Page 244 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 239

Polygons may follow linear soil boundaries such as abandoned lakes or river
channels, in which case they are rectangular.

11.10.4 Drainage in Permafrost Areas

Permanently frozen ground does not allow infiltration of surface water, so
permafrost areas are poorly drained and marshy during melt seasons, even though
the annual precipitation is that of a desert. During thaw seasons the tops of ice
wedges melt down, and low-centered polygons fill with meltwater. Small streams
tend to follow ice wedges, and where wedges meet they melt out pockets that may
be meters deep and appear like beads—hence the name ‘‘beaded streams.’’

11.10.5 Artesian Pressure from Confined Drainage

Water seeping downslope through the active zone becomes confined when the top
of the soil freezes during the onset of winter. This can lead to some extraordinary
consequences. A pingo is a large mound, of the order of 30–45 m (100–150 ft) in
diameter and 8–9 m (25–30 ft) high, where a frozen soil layer has been pushed up
by hydrostatic pressure and contains a core of ice. A palsa is similar to a pingo,
but consists mainly of peat soil and occurs most frequently near the southern
fringe of the discontinuous permafrost.

An abandoned building can delay ground freezing so confined downslope seepage
finds an outlet inside the building, which then fills with water so that ice cascades
out of the windows.

11.10.6 Thawing of Permafrost
Permafrost is sensitive to changes in the thermal regime and, as shown in
Fig. 11.20, will thaw underneath a heated building unless steps are taken to
prevent it. Destruction of ground cover such as by clearing of trees promotes
thawing, and tilting of the trees as ice wedges melt away creates an effect known
as ‘‘drunken forest.’’ Several years are required after clearing for ground
temperatures to warm up sufficiently for agriculture.

Lakes dot the high Arctic and change the soil thermal regime sufficiently to thaw
the upper part of the permafrost. This causes subsidence as ice lenses and wedges
melt, so lakes are perpetuated. Large areas of the Arctic coastal plain of North
America therefore present a patchwork of oblong thaw lakes that are oriented in
the direction of summer prevailing winds. As the lakes migrate and find outlets
they drain and the cycle starts over again.

Irregular pock-marked topography resulting from thawing of permafrost and
associated ice masses is called ‘‘thermokarst,’’ analogous with limestone sink
areas.

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