The development of a blasthole stemming performance evaluation model using a purpose built testing facility

Abstract

The ability of an explosive to break rock is influenced considerably by the extent of confinement in the blasthole and it is believed that confinement is improved by the use of stemming. The aim of this paper is to present the first and second stages of results in developing a stemming performance testing and evaluation facility for small diameter boreholes. The results showed that different stemming products have differences in terms of their functionality, which can have a major impact on the efficiency of rock breaking. Two test procedures were used, one through the exclusive use of compressed air and the second using a purpose built high pressure test rig with small quantities of explosives. Both tests were used to identify and evaluate the ability of various stemming products to resist the escape of explosive gas through the collar of a blasthole. Extensive research was conducted to determine the types of stemming products most commonly used in South African underground hard rock mines, and the differences in design between the various products are discussed. The first stage of tests using compressed air only did not prove adequate to predict with certainty the pressure behaviour in the borehole of a particular product under high pressure conditions. The purpose built high pressure test rig did not prove to be a very effective tool to test stemming products under high pressure conditions. The test rig only incorporated the effect of gas pressure on the stemming product and in doing so omitted to take the effect of the shock wave into account. This study proved that to only take the gas pressure generated in the blasthole into account in not sufficient to effectively test stemming product design. A more comprehensive study should include the effect of gas pressure in the borehole, shock waves generated by the explosive and also the coefficient of friction of both the surface of the stemming product as well as the inside of the blasthole.