Minneappolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 384, 1991 Jun; :1-2
Objective: Reduce rockfall fatalities and injuries by using innovative blasting practices to reduce damage to the rock at the excavation limits with little or no increase in cost. The Problem: Falls of highwall have been a leading cause of fatalities, injuries, and equipment damage in the surface mining industry. Although the number of highwall falls is not great, when such an incident does occur it is about 14 times more likely to be fatal or result in severe injury and damage to equipment than other types of surface mine accidents. Overbreak or damage to the rock from poor or improper blasting practices is a major factor contributing to rock falls. Presplitting or cushion blasting is the technique most often recommended for improved wall control. This type of blasting however, adds to drilling and blasting costs and is not universally accepted by the industry. Air gap presplitting techniques, also called air decks, are currently being used in some mines as a less costly alternative to conventional presplitting. Approach: Blasting can cause highwall damage, but proper blasting techniques have been shown to reduce the magnitude of rockfall problems. In preventing rock falls, it is important to maintain the natural strength of the rock structure to the greatest extent possible. Improved blasting practices have three main advantages: 1) they arc preventive rather than corrective in nature; 2) they are cost effective; and 3) they have a very low impact on the mining operation when properly implemented. New ways of evaluating highwall integrity, such as seismic refraction tomography, were investigated. Test Results: Bureau projects demonstrated that simple changes in blast design, such as repositioning and redistribution of explosive loads, can significantly improve highwal1 condition and reduce rockfalls in both production and presplit blast designs. Full scale tests were conducted at a surface coal mine using the presplitting technique before production blasting and at a quarry using production blast designs only. Air gap presplitting using repositioned and/or decoupled explosives was the technique used during the coal mine tests. Current research by the Bureau suggests that using air gap techniques with a fully-coupled charge produces overbreak and rockfall in weak rock situations. The Bureau's air gap tests involved additional decoupling from cardboard tubes, suspending bagged ANFO, and relocating the toe charge from a weak to a more competent rock layer. Periodic visual examinations and photographic surveys of the highwalls in all test areas revealed that as a result of these changes the amount of rockfall was reduced and the time before rockfalls began was increased. In the case of the quarry production blasts, a decoupled charge was used in the stemming zone of the blastholes that form the highwall. This was accomplished by using small diameter cartridged explosive in the stemming zone which promoted shearing between holes.
Minneappolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 384