Note: Descriptions are shown in the official language in which they were submitted.
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FLOATING FOUNDATION SYSTEM
Field of the Invention
This invention relates in general to
foundations, and more particularly to a floating
foundation module for use in discontinuous permafrost or
in areas having unstable soil conditions.
Background of the Invention
Regions of discontinuous permafrost such as
found in far Northern settlements, are typically
characterized by an upper layer of 2-3', in depth that
thaws in the summer and freezes in the winter, and a
lower layer that is permanently frozen.
Buildings that are erected on discontinuous
permafrost are commonly built on foundations of stacked
lengths of 8" x 8" timber, one or more levels of timber
being in an excavation in the aforementioned upper layer
and the remainder being above grade. Such buildings
are subjected to substantial settlement and movement
during annual temperature and precipitation variations,
and the life of such buildings is substantially less
than that of buildings erected in more temperate
climates.
One prior art approach to overcoming the
problem of erecting structures on unstable soils is
disclosed in U.S. patent 3,626,702 (Monahan). The prior
art patent discloses a flotation method involving back
filling of synthetic polymeric foams into a large
excavation for providing a sub-foundation. A second
rigid layer of concrete is then deposited on the
floating sub-foundation, and the dwelling or other
structure is erected on the second layer.
The system disclosed in the Monahan patent is
system very useful in marshlands for providing a stable
foundation platform on unstable sub-soil below the level
of the surrounding water table. The buoyant nature of
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the back-fill results in flotation which allows for the
construction of structures of greater weight and height
on a given area of land than was before possible.
However, the floating foundation of Monahan
suffers from the same problQms as the first mention prior
art buildings when construc~ed on discontinuous
permafrost. In particular the sub-foundation of foam is
subjected to substantial heaving and movement during
successive freezing and thawing seasons. Furthermore,
the continuous sub-foundati~n of foam in the Monahan
system is subjected to ice lensing whereby the foam
closest to the perimeter of the dwelling thaws first
while the sub-foundation under the center of the building
remains frozen. This results in excessive heaving of the
sub-foundation and consequent cracking or damage to the
overlying layer of concrete.
In accordance wit~ the present invention the
prior art problems of settl~ment and movement are
overcome by provision of a foundation comprising multiple
floatation modules connected to a frame that floats the
building during the time of the year when the upper layer
is soft and unfrozen. Each flotation module is
preferably connected to the network via a pivoted
resilient support for accomnodating soil movement due to
ice lensing, etc.
SummarY of the Invention
In accordance with an aspect of the present
invention there is provided a foundation supporting a
structure on land which is ~;usceptible to the formation
of discontinuous permafrost, wherein the structure is
characterized by a predetermined bearing load. The
foundation comprises a substantially horizontal frame
carrying the structure, a plurality of flotation modules
resting on the land which is susceptible to formation of
discontinuous permafrost, and a plurality of resilient
pivotal joints connecting respective ones of the
flotation modules to the frame in a distributed
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arrangement, for maintaining the frame and the structure
supported on the land which is susceptible to
discontinuous permafrost in a substantially horizontal
plane.
Description of the Drawings
A preferred embodiment of the present invention
will be described in detail below with reference to the
following drawings, in which:
Figure 1 is a perSpective view of a single
story family dwelling supported by a floating foundation
in accordance with the pres~nt invention;
Figure 2 is a perspective view of a frame
for the floating foundation of Figure 1;
Figure 3 is a plan view of the frame shown in
Figure 2 with a plurality of flotation modules connected
thereto;
Figure 4 is a perspective view of a flotation
module in accordance with a preferred embodiment of the
present invention; and
Figure 5 is a perspective view of a flotation
module having a modified pi~otal connection in accordance
with an alternative embodiment of the present
invention.
Detailed Description of the Invention
Turning to Figure 1, a standard single story
family dwelling 1 is shown ~ounted on a frame 3 supported
by a plurality of flotation modules 10
pivotally connected to the frame 3.
The floating foundation rests on the surface of
the active layer of discontinuous permafrost requiring
only a rough levelling of the active layer. This
eliminates any requirement for excavation or heavy
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equipment and also prevents the destruction of natural
drainage patterns.
A unique property of the floating foundation
comprising frame 3 and flotation modules 10 is the
ability to support the load of the structure or dwelling
although the bearing capacity of the active layer of
permafrost may be reduced significantly during seasonal
thawing cycles. Differential settlement, frost heave
and racking are completely avoided since the floating
structure is able to sustain constant and adequate
structural support by distributing the load and self-
adjusting to vertical displacements resulting from
seasonal thawing and freezing cycles.
With reference to Figure 2, the frame 3
comprises of a pair of parallel double ply header beams
12 each preferably comprising 1 3/4" x 14" x 36' boards
and six 24' three ply spanning beams 14, each ply
preferably comprising 1 3/4" x 14" x 24' boards.
To match the frame design and provide an equal
loading pattern, a twelve point support system is used
for connecting the flotation modules 10, as shown in
Figure 3.
The flotation modules 10 are connected at the
twelve points along respective joints between header
beams 12 and spanning beams 14. Thus, the frame 3
carries the dwelling 1 (Figure 1) without any central
supports, thereby avoiding the consequences of
differential thawing patterns and ensuring that each
flotation module 10 is easily accessible for servicing
or inspection.
The loads for a typical Northern housing unit
measuring 26' x 24' and located in the discontinuous
permafrost zone, are
as follows:
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Dead load = 20 psf
Live load = 40 psf
Snow load = 36 pst
Total = 96 psf (approximately 100 psf)
Therefore, the design load of the structure is
= 100 psf x 36 foot x 24 foot
= 86,400 lbs.
Number of support points = 12
Therefore, the loading per support point = 7,200 lbs.
The bearing capacity of soft clay has a
maximum allowable pressure of 835 lbs. per square foot,
(National Building Code). However, with the
introduction of water during the thawing process this
can be greatly reduced. Assuming that 60% of the
allowable bearing capacity is lost due to water, the
active layer of discontinuous permafrost would be
characterized by a bearing capacity of approximately 335
lbs. per square foot.
Therefore, the size of each flotation module
should be 7,200 lbs./335 lbs. per square foot = 21.5
square feet.
With reference to Figure 4, the flotation
module 10 is shown in greater detail comprising a float
pad frame 16 comprising a plurality of preferably 2' x
6' planks 17 connected via brackets in a straightforward
manner. Four foam filled flotation elements 18 are
connected to predetermined ones of planks 17 on the
underside of the frame 16 via mechanical fastening such
as washer and lag screw, strapping, etc.
The float plank frame 16 operates to transfer
the bearing load from the house frame 3 to the floats
18.
Each flotation element 18 is preferable in the
form of blow molded or rotary molded polymer block
foamed in place with polystyrene to provide buoyancy.
Other buoyant materials could be used in place of
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polystyrene, e.g. urethane, and it also would be
possible to employ a hollow housing filled with air.
The latter is not preferred, however, since it could be
punctured as a result of vandalism or through an
accident resulting in a loss of buoyancy for the
housing.
According to the successful prototype of the
invention, the four floats or flotation elements 18 on
each module 10 provide a surface area of 24' x 24' which
is more than the aforementioned required 21.5 square
feet for the standard Northern housing design.
Furthermore, the floats 18 provide an additional uplift
force due to buoyancy of up to approximately 430 lbs.
per float. Thus, the preferred four floats or floating
elements 18 per module 10 provide adequate support with
a significant safety margin.
A central reinforced portion 20 of the pad
frame 16 has a resilient pivotal connection 22 mounted
thereon. According to the preferred embodiment, the
pivotal connection 22 comprises a pivot joint biased by
a compression spring. The spring consists of a 1-1/8
bar stock material which is capable of supporting 7,200
lbs. at full compression and a change in vertical
displacement of approximately 3.5".
An elastomeric hinge 24 is connected by a
corresponding bracket to each of the four outside
corners of the pad frame 16 for inter-connecting
adjacent ones of the 14 flotation modules 10.
The elastomeric hinges 24 allow each
individual flotation module 10 to move vertically for
adjusting to seasonal soil conditions. The hinges 24
also allow for distribution of the bearing load among
successive ones of the modules 10. At the same time,
the hinges 24 inhibit the float pad frame 16 from
moving horizontally.
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Turning briefly to Figure 5, an alternative
embodiment of the pivotal connection 33 is shown
comprising a compressible columnar enclosure 32 filled
with one of either hydraulic fluid or air, and a pair of
hydraulic coupling tubes 34 for inter-connecting
respective enclosures 32 on adjacent modules lO.
In operation, upon compression under load of a
predetermined one of the enclosures 32, hydraulic fluid
or air passes via couplings 34 to adjacent modules lO
thereby permitting vertical deflection of individual
ones of the modules lO while maintaining even load
distribution throughout the foundation. It is
contemplated that the hydraulic fluid may be selected so
as to freeze during the cold season for maintaining a
rigid foundation until spring thaw begins.
In summary, the floating foundation of the
present invention is characterized by the advantages of
low cost construction, small and lightweight components
requiring limited air transport and easy assembly using
common carpentry tools. The load on each module lO is
distributed equally for avoiding uneven settlement
during spring thawing of discontinuous permafrost.
Central supports under the middle of the housing units
are eliminated for avoiding differential thawing
patterns (e.g. ice lensing) and to ensure that each
module is accessible for inspection or servicing.
Furthermore, the floating foundation of the present
invention may be installed with minimal excavation,
thereby avoiding the requirement for heavy equipment as
well as preserving natural drainage patterns.
Other embodiments or variations are possible
without departing from the sphere and scope of the
present invention as defined by the claims appended
hereto.
