pact area were measured. The ponds in the area of
to have compressed the soft pond bottom sedi-
impact are very shallow. The ice sheet covering
ments into a crater-shaped depression without lift-
the ponds was either partially grounded on the
ing or throwing sediment out of the hole. The
pond bottom or barely afloat. Because of the un-
center depth of the craters given in Table 4 is the
evenness of the pond bottom sediment and the
total depth from the top of the original snow sur-
very soft nature of the sediment, it was difficult to
face to the bottom of the depression in the pond
determine the exact water depth beneath the ice
bottom sediments. This averaged 0.68 m for the
cover. In part, the determination that an ice sheet
six craters.
was floating or not was based on the water surface
level in the center crater. The water surface levels in
four craters were obviously below the ice buoy-
ANALYSIS AND DISCUSSION
ancy point, indicating the ice sheet was grounded
on the bottom at that location. The ice sheet at the
The analysis of the craters formed by the 105-m
other two crater locations was floating, although
howitzers and 60- and 81-mm mortars and com-
the water depths below the bottom of the ice sheet
parison of these crater data with published data
were only a few centimeters. The measured vari-
on crater formation is complicated because these
ables for these six craters are summarized in Table 4.
were formed by impacting projectiles, while much
All the craters had a large, shallow outer crater
of the published data are for stationary explosive
formed in the overlying snow cover. All had a
charges. Also, the published data are for a uni-
smaller inner crater consisting of a hole blasted
form medium, while these craters were formed in
through the underlying ice sheet. The longest axis of
a three-layer medium with quite different densi-
the outer craters ranged from 3.95 to 4.90 m, and
ties and structural properties. The upper layer was
the shortest axis ranged from 2.30 to 3.15 m. The
snow, 0.150.19 m thick. The middle layer con-
mean lengths for maximum and minimum axes
sisted of a 0.30- to 0.60-m-thick ice cover, and the
were 4.04 and 2.84 m, respectively. This gives a
bottom layer was a seasonally frozen, saturated
mean apparent diameter of 3.44 m, or a mean
fine-grained soil. The explosion of point-detonat-
apparent radius Ra of 1.72 m. The depths of the
ing mortar projectiles, both 60 and 81 mm, only
outer crater averaged 0.17 m, which is the same as
removed the snow and rarely penetrated the sur-
the average snow depth.
face of the ice cover. The explosion of point-deto-
The longest axis of the inner crater or hole in
nating 105-mm howitzer projectiles removed the
the ice sheet ranged from 1.15 to 1.68 m, and the
snow cover and at least part of the ice cover. In at
shortest axis ranged from 1.15 to 1.46 m. This gives a
least one case the entire ice cover was removed, but
mean apparent diameter of 1.36 m, or a mean
the crater did not penetrate into the underlying fro-
apparent radius Ra of 0.68 m. In the pond bottom
zen soil layer.
sediments directly beneath the hole in the ice was
a crater-shaped depression. However, at none of
105-mm howitzer test results
the craters was there any sediment displaced out
The mean apparent radius of the seven craters
of the hole and scattered on the surface. The shock
resulting from the point-detonating projectiles was
wave of the explosion on the ice surface appeared
1.41 m. The mean scaled apparent radius of the
Table 4. Measurement data from craters on pond ice (105-mm howitzer).
Water
Water
Max.
Min.
Max.
Min.
Center
Snow
Ice
depth in
below
axis
axis
axis in
axis in
depth
depth
thick.
crater
ice sheet*
Crater no. (m)
(m)
ice (m)
ice (m)
(m)
(m)
(m)
(m)
(m)
Notes
1
3.95
2.75
1.58
1.40
0.92
0.20
0.27
0.60
<0.01
ice partially grounded
2
4.90
3.05
1.20
1.20
0.50
0.14
0.25
0.29
<0.01
ice partially grounded
3
3.40
2.75
1.15
1.15
0.60
0.19
0.29
0.39
0.02
ice floating
4
3.95
3.05
1.43
1.37
0.63
0.16
0.22
0.40
<0.01
ice partially grounded
5
3.95
2.30
1.68
1.46
0.76
0.18
0.25
0.53
<0.01
ice partially grounded
6
4.10
3.15
1.43
1.22
0.66
0.16
0.25
0.48
0.03
ice floating
Ave.
4.04
2.84
1.41
1.30
0.68
0.17
0.26
0.45
* The water depth below the bottom of the ice sheet is very difficult to measure because of the very soft pond bottom
sediments.
11
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