Chapetta

New Blasting Technique to Eliminate Subgrade Drilling, Improve Fragmentation, Reduce Explosive Consumption and Lower Ground Vibrations

by

R. Frank Chiappetta Explosives Applications Engineer

BLASTING ANALYSIS INTERNATIONAL, INC. Allentown, Pennsylvania, USA

Abstract A new blasting technique has been developed by International Technologies, LLC. to eliminate subgrade drilling, lower ground vibrations, improve fragmentation and reduce explosive consumption. The new technique utilizes a uniquely designed borehole plug, with a bottom hole air deck and a predetermined stemming mass on top of the plug. This combination is referred to as the Power Deck�. Single holes and full scale tests were conducted by Blasting Analysis International, Inc. (BAI) to determine how the system works, the mechanisms behind its success and the best design parameters to use for the given rock conditions. Results have shown reduced or eliminated subgrade drilling, ground vibration reductions of up to 33%, reduced explosive consumption by 16 to 25%, and improved fragmentation by up to 25%. This paper summarizes the methodology, field test setups, analyzes procedures and results for one series of single hole and full-scale tests which were performed in a Pennsylvania, USA quarry. Introduction In today's challenging and competitive mining industry, mine operators are always striving to lower their total mining system costs and improve their bottom line. International Technologies, LLC. has assisted mine operators in achieving these objectives with the Power Deck� blasting system. While the use of air decks in boreholes is not new, the concept of merging a specially designed borehole plug with a predetermined stemming mass on top of the plug and a calculated bottom hole air deck with no subgrade is relatively new. Air decks were first used in 1940 in the former Soviet Union by Mel' Nikov(1) and by Marchenko (2) in 1954. Both parties reported improved fragmentation and increased burden movement with the air decks. Air decks were also introduced by Mel' Nikov(3) in the USA in 1961, but did not generate any overwhelming interest from the mining/explosives industry, other than their follow up use in some presplitting techniques for wall control. Chiappetta et al, (4) 1987 studied the use of air decks in full-scale blast environments and reported some very unusual and promising results when very small charges were combined with air decks. The results of some of these early air deck tests, in conjunction with the demand from coal operators to minimize coal damage and dilution, provided the "germination seed" for developing the Power Deck� blasting technique to its present day form.

1 of 27Copyright © 2004 International Society of Explosives Engineers

2004G Volume 2 - New Blasting Technique to Eliminate Subgrade Drilling, Improve Fragmentation, Reduce Explosive Consumption and Lower Ground Vibrations

Single Hole Characterization Tests Hole load cross sections for the three full-scale single hole tests are illustrated in Figures 1 to 3. All of the single holes were 6 1/4 diameter, drilled to average depths of 48 feet and used 12 feet of 1/4 to 1/2 inch crushed rock for the top stemming. The explosives used were a consistent combination of an emulsion/Anfo blend and Anfo. Burdens were maintained between 14 to 18 feet along the bench height. Figures 1 depicts how the holes were normally loaded at the quarry with a full column of explosives, 3 feet of subgrade and 12 feet of top stemming. The test hole shown in Figure 2 was loaded with a smaller column of explosives, 12 feet of top stemming, a 3 foot air deck at the hole bottom and no subgrade. In Figure 3, the hole load consisted of two equal lengths of explosive decks separated by a 3 foot mid-column air deck, 12 feet of top stemming, a 3 foot bottom hole air deck and also no subgrade. Here both the top and bottom explosive decks were fired simultaneously. A mid-column air deck separated by two explosive decks requires precise simultaneous detonation of the two explosive decks.

Figure 1 – Typical continuous explosive column load with subgrade drilling

Figure 2 – Continuous explosive load with a 3-foot bottom hole air deck and no subgrade.



Figure 3 – Two explosive decks separated by a 3-foot mid-column air deck, a bottom hole 3-foot air

deck and no subgrade.

Note that the total explosive quantity was reduced by 17% for the hole load with the bottom hole air deck (Figure 2), and by 25% for the double air deck hole (Figure 3), relative to the typical normal hole loads used at the quarry as shown in Figure 1. The purpose of the single hole tests was to:

- Establish the initial control measures by eliminating many of the blast design variables inherent in full scale shots.

- Verify that the total explosive and instrumentation systems functioned as expected

and,

- Verify that the Power Deck� functioned reliably as designed and remained in place in the borehole during loading operations.

But the most important aspect of monitoring the single holes was to quantify the velocity of the Power Deck� and the gas front velocity traveling through the bottom hole air decks. In order to achieve this, it was necessary to drill a smaller 3 inch cable hole, starting near the toe of the free face until the hole intersected the vertical 6 1/4 inch hole as illustrated in Figure 4. The aim was to have the small 3 inch holes break through to the bottom of the vertical holes and to the bottom of the 3 foot air decks.



Figure 4 – Example of a fully instrumented 6 1/4 inch diameter hole and

the smaller intersecting 3 inch cable hole. Full-Scale Blasts Two 30-hole full-scale shots were then monitored to evaluate the blast result differences between the normal hole loads as shown in Figure 1 and the hole loads as shown in Figure 2. The boreholes in the normal full-scale shot were drilled with a 3 to 4 foot subgrade and loaded with a full column of explosives. The boreholes in the Power Deck� shot were drilled with no subgrade, but loaded with 16% less explosive and 3 foot bottom hole air decks. The top stemming was maintained the same at 12 feet for both shots. Floor breakage, fragmentation, ground vibrations and the muckpile profiles were the main focus of interest in the results for the full-scale blasts. All of the other blast design variables for the normal and Power Deck� shots were kept constant. Figure 5 illustrates a plan view of the drill hole layout and the cumulative firing times used. The general design parameters for both full scale shots were; number of holes = 30, number of rows = 2, drill pattern (B x S) = 14 ft. x 16 ft., hole depth = 47 – 51 ft., and hole diameter = 6 1/4 in. Both shots were fired on the same bench, one behind the other, in order to eliminate any structural geology influences. Both shots were also instrumented to monitor the exact firing time of each hole and the resulting kinetics at the free face. Figure 6 illustrates one of the full-scale shots which has been fully instrumented and readied to be fired. It is important to note that prior to firing the two full-scale shots, the blast design parameters and bench face geometry were fully optimized.



Figure 5 – Plan view of a typical full-scale shot, illustrating the cumulative firing times.

Figure 6 – Example of a fully instrumented, 30-hole, full-scale shot ready to be fired.

Instrumentation Systems Used for Monitoring A number of sophisticated, state-of the-art blast instrumentation systems were used by BAI to monitor the single holes and full-scale shots. Instrumentation consisted of an HRS-1 video borehole inspection camera system, two VODR-1 systems, a MotionMeter 1000 high-speed video system, a LOCAM high-speed film camera, the SPLIT-Desktop� digital fragmentation analysis system, a conventional and laser surveying system, and a number of White Industrial Seismology's digital seismographs. Each monitored test hole was probed and recorded using the HR-1 Borehole Inspection system. Refer to Figures 7 to 10



Figure 7 – Portable borehole inspection camera system used to probe and document the condition of each

monitored test hole.

Figure 8 – Borehole camera is equipped with integral built-in lights which are adjustable for the lighting

intensity required. Different camera heads can accommodate inspections in hole diameters from 2 to 12 inch diameters in B/W or color, and up to 2,000 foot depths.

Figure 9 – A standard camcorder is used to record the video sequence for each inspected borehole for

archiving and later analysis



Figure 10 – Example of the inside borehole wall for one of the test holes.

The main purpose of inspecting each borehole was to assure the integrity and consistency of the rock. Factors such as major faults, discontinuities, wall slough offs and voids could drastically alter the results. The borehole inspections were also an excellent tool to verify any major changes in the structural geology within the borehole and blast block. Any test boreholes which differed drastically from the other comparison holes were discarded and new holes were drilled. Also, each single hole test was spaced at least 50 feet apart to assure that the rock mass was not affected by the effects of an adjacent detonated borehole. Both conventional and laser surveying were used on each test shot/blast setup. Conventional surveying was used to line up the small 3-inch horizontal hole to intersect the vertical 6 1/4 inch face hole. Refer to Figure 4. This was accomplished by setting up a theodolite on the floor of the quarry and shooting the center of a plumbed survey rod which was placed over the center of the 6 1/4 inch vertical face hole. Once the survey rod was leveled and in-site, a vertical line was brought down to the toe near the floor level, and a collar point was established. Because it was difficult to establish a collar point on the face right at the floor level, the small intersecting hole was actually started 2 to 3 feet above the floor level. Trigonometric calculations were then performed to assure that the small intersecting hole broke through to the bottom of the vertical 6 1/4 inch hole, given the toe burden, hole depth, slope distance and how high the collar of the small intersecting hole was above the floor elevation. Refer to Figures 11 to 14.



Figure 11 – Field setup showing how the collar of the small 3-inch intersecting hole was established to break

through precisely to the bottom of the 6 1/4 inch vertical hole.

Figure 12 – Survey rod was placed and stabilized over the center of the 6 1/4 inch diameter vertical hole to

establish a vertical control line.

Figure 13 – Plumbed survey rod. It was essential to have the survey rod plumbed accurately before a

reference line could be shot and transferred down to the toe.



Figure 14 – Small intersecting hole is collared 2 to 3 feet above floor elevation and angled down appropriately

to intersect the bottom of the 6 1/4 inch vertical hole.

Laser surveying was performed to establish the true burdens along the bench face from the crest to the toe. This was required in order to normalize the data results, since burdens in some areas along the bench face varied by up to 4 feet. Refer to Figure 15.

Figure 15 – Laser surveying was used to establish true burdens along the face from the crest to the toe.

Laser surveying was also used to establish where the face markers were placed on the bench face relative to the vertical hole, bench height and true burdens. Face markers were used to accommodate the high-speed video camera for tracking purposes. Refer to Figure 16.



Figure 16 – Laser surveying was also used to establish the coordinates of the face markers relative to the

vertical hole, bench height and true burdens.

A high-speed video camera, high-speed 16 mm film camera and a standard digital camcorder were used on each test shot to quantify the shot dynamics, throw and the delay timing. Refer to Figure 17.

Figure 17 – High-speed video camera, high-speed 16 mm film camera and camcorder field setup.

In order to analyze full-scale blasts accurately, both dimensional and time controls needed to be established for each test shot. For 2-D analysis, a minimum of four control points with known coordinates forming a distinct quadrilateral must be available in the field of view. This allows one to calculate 8 calibration constants with a system of linear equations in 8 unknowns. The calibration constants correct for vertical, horizontal and elevation coordinates of the camera's location in relation to the face markers; automatically adjusts for the zoom focal length, lens aberration and screen curvature; corrects for motion towards or away from the optic axis; and also accommodates for any ground vibration disturbances in the recorded field of view. The



analytical software (MotionTracker™-2D) was developed by BAI and it is the most accurate and easiest to use on the market today. Refer to Figure 18. Those in the industry who use only a single vertical and horizontal scale for high-speed motion analysis can end up with large cumulative errors which can lead to erroneous conclusions.

Figure 18 – Placement of bench face markers for tracking purposes and how the four control points were

established for accurate motion analysis.

Each bench face marker which was used for tracking purposes was hung over the face with an independent separate line. Refer to Figures 19 and 20.

Figure 19 – Bench face markers were hung down the face for tracking purposes and to measure the initial

velocity, ejection angle, TMIN (rock response time) and cast distance.



Figure 20 – Illustrates the separate lines for each bench face marker that were hung over the face. All of the lines are then bunched together and wrapped around a surface detonator with the same firing time as the in-

hole delay. When the hole goes off, all of the lines holding up the markers are simultaneously cut, so that during motion the markers move freely and do not move as a pendulum fixed at one end (i.e., at the rock).

The second thing required for motion analysis is time control. This was established by tying shock tube to the bottom hole primer of the control hole, brought up through the explosive column and the stemming, and thrown over the face. All of the other holes had the shock tube placed on a stake in the shape of a coil. Refer to Figures 4, 6 and 21.

Figure 21 – Close-up of how the shock tube was wrapped in a coil and placed on a stake which was located at or near the hole. This setup allows accurate measurements of the firing times of each hole, when the length of

shock tube and VOD of the explosive are taken into account.



VOD measurements were obtained using two VODR-1 systems which are TDR (Time-Domain Reflectometry) systems. Refer to Figure 22.

Figure 22 - The VODR-1 System ( a TDR based VOD instrument) was used for all measurements. A special foam dielectric coaxial cable was used to measure the Power Deck� speed and gas front velocity in the air

decks. Standard RG-58 coaxial cable was used to measure the VOD of the explosive columns.

Although BAI had the choice of using the SLIFER, resistance wire or fiber-optic based VOD systems, the TDR based VODR-1 system was selected for its reliability and flexibility in obtaining field measurements. Other VOD systems, (particularly the resistance wire systems), were found unreliable, since they tended to generate more questions than answers. This was especially true for measurements involving wet hole conditions, air decks, low order detonations and deflagrations. In addition, the other VOD systems did not provide reliable disturbance velocities through stemming or air decks. The most critical measurements were in determining the velocity of the Power Deck� plug and the gas front traveling through the bottom and mid-column air decks. For this purpose, one of the VODR-1 systems used only the FSJ1-50A coaxial cable. This is a special foam dielectric coaxial cable with a low crush threshold, which can measure any disturbance front in a borehole down to as low as 300 ft/sec. A typical field setup to achieve this is illustrated in Figure 4. By measuring the velocity of the Power Deck� plug, the KE (Kinetic Energy) impacting the bottom of the hole could be reliably and easily calculated. To our knowledge, this is the first time a shock front velocity was measured through a bottom hole air deck in a full scale blast environment. Vibration/airblast measurements were obtained with 4-channel, full waveform, digital seismographs. A linear seismic array consisting of 4 seismographs was placed behind each test shot at distances varying from approximately 200 to 2,000 feet. Refer to Figures 23 and 24. This was necessary to establish the site-specific attenuation characteristics of the vibration and airblast from the single hole and full-scale shots. It was also very important for the single hole signature analysis when coupling air decks with the use of precise electronic detonators.



Figure 23 – Seismic array line in relation to the test shot area. The seismograph placed at the 2,000 foot

distance was in the middle of a small residential subdivision.

Figure 24 – Two types of digital seismographs were used. The close-in geophones to the test shots had a sand

bag placed on top of them to assure good coupling with the ground, and to minimize any airblast disturbances which could affect the geophone.

Once the seismic data were analyzed, it was normalized in order to compare the normal shot against the Power Deck� shot. Digital fragmentation analysis was performed on both full-scale shots with the Split-Desktop® Software. A total of 32 to 36 digital images were taken of each muckpile to assure statistical significance when comparing the results from the normal shot to the Power Deck� shot. In spite of what all fragmentation analysis software developers claim regarding the accuracy of their software, digital fragmentation analysis can be highly subjective, and is prone to numerous experimental, systematic, sampling and analyses errors. Unless extreme measures are taken to keep the analysis parameters consistent, particularly in the sampling technique, one could easily skew the analysis to generate any results so desired.



Thus in this series of tests, over 2 1/2 months were taken to calibrate the fragmentation analysis system in order to minimize or eliminate the inherent cumulative errors. For each muckpile, the following analytical procedures were implemented for consistency. Oversize generally results from the stemming area and ends up on the top of the muckpile. The thickness of the oversize in reference to the cross-sectional dig height of the muckpile in our tests ranged from approximately 10 to 25%. For example, at the highest muckpile height, 25% of the total muckpile images were taken from on top of the muckpile, and the remaining 75% were taken over 4 cross-sectional digs throughout the muckpile from the beginning to the end of the digging cycle. As the muckpile height dropped, the ratio of images taken from the top to the cross-sectional images was adjusted accordingly. Refer to Figures 25 to 26.

Figure 25 – Sampling at the highest part of the muckpile used 25% of the digital images from on top of the muckpile, and the remaining 75% were taken over 4 cross-sections throughout the muckpile. This ratio of

images was adjusted accordingly for lower muckpile heights.

Figure 26 – Dimensional markers were strategically placed on top of the muckpile on a surveyed grid system.



Extreme care was taken in avoiding duplicating the rock images, particularly along the top of the muckpile and along the quarry floor, (i.e., toe of the muckpile). During digging, it was inevitable that the oversize material from on top of the muckpile would roll down and collect along the floor. Thus, all of the cross-sectional images were strategically collected to avoid duplication counts of the oversize pieces. Refer to Figure 27.

Figure 27 – Cross-sectional sampling technique used to avoid oversize duplication counts.

Prior to any of the fragmentation analysis, numerous tests were conducted on a number of known but different fragment sizes of processed rock samples. The purpose was to determine the best dimensional control needed for measuring the rock fragmentation of interest in the muckpile. Refer to Figures 28 and 29.

Figure 28 – Example of poor rock size resolution for the wide angle field of view in relation to the large

dimensional control size.



Figure 29 – Example of good rock size resolution for the close-up view in relation to the dimensional control

size. Here the white circular dimensional control was an 18 inch disk and the known rock size was 4 to 5 inches. Fragmentation analysis on this frame produced a P50 passing size of 4.4 inches with a very narrow

(i.e., steep) cumulative percent passing curve.

Based on the fragmentation calibration tests, it was determined that the length/width of each image view should be somewhere between 5 to 12 times the size of the dimensional controls. Given this criteria, the field of view per image was limited to approximately 7 to 18 feet. This approach worked very well for the expected rock size distribution which was the feed to the primary crushers. An example of this sampling technique is illustrated in Figure 30, and the accompanying rock size analysis is shown in Figure 31.

Figure 30 – Illustrates the sampling technique which was used to achieve the correct ratios of the image field

of view to the dimensional control (disc size) and desired rock size resolution.



Figure 31– Example analysis of the unedited rock image shown in Figure 30

Another thing that was seriously considered before any of the fragmentation measurements was the rock shadows from sunlight. To minimize this error, all digital images of the muckpiles were always taken at the same time of the day between 3:00 to 4:00 P.M. Once all of the individual rock images were analyzed, they were merged together to develop a histogram and cumulative percent passing curve for the entire muckpile. The circular disc which was used in each rock image was not counted in the fragmentation analysis. Single Hole Results The blast results from the three single holes illustrated in Figures 1 to 3 showed no significant differences in terms of the fragmentation, breakage to the quarry floor, breakage in the collar area, the degree of muckpile throw and for the muckpile shape. This was quite significant in view of the fact that the holes containing the air decks used 17% and 25% less explosive with no subdrilling. Figure 32 illustrates a typical velocity record as a displacement versus time plot for the Power Deck� plug and gas front velocity traveling through a typical 3 foot bottom hole air deck in the 6 1/4 inch diameter vertical hole, and then continuing on through the smaller 3 inch intersecting bench face hole. In this case, the Power Deck� plug velocity was 11,000 ft./sec., and the expanding gas front through the smaller 3 inch intersecting bench face hole was approximately 1,500 ft./sec. Given an explosive type, borehole diameter, rock type in terms of its integrity and strength, air deck length, and the amount of stemming mass on top of the plug, the plug velocity (or gas front velocity) through the bottom hole air deck can be made to vary from approximately 1,000 to 12,000 ft./sec. The gas front velocity through the smaller 3 inch bench face hole can vary from just under 1,000 to 4,000 ft./sec. To our knowledge, this is the first time anyone has made these types of measurements in a full-scale blast environment. The significance of these measurements will be explained later.



Figure 32 - Displacement versus time graph. Recording starts at the primer which was located on top of the Power Deck� plug (0.0 feet), progresses through the 3-foot bottom hole air deck and continues on through

the smaller 3-inch intersecting face hole. Refer to the hole load shown in Figure 2.

Fragmentation Results Fragmentation from the two full-scale shots in terms of the standard cumulative percent passing sizes are listed in Table 1.

TABLE 1 – Fragmentation Results for the Normal and Power Deck� Shots

Normal Shot

Power Deck�

Shot

Percent Reduction with the Power Deck�

Shot

Number of Combined Images 32 37 Minimum Size Measured 2.50 in 2.10 in –– P20 size 2.86 in 2.17 in 24% P50 size 6.53 in 4.90 in 25% P80 size 11.33 in 8.97 in 21% Top size measured 25.13 in 24.86 in ––



The greatest significant difference in the fragment size distribution was found in the P20, P50 and P80 passing sizes. In all cases, the Power Deck� shot resulted in a fragment size reduction of approximately 24% for the P20 passing size; 25% for the P50 passing size; and 21% for the P80 passing size. Thus the fragment size distribution was reduced substantially for the Power Deck� shot. No significant difference was found in both shots for the larger size range between 24 to 25 inches, because the top size was heavily dictated by the major structural joint system. For our sampling setup, the digital fragmentation system was insufficient to resolve the fines content below 2 inches. Ground Vibration Results Ground vibration results comparing the Normal and Power Deck� shots are illustrated in Figure 33 as a plot of particle velocity versus scaled distance. Scaled distance here is defined as the distance divided by the square root of the maximum amount of explosives per delay. This plot is a very good way to normalize the data for comparison purposes since the distances from the test shots to the seismograph locations, and the maximum weight of the explosives varied slightly. Seismic locations from the shots varied from 200 to 2,000 feet in a linear array. Vibration amplitudes were reduced by an average of 33% for all locations, given a distance and maximum weight of explosives per delay for the Power Deck� shot. A 33% reduction in the vibration amplitudes is quite significant. Also, the Power Deck� shot did not trigger the seismograph which was stationed farthest from the shot at 2,000 feet away, while the Normal shot did.

Figure 33 – Peak Particle Velocity versus Scale Distance graph for the two full-scale shots.



Muckpile Displacement The muckpile shapes and cast distances were measured for both the normal and Power Deck� shots. The comparison results are illustrated in Figure 34. The normal shot spread the muckpile over a distance of 300 feet, and the Power Deck� hot spread the muckpile over a distance of 280 feet. Basically, there was no significant difference in the muckpile throw between the normal and Power Deck� shots even though the Power Deck� shot used 16% less explosives per hole.

Figure 34 – Muckpile profiles for both full-scale shots.

The center of gravity for each muckpile was basically the same at approximately 80 feet. Although the muckpile profiles varied slightly, the maximum height of each muckpile was basically the same, although the Power Deck� shot produced a little more of a power trough against the highwall. Level of Quarry Floor Both the normal and Power Deck� shots resulted in flat floors with no significant differences. But, it was very significant that the Power Deck� shot used no subgrade (i.e., 3 to 4 feet less drilling per hole) and 16% less explosives in each hole. In addition, the Power Deck� shot did not disrupt the collar zone of the next underlying bench.



The Power Deck� System Design and How it Works The Power Deck� concept was developed by International Technologies, LLC. over three years ago. Based on many field trials and feedback from around the world, it has undergone many unique modifications to its present day design, which is illustrated in Figure 35. The Power Deck� plug is very simple, easy and quick to use in any hole ranging from 2 1/4 to 12 inch diameters. For underground use, the principle of operation is the same, but the Power Deck� plug design is somewhat different. Basically the Power Deck� plug is attached to a precut length of wooden stick that defines the bottom hole air deck length, and a small amount of drill cuttings or crushed rock is placed into the Power Deck� plug 's specially designed holding chamber. This assembly is dropped into the borehole without any other accessories as shown in Figure 35. Unlike airbags which possess extremely high buoyancy, the Power Deck� plug works equally well in wet or dry holes. Once dropped into a borehole, an appropriate calculated amount of stemming is placed on top of the Power Deck� plug to satisfy the blasting objectives. Refer to Figures 36 and 37.

Figure 35 – Power Deck� plug design and assembly prior to loading in a borehole.

Figure 36 – Power Deck� plug when dropped down a borehole. A precut 3-foot wooden stake in conjunction

with the Power Deck� plug design holds the assembly in place.



Figure 37 – An appropriate amount of stemming mass (drill cuttings or crushed rock) is placed on top of the

plug prior to explosives loading. The stemming mass is selected based on the blasting objectives and how much pressure or KE is required at the hole bottom.

What happens at the bottom of a hole with the Power Deck� system can be explained in terms of the pressure and/or Kinetic Energy. When an explosive detonates in a borehole, the high temperature by-products of the detonation will always take the path of least resistance. The bottom hole air deck will first be subjected to an intense shock wave traveling through it. When the initial shock wave front hits the bottom of the hole, the shock wave speed decreases, reflects from the hole bottom and increases the pressure at that point. At this instant of time, a separate secondary impact from the explosion products adds another impulse to the bottom of the hole. The combined effect is that the resulting pressure P2 at the hole bottom can be increased from 2 to 7 times relative the initial pressure, P1. . Refer to Figure 38. The increased point source pressure is sufficient to create a planar split and fragmentation at the hole bottom. In essence, the sum of the primary shock wave energy and secondary explosion products are much more efficient than a concentrated continuous cylindrical charge at the hole bottom, but only when the bottom hole air deck length and Power Deck� plug mass are properly designed for the given rock conditions and explosives system.



Figure 38 – Pressure increase illustration of what happens at the bottom of a borehole with an air deck.

When non-electric shock tube was first introduced into the industry in the 1970's, the same mechanisms in the air gap effect caused the crimp end of the shock tube which was adjacent to the detonator to blow out, causing premature detonations on many shots. Here the side blowouts bypassed the delay element by prematurely detonating the primer. Today this problem is non-existent, but the failure mechanism in the early years was identical to the effects which occur in a bottom hole air deck. Refer to Figure 39.

Figure 39 – Pressure increase illustration of what happens at the crimp end or at the shock tube/detonator

interface for early nonelectric shock tube systems. The failure mechanisms are similar to what happens in a bottom hole air deck.



Another way to look at this phenomena is in terms of the KE (Kinetic Energy) which is imparted at the hole bottom. Refer to Figure 40. The KE can be calculated by measuring the mass of the stemming which is placed on top of the Power Deck� plug and by measuring its velocity through the bottom hole air deck. This is why it was important to have drilled the small 3-inch intersecting face hole in the single hole tests. This allowed measurement of the shock front velocity through the bottom hole air deck. The KE at the hole bottom can be in the order of 50 to over 100 times greater with the correct selection of the Power Deck� plug mass and correct air deck length.

Figure 40 – Kinetic Energy increase illustration of what happens at the bottom of a borehole with an air deck.

Because this new air deck blasting technique allows one to vary and control the intensity of the pressure or KE at the hole bottom as needed, it can be used in very soft to very hard rocks, including steeply dipping formations. To date, the Power Deck� system has been used all over the world in a variety of diverse field conditions with over a 95% success rate. The proper Power Deck� plug mass and air deck length is dependent on the type of explosive, borehole diameter and rock strength. Failure to select the correct design parameters can lead to very poor blast results, and thus a representative to advise with the initial starting design selection is highly recommended. Once correctly implemented, the system can reap tremendous financial benefits, as many users worldwide have already realized.



Conclusions

1. No significant differences were found with the single hole characterization tests in terms of the fragmentation, muckpile throw and breakage at the toes or at the collars. But the two single holes with the single and double air decks achieved the same results with no subgrade and with 17 to 25% less explosives, respectively. This implies that both air deck loaded holes with less explosives and no subgrade were considerably more efficient than a continuous column of more explosives with subgrade.

2. In reference to the Normal and Power Deck� full-scale blasts, the Power Deck� shot

resulted in 33% less vibrations at all of the monitored locations from 200 to 2,000 feet away. This reduction allows mine operators to maintain a higher compliance safety margin or allows larger quantities of explosive per delay while maintaining the same vibration level. In other parts of the world, ground vibrations were successfully reduced from 10 to 75%.

3. In reference to the fragmentation, the Power Deck� shot resulted in a 21 to 25%

improvement between the P20 to P80 cumulative percent passing sizes. This can result in substantial savings in utility costs, less wear and tear on the crusher linings and increased throughput. No significant differences were found between the two full-scale shots in the larger fragment sizes over 24 inches, because this was heavily influenced by the major structural joint systems in the rock mass.

4. In reference to the quarry floor, no significant differences were found between the

Normal and Power Deck� shots. Both shots achieved equivalent flat floors. The muckpile shape and maximum throw distance for both shots were essentially the same. The fact that the Power Deck� shot used 16% less total explosives and no subgrade drilling (3 to 4 feet less per hole) clearly indicates that the Power Deck� shot was considerably more efficient than the Normal shot.

5. For a given rock strength, explosive and borehole diameter, the total pressure at the

bottom of a hole with an air deck can be controlled with the Power Deck� system to vary from 2 to 7 times greater than that created by a full column of explosives; but only when the proper plug mass and air deck length are properly calculated. The Kinetic Energy imparted at the bottom of a hole for the same conditions can be varied from approximately 50 to 100 times greater. This allows the new blasting system to be used in very soft to very hard rock formations.

6. Since this series of tests was completed 2 1/2 years ago, mine operators around the world

have realized substantial cost savings and productivity gains. Subgrade drilling has either been completely eliminated or substantially reduced, thus lowering the drilling costs and increasing the drill availability. Another significant advantage when eliminating the subgrade is that the collar zone of the next underlying bench remains completely intact. By leaving the collar intact, the risk of flyrock, airblast and dust is proportionately decreased or eliminated, allowing operators to more easily and confidently comply with any prevailing environmental regulations. Because the



subgrade can be eliminated, mine operators can target the bottom hole breakage to a specific mineralization zone (such as in coal) without damaging or diluting it.

7. The Power Deck� blasting system has been developed for mine operators to reduce

ground vibrations, improve fragmentation, eliminate subdrilling and reduce explosive consumption. In one South American mine, the explosive consumption has now been reduced by up to 50% without any significant changes in the overall blast results. But the average explosive reduction is usually between 10 to 40%.

8. Since completion of this field research, the Power Deck� system has been successfully

applied to the bottom, mid column and at the top of the same borehole. Field evaluations are now continuing with the use of multiple air decks within a single explosive column, precise electronic detonators and small strategically placed charges. Preliminary tests have shown that when the Power Deck� system is used with electronic detonators, substantial cost savings will be realized.

Acknowledgements The author is very grateful for the field assistance, cooperation and advice received from Eastern Industries, Inc. – Ormrod Quarry management and personnel; Maurer & Scott Sales, Inc.; J. Roy, Inc.; Power Deck Company; and International Technologies, LLC. This work was performed by Blasting Analysis International, Inc. (BAI) under a research contract from International Technologies, LLC. R. Frank Chiappetta is an Explosives Applications Engineer and International Blasting Consultant with Blasting Analysis International, Inc., Allentown, PA, USA. References

1. MEL'NIKOV, N.V, Utilization of Energy of Explosives and Fragment Size of Rock in Blasting Operations, Gorn, Zh., No. 5 (1940).

2. Marchenko, L.N., Increasing the Energy Utilization Factor of Explosives in Ejection Blasting, Tr. IGD Akad. Nauk SSSR, 1, Moscow (1954).

3. MEL'NIKOV, N.V., Influence of Explosive Charge Design on Results of Blasting, Int'l Symposium on Mining Research, Proceedings Volume 1, pp 147 - 155, Pergamon Press (1961).

4. Chiappetta, R.F. and Mammele, M.E., Analytical High-Speed Photography to Evaluate Air Decks, Stemming Retention and Gas Confinement in Presplitting, Reclamation and Gross Motion Applications, Proc. Second International Symposium on Rock Fragmentation by Blasting, pp 257 - 301, Published by Society of Experimental Mechanics USA (1987).



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