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Flyrock Reduction at Coal Mine Using Varistem® Stemming Plugs

Table of Contents

Varistem® Stemming Plugs Trial


  • The intent of the Varistem® trial at Coal Mine X was to retain blast energy so as to minimize flyrock, noise and airblast. The trial block is situated in close proximity to a neighboring farm that falls within the 500m blast radius.
  • Using actual drilling and charging data for individual holes, the Scaled Depth of Burial (SDoB) was calculated in order to understand what the expected levels of flyrock should be under normal circumstances (without using Varistem®).
  • The average SDoB for the trial block was 0.53 – which based on theory should have resulted in the vast majority of holes producing violent flyrock.
  • Contrary to the expected violent flyrock, the actual blast result showed excellent energy retention (in context of the very low SDoB value), with the furthest visible flyrock estimated to have reached no further than 100m away from the blast.
  • Given the challenging nature of the block (average hole depth of 2m and average stemming height of 1m, with drill chippings as stemming), the blast result is highly satisfactory and indicative of the Varistem® plugs offering great assistance in terms of mitigating violent flyrock.


Flyrock is a significant safety and environmental concern in coal mining, particularly when blasting occurs near inhabited areas. This study aims to evaluate the effectiveness of Varistem® stemming plugs in reducing flyrock at Coal Mine X. The trial was conducted near a neighboring farm within a 500m blast radius, presenting a challenging scenario due to the proximity of the blast. By utilizing actual drilling and charging data to calculate the Scaled Depth of Burial (SDoB), this trial sought to assess whether Varistem® could effectively minimize flyrock, noise, and airblast, thereby enhancing safety and operational efficiency.

Trial Setup 

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  • Number of holes: 218
  • Average drill depth: 2.08m [shortest hole 0.4m]
  • Average stemming height: 1.04m
  • Hole Diameter: 171mm
  • Average scaled depth of burial: 0.53
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Based on the scaled depth of burial values, the expectation would be that a large number of holes during this trial blast will produce violent flyrock. Note: SDoB values were calculated using actual charging and drilling data. For access to the calculation sheet, please contact us.

Results | Blast Progression-Block Movement

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Results | Flyrock Velocity And Travel Distance

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  • Based on the calculated SDoB values and the accompanying theory, violent flyrock from the  vast majority of holes in the trial block was expected.
  • Reviewing the footage shows, contrary to the expectation of violent flyrock, that there were relatively low levels of flyrock throughout the entirety of the block (with the furthest flyrock estimated to have reached approximately 100m away from the block.
  • Taking a closer look at the block footage and the mechanism of blast propagation for individual holes, it can be seen that the holes produced more of a “violent heave” than violent flyrock, with no flyrock of significant size or velocity visible.
  • Based on the above, it can be concluded that the use of the Varistem® plugs contributed to the reduction of flyrock, and effectively increased the SDoB values by retaining more energy inside the rock mass.
  • However, it must be noted that more definitive conclusions can be drawn if we are able to compare this trial blast result to a block with similar characteristics (not using Varistem®).
  • In summary, the team at ERG Industrial is highly satisfied with the trial outcome and believe that the objective was achieved and Varistem®’s capabilities demonstrated on a challenging blasting block.


The trial at Coal Mine X demonstrated the efficacy of Varistem® stemming plugs in significantly reducing flyrock, even under challenging conditions with a low average SDoB of 0.53. Contrary to theoretical expectations of violent flyrock, the blast results showed excellent energy retention, with flyrock reaching no further than 100m from the blast site. The use of Varistem® plugs not only mitigated violent flyrock but also contributed to a more controlled blast, producing a “violent heave” rather than dangerous flyrock. These findings suggest that Varistem® effectively increases SDoB values by retaining more energy within the rock mass, enhancing safety and operational outcomes. While further comparisons with non-Varistem® blocks are recommended for definitive conclusions, the results of this trial are promising and affirm the potential of Varistem® stemming plugs in improving blast safety and efficiency in coal mining.

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