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-
these computations. The extent of the ice jam must
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.
Example 1. An ice jam one mile in length is to
Example 2. An ice jam 2 miles long needs to be
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
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