Controlled blasting in hard intense jointed rock in tunnels

CIM Bulletin, Vol. 78, No. 884, 1985

AGNE RUSTAN, TORBJORN NAARTTIJARVI* and BENGT LUDVIG, Department of Mining and Underground Construction, Lulea University of Technology, Lulea, Sweden

Falling rock is one of the major causes of serious injuries and deaths in Swedish mines. The research at the Division of Mining and Underground Construction was therefore directed toward controlled blasting in hard intense jointed rock. The project was started with an extensive literature review. Model tests with a new equivalent model material have been initiated to find ways of improving blasting techniques. The result showed that long cracks arise at the perimeter holes when there is a long time delay between adjacent holes. This led to the theoretical development of a new initiation method with ultra short delays (1,5 ms)for controlled blasting, called "Cutblasting". Full-scale tests have been done at LKAB in Malmberget in hard intense jointed magnetite ore. Four different types of perimeter charges have been tested, namely: tube charges, ANFO mixed with plastic beads, detonating cord and linear-shaped charges. Three types of initiation of the perimeter holes have been used, conventional (half-second delay detonators), instantaneous and ultra short "cutblasting initiation" (1,5 ms delay). The change in rock strength before and after blasting has been studied with bore hole periscope, vibration measurement and ocular observation of the rock surfaces. The full-scale tests showed that "cutblasting" with detonating cord in the perimeter holes gave the smallest damage to the surrounding rock. A combination of cutblasting initiation and tube charges are anticipated to give an even better result. A new method of controlled blasting using linear-shaped charges has also been tested, however, this charge type needs further development. The full-scale test also showed that the damage which can be seen on the rock surface is about the same for different controlled blasting methods but the not-visual-damage zone varies as much as from 0,1 to 0,7 m. A new classification system for controlled blasting regarding the damage to the surrounding rock has been devised.
Keywords: Rock mechanics, Controlled blasting, Hard intense jointed rock, Rock stability, ANFO, Explosives, Cutblasting, Blasting, Drilling, Initiation techniques, Vibration measurements.