SUMMARY:

Overbreak is least in strong rocks, and especially in those which contain few discontinuities. Controlling overbreak and ground vibrations demands that blastholes have the optimum orientation and inclination. Overbreak can be largely but not totally controlled by applying the smoothwall blasting technique (i.e., by using low explosion energy concentrations in perimeter blastholes and by firing these charges last, as simultaneously as possible, and with a burden: spacing ratio of 1,25 - 1.40). If a primary blast is poorly designed and/or carried out, there is a high probability that overbreak will occur even with the application of smoothwall blasting. This is especially the case where fewer largerdiameter blastholes are used in an effort to reduce both the cost and period of excavation. However, in strong rocks, larger-diameter blastholes and acceptable overbreak can be surprisingly compatible, provided that progressive relief of burden is achieved by carefully selecting the energy factor, initiation sequence and delay timing for each charge. Where large-diameter blastholes are used in horizontal benching and especially in downhole benching, it may well be preferable to presplit the walls of the cavern,

ZUSAMMENFASSUNG:

Das Überprofil ist in solidem Gestein am kleinsten, insbesondere in Gestein, welches wenige natuerliche Unregelmaßigkeiten aufweist. Die Kontrolle des Überprofils and der Erdschwingungen erfordert die bestmögliche Ortung and Neigung der Sprenglöcher. Das Überprofil kann weitgehend, jedoch nicht völlig, durch Anwendung der Glattmauer-Sprengmethode unter Kontrolle gebracht werden, d.h, durch Anwendung niedriger Sprengenergiedichte in Kranzsprengldchern, and durch Zuendung dieser Sprengladungen zuletzt and so gleichzeitig wie möbglich mit einem Uorgabe: Abstandsverhaltnis von 1,25 - 1,40. Falls eine Hauptsprengung mangelhaft entworfen oder ausgefuehrt worden ist, ist es höchstwahrscheinlich, daß eis Überprofil auftreten wird, sogar bei Anwendung der Glattmauer-Sprengmethode. Dies ist insbesondere der Fall, wenn weniger Sprenglöcher von größerem Durchmesser_angewandt werden, um die Kosten and auch die Ausschachtungszeit zu verringern. Jedoch können in solidem Gestein Sprenglöcher größteren Durchmesserszu gleicher Zeit mit hinnehmbarem Überprofil existieren, solange die schrittweise Entlastung der Vorgabe durch die sorgfëltige Auswahl des Energiefaktors, der Zuendreihenfolge

RESUME:

L''exces de fissurage au dela de l''excavation est moindre dans la roche dure, particulierement dans cello qui contient peu de failles. Le controle du fissurage et des vibrations du sol necessite que les trous de mine aient une orientation et une inclinaison optimales. L''exces de fissurage peut être controle en grande partie, mais non completement, par la technique de "fracture lisse" (c''est à dire en plaçant une faible concentration d''energie explosive dans chaque trou de mine sur la peripherie et en amorgant ces charges en dernier et, autant que possible, simultanement; avec un rapport distance a la face libre/ecartement des trous de 1,25 a 1,40). Si un tir primaire est mal prepare et/ou mal execute, it est probable que des fissures excessives se produiront au dela de l''excavation malgre l''application des charges peripheriques decrites.ci-dessus. C''est en particulier le cas quand le Hombre de trous de mine a ete reduit et hour diametre aggrandit afin de reduire le cout et la duree du forage. Cependant en roche dure, les charges de grand diametre peuvent être compatibles avec un degre de fissuration acceptable pourvu que l''abattage progressif de la roche soit obtenu par le choix soigneux du facteur d''energie, de la sequence d''amorçage et du retard d''armorcage de chaque charge. Quand des trous de mine a grand diametre sont employes pour

INTRODUCTION

The last two decades have witnessed the excavation of many large caverns for both hydro-electric power projects and the storage of crude oil. There are strong indications that, in the remaining years of this century, far greater numbers of such underground openings will be required for these and additional purposes. In the stronger rocks, at least, caverns. will be excavated by drilling and blasting for many years to come. As is to be expected, there are appreciable incentives to fire larger blasts, since these effectively lessen the cyclicity of the operation and, hence, reduce both the period and cost of excavation. However, unless carefully designed and executed, larger blasts can create strains which produce more cracking beyond the design perimeter of a cavern. Minimum overbreak is desirable because 1. it promotes safety by reducing the probability of rockfalls, 2. it reduces the rock quantities to be removed and the cost of support or lining, and/or 3. it restricts leakages both into and out of completed unlined caverns. On the other hand, achieving minimum excavation costs demands that conventional blasting techniques be used as close as possible to the final rock surfaces. That the factors which influence blast-induced overbreak be fully understood and then controlled to the optimum levels is, therefore, of considerable importance.

OVERBREAK MECHANISMS

There are at least four identifiable mechanisms by which blast-induced overbreak can occur, viz.

  • radial fracturing,

  • internal spalling,

  • gas extension of natural discontinuities and strain wave-induced cracks, and

  • release-of-load fracturing.

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