The 4 most costly blasting mistakes you can make

By |  April 9, 2021

3. Stiffness ratio

The stiffness ratio is one part key performance indicator, one part blast design tool. It is one of the most important parts of a blast.

In our No. 1 mistake – powder factor design – it was discussed that the geometry of the charge was one of the most important components for blast performance and blast design. The stiffness ratio is the metric used to determine the geometry of the charge.

In mistake No. 2 – key performance indicators – the violence factor was discussed as a KPI to monitor entire blast performance. I have examined more than 10,000 blasts, discovering that one of the major indicators of an appropriate violence factor was the stiffness ratio.

This stiffness ratio is simply a comparison of the true burden to the bench height. The reference class for stiffness ratio is typically 3 to 5. This produces the best performance and the lowest blasting costs.

Many sites have been trending to larger-diameter boreholes due to a decrease in cost. While the overall bulk explosive cost hardly changes, the initiator and drilling costs decrease as the borehole diameter increases. This is because as the borehole diameter increases, the burden increases in a linear fashion.

Still, the increase in burden ends up decreasing the stiffness ratio – often to a value of 2.5 to 2.0. Below 2.0, most blasts are uncontrollable and perform extremely poorly.

Now, I’ve mentioned that a stiffness ratio of 3.0 to 5.0 not only represents the best performance, but also the lowest cost. How can that be true if going to a larger-diameter hole increases burden and decreases cost?

This goes back to the No. 1 mistake of this article: powder factor design. Most sites use the same burden and spacing dimensions (i.e., if the burden is 10 ft., then the spacing is set at 10 ft. to achieve a certain powder factor). However, blasting literature (and practice) is clear that if the stiffness ratio is above 1.0, then the spacing should be larger than the burden.

A stiffness ratio of 4.0 or above results in no increases to spacing. The increase in spacing leads to a decrease in powder factor, as the stiffness ratio increases (above a 2.5) and also leads to a decrease in the drilling and initiators presenting the lowest blasting cost at a stiffness ratio of 3.0 to 5.0 – depending on site-specific costs. This is often completely ignored when comparing the size of drills.

The result of ignoring stiffness ratio is poor performance and high costs. When appropriate stiffness ratios are applied, then the blast has both good performance and low costs. Keeping stiffness ratio in mind and using appropriate spacings based on the stiffness ratio is how to overcome our No. 3 mistake.

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