frank chiappetta

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, AustraliaDrill and Blast 2010

October 12 - 14 , 2010

2010, Blasting Analysis International, Inc. All Rights Reserved.

c

What are the 3 most important things in blasting?

1 ) Drilling Controls

2 ) Drilling Controls

3 ) Drilling Controls

Mine

• Explosive energy• Energy distribution in rock mass• Timed and controlled release of energy

Mill

1

For illustrative purposes only

Major Sources of Oversize

6

1

2

3

4

5

Stemming

Explosive column

Subgrade

Bench top

Collar / Top Stemming

Free Face

Toe & Subgrade

Corners & Irregular Bench

Intact Massive Seams

Imbedded Conglomerate Boulders

13

2

4

5

8

6

7

9

10

11

12

Pit floor

Stemming volume can be 20 – 50% of blast volume

1

Shot Muck Pile

Original collar or top stemming

Oversize and Poor Fragmentation

Excellent Fragmentation

Shot Muck Pile

Oversize from collar (top stemming zone)

Boulders require secondary drilling and

blasting

Shot Muck Pile

Oversize from collar (top stemming zone)

Purposely Masked

Multiple dozers skim oversize and push it over bench face for shovels to dig – Tremendous re-handling of oversize!

Shot Muck Pile

Purposely masked

Shovels re-handle oversize for loading into haulage trucks!

Shot Muck Pile

Oversize Galore!

Normal Stemming

Explosive column

Decreased Stemming

Decreased Stemming

But, there are definite limits as to how much the top stemming can be reduced!

Decreasing stemming is often used to increase fragmentation in the collar zone (top stemming).

Uncontrolled Shot - Severe Flyrock & AirblastTop stemming too small

Unintentional Flyrock Damage

Secondary BreakageDrilling & Blasting Shaped Charge

Impact Hammer Drop Weight

Special Drop Weight

Special Drop Weight

Special Drop Weight

Secondary Blasting

Extreme Secondary Blasting

SD - Scaled Depth of Burial Calculations

SD = D/W1/3

D = SD * W1/3

W = ( D/SD)3

W = Mass of explosives equivalent to 10 explosive diameters.

D = Distance from surface to center of stem charge.

SD = Selected Scaled Depth of Burial.

D

W

Stemming

Length of W D = Distance from surface to center of W

W = Weight of explosive occupied in top of explosive column, equivalent to the length of 10 borehole or explosive diameters

Surface

L

If borehole diameter is less than 4-in (102 mm), use 8 explosive diameters to calculate W.

D

W

Stemming6.0 m (19.6 ft)

= 2.70 m (8.9 ft)

Length of W D = Distance from surface to center of W

W = Weight of explosive occupied in top of explosive column, equivalent to the length of 10 borehole or explosive diameters

Borehole diameter = 270 mm (10 5/8 in)

Explosive density = 1.27 g/cc

Surface

Imperial Units Metric Units

Explosive density = 1.27 g/ccExplosive diameter = 10.625 in

Stemming = 19.7 ft

Length of 10 borehole diameters = (10.625 in/12 in) x 10 = 8.9 ft

One linear foot of explosives at a density of 1.27 g/cc in a 10.625 in hole weighs 50.3 lbs.

Thus, W = 8.9 x 50.3 = 445 lbs

W1/3 = 4451/3 = 7.62 lbs1/3

D = Stemming + ½ (L)

SD =DW1/3

24.2

7.6= 3.18=

and

Explosive density = 1.27 g/ccExplosive diameter = 270 mm

Stemming = 6.0 m

Length of 10 borehole diameters = (270 mm/1000 mm) x 10 = 2.7 m

One linear meter of explosives at a density of 1.27 g/cc in a 270 mm hole weighs 72.5 Kg.

Thus, W = 6.0 x 72.5 = 201 Kg

W1/3 = 2011/3 = 5.84 Kg1/3

SD =D

W1/3

7.35

5.84= 1.26=

and

= 19.6 + ½ (8.9) = 24.2

D = Stemming + ½ (L)

= 6.0 + ½ (2.70) = 7.35

Alternatively, D = SD x W1/3 and W = (D/SD)3

L

© 1990,Blasting Analysis International, Inc.All Rights Reserved

SD = D/W1/3

Significance of SD value is illustrated in next slide.

SD = 0 – 0.600.64 – 0.88

0.92 – 1.40

1.44 – 1.80

1.84 – 2.402.40 +

Metric Units(m/Kg1/3)

SD = 0 – 1.5 1.6 – 2.22.3 – 3.5

3.6 – 4.5

4.6 – 6.06.0 +

Imperial Units

(ft/lb1/3)

Significance of SD (Scaled Depth of Burial)© 1990, 2008 Blasting Analysis International, Inc.

All Rights Reserved

No fallback

Some fallback

Uncontrolled Energy

Violent flyrock, airblast, noise and dust.

Very fine fragmentation.

Good craters.

Controlled EnergyGood fragmentation.

Maximum volume of broken rock in collar zone.

Acceptable vibration/airblast.

Good heave and muck pile mound.

Larger fragmentation.

Reduced volume of broken rock in collar zone.

Acceptable vibration/airblast.

Reduced heave and muck pile mound.

No flyrock.

Very Controlled Energy

Small surface disturbance

Insignificant surface effects

Minimal Surface Effects

No breakage

zone

SD - Scaled Depth of Burial equations can be used to calculate:

Top stemming

Stab charge quantity

Stem charge quantity

Top Stemming = (SD x (Ø 3 x ρ /127500)1/3) – (Ø /200)

Where: SD = Scaled depth of burial (Kg/m1/3)Ø = Explosive diameter (mm) ρ = Explosive density (g/cc)

SD = 0.92 – 1.40 m/Kg1/3

Example Calculation

Hole diameter = 229 mm

Explosive density = 1.25 g/cc

Top stemming

Metric Units

Top Stemming when SD is selected as 1.2

= (1.2 x (2293 x 1.25/127500)1/3) – (229/200)

= (1.2 x (12008989 x 1.25/127500)1/3) – (1.15)

= (1.2 x (117.74)1/3) – (1.15)

= (1.2 x 4.89) – (1.15)

= (5.87) – (1.15)

= 4.72 5 m

= (SD x (Ø3 x ρ /127500)1/3) – (Ø /200)

Imperial Units

Top Stemming = (SD x (Ø3 x ρ x 0.284)1/3) – (0.417 x Ø)

Where: SD = Scaled depth of burial (lb/ft1/3)

Ø = Explosive diameter (in) ρ = Explosive density (g/cc)

SD = 2.3 – 3.5 ft/lb1/3

Example Calculation

Hole diameter = 9 inExplosive density = 1.25 g/cc

Top Stemming when SD is selected as 3.0

= (3.0 x (93 x 1.25 x 0.284)1/3) – (0.417 x 9)

= (3.0 x (729 x 1.25 x 0.284)1/3) – (3.75)

= (3.0 x (258.80)1/3) – (3.75)

Top stemming

= (3.0 x 6.36) – (3.75)

= (19.08) – (3.75)

= 15.3 ft

= (SD x (Ø3 x ρ x 0.284)1/3) – (0.417 x Ø)

For illustrative purposes only

Distributing More Energy in Collar Zone

2

3

Explosive column

Subgrade

Bench top

Cartridge or Decoupled Charge in Stemming

Stab or Pilot Hole

Stem Charge

Pit floor

1

Stemming

1 23

Cartridge or decoupled explosives placed

within the top stemming

Method 1

Cartridge or decoupled explosives in top stemming

introduce too much risk as a top load in creating severe

blowouts, flyrock and airblast.

Cartridge or Decoupled Explosives

Laser Profiler

Method 1

Production Holes

Stab Hole

Subgrade

Stab or Pilot Hole – Placed In Between Production Holes

Stab holes are generally not popular

because…

* Requires additional holes & drilling.

* Restricts vehicle traffic on blast block.

* Unsafe to drive over loaded/unloaded holes.

Method 2

Could be illegal in some countries.

Production Holes

Stem Charge

Subgrade

Stem Charge – Placed Inside Stemming Column

Stemming

D = 50 – 65% of Stemming

Good distribution of energy in the collar zone.

Stab hole depth is generally drilled 50 – 65% of the normal stemming column.

Stem charge must be calculated precisely with SD = 1.0 – 1.6. A good starting point is 1.3 for typical applications.

If using electronic detonators, the stem and main charges should be fired instantaneously.

If using Nonel, it is critical that the stem charge is always fired before the main charge.

D

Method 3

Increase Stemming When Using a Stem Charge

Normal Stemming

Explosive column

Increased Stemming Stem Charge

Method 3

Uses less explosives per hole.Much better fragmentation in collar zone.Controlled flyrock, airblast and dust.

SD = 0 – 0.600.64 – 0.88

0.92 – 1.40

1.44 – 1.80

1.84 – 2.402.40 +

Metric Units(m/Kg1/3)

SD = 0 – 1.5 1.6 – 2.22.3 – 3.5

3.6 – 4.5

4.6 – 6.06.0 +

Imperial Units

(ft/lb1/3)

Significance of SD (Scaled Depth of Burial)© 1990, 2008 Blasting Analysis International, Inc.

All Rights Reserved

No fallback

Some fallback

Uncontrolled Energy

Violent flyrock, airblast, noise and dust.

Very fine fragmentation.

Good craters.

Controlled EnergyGood fragmentation.

Maximum volume of broken rock in collar zone.

Acceptable vibration/airblast.

Good heave and muck pile mound.

Larger fragmentation.

Reduced volume of broken rock in collar zone.

Acceptable vibration/airblast.

Reduced heave and muck pile mound.

No flyrock.

Very Controlled Energy

Small surface disturbance

Insignificant surface effects

Minimal Surface Effects

No breakage

zone

W = (D/SD)3

Example Stem Charge Calculation

= (3.9/1.2)3

= (3.25)3

= 34 Kg

D = 65% Stemming = 3.9 m

W = Stem Charge Stemming

6m

Surface

SD is chosen as 1.2

SD = D/W1/3

D = SD * W1/3

Important Stem Charge Cautions

6 mD = 3.9 m

34 Kg

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

Hole Delay = 1 – 3 ms

Row Delay = 100 – 300 ms

New Delays with Precise Electronic Detonators

0 ms 25 ms 50 ms

0 ms 2 ms 4 ms

Delay Timing

VpVs

VpVs

VpVs

Conventional

Electronic

No interaction of shock/stress

waves

Maximum interaction of shock/stress

waves

0 ms 2 ms 4 ms

= fn (Shock Wave, Vp, Vs, Gas pressure & crack velocity)

Vp Vp

Vs Vs

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

No. 1

Normal shot design. Modified shot design.

Hole delays = 17 – 42 ms.Row delays = Constant 42 or 67 ms.

Normal top stemming.

Nonelectric detonators.

Hole delays = 2 ms.Row delays = Increasing 125 – 250 ms.

Stem charge & mid-column air deck.

Electronic detonators.

Normal Modified Excessive oversize

Very little oversize

Dividing line between normal and modified

blasts.

Case History No. 1

Copper Mine in Chile

Normal Blast Design Results

Case History No. 1

Hole delay = 2 ms Row delays = 125 - 250 ms

Stem chargeMid-column air deck

Digging rates increased 50 -100% due to oversize reduction.

Modified shot

South Africa Coal

Normal shot design results

Expl./delay increased 17-fold.Peak vibrations - Unchanged

Case History No. 2

Hole delay = 1 ms Row delays = 100 - 300 msStem charge

Eliminated all oversize in

collar

Oversize

South Africa Coal

Case History No. 2

Quarry – Maryland, USA

Expl./delay increased 4-fold.Peak vibrations - Reduced

Case History No. 3

Hole delay = 2 ms Row delays = 125 -250 msStem charge

Improved fragmentation in collar by 10-fold

Massive granite

Stem charge alone eliminated oversize in collar for the dragline

Stemming

Main Charge

Coal Mine – South Africa

Case History No. 4

Coal

Massive sandstone

Shale

Quarry – Alabama, USA

Expl./delay increased 4-fold.Peak vibrations - Unchanged

Case History No. 5

Hole delay = 2 ms Row delays = 100 -250 msStem charge

Oversize in collar reduced 5-fold

Flyrock controls needed because shot was directly underneath power lines.

Normal stemming.

Copper Mine in Chile – Test Shots Done on Same Bench and with Same Orientation.

Case History No. 6

Hole delay = Nonel 42 ms.Row delays = 92 ms

Stem charge.Hole delay = 2 ms.Row delays = 100 - 300 ms

Oversize in collar completely eliminated

Copper Mine in Chile

Case History No. 6

Electronic Detonators, 2 ms Hole Delay, Stem Charges and 100 – 300 ms Row Delays

Quarry 1 – Pennsylvania, USA

Expl./delay increased 4-fold.Peak vibrations - UnchangedCase History

No. 7

Hole delay = 2 ms Row delays = 150 - 250 msBottom hole air deck

No back spill

Good power trough

CAP ROCK PROBLEM

Quarry 2 – Pennsylvania, USACase History No. 8

BEFORE

Quarry 2 – Pennsylvania, USA

Case History No. 8

After

Quarry 2 – Pennsylvania, USA

Expl./delay increased 8-fold.Peak vibrations – Increased only 25%

Case History No. 8

Hole delay = 2 ms Row delays = 90 - 300 msStem charge

Quarry 3 – Pennsylvania, USA

Before

Case History No. 9

After

Quarry 3 – Pennsylvania, USA

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

Blasting Analysis International, Inc.Allentown, Pennsylvania, U.S.A.

Perth, AustraliaDrill and Blast 2010

October 12 - 14 , 2010

2010, Blasting Analysis International, Inc. All Rights Reserved.

c