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

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

BLASTING ROCK Practical Exercise
Blast Design

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

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.

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

STEP 1: BURDEN DISTANCE ?

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

STEP 1: BURDEN DISTANCE SGe ANFO = 0.8 SGr Limestone (Table 13.2)?
De = ? try 6 in. the maximum

STEP 1: BURDEN DISTANCE SGr Limestone (2.4 - 2.9)
Lets use an average number of 2.7

STEP 1: BURDEN DISTANCE B = 12.6 ft

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

STEP 2: BURDENcorrected
Burden distance corrected for geological conditions. Bcorrected = 13.8 ft

STEP 3: STIFFNESS RATIO Need a Stiffness Ratio of 3 or greater.
Bench height = 24 ft Burden corrected = 13.8 ft

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

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

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

STEP 5: EXPLOSIVE DIAMETER

STEP 5: EXPLOSIVE DIAMETER
3 1/2 in.

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.

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

Explosive diameter and blasthole size are the same.
ANFO

Worked with a wooden pole to prevent bridging.
ANFO

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

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

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

STEP 9: SPACING L (bench height) = 24 ft. B (burden distance) = 8 ft.
Spacing = 10 ft

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

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.

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.

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.

STEP 9: SPACING don’t forget -- SAFETY
Throw rock

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

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

STEP 10: POWDER COLUMN Powder column = 240 in. ANFO - 16” dynamite
or 18.6 ft

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

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

STEP 11: LOAD DENSITY Dynamite Cartridge

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

STEP 12: AMOUNT of EXPLOSIVE

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

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:

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

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

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.

STEP 15: VIBRATION How many holes at one time.

STEP 15: VIBRATION Ds = 55

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

Shoot 9 holes at one time.

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

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

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

FIRE IN THE HOLE

FIRE IN THE HOLE