Geography Reference
In-Depth Information
The near-surface layer in which thawing and freezing occurs is
called the active layer. This layer can be a shallow few centimetres
in continuous permafrost to several metres deep in discontinuous
permafrost zone. Freezing of water causes an expansion of 9 per
cent in volume and so freeze-thaw processes in the active layer
cause movements of the ground surface. In fact this can lead to
major problems for infrastructure such as roads, pipelines and build-
ings which, if not properly engineered to cope with the conditions,
will collapse, buckle and subside. Human-occupied buildings trans-
mit heat to the ground and can melt permafrost, leading to subsid-
ence. Therefore, buildings must be well insulated at ground level
or even have refrigeration units to cool the ground although this
can be prohibitively expensive. Ensuring that buildings are
anchored to deep piles that are embedded to the bedrock is import-
ant and this means that the building could be supported off the
ground if the ground subsides. The trans-Alaskan oil pipeline is a
classic example of engineering which takes account of permafrost.
The pipeline carries oil at 65°C across 1,285 kilometres of perma-
frost terrain. The hot temperature of the pipeline would thaw the
ground if the pipe were buried or at the ground surface and this
would result in subsidence and damage to the pipeline. Therefore,
it is elevated above the ground, for a large proportion of its route,
upon racks. It is also deliberately built with bends in it to allow the
pipeline to expand and contract, move sideways and vertically
without cracking. The vertical supports for the pipe are also
equipped with devices to cool the permafrost. Animals can cross
the pipeline at certain locations where the pipe is buried with
refrigeration units.
Landforms in periglacial regions
Periglacial
environments are those that are cold and subject to
intense frost action but which are non-glacial. Permafrost regions
are periglacial but permafrost is not a pre-requisite for a periglacial
region. Many periglacial landforms are related to the freezing of
water in soils and sediments. Frost heaving and thrusting are the
vertical and horizontal movement of sediment due to the forma-
tion of ice. Heaving generally dominates because ice crystallisation
tends to happen in a direction parallel to the temperature gradient
