Permafrost
Permafrost, or permanently
frozen ground, underlies one-fifth of the world's land, and 85 percent
of Alaska lies within the permafrost region. Permafrost restricts use
of the ground surface, and limits the locations of roads, buildings,
communities, pipelines, airfields, and bridges.
Where the ground is frozen
and there is little water in the soil, there are no difficulties in
building new structures. However, if permafrost with lots of ice is
allowed to thaw, major problems can develop, because the ground can
lose most of its strength. Consequently, a structure built on the permafrost
will be damaged. Generally, the thawing permafrost causes structures
to melt unevenly (see photograph of the "roller coaster railroad").
The best solution to avoiding
permafrost problems is to avoid locating structures on ice-rich permafrost.
Unfortunately, this is not always possible. If areas of ice-rich permafrost
cannot be avoided, then special studies should be made to determine
how to overcome the permafrost problems. Common procedures used to avoid
thawing the permafrost include covering the ground surface with a thick
layer of insulating gravel, refrigerating the ground, leaving the vegetation
mat on the ground intact, and, for heated buildings, elevating them
on piles.
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| The
"roller coaster railroad" (the abandoned Copper River
Northwestern Railroad) near Strelna in south-central Alaska, illustrates
how building the railroad made the underlying ice-rich permafrost
thaw and caused the railroad bed to settle unevenly. Less disruption
of the ground surface during construction and a thicker insulating
blanket of gravel fill should have been used. |
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