Varistem® Stemming Plugs Trial – Cast Improvement Coal Overburden

Executive summary

• The intent of the trial was to demonstrate the energy retention capabilities of the Varistem® blast stemming plugs through conducting a split blast on one of Coal Mine X’s overburden production blocks, in order to determine the difference in cast gain between Varistem® and Non-Varistem® sides of the block.
• The Varistem® side of the block had a deeper and
• wider power trough.
• The cast analysis showed that the portion of the block that used Varistem® cast the material, on average, 87% further than the portion that did not use Varistem®.
• The blast outcome was satisfactory with Varistem®
• performing as expected.

Trial setup

Information and scaled depth

• Number of holes: 218
• Average drill depth: 12.38m
• Burden: 5.2m
• Spacing: 6.1m
• Geology: Sandstone
• Average stemming height: 2.5m
• Hole Diameter: 171mm
• Average scaled depth of burial: 0.9 (as calculated by
• ENAEX)

Results – block movement & stemming activity

Results – muckpile comparison

Results – cast analysis

The main focus of the trial was to establish the improvement of the cast by utilizing Varistem® Plugs. The analysis is achieved through the use of drones equipped with survey sensors:

• A surface, with geological and spatial data, is obtained of the block prior to the blast.
• A surface of the muckpile is
• obtained post blast
• The block is divided into area sections (polygons) as indicated (to the right)
• The two surfaces (pre and post blast) are then superimposed to calculate volume differences for a specific area/polygon.

Cast blast analysis – volume moved per

Cast blast analysis – percentage cast (%)

Results – cast analysis (%)

Results – cast Analysis – average muckpile profile

Results – velocity of detonation comparison

ENAEX Summary: Five drilled blast holes on the crest, initiated directly after one another from the initiation point, were identified and instrumented to measure the velocity of detonation of the Vertex A explosives type. In three of these holes stemming plugs were inserted and an average VOD of 5 774.00m/s was measured. The remaining two holes were stemmed with drill chips as usual. The VOD trace in the unplugged hole indicated deflagration which possibly could have been a result of column shift (as witnessed in borehole tracking results of hole #5). The geology around the instrumented hole was cracked and in-situ material badly damaged as a result of back break from the previous blast. The VOD results captured in this hole is not a good representative result for the given explosives type and geology.

Conclusions

Block movement & stemming Activity

From the visual analysis conducted, it appears that the holes on the Varistem® side of the block had a longer time-to-stemming-movement as compared to the non-Varistem® side of the block. This can be seen by comparing the visuals of the surface of the block at different points in time. Although the block still showed a number of ejections, no significant flyrock was present. The type and number of ejections was expected, based on the scaled depth of burial values.

Pre-blast to post-blast transition & muckpile comparison

When looking at the shape and peak of the muckpile following the blast, it is evident that the Varistem® side of the block had a deeper and wider power trough, and a muckpile peak that was further away from the highwall. In terms of fragmentation, visual observations reveal that the fragmentation was finer on the Varistem® side of the block.

Cast analysis

The cast analysis showed that the portion of the block that used Varistem® cast the material, on average, 87% further than the portion that did not use Varistem®. Although this is a significant increase, it must be noted that there were some differences in timing (due to the sensitive nature of the blast) that may have had an impact in the percentage of material cast by the non-Varistem® side. However, it is unlikely that this difference was wholly responsible for the entire difference of 87%, and it is likely that there would still have been a significant difference with a “normal” timing pattern.

Velocity of detonation comparison

A total of 5 holes were used for VOD measurements (3 x holes with Varistem®, 2 x holes without Varistem®). On average, the Varistem® holes had a VOD of 5 774.00 m/s, with a maximum VOD of 7 420.8 m/s. Of the 2 x non-Varistem® holes, only one registered during the blast, and this hole was deemed to be not representative due to a likely column shift. The 5 x holes selected for the VOD analysis were subject to adverse geology, and thus the conclusion was reached that the VOD results could not be deemed as representative.