche tampon est present, une relaxation progressive du roche est empechee et par consequent il se produit des cassures assez loin du chantier et des tremblements plus sensibles. ZUSAMMENFASSUNG: Der Abbruch ausserhalb der Grenze des gewuenschten Flachenraumes und die Grundvibrationen können verhindertwerden und die Grubenwandstabilitat kann erhöht werden durch bessere technische Durchfuehrung des Sprengungsplanes. Die durch Sprengung verursachten Schaden können wesentlich verringert werden durch den maximalen Gebrauch von Boden-gezuendeten Ladungen von ANFO. Wenn es notwendig ist, dass man getrennte Ladungen oder Lagen-Ladungen benuetzen muss, muss das Ausmass der Sprengwand verringert werden in demselben Verhaltniss, wie sich die Menge der Energie in jedem einzelnen Sprengloch verringert. Getrennte Ladungen sind besonders wertvoll in den oberen Bereichen der letzten Reihen von Sprenglöchern. Die Anwendung von schraeg gebohrten Sprenglöchern ist gewöhnlich viel wirksamer als Sprenglöcher, die in senkrechter Richtung gebohrt sind. Das wichtigste ist aber, dass die Sprengladungen in der hinteren Reihe in der Lage sind, die Sprengwand zu brechen, und diese mit gewisser Leichtigkeit in waagrechter Richtung nach vorne zu bewegen. Übergrosse Sprengwande, sowie unzureichend verzögerte Zeitabstande zwischen den Sprengreihen und/oder das Vorhandensein von Puffersteinen verhindert die gewuenschte stuffenweise Ablösung der Sprengwand und tragt damit bedeutend bei zum grösseren Abbruch ausserhalb der Grenzen des gewuenschten Flachenraumes und auch zur grösseren Heftigkeit der Grundvibration. 1. INTRODUCTION The stabilities of newly-exposed rockwalls in open pit, strip mining and quarrying operations are often critical to productivity and profitability. Large failures create hazards and can result in appreciable delays to operating schedules. Where there is insufficient engineering design effort, blasting can contribute appreciably to pitwall instability. Because the nature and effects of blasting can be controlled within wide limits, operators have the ability to promote stability by minimising both blast-induced damage and ground vibrations. In order to fully appreciate the relationship between stability and blast design, of course, it is first necessary to understand how each of the various blast parameters affectthe extent of over break and the intensity of ground vibrations 2. EFFECTS OF BLAST PARAMETERS ON OVERBREAK AND GROUND VIBRATIONS 2.1 Effects of explosive type For a fully-coupled charge of bulk ANFO or a pumped non-aluminised water gel, the peak blasthole pressure WBP) exerted by the explosion gases can be calculated from the equation. For a typical non-aluminised water gel explosive, equation (1) gives a PBP value that is about 1.7 times that for (lower density, lower velocity) bulk ANFO. A typical aluminised water gel containing 10% Al exhibits a PBP value (calculated using an equation of greater complexity) that is about 2.2 times greater than that for bulk ANFO. As one would expect, the intensity and amount of over break brought about by strain wave-related mechanisms decrease with PBP. The levels of ground vibrations produced by blasting also decrease with PBP. Work at a large Australian underground mine has shown, for example, that the vibrations produced by a highly-aluminised water gel explosive are about twice those resulting from an equal weight of ANFO (Hagan and Kennedy, 1977).
- Materials > Metals & Mining (1.00)
- Energy > Oil & Gas > Upstream (0.68)
SUMMARY: Rock mass deformability can be expressed with empirical correlations or analytical component models. The paper discusses currently used empirical correlations and analytical models and points out their limitations. A new statistically based analytical model is proposed and the relations between results of the model and current empirical correlations are discussed. The new approach improves understanding of rock mass behavior and appreciation of variance of observations. SOMMAIRE: La deformabilite des massifs rocheux peut être exprimee par des correlations empiriques et modèles analytiques. On presente et discute, dans cet article, les correlations empiriques et les modèles analytiques disponibles à l'heure actuelle. On propose ensuite un modèle analytique nouveau base sur des statistiques. On explore, enfin, les liens entre le nouveau modèle et les correlations empiriques. Le nouveau modèle permet de mieux comprendre le mecanism de la deformation des massifs rocheux et d'expliquer la variance des observations. ZUSAMMENFASSUNG: Die Gebirgsverformbarkeit kann mit empirischen Beziehungen oder analytischen Komponenten-Modellen ausgedrueckt werden. Gegenwartig im Gebrauch stehende empirische Beziehungen und Komponenten- Modelle werden besprochen und ihre Schwachen aufgezeigt. Der Artikel beschreibt dann ein neues analytisches Modell, das auf statistischen Prinzipien beruht. Ausserdem werden die Resultate, die mit dem Neuen Modell erzielt werden, mit den gegenwartig in Gebrauch stehenden empirischen Beziehungen und die Varianz von Beobachtungen zu erklaren. 1. INTRODUCTION Rock mass deformabiltiy affects the performance of essentially all structures in and on rock, from underground openings and excavations to foundations. Thus, the prediction of deformabiltiy is an important part of rock engineering. The most direct way of estimating deformability is through field testing. However, for meaningful results, field tests must subject large volumes of rock to significant stress. Therefore, the tests are expensive, time consuming, and must be limited in number. To supplement direct testing and to provide estimates of deformability when field tests are impractical, other procedures have been introduced. Broadly, these derive from empirical correlations, on the one hand, or analytical decompositions, on the other. Many such procedures have been introduced. Empirical correlations attempt to statistically relate deformability to index properties, such as RQD, or to descriptive rock mass classifications. Analytical decompositions attempt to predict deformability by summing deformations over elements of the rock mass, such as intact blocks and joints. Both approaches suffer limitations. Correlations are limited by the character of the case studies from which the baseline data come. Decompositions are limited by an inability to measure and specify parameters of the models. Improvement of these techniques is needed. To improve predictions of deformability either more and better data are required, better information is required on joint stiffness and geometries, or a tie-in is required between correlations and decompositions. A tie-in between correlations and decompositions would allow information of each type to, in part, compensate inadequacies in the other, and would allow extrapolations of correlations through decompositions. The present work indicates a connection between correlations and decompostions, which becomes increasingly apparent as the strict geometric assumptions underlying decompositions are relaxed. 2. PRESENT TECHNIQUES FOR PREDICTING DEFORMABILITY 2.l Empirical Correlations Possibly the best known correlation of deformability to indices or descriptions is that of Deere (1967), using RQD. However, others have been proposed, ranging from the refined descriptions of the German-Austrian school (Muller, 1963; Terzaghi, 1946; Stini, 1950), to intricate quantitative descriptions of Barton (1977) and Bieniawski (1975). Indices or descriptions are correlated:to deformability by correction factors on material properties that are easily determined (e.g., intact modulus), directly to a rock mass deformability, to design features (e.g., structural dimensions). Obviously, correlations are based on field studies, and are limited by the geologic richness of the calibrating cases. 2.1.1 RQD - Modulus Ratio Relations Deere's work (1967) in conjuction with co-workers was originally based on field studies at Dworshak Dam(1964). Field plate loading tests were compared with intact modulus and RQD to arrive at the empirical correlation of Figure 1. Further data were added by Coon and Merritt (1970) from other sites. All of the data, however, were from good quality rock and the lower portion of the curve is therefore poorly defined. Although Coon and Merrit noted the inadequacy of their data base, little additional data has been publicly presented. A problem with these correlations is that they are based upon jacking tests of limited load and zone of influence. Further, some RQD's are obtained indirectly by correlation with seismic velocity ratios. Although RQD is often assumed to equal the velocity ratio, this is in fact only an approximation. Finally, only a limited number of tests and sites form the basis for the correlations. 2.1.2 Direct Correlations between Descriptors and Rock Mass Deformability. Quantitative geologic descriptors like fracture spacing and qualitative descriptors of structural features and weathering have been related directly to the deformability observed under one or several structures. Boughton(1968), for example, produced a correlation at dam sites.
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SUMMARY: Thirteen carefully scaled physical models (scale 1: 300) were used to study the two-dimensional deformation resulting from excavation of very large span openings in near-surface rock masses. The openings were excavated in stages up to final simulated spans of 50 metres. In some cases more than one opening was excavated in parallel. The models consisted of at least 20,000 discrete blocks. Joint orientations and stress levels were varied, and some models were dynamically loaded to simulate strong earthquakes(peak horizontal acceleration 0.2 - 0.7 g). RESUME: A l'aide de treize modèles soigneusement mis à l'echelle 1: 300, le present article etudie les deformations bidimensionnelles suite à l'excavation d'ouvertures à très grandes travees situees à faible profondeur dans un massif rocheux. Les ouvertures furent amenagees par etapes jusqu'à simulation de travees finales de 50 mètres. Pour certains essais, plusieurs ouvertures parallèles furent pratiquees. Les modèles, discretises au moyen d'un minimum de 20,000 blocs et avec diverses orientations des joints, furent soumis à des niveaux de contrainte variables. Afin de simuler de violents tremblements de terre, quelques modèles furent soumis à des charges dynamiques (acceleration horizontale maximale de 0.2 à 0.7 g). ZUSAMMENFASSUNG: Dreizehn sorgfaltig skalierte fysische Modellen (Geometrischer Masstab 1: 300) wurden benutzt um die zweidimensionalen Formanderungen zu studieren, die durch den Ausschub von sehr weitgespannten Hohlraumen inoberflachennahen Felsmassen entstehen. Die Hohlraume wurden stufenweise erweitert, bis zur vollen simulierten Spannweite von 50 Meter. In einigen Fallen wurden mehrere Hohlraume in parallell abgeraumt. Die Modellen bestanden aus mehr als 20,000 Einzel- Blocke. Die Orientierung der Kluftebenen und die Spannungshöhen wurde variert. Einige Modellen wurden dynamisch belastet um starke Erdbebungen zu simulieren. (Grösste horizontale Beschleunigungen waren dabei 0.2 - 0.7 g). 1. INTRODUCTION 1.1 Typical deformation magnitudes The engineering performance of large rock caverns has traditionally been learned from mining and hydro power projects, where the depth below surface is most often many times greater than the 15–35 metres span of the openings. Deformations measured in the walls and roofs of hydro power caverns generally range from about 10–40 mm, though there is a documented case where a wall moved in 126 mm (Eristov and Khechinov, 1972), and another where the arch moved down 147 mm (Imrie and Jory, 1968). 1.2 Unknown influence of free-surface The accelerating Interest in the use of near-surface underground space for purposes as diverse as food storage and nuclear power generation, forces designers to recognize the potential influence of the free surface. It is a matter of some interest whether large excavations close to the surface will SUffer similar or very different deformations when compared with their deep-seated relations. Since elastic continuum models of rock mass behaviour are increasingly suspect as the jointed near-surface is approached, it is generally necessary to consider jointing, Whether this be simulated in numerical or in Physical modelling. 1.3 Comparison of physical and numerical models The objective of these model studies is to provide deformation data to compare with monitored data from future engineering projects (i.e. underground nuclear power stations) and for comparison with some sophisticated numerical modelling that is in progress. Both numerical and physical models can be invaluable in correctly interpreting the results of displacement monitoring of important large span near-surface openings. Physical models have a double role in that they provide a check for numerical model results. The number of blocks that can be physically modelled is closer to that in practice, generally perhaps one to two orders of magnitude larger than the number that can be handled in presently existing jointed finite element programmes. 2 MODEL REQUIREMENTS The principle requirement for a model of a near surface opening is that it should be gravity loaded, besides any additional horizontal loading. It should also be large enough to simulate several hundred metres of rock mass so that boundary effects are minimized. Consequently a material of high density and low strength needs to be used. In this study a model-prototype geometric scale factor of 1: 300 was adopted, and this made it possible to simulate horizontal dimensions of approximately 350 metres and a depth of approximately 250 metres in a model measuring 120 × 80 cm. This model was "two-dimensional", having a wall thickness of 2.5 cm. A photograph of one of the models is shown in Figure 1. A two dimensional model is equivalent to the unfavourable situation in which a cavern is excavated with its long axis parallel to the strike of the predominant jointing. It is well known that such a condition leads to large deformations. (Cording, Hendron and Deere, 1972). Measured deformations should be interpreted in the light of this limitation. 2.1 Model material The model material consisted of an oven cured mixture of red lead-sand-ballotini-plaster- water. The physical properties of a range of these weak brittle materials have been described in detail by Barton (1970).
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (0.89)
- Energy > Power Industry > Utilities > Nuclear (0.74)
- Energy > Renewable > Hydroelectric (0.69)
SUMMARY: Tunnelling machine performances in unjointed rock can be calculated, based on some fundamental equations for the penetration of a single disk cutter. The influence of joints (and other discontinuities of the rock mass) on the penetration rate depends on the frequency and type of joints. The joint frequency is determined by the total joint area per unit volume of excavated rock, i.e. square meters of joints per cubic meters of rock. Healed joints, tight joints and fissures produced by tensile stresses in the rock mass have no influence, but the specific penetration is significantly increased by mylonites, fissile zones and joints with gauge material, produced by shear stresses. This improvement depends also on the rock type. RESUME: Pour des roches non-fissurees, la performance d'un tunnellier peut etre determinee à base de quelques formules pour la penetration d'une seule molette à disques. L'influence des fissures (et d' autres discontinuites du massif rocheux) depend de la frequence et du type des discontinuites. La frequence s'exprime par: surface totale des fissures par unite de volume. Les fissures et joints fermes, produits par tension dans la roche, ont presqu'aucune influence sur la penetration. Les mylonites, zones fracturees et joints produits par contrainte de cisaillement peuvent augmenter considerablement la performance du tunnellier. L'ordre de grandeur de cette amelioration depend aussi du type du rocher. ZUSAMMENFASSUNG: In ungeklueftetem Gebirge kann die Vortriebsleistung einer Tunnelvortriebsmaschine berechnet werden aufgrund einiger Formeln fuer das Eindringen eines einzelnen Diskenmeissels. Der Einfluss von Klueften (und anderer Störungsflachen im Gebirge) hangt ab von der Haufigkeit und Art der Kluefte. Die Klufthaufigkeit wurde bestimmt als Kluftflachen je ausgebrochene Volumeneinheit. Verheilte una geschlossene Kluefte (Risse), welche durch Zugspannungen im Gebirge entstanden, beeinflussen die Penetration nur wenig. Mylonite, Ruschelzonen und Kluefte mit Fuellmaterial (durch Scherspannungen entstanden) fuehren zu einer signifikanten Erhöhung der Netto-Vortriebsleistung. Der Betrag dieser Erhöhung ist zudem gesteinsabhangig. 1. INTRODUCTION During the last few years, mechanical excavation of tunnels has become more and more important. The application of tunnelling machines appears even to increase in the near future: from the total of 484 km of planned tunnels in Switzerland only 15% are restricted to drill-and-blast methods. 30% are definitive assigned for tunnelling machines, and for the rest the excavation method will be mixed or is not yet decided. An extensive literature, including more than 1000 publications, deals with many aspects of mechanised tunnelling. Particularly well examined is the application of drag bits in soft rock in connection with the planning of some major projects (e.g. the Channel Tunnel). Up to the present, most contractors hesitated to apply a tunnelling machine in very hard rock, due to some failures when the rock hardness was much higher than expected (e.g. inclined shafts of Kaprun and Motto di Dentro). Nowadays a better technology and new designed machines allow thrusts of up to 200 kN per cutter. Consequently a much wider range of hard rock can now be excavated by disc cutters. The mechanical excavation of hard rock is highly influenced by joints, bedding planes and other discontinuities of the rock mass. In many cases this method is not efficient when such discontinuities are absent. Therefore quantitative indications on the influence of discontinuities are particularly important when planning a mechanical excavation method in hard rock. 2. OBJECT OF INVESTIGATION The excavation of soft rock by drag bits depends not significantly on the jointing of the rock mass. Drag bits and consequently rod headers are therefore not considered for the present investigation. In extremely hard rock tunnelling machines have often been equipped with roller cutters with tungstene carbide button inserts, called "strawberry cutters" or "pineapple cutters". The wear of this cutter type usually was high and the performances low. With the development of disk cutters for high thrusts strawberry cutters seem to disappear gradually, as well as cutters with multiple disk rings. Besides the jointing, the anisotropy of the rock and the rock mass also proved to influence the performances of tunnelling machines. This is subject of a separate study (in preparation). The rock mass is - for this publication - assumed to be isotropic. 3. FORMATION OF ROCK CHIPS BY DISK CUTTERS The indentation of a disk cutter into a rock surface is similar to the indentation of a wedge. Roxborough and Phillips (1975) have studied these problems at the University of Newcastle upon Tyne. The formulas have been extended by Ozdemir, Miller and Wang (1977) at the Colorado School of Mines. The section below follows partially these two publications. Observations of the wedge indentation into rock showed that the rock chips are formed by shearing off. Once the disk has penetrated into the rock, shear forces exceeding the rock shear strength are induced. 3.1. -Cutter indentation For the indentation of the disk into the rock surface, the rock under the cutter must be crushed.
- Europe > Switzerland (0.49)
- North America > United States > Colorado (0.26)
- Europe > United Kingdom > England > Tyne and Wear > Newcastle (0.24)
- Geology > Rock Type > Metamorphic Rock (0.77)
- Geology > Geological Subdiscipline > Geomechanics (0.69)