This is a general report on the related subjects of rock breaking by blasting, mechanical cutting, and in situ fracturing. It presents briefly the recent general development within the subject area and highlights specifically some recent developments. These include a new Swedish method for predicting rock damage due to blasting, both in the near and far region, and a Norwegian and an American technique for predicting rock drillability. The status of water jet assisted rock cutting is briefly reviewed, and a short survey of progress in situ fracturing concludes the report.
Im Folgenden wird ueber verschiedene Methoden des Bergabbaus berichtet, wie Sprengung, mechanischen Ausbruch und Zerklueftung in situ. Auf Hintergrund einer kurzen Prasentation der letzten Fortschritte in diesem Bereich wird ueber einige der letzten Entwickelungen mehr eingehend berichtet. Somit handelt es sich teils um eine neue schwedische Methode Sprangbeschadigungen im Gebirge vorauszusehen, sowie in der unmittelbaren Nahe wie in einem weiteren Umkreis, teils um eine norwegische und amerikanische Technik fuer die Bestimmung von Bohrbarkeit. Die Verwendung von Wasser-jets beim Bergabbau wird fluechtig beruehrt und der Bericht schliesst mit einer kurzen Übersicht der Fortschritte in Zerklueftung in situ.
Le present est un rapport sur ies differentes methodes de creusment au rocher, comme tirs des mines, excavation mechanique et fracturation in situ. Sur le fond d'une courte presentation du developpement general dans ce domaine quelques nouveautes recentes sont presentees plus en detail. II s'agit donc d'une methode suedoise de determiner à l'avance l'endommagement apporte au roches par une charge explosive dans le voisinage comme à distance, et d'une Technique norvegienne et americaine pour determiner la penetrabilite de forage. La question de l'emploi de Jets d'eau dans les travaux d'excavation et touchee superficielment et le rapport s'achève par une courte presentation des progrès dans le domaine de la fracturation en situ.
The use of explosives as a tool to remove rock requires controlled blasting to minimize damage to the remaining rock walls and neighbouring structures. In present-day open pit mining with Shothole diameters in the range 250–500 mm (10 to 20") each shothole may contain one or two tons of explosive and a whole blast may involve the detonation of 200–500 tons of explosive. In underground mining, large shothole diameters in the range 150 to 200 mm are increasingly being used. In tunnelling, larger diameter (50–100 mm) and long (3–6 m) shotholes are also common. The development in all these areas towards larger blasts gives great savings in the cost of excavation, but also makes greater demands on methods to avoid damage. In open pit mining, the stability of the pit slopes and the corresponding slope angles have a tremendous influence on the economy and safety of the operation. Two papers in this conference deal with methods to produce steeper slopes by controlled blasting that leaves the remaining rock strong enough for the increased stresses that result. Even steepening the slope by one degree means saving a considerable amount of waste rock removal in a deep and large open pit mine. Similar savings can be realized by introducing controlled blasting in underground mining and tunnelling where damage to nearby tunnels and building structures representing large sums of money must be avoided. Controlling blasting damage requires an understanding of stress waves in rock.
The detonation of an explosive charge in a borehole in rock gives rise to stress waves in the surrounding rock. Further out, the conditions are favourable for the formation of radial cracks. As the wave moves radially out from the borehole, the amplitude (pressure) decreases and the wave becomes purely elastic. As a result of the interaction with the free surface, the different types of waves that we know from seismology develop, the p-wave, the s-wave, and the Rayleigh--wave. In this area, the structure of joints or fissures in the rock begin to influence both the wave propagation (wave velocity and stress amplitude) and the degree of fracturing more than in the region close to the drillhole. When we are discussing wave strength in this far-field region, it becomes useful to use the peak particle velocity as measure. Figure 1 shows the approximate decrease of the peak particle velocity with distance away from a 15 m long charge in a 250 mm diameter borehole.
Granite may be expected to fail in dynamic tension at a stress of perhaps 30 MPa, corresponding to a strain of ≈ 1 %, that is a particle velocity between 1000 and 2000 mm/sec depending on the wave type. But normal fissured rock will undoubtedly show tensile damage in the joints at lower stress levels(say around 700 mm/sec). For soft, sedimentary rocks, with relatively weak joints, damage may occur at a particle velocity of 400 mm/sec or less.
