Page 245 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
240 Geotechnical Engineering

Figure 11.20
Settlement from
thawing of
permafrost by a
heated building,
Big Delta, Alaska.

11.10.7 Premafrost and Engineering
The most direct approach to prevent problems from melting of permafrost is to
keep it frozen, but this is becoming more difficult because of the sensitivity of
northern climates to change, with the result that these areas are the first to feel
major impacts from global warming. The moisture content at which a soil
becomes liquid is called the ‘‘liquid limit,’’ and when permafrost melts, the soil
moisture content usually exceeds the liquid limit and is trapped until it can drain
away or evaporate.

Seasonal thawing can be prevented by supporting structures on piles extending
into the permafrost layer and allowing winter air to circulate underneath the
structures in the winter. Another method uses ducts in soil under a structure in
order to force-circulate cold winter air.

Oil pipelines must be protected from loss of support by thawing permafrost in
order to prevent environmentally disastrous oil spills. The scheme shown in
Fig. 11.21 was used for the Trans-Alaska pipeline, and employs a passive heat
transfer mechanism with a vaporizing refrigerant. During winter when the soil is
warmer than the air, refrigerant in the soil zone evaporates, taking heat with it,
then rises and condenses in the colder ends of the tubes, releasing heat to the air.
The refrigerant then flows down by gravity to replenish the supply at the bottom.
During summer the cycle is broken because the refrigerant remains at the bottom
of the tubes. A similar, less efficient, but less costly system uses convection, as air
inside a pipe is warmed by the soil and moves up and out. The air is replenished by
cold winter air from the ground surface and conducted downward through a
center pipe.

Roads can pose a difficult problem in permafrost areas because, if they are built
up for better drainage, permafrost can rise in the road embankment and create a

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