Geology Reference
In-Depth Information
high pore-water pressures that are generated favor slope instability and slumping. These
processes are described in Chapter 8. A related problem is that the physical properties of
icy sediments, in which soil particles are cemented together by pore ice, may be consider-
ably different to those of the same material in an unfrozen state. For example, in uncon-
solidated and/or soft sediments there is often a signifi cant loss of bearing strength upon
thawing. Beneath heated buildings, therefore, it is often essential to maintain the frozen
state of the underlying material in order to support the structure.
As explained in Chapter 5, the hydrological and groundwater characteristics of perma-
frost terrain are different from those of non-permafrost terrain. For example, the presence
of both perennially- and seasonally-frozen ground prevents the infi ltration of water into
the ground or, at best, confi nes it to the active layer. At the same time, subsurface fl ow is
restricted to taliks. A high degree of mineralization in subsurface permafrost is often
typical, caused by the restricted circulation imposed by permafrost and the concentration
of dissolved solids in taliks. Thus, frozen ground eliminates many shallow depth aquifers,
reduces the volume of unconsolidated deposits or bedrock in which water is stored, infl u-
ences the quality of groundwater supply, and necessitates that wells be drilled deeper than
in non-permafrost regions.
Engineers adopt a number of approaches that deal with these problems. If the site is
underlain by hard igneous and metamorphic rock, as is the case for some regions of the
Canadian Shield, ground ice is usually non-existent and permafrost problems can be
largely ignored. In most areas, however, this simple approach is not feasible since an over-
burden of unconsolidated silty or organic sediment is usually present.
14.2.2. General Solutions
Modern construction techniques aim to maintain the thermal equilibrium of the perma-
frost and avoid the onset of thermokarst. The most common technique is the use of a pad
or some sort of fi ll which is placed on the ground surface (Figure 14.2). This compensates
for the increase in thaw which results from the warmth of the structure. By utilizing a pad
of appropriate thickness, the thermal regime of the underlying permafrost is unaltered.
It is possible, given the thermal conductivity of the materials involved and the mean air
and ground temperatures at the site, to calculate the thickness of fi ll required. Too little
fi ll, plus the increased conductivity of the compacted active layer beneath the fi ll, will
result in thawing of permafrost and subsidence (Figure 14.2A). On the other hand, too
much fi ll will provide too much insulation and the permafrost surface will aggrade on
account of the reduced amplitude of the seasonal temperature fl uctuation (Figure 14.2B,
C). In northern Canada and Alaska, gravel is the most common aggregate used since
it is reasonably widely available and is not as frost-susceptible as more fi ne-grained
sediment.
Where large quantities of high-quality non-frost-susceptible aggregate are scarce and
the structure justifi es the cost, more sophisticated technologies are sometimes used. For
example, the Yukon Government constructed two 350 m
2
multi-purpose municipal build-
ings in the communities of Ross River and Old Crow, using heat-pump chilled foundations
(Goodrich and Plunkett, 1990). The aim was to prevent the thaw of permafrost beneath
the buildings. At Ross River, located in the discontinuous permafrost zone, the permafrost
is marginal and the mean annual ground temperature is between 0 °C and
0.5 °C. To
prevent thaw, heat exchangers were placed in a sand layer within the granular fi ll used to
level the site (Figure 14.3A). Heat fl owing down through the fl oor is then captured by the
heat exchangers and pumped back into the building. Thus, while the building is being
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