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Chapter 13 Blasting Rock Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**BLASTING ROCK Practical Exercise**

Blast Design

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PRACTICAL EXERCISE Design a delayed initiation blast (would like to shoot a minimum of 10 holes per delay). Base the plan on the following: Rock - limestone, in thin layers w/tight joints Bench height - 24 ft

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PRACTICAL EXERCISE Explosive - bulk ANFO, SGe 0.8 dry holes, no liner required. Explosive - dynamite, SGe 1.4 Cartridge size: 2 to 3.5 inch diameter in ¼ in. increments, by 16 in. long.

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PRACTICAL EXERCISE Drills can use 6 in. bits. Regulatory scale distance factor 55 Nearest structure is 1,400 ft away

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STEP 1: BURDEN DISTANCE ?

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**Explosive density empirical formula for burden distance:**

STEP 1: BURDEN DISTANCE Explosive density empirical formula for burden distance: Equ. 13.1

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**STEP 1: BURDEN DISTANCE SGe ANFO = 0.8 SGr Limestone (Table 13.2)?**

De = ? try 6 in. the maximum

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**STEP 1: BURDEN DISTANCE SGr Limestone (2.4 - 2.9)**

Lets use an average number of 2.7

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STEP 1: BURDEN DISTANCE B = 12.6 ft

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**STEP 2: BURDENcorrected**

Burden distance corrected for geological conditions (Table 13.3). Kd = 1.00, no information assume “other cases.” Ks = 1.10, thin layers w/tight joints

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**STEP 2: BURDENcorrected**

Burden distance corrected for geological conditions. Bcorrected = 13.8 ft

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**STEP 3: STIFFNESS RATIO Need a Stiffness Ratio of 3 or greater.**

Bench height = 24 ft Burden corrected = 13.8 ft

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STEP 4: BURDEN ADJUSTED Want a Stiffness Ratio of 3 therefore:

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STEP 4: BURDEN ADJUSTED Do not complicate the field work, try to use one foot or half foot increments.

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STEP 4: BURDEN ADJUSTED Burden distance corrected for geological conditions.

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**STEP 5: EXPLOSIVE DIAMETER**

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**STEP 5: EXPLOSIVE DIAMETER**

3 1/2 in.

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**STEP 6: BLASTHOLE SIZE Use a 3 1/2 in. size bit,**

Bulk ANFO dry holes, no blasthole liner, explosive diameter and blasthole size are the same. Use a 3 1/2 in. size bit, therefore 3 1/2 in. borehole.

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STEP 6: BLASTHOLE SIZE Bulk ANFO, explosive diameter and blasthole size are the same, i.e. there is no liner. EXPLOSIVE BORE HOLE

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**Explosive diameter and blasthole size are the same.**

ANFO

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**Worked with a wooden pole to prevent bridging.**

ANFO

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STEP 7: STEMMING Stemming depth (T) = 5.6 use 6 ft

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STEP 8: SUBDRILLING Subdrilling depth (J) = 2.4 use 2 ft

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STEP 9: SPACING Spacing controlled by initiation timing and stiffness ratio. Delayed initiation: SR greater than 1 but less than 4 (Equ. 13.8)

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**STEP 9: SPACING L (bench height) = 24 ft. B (burden distance) = 8 ft.**

Spacing = 10 ft

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**STEP 9: SPACING 8.5 ft 11.5 ft Calculated spacing -15% +15%**

Spacing in the field should be within plus or minus 15% of the calculated value. Calculated spacing -15% +15% 8.5 ft 11.5 ft

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**STEP 9: SPACING Spacing = 9, 10, 11 ft?**

If this is an estimate for a bid it might be better to be conservative and use an 9 ft spacing. That would mean more holes and a higher drilling cost.

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**STEP 9: SPACING Spacing = 9, 10, 11 ft?**

If we are in the field drilling for the first shot we might try a 10 ft spacing. This would lower cost.

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STEP 9: SPACING Pull the first shot and evaluate the results: fragmentation etc. If it is ok, use 11 ft, if it is not satisfactory reduce the spacing.

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**STEP 9: SPACING don’t forget -- SAFETY**

Throw rock

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STEP 10: POWDER COLUMN Powder column length is the blasthole depth minus the stemming depth. Blasthole Depth = 26 ft Bench height (24 ft) + Subdrilling ( 2 ft) Powder column = 26 ft - 6 ft = 20 ft

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**STEP 10: POWDER COLUMN Powder column = 20 ft or 240 in.**

Dynamite cartridge size: 2 to 3.5 inch diameter in 1/4 in. increments, by 16 in. long. Dynamite: 16 in or 1.3 ft

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**STEP 10: POWDER COLUMN Powder column = 240 in. ANFO - 16” dynamite**

or 18.6 ft

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STEP 11: LOAD DENSITY Have to prime the dynamite to fire the shot.

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**STEP 11: LOAD DENSITY 3 1/2 in. borehole**

Dynamite: Cartridge size: 2 to 3.5 inches in 1/4 in. increments. Use 3 in. diameter cartridges, will fit easily into the hole. ANFO: 3 1/2 in. diameter

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STEP 11: LOAD DENSITY Dynamite Cartridge

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**STEP 11: LOAD DENSITY Dynamite: 3 in dia. SGe = 1.4**

ANFO: 3 1/2 in dia. SGe = 0.8 Explosive Loading Density Table 13.5 Dynamite: 4.29 lb/ft ANFO: lb/ft

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**STEP 12: AMOUNT of EXPLOSIVE**

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**STEP 12: AMOUNT of EXPLOSIVE**

Amount of explosive (lb) = powder column (ft) X loading density (lb/ft) Dynamite: 1.3 ft X 4.29 lb/ft = lb ANFO: ft X 3.34 lb/ft = 62.1 lb Explosive per blasthole lb

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**STEP 13: AMOUNT of ROCK Rock per blasthole: Amount of rock (cy) =**

pattern area (ft2) X depth to grade (ft) X 1/27 (ft3/cy) Rock per blasthole:

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STEP 14: POWDER FACTOR Powder Factor (lb/cy) = Powder Factor:

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**STEP 15: VIBRATION Check vibration formula 13.12 Ds = 55 d = 1,400**

Want to shoot 10 holes per delay. W = 10 X 67.7 lb or 677 lb

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STEP 15: VIBRATION Ds = 55 A scaled distance value of 55 or greater is required for this location. 53.8 55 Therefore, can not shoot 10 holes at one time.

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STEP 15: VIBRATION How many holes at one time.

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STEP 15: VIBRATION Ds = 55

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STEP 15: VIBRATION Therefore, can only shoot 9 holes at one time.

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Shoot 9 holes at one time.

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PRESPLITTING ROCK To limit overbreak, develop a presplitting blast plan, for the previous example.

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STEP 1: EXPLOSIVE LOAD Equation 13.10 Explosive load = lb/ft

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**STEP 2: AMOUNT of EXPLOSIVE**

Explosive load = lb/ft Amount of explosive / hole = (24 ft X lb/ft) + (3 X lb) = 11.8 lb/hole

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STEP 3: HOLE SPACING Hole spacing = 10 X 3 ½ in. = 35 inches

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FIRE IN THE HOLE

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FIRE IN THE HOLE

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Time to load and haul.

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