Note: Descriptions are shown in the official language in which they were submitted.
CA 02271442 1999-OS-19
Title: "Method and apparatus for thawing ground"
Backcround of the Invention.
l.Field of the Invention
The present invention relates to method and apparatus for
5_ thawing ground, especially in preparation for construction of
buildings such as houses.
2.Prior Art
If construction is to be carried out in the winter in cold
climates, it is usually desirable to thaw the ground before
digging operations begin. This avoids problems with heavy
ripping equipment needed to dig into frozen soil, and also
means that a contractor need not worry that there is frost
under the footings.
Hitherto, ground thawing prior to construction has usually
been done by apparatus which is laid upon the frozen ground and
which heats it from the top. Such apparatus is shown, for
example, in the following patents:
U.S.Pat.No.3,868,825, which issued Mar.4, 1975 to Boyce;
U.S.Pat.No.4,349,010, which issued Sept.l4, 1982 to
Bentham;
U.S.Pat.No.5,033,452, which issued Ju1.23, 1991, to
Carriere;
U.S.Pat.No.5,441,038, which issued Aug.l5, 1995 to Ohmann;
and
U.S.Pat.No.5,820,301, which issued Oct.l3, 1998 to
Bruckelmyer.
I have found that such apparatus, which heats the ground
1
CA 02271442 1999-OS-19
from the surface, is inefficient and cannot produce sufficient
depth of thawing for construction, over the kind to area needed
for a house, say 1,200 sq.ft., in reasonable time. Firstly,
with such apparatus, at low temperatures such as -30° or -40°
5_ F, much heat supplied to such probes is lost to the air.
Secondly, since heat rises, there is little natural tendency of
the heat to penetrate into the ground. This problem is made
worse by the drying of the upper layers as they are heated,
since the dry air acts to insulate the lower layers from the
heat source. I have found that such surface heaters, in cold
weather conditions, can only thaw the ground to a depth of 2 to
2.5 feet in one week. The time (of around 2 to 3 weeks) and
energy required to thaw to a depth of 5 to 6 feet, as is needed
in house construction, is unacceptable.
Some of the designs shown in the patents have other
problems. Thus, the Bentham system needs sand or other material
as a cover; in really cold weather there is little such
material which is available and not frozen solid. This patent
also uses gas burners, and such burners need attention or they
can become hazardous. With this Bentham arrangement, if one of
the burners went out the raw gas would tend to settle, and
other burners could ignite the gas . The LP gas container is
exposed, and for safety reasons such an arrangement should not
be left unattended. Similar comments apply to the Carriere and
Ohmann patents which also use gas.
The Bruckelmyer patent has the problem that the moisture,
when thawed, will create wet and muddy conditions which make it
hard for equipment to move around.
Apparatus has been suggested which involves placing
heating tubes vertically in the ground. Thus, U.S.Pat.No.
2
, CA 02271442 1999-OS-19
3,293,863 to Cox et al. describes apparatus for thawing ground
in the vicinity of a sub-surface utility installation,
including a tubular heater which is inserted into a vertical
bore. The heater is supplied with L.P. gas which is burnt in a
5_ burner near to the bottom of the heater. Exhaust gas leaves the
probe via vents at the top of the heater. The heater probe is
designed to have its own gas tank, and the arrangement is not
suitable for thawing a whole large area, as needed for
construction of houses . Again, a probe using gas in this way is
1Q likely to need constant supervision in order not to be
hazardous.
U.S.Pats.Nos.5,181,655 and 5,449,113 of Bruckelmyer show
water heated probes primarily intended for thawing a frozen
sewer or water line. The ' 655 patent suggests that these probes
15 can also be used to thaw construction materials, for example
being inserted into holes in bricks. The holes in bricks are
usually less than 1.5 inches in diameter, meaning that
Bruckelmyer's probes must be quite small, well under 1.5 in
outer diameter. The length is not given, but considering the
20 proportions shown in the drawings it seems that the length
would be 18 inches or less. The upper ends of the probes have
standard pipe fitting type elbows which have diameters similar
to those of the probes, and which, given the small diameter of
the probes, necessarily extend out way beyond the probe
25 surfaces. Such projecting elbows would easily be damaged if a
group of the probes were to be stacked on top of each other.
These patents suggest using an anti-freeze solution which is
likely to become lost if the probes are disconnected from the
hoses when moving the apparatus.
A system for use in mining is known from Can. Pat.
No.208,350, which issued to Evans on Feb.B, 1921, and which
3
CA 02271442 1999-07-15
uses vertical pipes feeding water into the ground. If used for
construction work, the large amount of water put into the
ground would result in very soft and muddy ground conditions.
Such a system also needs a constant supply of water.
Summary of the Invention
The present invention provides a system for thawing ground
to make it suitable for construction, and which may be used
when ambient temperatures are well below freezing, and which is
safe with little supervision. The probes are long enough to
thaw ground to the kind of depth needed for basements, say to
5 or 6 feet . While the system leaves moisture in the ground, it
does not add to it so as to cause the ground to become overly
soft . The probes have means to prevent loss of anti-freeze when
hoses are detached.
In accordance with one aspect of the invention, a method
for thawing an area of frozen ground comprises:
drilling a series of generally vertical bores at a
generally even spacing of between about 3 and about 6 feet;
inserting into each bore a heating probe comprising an
outer tubular casing and an inner delivery pipe extending
within the casing and having an outlet communicating with the
lower end of the casing, the casing having an outer diameter of
at least 2.5 inches and a length of at least 4.5 feet, and
preferably at least 5 feet,
connecting the inner pipe to a source of hot liquid and
connecting the casing to an outlet for cool liquid; and
pumping hot liquid from the source to each inner pipe so
that it flows through the pipe and tubular casing of each
heating probe and exits through the outlet and returns to the
4
CA 02271442 1999-OS-19
source, thereby thawing the ground surrounding the bores.
Preferably, several of the heating probes are connected in
series, with the outlet of one being connected to the inlet of
the next. A series of the heating probes may include three or
5_ more such probes forming a first row. A second row of several
of said heating probes may also be connected in series, a first
heating probe of the first row and a first heating probe of the
second row both being directly connected to the source of hot
liquid, so that the rows themselves are connected in parallel.
The bores are usually drilled on a grid pattern and spaced
between 4 and 5 feet apart. The tubular casings are preferably
at least 3.0 inches in outer diameter.
In accordance with another aspect of the invention,
apparatus for thawing ground includes a series of heating
~5 probes, wherein each heating probe comprises:
an outer tubular casing having a diameter of at least 2.5
inches, preferably 3 inches, and a length of at least 4.5 feet
and having a closed lower end, and
an inner pipe extending within the casing and having an
open lower end communicating with the outer tubular casing near
its lower end.
The tubular casing has an upper end closure provided with
first and second apertures, apart from which the casing is
closed. The first aperture accommodates and is sealed around an
upper end portion of the inner pipe, and the second aperture is
in sealed communication with an outlet pipe element. An inlet
pipe connector is provided for inlet of hot liquid at the upper
end of the inner pipe, and an outlet pipe connector is provided
at the outer end of the outlet pipe element.
5
CA 02271442 1999-OS-19
Preferably, the inlet pipe connector is joined to the
inner pipe by a first elbow, and the outlet pipe element
includes a second elbow, both the elbows and pipe connectors
attached thereto being arranged so as to be entirely within a
5_ cylindrical surface corresponding to an extension of the
tubular casing. This allows a large number of the probes, such
as the number required for thawing a house site, to be stacked
up without any of the elbows or pipe fittings attached to the
elbows extending out beyond the cylindrical surfaces of the
respective probes and thus protects such elbows and fittings
from damage.
Preferably also, the connectors include valves which close
automatically when hoses are disconnected therefrom.
Brief description of the drav~inqs.
A preferred embodiment of the invention will now be
described by way of example with reference to the accompanying
drawings, in which;
Fig.l is an elevation of a heating probe;
Fig.2 is a view of the heating probe on lines 2-2 of
Fig. l;
Fig.3 is a plan view of the same heating probe, on lines
3-3 of Fig. l;
Fig.4 is a perspective view of the apparatus in use
thawing an area of ground, and
2_~ Fig.5 is a vertical section through ground being thawed by
one of the heating probes.
Detailed Description.
Figs.l to 3 show one of the heating probes 10; typically
6
CA 02271442 1999-07-15
100 such probes will be used to thaw an area of ground, say of
1,200 sq.ft., suitable for a house.
Each probe has an outer tubular, cylindrical casing 12,
closed at its bottom end by a welded plate 13. The casing is
formed from standard 3 inch (internal) diameter pipe, having an
outer diameter of 3.5 inch. The length of the probe will be
typically be between 5 and 8 feet long.
The top end of the casing 12 is closed by a top plate 14
which has two apertures providing the only inlets or outlets
into the casing. A first aperture accommodates and is sealed
around an upper end portion of a delivery pipe 16, and a second
aperture is sealed around the lower end of an outlet pipe
element in the form of elbow 18 which provides the sole outlet
from the space within the casing. The upper end of pipe 16 is
also provided with an elbow, indicated at 19, through which hot
liquid can be fed to the probe via the pipe. The pipe extends
within the casing, spaced away from the casing walls, and a
lower, open, end of the delivery pipe, which is located near to
the bottom end of the casing, is held in a radially central
position by means of a spacer tab 20 welded to the side of the
pipe near its lower end.
The upper plate 14 extends slightly beyond the cylindrical
surface of the casing so as to limit its descent into a close
fitting bore drilled for it, as will be described. A lifting
loop 22 is provided above the plate 14 to allow extraction of
the probe.
Each elbow 18 and 19 is provided with an internally
threaded pipe connector 24; these are of the "ball valve
connector" type which include a ball valve which closes the
7
CA 02271442 1999-07-15
connector when it is not attached to a hose. These connectors,
and the elbows themselves, are arranged to be entirely confined
within a cylindrical surface corresponding to an extension of
the casing outer surface, so that the probes can be rolled
around and stacked without these parts coming into contact with
and being damaged by the ground or by other probes.
The diameter of the casing is many times that of the
delivery pipe 16, which is typically a 1/2 inch internal
diameter pipe, so that when heated liquid is supplied to the
pipe and leaves its lower end the hot liquid passes up the
casing relatively slowly. Preferably the internal area of the
casing surrounding the pipe is more than 20 or 30 times the
internal area of the pipe, and in the preferred embodiment is
about 40 times the internal area of the pipe, so that the
residence time in the casing is relatively long.
Figs.4 and 5 show the probes in use. The ground to be
thawed has been prepared by having vertical bore holes H
drilled into it on a square grid pattern, the bore holes having
a diameter of about 3.5 to 4 inches, and having a 4 to 5 foot
spacing. The probes are placed within the holes, and connected
by hoses to a mobile boiler 30 in a trailer which includes a
pump and which supplies hot liquid to the probes . The boiler is
arranged to be locked when not being under supervision, to
avoid any tampering or accidents. As shown, the arrangement
includes a series of rows of heating probes indicated A, B, C,
D. Normally, more (say 100 or so) heating probes will be used
to thaw an area of say 1, 200 square feet suitable for a typical
house.
The heating probes 10 are all connected to the boiler 30
by insulated hoses comprising outlet conduits 32 and return
8
CA 02271442 1999-07-15
conduits 34, attached to the connectors 18 and 19. The conduits
connect the heating probes of each row in series, the outlet of
one being connected to the inlet of the next, so that the hot
liquid flows through each probe of the row in turn. The first
probe of each row is directly connected to the outlet from the
boiler via conduits 32a, 32b, etc., and the last probe of each
row is connected to a return conduit 34a, 34b, etc . leading
back to the boiler, so that the rows themselves are connected
in parallel.
In operation, the area being thawed by the probes is
covered with an insulated tarp, a portion of which is shown at
36. Heated liquid containing anti-freeze is pumped from the
boiler through the conduits 32, 34 and into successive delivery
pipes 16, passing to the bottoms of the probe casings and
moving up the probes while radiating heat into the surrounding
ground. Each probe will thaw an area around it, reaching out to
a radius of about 2 to 2.5 feet; the pattern of thawing is
shown in Fig.5, where FS is frozen soil and TS is thawed soil.
The method allows moisture to remain in the ground, but does
not add to it . Vehicles can travel on the thawed ground without
encountering mud or soft areas . The ground is easy to excavate .
The system is also safer than those which use gas burners
to produce heated gas. The heating apparatus, i.e. the boiler,
remains in a trailer which can be locked. The hot probes are
submerged, and the hot liquid passes through insulated hoses.
The system does not require superheated liquid, or high
pressure, so there is little chance of injury even if leaks
occur. To move the apparatus, the probes are simply pulled up,
the tarps are wrapped up, and hoses coiled for transport.
The valued connectors on the probes prevent anti-freeze
solution being lost when the hoses are detached therefrom.
9
