Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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VARIABLE CONDUCTIVITY TRAFFICKED
SURFACE FOR PER~IAFROST REGIONS
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to road construction
in general, and in particular to road and the like
construction in cold and permafrost regions. More
particularly still it relates to layered construction
wherein one of the deeper layers under the trafficked
surface exhibits relatively variable heat conductivity
(K) from summer to winter. As a result, the annual mean
temperature of the underlying permafrost layer is
lowered, in contrast to merely decreasing the amplitude
of the normal seasonal temperature variations which
occurs with insulating or heat-sinking layers.
The construction of engineering structures such as
roadways and airstrips on permafrost may lead to long
term warming of the natural subgrade. The consequences
of such warming are particularly debilitating in the
vast regions of ice laden discontinuous permafrost which
are characte~ristic of the major zones of northern
habitation and industrial activity, such as the
MacKenzie Valley of the Canadian Northwest Territories
and Central Alaska in the U.S.A. In such soils, melting
(permafrost degradation) leads to continued settlement
and distress to the embankment structure.
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These are also the permafrost areas with the
greatest extent of existing roads and airfields. If the
recently suggested climatic warming predictions for -the
North are verified it wilL be essential to provide
cooling, and not merely insulation, to protect the
existing infrastructure.
Pr-ior Art of the Invention
The mechanical strength of frozen soil or
permafrost is typically greater than its unfrozen
~0 counterpart. Also a frost susceptible soil will tend to
heave when it freezes and settle when it thaws. Hence,
considerable effort in the engineering of northern
infrastructure has in the past been, and will continue
to be, directed towards avoiding seasonal thaw of frost
lS susceptible per~nafrost, thereby, taking advantage of
increased strength values for design purposes, all the
while precluding damaging thaw settlement effects.
Currently accepted construction techni~ues in
permafrost regions consist of building gravel
embankments whose thickness is chosen to be greater than
the anticipated depth of annual thaw. In some cases
layers of rigid polystyrene insulation are included
within the embankment for the purpose of limiting
seasonal tilaw while maintaining embankment thicknesses
within acceptable limits.
The closest prior art known is United States patent
3,722,378 granted to John S. Best March 27, 1973. The
patent pertains to trafficked surfaces built on
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foundations which rernain substantially undisturbed
during seasonal climatic cycles, particularly in
permafrost and near permafrost regions where
considerable disturbance of the ground beneath
foundations is otherwise common. The foundations
include combinations of insulation layers, heat sinks
and/or thermal bleeds which dampen and prevent the
cyclic climatic seasonal variations from affecting the
earthen support under the foundations, in both cut and
fill sections, and in embankments and backfills adjacent
the sections.
This prior art patent recognizes the problems
associated with permafrost road construction and
structure, and in particular recognizes that mere
provision of an insulating layer above the permafrost
layer is not sufficient. This and the proposed solution
are discussed in conjunction with figure 2 of the
patent, at column 29 line 59, as follows:
"Because of the sloping embankment 28 the
insulation layer 18 extends downwardly along
the embankment incline and a generally greater
capacity (depth) of heat sink 20 is employed
to achieve the desired effect. T~e purpose of
section lO is primari]y to prevent permafrost
2~ from thawing during the summer months which
would o-therwise occur during the summer
climatic cycle. In some instances permafrost
can melt to a depth of several feet, from
solar heat. Such melting of permafrost
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generally makes for a completely impassable
condition. Section 10 prevents such thawing
or can be used to control thawing where sorne
thawing can be tolerated. The use of an
S insulation layer alone is not practical in
many places to sufficiently prevent such
thawing. The present invention comprises in
its preferred embodiment the combination of a
heat sink with an insulation layer wherein the
thermocell holds a trapped solution which is
substantially frozen at the beginning of the
summer season, whereby the heat of fusion of
the solution then becomes addi-tional heat sink
capacity, supplementing that of the permafrost
supporting the same. The capacity of the
thermocell heat sink is designed such that it
will no-t achieve complete melting until the
end of a summer season. Thereafter the winter
season refreezes the heat sink solution such
that it is ready again for the next summer
season. The insulation layer on top of the
heat sink prevents an undue quantity of heat
from getting to the heat sink, permitting each
to be of a practical design. However, during
the winter season the insulation layer dampens
the freezing effect of the colder season from
regenerating the heat sink. But because in
permafrost and near permafrost regions the
winter season is so severe and longer lasting
than the summer season, proper designing of
the insulation layer still permits
regeneration of the heat sink in the winter
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while preventing undue heat penetration during
the summer."
The United States patent 3,722,378 is a generally
useful background to the present invention and is
incorporated herein by reference.
SUMMARY OF T~IE INVENTION
In its broadest aspect the present invention
recognizes that it is not merely desirable to minimize
the amplitude of annual temperature variations of the
permafrost but to lower its annual mean temperature.
Such desideratum has been achieved by the inclusion of a
variable heat conductivity (K) layer between the
permafrost and the traeficked surface.
In a narrower aspect of the present invention, the
variable K layer is more conductive of heat in the
winter than in the summer.
In a narrower aspect still, the variable K layer
has seasonally variable moisture content; dryer in the
summer, and hence having lower K, and more moist in the
winter thereby providing higher K due to its frozen
water content.
The high K in the winter lowers the permafrost
temperature because of the better heat conduction
between the permafrost and the cold atmosphere. Thus,
in many specific situations the annual mean temperature
of the permafrost layer mav be lowered -from year to year
as a result until it reaches a new equilibrium value.
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Thus, according to the present invention there is
provided a permafrost road or the like construction of
the type having a layered structure above the permafrost
characterized by a variable heat conductivity (~) layer,
K ~eing larger in winter than in summer.
The thermal conductivity K of a material is a
quantifiable property which relates the propensity for
heat to flow through the material under a thermal
gradient. Thermal conductivity is the ratio of rate of
heat flow to the thermal gradient and is expressed in
units of W/rnK (~atts/metre-Kelvin).
The thermal conductivity of a wettable material
varies as a function of moisture content, The thermal
conductivity may also vary with temperature, notably if
lS material structure or phase composition are affected.
The effect water has on the thermal conductivity of
a base material to which it is added can be related to
the relative ~hermal conductivity values of the
constituents, i.e., adding water to a base material
which is a poorer conductor than water will increase the
thermal conductivity of the whole, Inversely, as this
material dries out, it will become a better thermal
insulator, Further, freezing a partially to fully
saturated material will increase its thermal
conductivity since water in the solid statP is a better
conductor than in the liquid state. Hence, the thermal
conductivity of a given material in a dry unfrozen state
may be substantially less than that in the frozen
saturated state.
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BRIEF DESCRI~TION OF THE DRAWINGS
The preferred reference of the present invention
will now be described in detail in conjunction with the
annexed drawings, in which:
Figure 1 is a schematic representation of the
layered construction of road construction, including one
shoulder, on a permafrost foundation;
Figure 2 is an alternative construction to that
shown in figure 1 using a high conducting fin at the
shoulder;
Figure 3 is another alternative using a wick--type
: geofabric at the shoulder;
Figure 4 is an alternative construction using
geotextile wick at the shoulder;
Figure 5 is a computer simulation slowing -thermal
: 1~ behaviour of permafrost roads constructed using a normal
: insulation layer;
Figure 6 is a compu-ter simulation showing thermal
behaviour of permafrost roads constructed using a heat-
sink layer; and
Figure 7 is a computer simulation showing thermal
behaviour of permafrost roads constructed using a
variable K layer according to the embodiments of Figure
1 to 4.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure 1, which shows a transversal
half section including one shoulder (hatching has been
omitted for clarity), a sub~rade or permafrost layer 10
has a layer 11 of coarse grained material such as gravel
S laid on top. On top of layer 11 a variable K layer is
constructed which cornprises an impermeable membrane 12
on top of which is a layer 13 consisting of a free
draining porous thermal insulating strands, pellets or
the like such as superexpanded polystyrene beadboard
pellets manufactured by BASF and identified by it as
drainage board. Extruded polystyrene strands or
flexible polyurethane foam are also useable. On top of
the layer 13 there is another coarse grained layer 1~
such as gravel, which provides the top surface layer of
the road construction. At the shoulder, zones 15 and 16
are constructed of conventional insulating materials
having conductivities not significantly dependent on
temperature . Zone 17 at the shoulder is a continuation
of the variable K layer 13 and, due to the provision
underneath of the insulting zone 16, follows more
closely the atmospheric (air) temperature, thereby
allowing it (zone 17) to block drainage of the layer 13
as temperatures drop at -the beginning of winter. The
water retained in the layer 13 therefore freezes and
causes the layer 13 to become highly conductive in
contrast to its previous low conductivity in summer.
In order to aid the draining blockage provided by
the zone 17, an improvement is added as shown in Figure
2, wherein a highly conductive fin or heat pipe 18 is
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g
provided to conduct heat ~rom close to the road surface
down to the zone l7. The in 18 is perforated at its
base (between the layer 13 and the zone 17) in order to
continue to allow drainage of the layer 13 in summer.
The fin 18 could be rrleta1~ or totally made of or include
wick-type geofabrics in conjunction with perforations to
enhance blockage.
Figure 3 shows an embodiment wherein the shoulder
drainage/blockage is achieved by means of a wick-type
geofabric layer 19 delimiting the variable K layer 13 at
the shoulder, and freezing due to its proximity to the
top surface thereby blocking drainage.
In Figure 4, the variable K layer 13 continues down
the shoulder of the embankment, but is interrupted by
lS geotextile wick barriei^s 20 and 21, which freeze due to
their contact with a similar barrier 22 that is close to
the exposed embankment shoulder 23 and the atmospheric
cold air in winter.
Figure 5 shows the result of a computer model
simulating equilibrium thermal behaviour of permafrost
construction using a conventional insulating layer in
place of the variable K layer 13. The dotted curve F
shows the protile after the first five years, while
curve E shows the profile of temperature vs. depth after
equilibrium has been reached.
In Figure 6 the thermal behaviour simulation is
that using a heat sink layer rather than an insulating
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layer as in Figure 5. As may be seen, the thermal
behaviour is quite similar at a depth of lm down to 8m,
where the equilibrium temperature is slightly above 0C.
Figure 7, on the other hand, shows the results of
simulation using the variable K layer 13 of the present
invention, where at 8m depth the equilibrium temperature
is below 0C, with an improved (lower) temperature
profile above, including a lower temperature than
surface temperature in contrast to the results of the
simulation as shown in Figures 5 and 6.
