The influence of cartridge length and tamping practices on the efficiency of packaged emulsion explosives in development blasting

摘要

The underground mining and civil construction industry requires the design, drilling, blasting, and excavation of many tens of thousands of metres of tunnel development per year. The majority of this tunnel driving is still carried out using pneumatically loaded bulk ANFO, with the use of cartridge emulsion explosives in the bottom row of 'lifter' holes being widespread. Occasionally however, the presence of running water or excessive quantities of mine water and humidity render the use of prilled AN based explosives impractical due to product loss through dissolution in water. Under these circumstances, the operator must use water-resistant products such as bulk or cartridge emulsions. The latter choice is to date more common due to ease of supply and the low tech nature of the required handling. Many operators report, however, that when cartridge emulsion products are used exclusively in wet faces, the incidence of unbroken hole length (butts) increases. This tendency has been observed independently of explosive manufacturer and product. Initially it was postulated that such a reduction in blast/explosive performance was due to dynamic desensitisation (dead pressing) of the solid sensitised emulsion explosive, particularly in the burn cut region of the round where inter-hole distances may fall below 25 centimetres. However vibration monitoring studies and direct observation of blast results calls into question this hypothesis. In its place, it has been suggested that inadequate coupling of the explosive cartridge with the borehole wall may reduce the explosive energy/rock transfer to such a degree that rock fragmentation is inhibited and inadequate round performance results. In the case that this were so, it was further postulated that any change in explosive characteristics/charging techniques which were to bring about an improvement in this coupling, should consequently cause an improvement in tunnel blast performance as measured by linear advance per drilled and charged metre. The following paper describes a series of experimental measurements designed to quantify the impact of cartridge length and tamping practices on the degree of explosive/rock coupling. The resultant coupling factors are shown to have a marked effect on anticipated explosive performance in the development blasting application.