create a crater of equal diameter as ice thickness

average 6 ft thick. What size of charge should be

increases. The crater diameter for a given charge

used and what is the expected cost?

is determined by finding the appropriate charge

Step 1. Determine the minimum charge size

weight on the left side of the figure and drawing a

that can be used.

straight line to the right until it intersects the curve

From Figure 23, the minimum charge size

representing the estimated ice thickness. A straight

that could be used is approximately 7

line is then drawn straight down to find the ex-

lb. This is less than the size of charge

pected crater diameter. The crater diameter is im-

planned, so the planned charge size can

portant because it determines the length and width

be used.

of the channel created through the jam. It also deter-

Step 2. Determine the cost of several charge

mines the distance between charges. It is recom-

sizes. Compare costs for charges of 20,

mended that charges be placed a distance apart equal

30 and 40 lb.

to the diameter of the crater those charges create.

Use Table 21 and fill in columns 1, 3, 7, 9, 11

Thus, if a charge creates a crater 50 ft in diameter,

and 12 for charge size, length of chan-

slightly over 100 charges would be required to

nel to blast, number of charges per trip,

clear a channel one mile long and 50 ft wide.

time for one round trip, cost of explo-

If blasting is to be done, some quick computa-

sives per pound and cost of helicopter

tions can help determine a proper charge size.

per hour.

Several items must be determined before doing

Use Figure 24 to estimate the crater diame-

ter. The crater diameter is 22 ft for the

be determined so that the length of channel to be

20-lb charge, 27 ft for the 30-lb charge

blasted is known. A plat map or USGS quadran-

and 31 ft for the 40-lb charge. Enter

gle map would be best for determining jam

these values in column 2 in the chart.

lengths, since the pilot channel to be blasted is not

You can now complete the chart to com-

likely to be straight. Since access to the jam is

pare costs.

likely to be poor, the charges will need to be placed

Complete column 4 by dividing column 1

from a helicopter. It would be necessary to know

by column 2. Column 5 is equal to col-

how many charges a pilot will allow on his heli-

umn 3 times column 4 times 5280. Com-

copter for either weight or space reasons. It is also

pute the number of charges, column 6,

necessary to know how long it takes to complete

by dividing column 5 by column 1, and

one round trip with the helicopter to load the

compute column 8 by dividing column

charges, drop the charges and return to the base

6 by column 7. Column 10 is equal to

of operations. This can be estimated either from

column 8 times column 9 divided by

experience or from the distance from the base of

60. The cost of the explosives is deter-

operations to the blasting site. The last two items

mined by multiplying columns 5 and

are the cost of explosive per pound, including the

11, the cost of the helicopter is deter-

booster charge, and the cost of the helicopter per

mined by multiplying columns 10 and

hour, including the use of the helicopter, the pi-

12, and the total cost is the sum of col-

lot's time and the blaster 's time.

umns 13 and 14. Any miscellaneous

Two examples are given below to illustrate the

costs can be added on to determine the

methodology used to estimate quantities and time.

true total cost.

These are only examples and should not be used

As can be seen, using 20-lb charges is the least

for an actual blasting event. Diagrams illustrating

expensive alternative by about 0, but it takes

the use of the two figures above are shown, as

the most time by at least 2.5 hours. If time is im-

well as a chart for computing explosive quantities

portant, it may be worthwhile to spend more for

and costs.

larger charges.

be blasted. The cost of explosives is
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.50 per

removed by blasting. You estimate the jam thick-

pound, and the use of helicopter is 0 per hour.

ness to be 8 ft in the lower 0.5 mile and 3 ft in the

You would like to use charges of about 2535 lb

upper 1.5 miles. Explosives will cost
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.25 per

and can take three charges per trip. The pilot esti-

pound and the helicopter will cost 0 per hour.

mates that one round trip will average ten min-

The helicopter pilot will allow 80 lb of explosives

utes. You think the charges will detonate just be-

each trip and says each trip will take 10 minutes.

low the ice bottom. The ice jam is estimated to

You think the explosives will explode just under

51