# Question: Problem 12.3) The project is in a low-silica marble formation. Company drilling experience in this area indicates that compressed-air rotary-percussion drills with 3-in. button bits can average 2.2 ft of penetration per minute. Assume that it takes 3 min to change a bit. Drilling efficiency is typically a 45-min hour. These drills use 10-ft drill steel. The best blasting pattern is 8 ft x 9 ft with 1.5 ft of sub drilling. The average excavation depth is 12 ft. How many drills will you recommend for use on the job? -Free Course Hero Question Answer.

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I need help understanding the following civil engineering mining question please.

Problem 12.3) The project is in a low-silica marble formation. Company drilling experience in this area indicates that compressed-air rotary-percussion drills with 3-in. button bits can average 2.2 ft of penetration per minute. Assume that it takes 3 min to change a bit. Drilling efficiency is typically a 45-min hour. These drills use 10-ft drill steel. The best blasting pattern is 8 ft x 9 ft with 1.5 ft of subdrilling. The average excavation depth is 12 ft. How many drills will you recommend for use on the job?

**Course Hero** **Answer & Explanation**:

**2 drills are required for use in this job **Step-by-step explanation

Depth of Hole

a)12

b)12.0 ft

sub-drill 1.5 ft/ drill 13.5 ft

Penetration rate 2.20ft/min Field test

Drill time 16.4 min 13.5 ft/2.20 ft/min

Change steel 1.00 min d= 14 ft

d > 10 ft

Therefore 0.5 min add ;0.5 min remove

Blow hole 0.14 min about 0.1 min

per 10 ft hole

14 ft/ 10 ft =1.35

Move 0.41 min 9 ft @ 5.280ft/mile 0.25mph

0.0017 miles

0.0068 hours 60 min /hr

Align steel 0.50 min not high mast

Change bit 0.03min 3 min

Table 12.10a marble ; low-silica

Drill bit 3in- button bits.

1,300 ft

Change frequency

14 ft/ 1,300 ft = 0.010

Total time /8.21 min

Operating rate 1.64ft /min 14ft/ 8.21 min

Production efficiency 45.00 min

Hourly production **74.0 ft/ hr**

45min/hr * 1.64 ft/min

Each hole products:

8ft 9ft grid *12 ft deep 864 cu ft

864ft /27 cu ft/cy = 32.0 bcy/ hole

74.0 ft/hr per drill /13.5 ft per hole = 5.5 holes/hr per drill

5.5 holes *32.0 bcy/hole= 175bcy/hr per drill

175bcy/hr per drill < 300 bcy/hr

300bcy/hr/175bc/hr < = 1.7

Therefore **2** drills are required

351bcy/hr > 300bcy/hr