Pit & Quarry, February 2019

The burden ratio of a blast can be used to help determine what an appropriate burden is compared to the hole or explosive diameter However the actual value of the burden will depend on a combination of the rock characteristics structural geology type of explosive and efficiency of the utilization of the explosive While the burden ratio can be monitored it is more critical to observe how variations of this ratio change the outcomes of the blast The burden ratio from 1963 to today typically ranges between 20 and 40 depending on the exact situation In the most basic form a blast that uses a burden ratio of 20 may experience significant throw and good fragmentation A blast that uses a burden ratio of 40 or more may experience some breakage typically into larger boulders and no movement This would be considered the maximum effective burden of a blast Richard Ash presented key ratios 56 years ago that could be applied throughout surface blasting conditions to give comparisons between multiple types of operations Typically a mine will not want to blast at the maximum effective burden because the rock will break into large boulders and be difficult to dig The blasters will then reduce the burden into a more normal range where proper movement and breakage occur In 1963 Ash acknowledged that many variables go into the burden and that the development of the burden ratio was to give the blaster an easy tool to design the burden in the field At the time burden formulas were complex and required knowledge of the rocks Youngs modulus and tensile strength as well as knowledge of the inter borehole pressures created by the explosive Not only did the blaster not have this information but in many cases the technical representatives and top explosive scientists of the time did not fully understand these Ash wanted a more appropriate and easy to use method to estimate burden that would actually be used Later Calvin Konya refined this method and developed easy to use equations that could be solved on the back of an envelope but keyed in the burden further while still scaling with the borehole diameter for bulk explosives and by explosive diameter for cartridge explosives Generally the use of a burden ratio of 30 typically provides the blaster with satisfactory results for the average field condition With the use of lower density explosives such as ANFO a burden ratio of between 20 and 25 was recommended Today the ratio of 24 is typically recommended for ANFO in average rock ALL OPERATIONS KS FREQUENCY KH FREQUENCY 00 09 0 10 13 0 10 19 43 14 17 5 20 29 70 18 21 13 30 39 56 22 25 51 40 49 45 26 29 74 50 59 22 30 33 66 60 69 22 34 37 44 70 79 11 38 41 20 80 89 4 42 45 7 90 99 2 46 49 4 100 109 8 50 53 0 110 119 0 120 129 1 Total 284 Total 284 Mean 30 Mean 40 Mode 38 Mode 26 Medium 29 Medium 34 ALL OPERATIONS BUT COAL STRIPPING KJ FREQUENCY KT FREQUENCY 01 019 0 02 029 6 03 039 12 00 009 15 04 049 18 01 019 18 05 059 18 02 029 27 06 069 25 03 039 26 07 079 19 04 049 25 08 089 13 05 059 2 09 099 6 06 069 6 10 109 14 07 079 2 11 119 7 08 089 0 12 129 7 13 139 3 14 149 2 15 159 2 Total 125 Total 152 Mean 028 Mean 074 Mode 024 Mode 065 Medium 027 Medium 067 A reproduction of Ashs table on standard blasting ratios for vertical holes pitandquarry com February 2019 PIT QUARRY 61