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.
Question Description:
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