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> Figure 55. Horizontal and vertical thin sections of core sample removed from the Pegasus runway surface at the 6000-ft zone
Figure 53. Failure planes in ice extending into sides of main cavity of failed area
Figure 55. Continued - M98_010069
M98_01
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heaving
and
blister
formation
(Fig.
35)
when
Application
of a
high
rate
of
loading
by
mov-
freezeup
occurred
in
the
prior
austral
autumn.
ing
vehicles
which
predicates
operating
Certainly
some
prefracturing
of
the
ice
occurred
within
the
brittle
failure
regime.
during
construction
of
the
runway.
Aggressive
Pre-existing
internal
damage
to
the
ice
(from
grading
was
used
to
level
the
ice
surface
to
bring
blister
development
during
refreezing
and
it
into
tolerance
for
aircraft
operations.
The
grad-
from
runway
construction
activities).
ing
introduced
cracks
in
the
near-surface
ice,
par-
The
presence of
many
included
impurities
ticularly
in
areas
where
former
blisters
(domed
in
the
ice
structure.
ice
with
radial
surface
cracks)
were
located.
En-
Thin
sections
of
the
ice
cores
were
taken
from
countering
a
blister,
the
grader
blade
often
caught
the
north
threshold
(0-ft),
and
at
the
6000-ft
and
the
radial
cracks
and
caused
them
to
propagate.
10,000-ft
areas
of
the
runway.
The
cores at 6000-ft
On
its
own,
the
grading
process
probably
intro-
and
5000-ft
locations
were
macroscopically
simi-
duced
new
near-surface
cracks
as
well.
lar
in
structure.
A
complete
profile
of
the
core
We
conclude
that
the
following
factors
contrib-
from
the
6000-ft
location
is
shown
in
Figure
55.
uted
to
the
brittle
failures
witnessed
in
the
Pe-
Clearly
this
ice
was
formed
by
flooding
with
snow-
gasus
runway
ice:
melt
water
during
runway
construction.
The
top
Large
grain
size
ice
in
areas
where
the
natu-
of
the
core is
characterized
by 2- to
3-mm
(0.08-
to
ral
glacial
ice
had
experienced
melting
and
0.12-in.)
diameter,
randomly
oriented
ice
grains
refreezing.
included
in 8- to
35-mm-
(0.3-
to
1.4-in.-)
equiaxed
a. 0- to
5-cm
horizon.
b. 5- to
18-cm
horizon.
Figure
55.
Horizontal
and
vertical
thin
sections
of
core
sample
removed
from
the
Pegasus
runway
surface
at
the
6000-ft
zone.
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