Typically, the loss of explosive energy through stemming ejection reduces the performance of the blast. The fundamental theory promoting the use of stemming enhancement plugs is that they could potentially improve the effectiveness of stemming material in the blast hole. This would, in turn, better contain the explosives energy within the rock mass, and yield a more controlled and efficient blast event.
The three keys to efficient blasting are:
1. Energy level ? the amount of available energy in the explosive product
2. Energy distribution ? the optimised linear distribution of the explosives
3. Energy confinement ? the optimised burden to energy ratio
The optimum stemming column height is determined so it provides proper energy confinement while still allowing for maximum explosive energy distribution.
The measure of the potential effectiveness that the available explosives energy has to break and displace the rock mass is directly proportional to the effective burden that energy must overcome. This relationship is a crucial element in basic blast design. An accurate controlled sequence of hole detonation is a fundamental design parameter, having a major direct effect on overall blast performance. Any variation in hole detonation timing, results in that hole being fired prior to or after its nominal firing time. The hole-to-hole detonation could still remain properly sequenced or holes could potentially detonate totally out of sequence. This will result in burden to energy relationships that can have adverse impacts on the performance of a blast.
The results of these impacts have been witnessed in the past as:
? Poor rock fragmentation
? Large amounts of oversize
? High ground vibration levels
? High air blast levels
? Flyrock incidents
? High downstream processing costs
Prior to the introduction of the Vari-Stem stemming plugs at the Better Materials’ Rich Hill Quarry (now a subsidiary of Hanson) in Pennsylvania, a previous study was conducted to quantify benefits of electronic detonators over pyrotechnic initiation systems. This exhaustive study yielded clear evidence demonstrating the increased blast performance and reduced overall mining costs through the use of electronic initiation.
Following this study, Rich Hill Quarry has converted its stone production blasting operations from a pyrotechnic initiation to a programmable electronic system. The ongoing use of the electronic detonators at this site has yielded excellent blast fragmentation and high primary crusher throughput data.
The Rich Hill Quarry site was selected for this study because it has achieved a high level of blast optimisation through their drilling and blasting practices. One of the key factors resulting in the quarry’s blast optimisation achievements has been that the negative effects of timing inaccuracy have been totally eliminated, by using the Daveytronic electronic initiation system.
The testing procedures at the Rich Hill Quarry were designed to provide data to quantify the stemming plug’s performance with the following parameters:
? Rock fragmentation
? Crusher throughput
? Blast control
A series of five production blasts were monitored. The first two blasts were initiated without using the plugs to establish baseline data, and three blasts were detonated using the plugs.
A high level of field control was maintained during the drilling, blasting and data-collection processes, to ensure the integrity and accuracy of the data throughout the testing procedure.
Blasts were symmetrical to one another in terms of geometry and loading parameters.
Several of the blasts were also filmed using a Redlake high-speed digital video recorder. This was done at a rate of 500 frames per second. The camera lens was zoomed to the surface of the bench, directly above the first hole detonated in the face row of the blast. A surface electronic detonator was programmed, with the same firing time as the in-hole detonators of the explosive column below. This surface detonator was inserted into a box and placed directly above the blast hole collar.
The purpose of this test was to determine the amount of time elapsed (Dt) between the detonation and the vertical heave, and gas venting above the blasthole. Any increase in the Dt would indicate a high energy containment. This enabled the expanding gasses to penetrate deeper into the micro-fractures of the rock, increasing fragmentation, before the gas pressure head reduced with burden movement.
Following each blast, the much pile dimensions were documented and an optical fragmentation analysis was conducted throughout the excavation of the shot rock.
The primary crusher throughput was also monitored. Previous studies have demonstrated a direct correlation between improved fragmentation and increased productivity.
Throughout the study, the emphasis was on maintaining high field controls. These controls were monitored and applied as they related to bench preparation, pattern layout, drilling, blasthole loading and post-blast data collection.
The implemented blast design during these test blasts involved a 170mm blasthole drilled to a bench depth of 17-18 metres. Four rows of holes were drilled on a 4.5 metre-by-6m staggered pattern. To ensure proper toe burden dimensions, set-back markers were placed prior to the detonation of each blast to ensure the correct placement of the following blast’s face row holes.
The production blasts were loaded using an Iremix, 40 per cent emulsion blend. An 18kg high-energy toe load of SEC Sluran-600 was placed at the bottom of each hole prior to the bulk loading of emulsion. Before the blast hole loading, each of the holes were again measured to verify the correct depth and the presence of water. If water was encountered, the holes were pumped prior to the introduction of explosives.
The holes in the front row of each blast were stemmed with three metres of crushed rock. The holes in the second, third and fourth rows were stemmed with two metres of crushed rock. In the blasts that used the stemming plugs, 76-100mm of stemming material (drill cuttings) were loaded above the powder column before introducing the plug, followed by the crushed rock stemming to the top of the hole.
The explosive column rise was carefully monitored at each blast hole to ensure the proper explosive column height and the designed amount of stemming material.
The fragmentation data during this study was processed using a digital image analysis system. The images were gathered using a Sony TRV-900 digital video recorder and a Sony Mavica CD-400 high-resolution digital camera, transferred to disc, and loaded into the image processor for delineation and size distribution analysis. The digital images were gathered during the excavation procedures, at locations throughout the resulting muck piles, to ensure the merged findings would be representative of the true level of blast induced fragmentation. The images were obtained at the primary crusher as each truck load was emptied.
The review of the blasts’ video recordings verified that a large percentage of oversize rock originates from the cap rock, above the level to which the explosives could be safely or efficiently loaded to maintain proper confinement levels. This oversize material is directly related to the geologic conditions and blast geometry. The percentage of oversize in the post blast muck pile (2 per cent) is the same for the plugged and unplugged blasts. Therefore, the fragmentation analysis was concentrated to the rock fragment sizes within the muck pile produced by the blast.
During the analyses of the images, the data files were saved in the system and used to create a merged analysis report. This report is very representative of the size distribution and uniformity of each of the resulting muck piles during the testing procedures. The merged fragmentation data analysis shows that the post-blast muck piles of the test blasts using the stemming plugs were composed of a higher degree of fragmented rock with a uniform size distribution.
The merged analysis of the blasts resulted in a 27 per cent reduction in the average mean size of rock and a 26 per cent decrease in the D90 (90 per cent passing) screen size, from the combined no-plug result of 310mm to 228mm. There was also a 24 per cent decrease in the D75 size from 209mm to 158mm. These numbers can be related to reductions in excavation and crushing costs.
The high speed filming of the surface swell above the opening hole indicated that the stemming plugs effectively contained the expanding gasses, roughly three times longer than the non-plugged holes, using only crushed stone as the stemming material.
The primary performance parameter monitored in this study was the crusher throughput. During the shifts when the primary crusher was operating, records were kept regarding the source and the total tonnage of the stone delivered to the primary crusher during each shift.
The average throughput of blasts loaded without stemming plugs was 772 tonnes per hour. The average primary crusher throughput of stone from the Vari-Stem plugged blasts was 797 tonnes per hour, representing a 3 per cent increase in stone throughput at the primary crusher.
The Vari-Stem plug trials at Rich Hill Quarry resulted in performance benefits in terms of improved fragmentation and increased primary crusher productivity.
The findings provide evidence of the benefits of stemming plugs for improved blast performance, as represented by a 27 per cent decrease in the mean size of rock in the post-blast muck pile from 200mm to 145mm, and a 3 per cent increase in the tonnage throughput at the primary crusher.
Testing was conducted to highlight the potential productivity increase and downstream savings that stemming plugs could provide. Similar test results could possibly be obtained using the same testing procedures with pyrotechnic initiation systems instead of electronic detonators. However, the use of electronic detonators to eliminate timing scatter increased the reliability of the study’s findings.
Douglas Bartley is president of US-based DBA Consulting.