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-
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
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