Note: Descriptions are shown in the official language in which they were submitted.
`- 21S3941 BAL~K~u~D OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a road construction,
particularly for use in a region having a permafrost layer in
the ground. More specifically, the present invention relates
to a roadway which allows natural convection of the pore air
in the porous embankment to occur during winter to remove heat
from the ground under the roadway, thus preserving the
permafrost layer throughout the year.
BACKGROUND ART
In northern regions designers are often forced to locate
roadway, railway, or airport embAnk-?nts in areas that are
underlain by permafrost. Permafrost in sub-Arctic regions
tends to be warm (i.e., near 0C) and therefore susceptible to
thaw. Often the permafrost layer contains inclusions of thick
ice lenses or ice wedges making consolidation likely if
thawing occurs. Beneath engineering structures, such as
roadway embankments or buildings, thaw consolidation typically
results in instability and failure of the structure.
Roadway embAnhm~nts constructed in permafrost regions
usually have a large influence on the thermal regime of the
ground. This occurs primarily because the embAnk~^nt modifies
the pre-existing surface conditions and the associated
2153941
ground-surface energy balance. The ground-surface energy
balance is a complex function of seasonal snow cover,
vegetation, atmospheric radiation, surface moisture content,
and atmospheric air temperature. These factors produce a mean
annual surface temperature (MAST) which may differ by several
degrees from the mean annual air temperature (MAAT). At
undisturbed sites the MAST is typically warmer than the MAAT
because of the large impact of the insulating snow layer
during winter months. Disturbance of the ground surface due
to construction of a roadway embankment often increases the
difference between MAST and MAAT, resulting in a warmer
surface condition. This often results in thawing of the
permafrost, causing settlement of the roadway embankment.
The lack of effective methods for preventing or
mitigating thaw settlement has left many northern communities
with large maintenance bills and sub-standard roadways.
Consequently, techniques for avoiding damage to roadway
emb~nkments in permafrost regions have great importance.
Research in the prior art has produced in a number of
techniques to protect embankments from thaw settlement. In
general, these techniques can be grouped into three
categories: (1) those that modify conditions at the
embankment surface in an effort to reduce the MAST; (2) those
that augment the removal of heat from the embankment structure
2153941
during winter months; and (3) those that employ insulation
within or beneath the emb~nkment. Surface modifications that
have been studied include painting the asphalt surface to
increase the albedo and the use of snow sheds and snow removal
on embankment side slopes. Other attempted solutions include
the operation of an experimental air duct system, use of
thermosyphons in roadway embankments, use of foam insulation
in roadway and airfield embankments, and use of insulating
materials in order to reduce embankment fill requirements in
Arctic regions. Each of these techniques has suffered from
some combination of limited effectiveness, high cost, high
maintenance, or safety concerns when applied in the field. As
a result, these methods have been employed only on an
extremely limited or research basis. Accordingly, the vast
majority of roadways constructed in sub-Arctic environments
still lack protection from damage or complete failure due to
thaw settlement.
A number of the techniques described above have been the
subject of previous Canadian and U.S. patents. The closest
known prior art is Canadian patent 2,051,024 granted to Laurel
E. Goodrich March 11, 1993. This prior art patent pertains to
trafficked surfaces located in permafrost regions and
discusses the problems associated with road construction in
such areas. In particular it discusses the fact that, in most
cases, mere provision of an insulating layer within the
21S39~1
emb~nkment is not sufficient to protect the permafrost
foundation and roadway from thaw settlement. This prior art
patent describes the use of a variable heat conductivity layer
within the embankment, the intention of which is to lower the
mean annual temperature of the permafrost foundation. The
variable conductivity layer is described as being achieved by
the inclusion of a variable heat conductivity (K) layer
between the permafrost and the trafficked surface. The patent
further specifies that the variable K layer is more conductive
of heat in the winter than in the summer. The patent
discloses that 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 basis of this patent is that
the high K in the winter lowers the permafrost temperature
because of the better heat conduction between the permafrost
and the cold atmosphere. It teaches that in many specific
situations the annual mean temperature of the permafrost layer
may be lowered from year to year as a result until it reaches
a new equilibrium value.
The patent then goes on to describe how the variable K
layer is to be constructed, consisting generally of a layer of
material in which the moisture content is altered on an annual
basis. Such alteration of the moisture content changes the
21539~1
.
heat conductivity of the layer thus providing a larger thermal
conductivity in winter than in summer.
The present invention improves on the prior art in at
least two specific ways. First, the present invention does
not rely on the ability to change material characteristics
(such as moisture content) throughout the course of the annual
cycle. This avoids the complexity of intricate layer
geometries, heat sinks, impermeable membranes, and the ability
to introduce and drain moisture, as described in the Goodrich
patent. Secondly, since the present invention relies on
natural convection, rather than variable heat conductivity to
increase the winter-time heat transfer out of the emb~nkment,
it is generally much more effective at reducing the mean
annual temperature of underlying permafrost. This is a
consequence of the fact that thermal convection can be a much
more effective heat transfer process than thermal conduction.
It is well known that thermal convection in a cavity of porous
material can increase the heat transfer by more than an order
of magnitude over that due to thermal conduction alone.
2153941
SUMMARY OF THE INVENTION
In accordance with the present invention a roadway is
constructed having an embankment containing a highly porous
material and a pavement structure placed on the top of the
embankment. The high permeability of the material comprising
the embAnk~ent allows natural convection of the pore air to
occur within the emb~nk~-nt during winter months when unstable
air density gradients exist therein. This convection removes
heat from the embAnkment and underlying ground during the
colder, winter months. During summer, density gradients in
the embankment are stable and no circulation occurs. Thus,
the emb~nkment works analogous to a one-way heat transfer
device, or thermal diode, which removes heat effectively from
the emb~nkr?nt and underlying ground during winter without
re-injecting heat during the subse~uent summer. This
increased cooling during the winter prevents thaw of
underlying permafrost in the summer, thus eliminating thaw
settlement. Analysis shows that winter-time convection in the
present invention can lower foundation ground temperatures
beneath such embankments by as much as 6C on an annual
average basis compared to standard sand and gravel
embankments.
To ensure sufficient buoyancy-driven pore air convection
occurs within the embankment, the material of the emb~nl Ant
must have both sufficient permeability and an adequate
2153941
vertical separation between the bottom and the top of the
embankment. The permeability of the emb~nkment is controlled
primarily by the particle size and porosity. To achieve
enhanced cooling it is necessary that this material have an
intrinsic permeability of 100,000 Darcy (approximately lxlO-
~cm~ or lxlO-~ ft2) or greater. These permeability requirements
can be met by using poorly graded alluvial, gravel, or crushed
rock with a low fines content.
The vertical separation between the bottom of the
embankment and the top of the embankment should be at least
one meter. However, the larger the vertical separation, the
better the convection. The minimum height requirement is
related to the permeability of the material being used in an
inverse relationship. That is, the greater the permeability
of the embankment, the less height of the emb~nkment that is
required and vice-versa.
The pavement structure, which is placed on the top of the
emb~nkment, has no special material requirements. Materials
typical of present construction practice, usually consisting
of compacted sand/gravel disposed underneath asphalt, can be
used. It is possible, however, to pave an asphalt layer
directly on the top of the embankment. The thickness of the
pavement structure should be ri n;r;zed. It is desired that
the temperature at the lower side of the pavement
21539~1
structure/top of the embankment be approximately as cold as
the upper side of the pavement structure which is exposed to
low ambient temperatures. If the thickness of the pavement
structure is too great, it will begin to reduce the strength
and effectiveness of the natural convection occurring within
the embankment because of a reduced temperature differential
existing between the bottom and top of the embankment.
It is important to substantially prevent entry of fines
into the embankment to maintain a high permeability. The
roadway, therefore, can optionally have separators positioned
intermediate the bottom of the embankment and the ground
surface, intermediate the top of the emb~nk~ent and the
pavement structure, or on the sides of the emb~nkr?nt. If
fines enter the emb~nkrent, they will reduce the permeability
and hinder natural convection.
BRIEF DESCRIPTION OF THE DRANING
The preferred reference of the present invention will now
be described in detail in conjunction with the annexed
drawing, in which:
Fig. 1 is a schematic representation of the component
parts of the present invention.
DESCRIPTION OF THE INVENTION
21539~1
Referring to Fig. 1, a roadway 10 of the present
invention is shown. This roadway 10 is designed to m;nim; ze
thawing of permafrost ground thereunder. The embodiment of
the roadway 10 illustrated in Fig. 1 can be used, for example,
as a driving surface for automobiles. Other configurations
can be used and are specifically contemplated in the
definition of "roadway. n Another use is an airport emb~nkment
which would have a much larger width compared to its height.
Likewise, the roadway 10 can be used to support railroad
tracks for trains.
The roadway 10 comprises a porous embankment 20 having a
bottom 22 adjacent the ground G, an opposite top 24, and two
sides 26 and a pavement structure 30 having a lower side 32
adjacent the top 24 of the embankment 20 and an opposite upper
side 34. The embankment 20 has a desired vertical separation
between the bottom 22 and the top 24 of the emb~nkme.^t 20 and
comprises material of sufficient permeability to allow
buoyancy-driven pore air convection to occur within the
embankment 20 when an unstable density stratification exists
therein due to a temperature differential between the ground G
and the top 24 of the e-m-bAnkm-nt 20. Thus, the roadway 10
promotes natural convection within the porous e-mb~nkmAnt 20
which enhances heat removal from the embankment 20 and
underlying ground G to preserve the permafrost layer
throughout the year.
21 5~941
More specifically, low ambient temperatures act on the
upper side 34 of the pavement structure 30 during winter
months. Similar low temperatures also exist at the lower side
32 of the pavement structure 30/top 24 of the emb~nkment 20 by
conduction heat transfer through the pavement structure 30.
This temperature difference between the top 24 and bottom 22
of the emb~nkment 20 creates an unstable density
stratification. Buoyancy-driven convection of the pore air
occurs as a result of the unstable density gradient. The
resulting convection enhances the upward transport of heat out
of the roadway 10 during winter months, thus cooling the lower
portions of the emb~nkrent 20 and underlying foundation ground
G. During summer months the density stratification is stable
and little, if any, convection occurs. Consequently,
summer-time heat transfer is ~9~; n~ted by thermal conduction
which transports heat less effectively. The roadway 10, in
other words, promotes enhanced winter-time cooling of the
embankment 20 and underlying ground G, thus avoiding the
thawing of permafrost and associated maintenance costs in the
warmer, summer months.
Two main, interrelated factors ensure sufficient
buoyancy-driven pore air convection can occur within the
embankment 20: ~1) using material having sufficient
permeability and (2) using a desired vertical separation
between the bottom 22 and the top 24 of the embankment 20.
21S39~1
The material of the e_bankment 20, which can also be referred
to as the e_b~nkr?nt core, lies beneath the pavement structure
30. The permeability of the material used in the emb~nkment
20 is controlled primarily by the particle size and porosity.
To achieve enhanced cooling, it is necessary that this
material have a m;n;mllm intrinsic permeability of 100,000
Darcy (approximately lxlO-- cm or lxlO-- ft~). In general, there
is no ideal range for the particle size. But, as a rule, it
is better to have a larger size of the material and a smaller
range, or size distribution. The larger the size of material,
the larger the permeability. The more uniform the size of the
material, the larger the permeability. If too large a range
of particle diameters are used, then the smaller particles
will fill the voids between the larger particles, thus
reducing the porosity and reducing the permeability.
The material requirements for adequate permeability can
be met by poorly graded alluvial or crushed rock with a size
distribution that meets the following specifications:
substantially all of the material of the embankment 20 has a
r;n;rllm diameter ~D~) of at least one centimeter and a maximum
diameter (D ) of up to five times D~ of the material of the
emb~nkment 20. More preferably, however, D~ is 5 centimeters
and the maximum diameter of up to three times D.. The cooling
effect can be enhanced by increasing D~ above five
centimeters, but material larger than three times D~ should be
2I539~1
excluded. These specifications would typically be met by
screening to remove material smaller than D~ or larger than
D
For the other factor, the vertical separation between the
bottom 22 and the top 24 of the embankment 20 should be at
least 1 meter. As a rule, the larger the vertical separation,
the better the convection. Although there is no maximum
height, in practical embankments the height is kept less than
the width for material stability reasons. In cases where
water is present above the surface of the ground G, the
vertical separation should be measured from the water surface
rather than from the bottom 22 of the embankment 20. Also,
the surface of the ground G may be lower than the level of the
ground adjacent to the embankment 20 wherein the vertical
separation is still measured between the bottom 22 and the top
24 of the embAnk~ent 20.
There is a certain minimum height required to achieve the
desired natural convection. The minimum height requirement is
related to the permeability of the material being used. This
interrelationship is mathematically represented by the
Rayleigh number. The Rayleigh number is defined as:
Ra Po g ~ K ~ ~r
~ a
21539~1
where pO is the density of the pore air, g is acceleration due
to gravity, ~ is the thermal expansion coefficient, K is
intrinsic permeability, ~T is the temperature difference
between the upper boundary (the top 24 of the embankment 20)
and lower boundary (the surface of the ground G/the bottom 22
of the emb~nkment 20), H is the vertical separation of the
emb~nk~ent 20, u is the dynamic viscosity, and a is the
thermal diffusivity of the medium. As can be seen, the
numerator of the Rayleigh nu_ber contains both the height, or
vertical separation, of the embankment 20 and the permeability
of the material of the embankment.
For Rayleigh numbers less than approximately 40, natural
convection is weak and heat transfer in the embankment is
dominated by thermal conduction. For Ra larger than this
value natural convection becomes important and increases the
heat transfer from the lower to the upper boundary over that
due to conduction alone. Larger values of Ra result in
stronger natural convection and are generally desirable in the
present application. In constructing the roadway 10, the
variables that can be adjusted in order to increase Ra are the
vertical separation of the embankment 20 (H) and the
permeability of the emb~nkr?nt 20 (K). It should be realized
that the height of the emb~nkr~nt 20 must be increased to
allow convection of sufficient strength if the material of the
2153941
embAnk~ent 20 has a lower permeability. Likewise, the height
can be lower if there is a higher permeability.
Also of note, Ra increases as the temperature difference
increases. Thus, when the upper side 34 of the pavement
structure 30 is cleared of snow in the winter, its surface
temperature decreases to ambient. This creates a greater
temperature differential, ~T, between the top 24 of the
embankment 20 and the ground G. Accordingly, this greater ~T
increases Ra (and convection), removing more heat from the
ground G.
As to the pavement structure 30, it normally consists of
the driving surface covering disposed above a supporting
layer. There are no special material requirements for the
pavement structure 30 and materials typical of present
construction practice, usually consisting of compacted
sand/gravel disposed underneath asphalt, can be used. It is
possible, however, to pave an asphalt layer directly on the
top 24 of the embankment 20.
The pavement structure 30 is not part of the embAnkment
20 and the materials required therefor. However, the vertical
separation between the upper side 34 of the pavement structure
30 and the top 24 of the emh~Ankment 20 should not exceed 0.5
meters for an asphalt-based pavement structure 30. If the
2153~ql
thickness of the pavement structure 30 is too large, there
will be a greater temperature difference between the upper
side 34 exposed to the low ambient temperatures and the lower
side 32. This relatively warmer lower side 32 will reduce the
strength and effectiveness of the natural convection occurring
within the emb~nkment 20 because of the decrease in the
temperature differential between the top 24 and the bottom 22
of the embankment 20 which drives the pore air convection. A
thickness of greater than 0.5 meters may be acceptable if the
heat transfer effectiveness of the pavement structure 30 is
greater than that of asphalt/gravel.
The roadway 10 can further comprise a means, disposed
intermediate the top 24 of the emb~nkment 20 and the lower
side 32 of the pavement structure 30, for preventing fines
from the pavement structure 30 from migrating into the
embank-~nt 20. When the pavement structure 30 consists of the
asphalt plus some fine sand/gravel material beneath it, the
preventing means is needed to keep this fine material from
dropping down into the embankment 20 and filling the pores,
thus reducing permeability. The preventing means is not
necessary if an asphalt layer is placed directly on the top of
the emb~nkr?nt 20 without using sand, small rocks, or other
similar materials intermediate the top 24 of the embankment 20
and the underside of the driving surface covering.
21 539~1
The preventing means can be any of a variety of
separators 40 typically used for the purpose of avoiding
material contamination by fines. The separator 40 is disposed
on the top 24 of the emb~nk~^nt 20 and stretches horizontally
from one side of the embankment 20 to the other. The
separator 40 can be thick geofabric sheeting or any other
separator which would be strong and capable of stopping fines,
such as sand, small rock particles, and the like, from
dropping into the embAnkm~nt 20. It is preferred that the
prevention means be a geotextile separator which consists of a
geotextile fabric material of sufficient strength and weave
tightness to ensure that fine material contained in the
pavement structure 30 does not fall or migrate into the
material of the embankment 20.
The roadway 10 can further comprise a means for covering
both sides 26 of the embAnkr^nt 20. The covering means
preferably is included in windy areas to prevent intrusion of
warm, summer-time air into the embAnkr^nt 20. The covering
means can be soil 27 and a side separator 28 which is disposed
intermediate the soil 27 and the side 26 of the embankment 20.
The soil 27 can consist of any combination of fine sand and
gravel or topsoil. The side separator 28 itself is not
required for proper operation of the embankment 20, but should
be included if soil 27 or any other slope covering is disposed
on the sides 26 of the embankment 20.
21 539ql
The side separator 28, preferably a geotextile separator,
prevents migration of fines into the embankment 20. The side
separator 28 must be of sufficient strength and weave
tightness to ensure that soil 27 contained on the sides 26 of
the embankment 20 will not fall or migrate into the em~b~nkment
20.
The roadway 10 also can further comprise a means,
disposed intermediate the bottom 22 of the embankment 20 and
the ground G, for isolating the embankment 20 from the ground
G. This prevents migration of fines upward into the
embankm~nt 20. The isolating means can be a bottom separator
50, preferably a geotextile separator. This bottom separator
50 requires sufficient strength and weave tightness to ensure
that fine material contained in the ground G does not migrate
upward into the embankment 20. It is not required for proper
operation of the embankment 20, but provides additional
assurance that the material in the embankment 20 will not be
contaminated with fines.
The ground, G, or foundation layer, normally corresponds
to the original ground surface after clearing of vegetation
and leveling. In general, the ground G consists of native
sands, gravel, or soil overlying permafrost. In some
instances, the ground G may consist of backfilled material
moved during construction to create a level surface.
19
2153941
In addition, the roadway 10 can further comprise an
impermeable separator 60 disposed vertically from the bottom
22 to the top 24 of the embankment 20 and also intermediate
the two sides 26 of the embAnkrent 20, preferably along the
centerline of the embAnkment 20. This impermeable separator
60 prevents surface water from flowing laterally through the
bottom 22 of the embAnk~ent 22. The material used for the
impermeable separator 60 can consist of an impermeable high-
strength plastic or rubber sheet. The impermeable separator
60 is not required for proper operation of the roadway 10, but
is preferably included if the embankment 20 is constructed in
areas where surface water is likely to move through the
embAnkre~t 20. Other impermeable separators 60 can be
included in the embankment 20 to shape the convection cells
and tailor heat transfer characteristics.
The present invention also encompasses a method of
constructing a roadway 10 for use on permafrost ground. The
first step is laying an embankment 20 of material having a
desired height and sufficient permeability to allow
buoyancy-driven pore air convection to occur within the
embankment 20 when an unstable density stratification exists
therein due to a temperature differential between the ground G
and the top of the embankment 20. The next step is installing
a pavement structure 30 above the embankment, whereby the
roadway 10 promotes natural convection within the porous
2153941
-
embAnkrent 20 which enhances heat removal from the emb~nkment
20 and underlying ground G to preserve the permafrost layer
throughout the year.
In addition, prior to the step of laying the embankment
20, the method can include the step of positioning a bottom
separator 50 on the ground where the emb~nk~^nt 20 is to be
located. This bottom separator 50 preferably is a geotextile
separator.
Moreover, after the step of laying the embankment 20, the
method can encompass the step of covering the sides 26 of the
emb~nkment 20 with a side separator 28 and then disposing soil
27 on top of the side separator 28. Furthermore, after the
step of laying the embankment 20 and prior to the step of
installing the pavement structure, the method can include the
step of placing a separator 40 over the top 24 of the
embankment 20. And, prior to the step of laying the
embankment 20, the method can also include the step of
positioning an impermeable separator 60 so that the
impermeable separator 60 will be positioned vertically from
the bottom 22 to the top 24 of the embankment 20 and
intermediate the two sides 26 of the embankment 20.
