Low Powder Factor Blasting to Take Advantage of Natural Rock Fissures is Uneconomical

摘要

In many quarries and construction sites visited by the authors over the last 50 years, the latter havernobserved muck piles from blasting with explosives, which included numerous large blocks, in somerncases of dimensions larger than the input to the crusher. The names of these sites cannot be given in thernarticle because the owners insist that data obtained on such visits be kept confidential. Often the blastersrnfelt that the natural fissures in the rock would compensate for the use of a low powder factor, and theyrnasked the authors to explain why such blocks were present. A detailed analysis of the situation oftenrnindicated that the cause of the blocks is that explained by figures such as 6 and 7 below in the article.rnIndeed it is true that some blasts do give adequate fragmentation as a result of a large number of naturalrnfissures in the rock mass, even if a low powder factor is used. However, a blast with adequaternfragmentation with low powder factor blasting is often followed by a blast with large blocks andrninadequate fragmentation size distribution, the reason being that the later blast happens to be inrncompetent rock with few natural fissures; it is not easy to evaluate visually the degree of natural fissuresrnin the rock mass.rnThe article demonstrates how low powder factor blasting may lead to inadequate fragmentation sizerndistribution even in a competent rock, as well as difficult mucking and difficult crushing. The articlernconcludes that in the long run the savings in explosive costs from the low powder factor blasting in rockrnthat has many natural fissures are more than cancelled by the periodic high crusher costs for the blastsrnwhen the rock is competent. In fact, whether the rock is fissured or competent, in both cases the lowrnexplosive costs due to low powder factor blasting is usually cancelled by the higher mucking costs duernto the inadequate rock movement resulting from low powder factor blasting. This is a practical articlernwhose aim is to dispel the dubious premise that low explosive costs always save money, an opinion,rnwhich the authors have found to be believed and followed by too many blasters, sometime under urgingrnby the owner of the quarry. The present article can assist blasters to convince the owner not to insist onrnlow explosive costs. In the long run, the most economical practice is usually to blast as if the rock massrnwere competent.