Combining Electronic Detonators with Stem Charges and Air Decks by R. Frank Chiappetta, MSc. P.Eng. Explosives Applications Engineer Blasting Analysis International, Inc. Allentown, Pennsylvania, U.S.A. Perth, Australia Drill and Blast 2010 October 12 - 14 , 2010 2010, Blasting Analysis International, Inc. All Rights Reserved. c
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Combining Electronic Detonators with Stem Charges and Air Decks
by
R. Frank Chiappetta, MSc. P.Eng.Explosives Applications Engineer
Stem charge quantity and placement must be fairly exact!
With electronic detonators, fire stem charge and main charge instantaneously.
With pyrotechnic detonators, always fire the stem charge first, before main charge.
The same in-hole pyrotechnic delays in the stem and main charges have too much scatter. If the main charge fires first, there is a risk that the stem charge could be ejected out with the top stemming.
Advantages of Using a Stem Charge
Decreased explosives per hole, but
Improves fragmentation 5 -10 fold or more in the
stemming zone.Doubling only the normal powder factor (without the use of a stem charge) will have no significant effect on the fragmentation in the collar zone. This was demonstrated with full scale test blasts in Chile to convince mine operators.
Mid column air deck results in a longer lasting pressure pulse on the surrounding rock.
Pressure pulse from a continuous explosive column load.
Time
Explosive deck
Explosive deck
Stemming
Air Deck - Rapidly expanding gasses collide in center of
air deckP
ress
ure
Effects of a mid-column air deck versus a full column load
Primers in each explosive deck must be placed equidistant from center of mid-column air deck.
Explosive deck
Explosive deck
Stemming
A single air deck placed anywhere in the explosive
column will:
Air Deck
Air Deck
Reduce ground vibrations and fines.
Bench Top
Floor
Subgrade
Unbroken Rock
End Charge Effects and Subgrade Drilling
P1
P2P2
On reflection at bottom of hole,
Pressure P2 = (2 – 7) x P1 due to combined effects of shock wave reflection at hole bottom and the immediate
gas pressure buildup.
Surface
Stemming
Explosive Column
Power Deck
1 m Air Deck
Coal No coal damage
Effect of Bottom Hole Air DeckReduces explosives, vibrations and fines.
Reduces/eliminates subgrade drilling.
Coal
Primer must be placed directly on top of air-deck to succeed in breaking to bottom of hole.
This is critical.
P1
P2P2
Surface
Stemming
Explosive Column
1 m Air Deck
Coal
Effect of Raising Primer Over Bottom Hole Air Deck
Coal
Solid, unbroken rockTargeted Floor
Shock wave has disappeared before reaching bottom of hole.
P2 is now less than P1, and also less than the compressive strength of the rock.
Poor fragmentation
Initial energy from primer and explosives migrating into the rock mass negates the bottom hole air
deck.
Surface
Coaxial cable to TDR VOD instrument
Coaxial cable to TDR VOD instrument
Stemming
Explosive
Subgrade
3-In (76 mm) diameter hole drilled from bench face to intersect bottom of hole.
Surface
Coaxial cable to TDR VOD instrument
Coaxial cable to TDR VOD instrument
Stemming
Explosive
No Subgrade
3-In (76 mm) diameter hole drilled from bench face to intersect bottom of hole.
3.3-ft (1 m) Air Deck
Power Deck Plug
Bottom primer 500 ms
Top backup primer 525 ms
Top backup primer 525 ms
Bottom primer 500 ms
Production Holes = 6½-in.
(165 mm)
Conventional Hole Load With Subgrade
Power Deck Plug at Bottom of Hole With No Subgrade
A B
(a)
(b)
(c)
Bottom Hole Air Deck Measurements.
Surface
Coaxial cable to TDR VOD instrument
Coaxial cable to TDR VOD instrument
Stemming
Explosive
No Subgrade
3-In (76 mm) diameter hole drilled from bench face to intersect bottom of hole.
3.3-ft (1 m) Air Deck
Power Deck Plug
Top backup primer 525 ms
Bottom primer 500 ms
Power Deck Plug at Bottom of Hole With No Subgrade
B
(a)
(b)
(c)
0.00
1.15
2.29
3.44
4.59
5.73D (m)
Primer
Bottom of hole
Gas front velocity through 3-in (76 mm)
hole = 1,500 ft/sec (457 m/s)
Shock wave velocity = 11,000 ft/sec (3354 m/s)
(a)
(b)
(c)
Typical Bottom Hole Air Deck Results from VODR System.
Courtesy of International Technologies and BAI.
1.12 2.29 4.86 6.72 8.59 10.46
Time (ms)
Hole Delay = 17 – 42 ms
Row Delay = 65 – 109 ms
Typical Delays with Conventional Non-Electric (Nonel) System
Vp = Compressional Wave (Sonic velocity of the rock)
Vs = Shear Wave Velocity.
Hole Delay Timing
Calculating Electronic Delay Time Between Holes
T = 0.6 (D/Vp) x 1000
Where:
T = Delay time between holes in a row (ms)
D = Distance between holes in a row (m)
Vp = Compression or sonic wave velocity (m/s)
Example Calculation
Assume hole spacing S = 7 m and Vp = 2800 m/s.
T = 0.6 (S/Vp) x 1000
T = 0.6 (7 m/2800 m/s) x 1000
T = 1.5 ms Future electronic detonator precision must be increased to 0.1 ms (100 us), because current electronic detonator timing can only be selected in increments of 1 ms. In this example, the choice is either 1 or 2 ms.
Vp & Vs are an important dynamic rock properties because they are a
direct function of:
• Young’s modulus (elasticity)• Poisson’s ratio (brittleness)• Rock Density (mass/unit volume)• RQD (Integrity of rock mass due to frequency of
discontinuities, joints, voids, etc.)
Increasing fragmentation with lower overall mining system
costs
Top stemming
Explosive column
500 ms
525 ms
0 ms
0 ms 0 ms
0 ms
0 ms
0 ms
Stem charge
1 m Air Decks
Combining Electronic Detonators, Air Decks & Stem Charges
0 ms
0 ms
Stem charge
1 m Air Decks
A B EDC
2009, Blasting Analysis International, Inc. All Rights Reserved
c
0 ms0 ms
Primary Objectives Were:
Improve fragmentation
Increase plant throughput
Required 87% of fragmentation @ ≤ 6 in.
Minimize vibrations on slopes
Copper Mine in Chile using 9 7/8 & 10 5/8 in holes.Case History
No. 1
Copper Mine in Chile
No explosives in collar.Represents 40 – 50%
of blast block.
Fragmentation here is OK
Subgrade
Explosive
Stemming
18 m (60 ft)
Normal energy distribution in a hole load resulted in excessive
oversize in collar.
Case History No. 1
Copper Mine in Chile
Stem charge 30 Kg
Mid-column air deck = 1 m
(3.3 ft)Subgrade
Explosive deck
Stemming
18 m (60 ft)
Mid-column air deck and stem charge provide a much better
energy distribution in blast block.
Explosive deck
Eliminated 90 – 95% of oversize in collar.Case History
Expl./delay increased 8-fold.Peak vibrations – Increased only 30%
This oversize came from
corner
Case History No. 9
After
Quarry 3 – Pennsylvania, USA
Expl./delay increased 8-fold.Peak vibrations – Increased only 30%Case History
No. 9
Hole delay = 2 ms Row delays = 100 - 300 ms
Stem chargeMid-column air deck
Quarry 4 – Pennsylvania, USA
Case History No. 10
Electronic detonators Mid-column air deck = 2 m
Explosives reduced 12 – 18%. No change in fragmentation.
Australia
Hole delay = 2 msRow delays 100 – 300 ms
Case History No. 11
Iron Ore
Digging rates increased 40 – 45%
No back break or back spill
Power trough in back of shot
Case History No. 11
AustraliaIron Ore
Deck delays = 0 ms Row delays = 100 - 300 ms
Stem chargesBottom hole air deck
Hole delay = 2 ms
Stab holes
2
4
6
8
10
12
14
16
18
20
H9 H10 H11 H12 H13 H14 H15 H16 H17 H18 H19
Powder factor = 0.60 Kg/m3 with stab holes. Hole diameter = 229 mm (9-in).Drill pattern = square or staggered
Section B Section C
Scale (Meters)
0 2 4 6 8 10
Stemming 5.0 m
Explosive column
Subgrade
2.0 m2.0 m
17.5 m
4.0 m
Stab hole
5.5 Kg 5.5 Kg
1.5 Kg
44 Kg 44 Kg
Target Elevation
Holes intersecting 5 m coal Holes intersecting 5 m and 2m coal
7.5
m
7.5
m
Coal
seam
Coal seam
Case History No. 13
Single Row Blast
Stemming
Explosive column
180 ft (55m)
500 ms
525 ms
525 ms
0 ms
0 ms
0 ms
0 ms
0 ms
Nonelectric Detonator Timing Electronic Detonator TimingHole Delay = 42 ms Hole Delay = 10 ms
Bottom Hole Initiation Multiple Point Initiation
A – Conventional loading and timing
Case History No. 13
Objective was to lower muck pile height for safety
Quarry 5 – Pennsylvania, USA
B – New loading and timing
Testing multiple point initiation
versus bottom hole initiation
Muckpile height of nonelectric blast.
Muckpile height of electronic blast.
A
B
Case History No. 13
Quarry 5 – Pennsylvania, USA Multiple point initiation and smaller hole delay (with same powder factor), results in greater cast and lower muck pile height.
In Conclusion – Improved BlastResults Depend on Combining:• Good drilling and field controls (Over 50%
of blasting problems).• Precise electronic detonators.• Stem Charges.• Very short delays between holes.• Long progressively increasing row delays.• Bottom & mid-column air decks.• Multiple point initiation within borehole.
Combining Electronic Detonators with Stem Charges and Air Decks
by
R. Frank Chiappetta, MSc. P.Eng.Explosives Applications Engineer