Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02650182 2015-06-09
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CORRUGATED PIPE WITH OUTER LAYER
DESCRIPTION
Related Application
[001] This application is related to U.S. Patent Application No.
11/078,323,
which was filed on March 14, 2005.
Field of the Invention
[002] The present invention relates generally to corrugated pipe having an
additional outer layer, and more particularly, to such a corrugated pipe for
use in the
drainage of soil and transportation of surface water by gravity.
Background of the Invention
[003] Corrugated pipe has been used in the drainage of water-saturated soil in
various agricultural, residential, recreational, or civil engineering and
construction
applications, such as for storm sewers. Traditionally, drainage pipe was made
from clay
or concrete, which caused the pipe to be heavy, expensive, and brittle. In
order to
improve the cost-effectiveness, durability, and ease-of-installation of
drainage pipes, t is
now common in the art to manufacture them from various materials including
various
polymers and polymer blends. Such polymer pipes are typically corrugated,
having a
molded profile with sides of the corrugation that are fairly steep and a top,
or crest, of
the corrugation that is fairly flat.
[004] There are two basic ways that polymer, corrugated pipe can fail in use:
by
deforming excessively or by fracturing. Stiffer material is less likely to
deform but more
likely to fracture under stress. Flexible material is more likely to deform
but less likely to
1
CA 02650182 2009-01-19
fracture under stress. Deformation is expressed as a ratio of elongation of
the material
to its original material length and is called "strain." Stress causes the
deformation that
produces strain. The modulus, or stiffness, of a plastic is the ratio of
stress divided by
strain, or the amount of stress required to produce a given strain.
[005] There are a number of ways to provide lower deformation of a pipe in
use:
(1) increasing pipe stiffness by using a stiffer material; (2) thickening the
pipe walls; or
(3) changing the wall design to increase the moment of inertia, which
increases the
overall stiffness of the pipe wall. Using stiffer material to make a
corrugated plastic pipe
is disadvantageous because the pipe must be able to deflect under load to a
certain
degree without cracking or buckling. A certain amount of elasticity is
therefore
beneficial in preventing brittle failures upon deflection.
[006] Thickening the pipe walls is also disadvantageous because it adds
material cost and increases weight to the pipe, which increases shipping and
handling
costs. Thus, it is advantageous to find a wall design that increases the
moment of
inertia of the pipe, while causing a minimal increase to the weight of the
pipe or the
stiffness of the material used to make the pipe.
[007] Increasing the moment of inertia of a pipe wall increases its resistance
to
bending. One example of a wall design that increases the moment of inertia,
and
therefore the stiffness, of a plastic corrugated pipe with minimal increase in
pipe weight
and material stiffness is illustrated in U.S. Patent No. 6,644,357 to Goddard.
In this pipe,
the ratio of the height of a corrugation to the width of that corrugation is
less than 0.8:1.0,
and the sidewall of the corrugation is inclined, with respect to the pipe's
inner wall, in the
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range of 75-800. This ratio allows the pipe to deflect to greater than 30% of
its original
diameter without exhibiting imperfections associated with structural failure.
[008] Pipe failure can be prevented by minimizing the maximum force exerted
on the pipe walls during the bending associated with deformation. If a sheet
of material,
such as plastic, is flexed, the outside of the resulting curve is deformed in
tension, and
the inside of the curve is deformed in compression. Somewhere near the middle
of a
solid sheet is a neutral plane called the centroid of the sheet. In the case
of corrugated
pipe, the "sheet" thickness comprises corrugations to achieve economy of
material.
Because the "sheet" is therefore not solid, the centroid may not be in the
middle of the
sheet, but rather is located at the center of the radius of gyration of the
mass (i.e., the
centroid is displaced toward the location of greater mass). The more offset
the centroid
is from the middle of the sheet thickness, the greater the maximum force will
be at the
surface farthest from the centroid during bending or flexure from deformation,
due to a
longer moment arm for certain acting forces. Thus, to lower the maximum force
caused
by pipe wall deformation, the pipe should be designed so that the centroid is
closer to
the middle of the sheet thickness. The closer the centroid is to the middle of
the sheet
thickness, the more desirably uniform the stress distribution will be. Thus,
the maximum
stress upon deformation will be minimized to prevent pipe failure due to
shorter moment
arms for acting forces.
[009] Figure 1 illustrates a vertical cross-section of a sidewall section of
one
type of prior art double-wall corrugated pipe. The illustrated section
includes a smooth
inner wall 100 and a corrugated outer wall 110. The corrugated outer wall
includes
corrugation crests 120 and corrugation valleys 130.
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[010] In use, it is the deflection and integrity of inner wall 100 that is
critical to
pipe performance. Deflection of the outer wall 110 is greater than deflection
of the inner
wall 100 in use, but a certain amount of deflection of the corrugated outer
wall 110 is
acceptable because, although maintaining the integrity of the outer wall 110
is
advantageous, its integrity can be sacrificed to a certain extent without
affecting pipe
performance, as long as the integrity of the inner wall 100 is maintained.
Thus, it is
advantageous to provide some flexibility in the outer wall 110 so that it can
deflect in
use without that deflection translating to the inner wall 100. Although the
double wall
pipe illustrated in Figure 1 may have sufficient flexibility, its centroid is
too far from the
middle of its sheet thickness to provide sufficiently uniform stress
distribution during
deformation. Moreover, the double wall pipe profile provides insufficient
resistance to
pipe buckling, for a given amount of raw material. Therefore, the double wall
pipe may
not be stiff enough to provide installation insensitivity and long-term
durability.
[011] Accordingly, it would be advantageous to provide a corrugated polymer
pipe having an additional outer layer that increases the moment of inertia so
the pipe
experiences less deformation in use, and greater resistance to buckling.
Summary of the Invention
[012] The objects and advantages of the invention may be realized and attained
by means of features and combinations particularly pointed out in the appended
claims.
[013] One exemplary embodiment of the present disclosure provides a pipe
having an axially extending bore defined by a smooth inner wall fused to a
corrugated
outer wall. The corrugated outer wall has axially adjacent, annular, outwardly-
extending
crests separated by valleys. The pipe further includes an outer layer fused to
the outer
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wall, the outer layer having adjacent concave portions and convex portions,
the concave
portions being aligned with corrugation valleys of the outer wall so that each
concave
portion of the outer layer extends between at least two corrugation crests.
[014] Another exemplary embodiment of the present disclosure provides a
method of improving the resistance to deformation of a corrugated pipe having
a
smooth inner wall fused to an outer wall defined by annular crests and
valleys. The
method includes: fixing an outer layer having adjacent annular concave
portions and
convex portions to the outer wall with the concave portions being aligned with
corrugation valleys of the outer wall so that each concave portion of the
outer layer
extends between at least two corrugation crests.
[015] In this respect, before explaining at least one embodiment of the
invention
in detail, it is to be understood that the invention is not limited in its
application to the
details of construction and to the arrangements of the components set forth in
the
following description or illustrated in the drawings. The invention is capable
of
embodiments in addition to those described and of being practiced and carried
out in
various ways. Also, it is to be understood that the phraseology and
terminology
employed herein, as well as the abstract, are for the purpose of description
and should
not be regarded as limiting.
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[015a] According to another aspect of the present invention, there is provided
a
pipe having an axially extending bore defined by a smooth inner wall fused to
a
corrugated outer wall having axially adjacent, annular, outwardly-extending
crests
separated by valleys; wherein the pipe further includes an outer layer fused
to the outer
wall, the outer layer having adjacent concave portions and convex portions,
the concave
portions being aligned with corrugation valleys of the outer wall so that each
concave
portion of the outer layer extends between at least two corrugation crests;
wherein a
radial distance between a peak of a convex portion of the outer layer and a
valley of a
concave portion of the outer layer is approximately 0.25 inches, wherein the
inner wall
and the outer layer each has a thickness of at least approximately 0.15
inches.
[015b] According to still another aspect of the present invention, there is
provided
a method of improving the resistance to deformation of a corrugated pipe
having a
smooth inner wall fused to an outer wall defined by annular crests and
valleys, the
method comprising: fixing an outer layer having adjacent annular concave
portions and
convex portions to the outer wall, thereby forming a three wall pipe, wherein
the concave
portions are aligned with corrugation valleys of the outer wall so that each
concave
portion of the outer layer extends between at least two corrugation crests,
wherein a
radial distance between a peak of a convex portion of the outer layer and a
valley of a
concave portion of the outer layer is approximately 0.25 inches, wherein the
inner wall
and the outer layer each has a thickness of at least approximately 0.15
inches.
[015c] According to a further aspect of the present invention, there is
provided a
pipe having an axially extending bore defined by a smooth inner wall fused to
a
corrugated outer wall having axially adjacent, annular, outwardly-extending
crests
separated by valleys; wherein the pipe further includes an outer layer fused
to the outer
wall, the outer layer having adjacent concave portions and convex portions,
the concave
portions being aligned with corrugation valleys of the outer wall so that each
concave
portion of the outer layer extends between at least two corrugation crests;
wherein the
outer layer has a thickness of approximately 0.20 inches, and the inner wall
has a
thickness of approximately 0.15 inches.
5a
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[016] As such, those skilled in the art will appreciate that the conception
upon
which this disclosure is based may readily be utilized as a basis for
designing other
structures, methods, and systems for carrying out the several purposes of the
present
invention. It is important, therefore, to recognize that the claims should be
regarded as
5b
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26934-29
including such equivalent constructions insofar as they do not depart from the
scope of the present invention.
Brief Description of the Drawings
[017] The accompanying drawings, which are incorporated in and constitute a
part of the specification, illustrate embodiments of the invention, and,
together with the
description, serve to explain the principles of the invention.
[018] Figure 1 illustrates a cross-section of a sidewall of one type of prior
art
double-wall corrugated pipe;
[019] Figure 2 illustrates a cross-section of a sidewall of an exemplary
embodiment of a three-wall, corrugated pipe consistent with the present
invention;
[020] Figure 3 illustrates a partial cross-section of the sidewall of Figure
2,
depicting the location of the centroid before and after addition of the outer
layer; and
[021] Figure 4 illustrates a cross-section of the three-wall, corrugated pipe
including an in-line bell and spigot formed therein.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[022] Reference will now be made in detail to the exemplary embodiments of
the invention, examples of which are illustrated in the accompanying drawings.
[023] Figure 2 illustrates a cross-section of a sidewall of an exemplary
embodiment of a three-wall, corrugated pipe consistent with the present
invention. The
illustrated section of pipe wall 200 preferably includes a smooth inner wall
210 and a
corrugated outer wall 220. The inner wall 210 has a smooth interior surface to
improve
the hydraulics of fluid traveling through the pipe. The corrugated outer wall
220
provides a high strength-to-weight ratio for the pipe wall 200.
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[024] The corrugated outer wall 220 includes corrugation crests 230 and
corrugation valleys 240. On top of the corrugated outer wall 220 is an outer
layer 250 of
the pipe wall 200 that includes convex portions 260 and concave portions 270.
The
concave portions 270 of the outer layer 250 are generally aligned with the
valleys 240
and extend between adjacent crests 230 of the outer wall 220.
[025] For the purposes of example and illustration, the present disclosure
will be
discussed with respect to two exemplary dimensional scenarios of the
illustrated
embodiment. For an exemplary embodiment of eighteen inch diameter corrugated
pipe,
an inner wall 210 may have a thickness of approximately .052 inches and an
outer wall
220 may have a material thickness of approximately .08 inches to approximately
.09
inches. In some cases, the thickness of the walls may not be completely
uniform. The
thickness of the outer layer 250 may be approximately .052 inches. The axial
distance
between the midpoint of adjacent corrugation valleys 240 may be approximately
2.617
inches. The radial distance between the top of the thickness that forms the
corrugation
valley 240 and the top of the thickness that forms the corrugation crest 230
may be
approximately 1.3566 inches. The radial distance between the peak of a convex
portion
260 of the outer layer 250 and the valley of a concave portion 270 of the
outer layer 250
("outer layer corrugation height" or "wave height") may be approximately .25
inches. In
some cases, the thickness of the outer layer 250 may not be completely
uniform.
[026] For an exemplary embodiment of forty-two inch diameter corrugated pipe,
an inner wall 210 may have a thickness of approximately .111 inches and an
outer wall
220 may have a material thickness of approximately .15 inches to approximately
.16
inches. In some cases, the thickness of the walls may not be completely
uniform. The
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CA 02650182 2009-01-19
thickness of the outer layer 250 may be approximately .1123 inches. The axial
distance
between the midpoint of adjacent corrugation valleys 240 may be approximately
5.1383
inches. The radial distance between the top of the thickness that forms the
corrugation
valley 240 and the top of the thickness that forms the corrugation crest 230
may be
approximately 2.9025 inches. The radial distance between the peak of a convex
portion
260 of the outer layer 250 and the valley of a concave portion 270 of the
outer layer 250
("Outer Layer Corrugation Height") may be approximately .25 inches. In some
cases,
the thickness of the outer layer 250 may not be completely uniform.
[027] The following chart provides some exemplary dimensions of a greater
variety of pipe sizes:
Pipe Diameter Pipe Diameter Inner Wall Outer Layer Outer Layer
(250)
(inside bore) (exterior) (210) (250) Corrugation
Height
Thickness Thickness
12" 14.59" 0.035" 0.040" 0.100"
15" 17.76" 0.039" 0.045" 0.133"
18" 21.38" 0.051" 0.050" 0.133"
24" 28.03" 0.059" 0.075" 0.160"
30" 35.40" 0.059" 0.080" 0.213"
36" 42.05" 0.067" 0.090" 0.267"
42" 48.06" 0.709" 0.095" 0.267"
48" 53.98" 0.709" 0.110" 0.267"
60" 67.43" 0.078" 0.130" 0.305"
[028] It is to be understood that these pipe dimensions are merely exemplary,
and that the present invention contemplates various pipes having a wide
variety of
dimensions. However, detailed experimental examples will be discussed below
with
respect to an exemplary embodiment of forty-eight inch corrugated pipe having
an outer
layer.
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[029] Specifically, two studies were performed on ADS standard N-12 design
48-inch, three-wall corrugated pipe. The studies examined the influence of the
thickness of the outer layer 250, the outer layer corrugation height, and the
thickness of
the outer wall 220, on overall pipe stiffness and buckling.
[030] The first study examined the effect of changing the thickness of the
outer
layer 250 (i.e., 0.12", 0.16", 0.20", 0.24", and 0.28") for four different
outer layer
corrugation heights (i.e., 0", 0.125", 0.25", and 0.375"), given a fixed
thickness for each
of the inner wall 210 and the outer wall 220. The twenty different cases are
represented
in the table below:
Case Number Outer Layer 250 Outer Layer 250
Corrugation Height Thickness (inches)
(inches)
1 0 0.12
2 0 0.16
3 0 0.20
4 0 0.24
0 0.28
6 0.125 0.12
7 0.125 0.16
8 0.125 0.20
9 0.125 0.24
0.125 0.28
11 0.25 0.12
12 0.25 0.16
13 0.25 0.20
14 0.25 0.24
0.25 0.28
16 0.375 0.12
17 0.375 0.16
18 0.375 0.20
19 0.375 0.24
0.375 0.28
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[031] The addition of the various thicknesses of outer layer 250 resulted in a
percent increase in pipe profile area, compared to a standard N-12 profile, as
represented by the chart below:
60L't
1"5
2 50%
<It
40% ___________________
9.
20%
-ct 10%
0%
0,1 0.15 0.2 0 25 0.3
Outside Liner Thickness (inch)
[032] Finite element analyses were conducted for the twenty cases to determine
the percent increase in pipe stiffness for each thickness of added outer layer
250,
compared to a standard N-12, 48-inch pipe, as represented by the chart below:
70%
0 60% Wave
Height
50% (inch)
ct. 40% ¨4-0
-AI¨ 0.125
',"" 30% ¨
D.26
a,
20%
¨X¨ 0.375
e 10%
0%
0.1 0,15 0.2 0,25 03
Outside Liner Thickness (inch)
[033] The results confirmed that, for most thicknesses of the added outer
layer
250, an increase in wave height may reduce the benefit of the added pipe
stiffness.
CA 02650182 2009-01-19
[034] Linear buckling analyses were also conducted on the profiles to
determine
the load per unit length sustainable by each of the inner wall 210 and outer
layer 250,
as compared to the load per unit length required to produce a 5% deflection in
the pipe.
The chart below depicts the predicted load per length necessary to produce a
5%
deflection (solid lines) and the buckling load of the inner wall 210 (dashed
lines).
Wave Height
(inch)
_________________________________________ "..##" 0
= 90
0.125
850.25
0375
-
a, 80
- = - Buck.
- ".= 0.125 Buck
7 5 t
025 Buck
70 = X ¨ 0.375 Buck
0.1 0.15 0.2 0.25
Outside Liner Thickness (inch)
[035] The results indicate that increasing the thickness of the outer layer
250
may substantially increase both the load at 5% deflection and the buckling
load of the
inner wall 210. However, a thickness of the outer layer 250 of less than 0.15"
may
result in a buckling load for the inner wall 210, which is less than that
required for a 5%
deflection of the pipe.
[036] The second study examined the effect of changing the thickness of the
corrugated outer wall 220 (i.e., 0.18", 0.20", 0.22", 0.237", and 0.260") for
the four
different outer layer corrugation heights (i.e., 0", 0.125", 0.25", and
0.375"), given a
thickness of the inner wall 210 of approximately 0.116" and a thickness of the
outer
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CA 02650182 2009-01-19
layer 250 of approximately 0.16". The twenty different cases are represented
in the
table below:
Case Number Outer Layer 250 Outer Wall 220
Corrugation Height (inches) Thickness (inches)
1 0 0.180
2 0 0.200
3 0 0.220
4 0 0.237
0 0.260
6 0.125 0.180
7 0.125 0.200
8 0.125 0.220
9 0.125 0.237
0.125 0.260
11 0.25 0.180
12 0.25 0.200
13 0.25 0.220
14 0.25 0.237
0.25 0.260
16 0.375 0.180
17 0.375 0.200
18 0.375 0.220
19 0.375 0.237
0.375 0.260
[037] The addition of the 0.16" outer layer 250 and changes to the thickness
of
the outer wall 220 resulted in a percent increase in pipe profile area,
compared to a
standard N-12 profile, as represented by the chart below:
12
CA 02650182 2009-01-19
1
50%
45%
ro
ii 40%
.ct #
cu 35%
30%
o .
a-- 25% ¨
U
'''' 20%
m
15% 1111 ¨ 1
u
.-G 10% r1
1
e
5% T 4
0% ___________________________________________________________ 1
0.18 0.2 0,22 0.24 0.26
Corrugation Thickness (inch)
[038] Finite element analyses were conducted for the twenty cases to determine
the percent increase in pipe stiffness for each thickness of the corrugated
outer wall 220
including the additional 0.16" outer layer 250, compared to a standard N-12,
48-inch
pipe, as represented by the chart below:
50%
Wave
,
= 4.e% -aso
141 Height
-S
l'-'- 35% ______________________________________________ ' (inch)
v**4 .....--
.2) ____________________________________________________ i
C 30% j":''' l.
cl. 25% ,
C ;'', ,' . ' e . r0 ¨1111¨
0.125
¨
ei 20%
9 0.25= 11
,,soo'õ,....o'
b c r ,..õ.
1.0% -----'. ¨x-0.375
e 1
5% i _.
0.18 0.2 0.22 0.24 0.26
Corrugation Thickness (inch)
[039] The results indicate that increasing the thickness of the corrugated
outer
wall 220 increases the overall pipe stiffness. It was determined that reducing
the
13
CA 02650182 2009-01-19
thickness of the corrugated outer wall 220 from the standard N-12 thickness of
0.237" to
0.220" would reduce the pipe profile area by approximately 6.0% and reduce the
pipe
stiffness by approximately 6.3%. Moreover, only an outer layer 250 corrugation
height
("wave height") approaching 0.375" would cause any substantial reduction in
pipe
stiffness.
[040] Linear buckling analyses were conducted on the twenty profiles to
determine the load per unit length sustainable by the inner wall 210 for each
thickness
of the corrugated outer wall 220 at a given outer layer 250 corrugation height
("wave
height"), as depicted in the chart below:
Wave
z 85 ght
, I (inch)
zz," 80
-4- 0
-0
"c 75
" -111- 0.125
,1 70 0.25
-c
= 0.375
60 _____
0.18 0.2 0.22 0.24 0.26
Corrugation Thickness (inch)
[041] It was determined that reducing the thickness of the corrugated outer
wall
220 from the standard N-12 thickness of 0.237" to 0.220" would reduce the
buckling
load of the inner wall 210 by about 4.5%.
[042] Linear buckling analyses were also conducted on the twenty profiles to
determine the load per unit length sustainable by the outer layer 250 for each
thickness
14
CA 02650182 2009-01-19
of the corrugated outer wall 220 at a given outer layer 250 corrugation height
("wave
height"), as depicted in the chart below:
300
Wave
250 Hht
¨0¨ 0
"c 150
-11/¨ 0.125
(11 , 0 . 2 s
100 - .`;4. =
20.375
0-
0.18 0.2 0.22 0.24 0.26
Corrugation Thickness (inch)
[043] It was determined that reducing the thickness of the corrugated outer
wall
220 from the standard N-12 thickness of 0.237" to 0.220" would reduce the
buckling
load of the outer layer 250 by about 3.5%.
[044] The buckling load of the corrugated, outer wall 220 of the three-wall
pipe
was also compared to the buckling load for corrugated wall of the standard N-
12 profile,
as depicted as a negative percent change in the following chart:
CA 02650182 2009-01-19
0% T
Wave
He ight
"Ft1 1
0
-10% f (inch)
Cs4
= = -15%
1
en = -20% -421-- 0.125
'V -25% '1` 0.25
- ¨X¨ 0375
L.)
-35% 4 _____________
0,18 0.2 0,22 0.24 0.25
Corrugation Thickness (inch)
[045] The results indicate that, over the profile dimensions considered,
adding
the outer layer 250 decreases the load at which buckling occurs in the
corrugated wall.
It was determined that reducing the thickness of the corrugated outer wall 220
from the
standard N-12 thickness of 0.237" to 0.220" would reduce the buckling load of
the outer
wall 220 by about 4.5%.
[046] Based on the results of these and other studies, it was determined that
in
an exemplary embodiment of the three-wall corrugated pipe, it would be
advantageous
to have the outer layer 250 and the inner wall 210 buckling at loads greater
than the
loads required for 5% pipe deflection. Accordingly, the outer layer 250 may
have a
thickness of approximately 0.15" or greater. For example, a thickness of 0.20"
for the
outer layer 250 may result in a 40% increase in stiffness. The inner wall 210
may have
a thickness of approximately 0.15" or greater, considering that an increase in
thickness
from 0.116" to 0.15" results in an additional 40 lb/in in buckling load per
unit length.
[047] Moreover, the studies indicated that in an exemplary embodiment of the
three-wall corrugated pipe, it would be advantageous to have an outer layer
250
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CA 02650182 2009-01-19
corrugation height ("wave height") between approximately 0.15 and 0.25 inches.
Specifically, it was found that an increase in outer layer corrugation height
from 0.0 to
0.25 inches provided a 40% increase in buckling load for the outer layer 250,
while
producing only a modest 3% decrease in stiffness.
[048] Accordingly, it was determined that the thicknesses of the outer wall
220
and the outer layer 250 could be adjusted in order to keep the overall pipe
profile area
relatively low, while providing increased stiffness and tolerable buckling
loads. In
particular, the corrugated pipe disclosed herein achieves reduced failure and
installation
sensitivity due to an increased moment of inertia (i.e., stiffness) of the
pipe wall, which
translates into increased resistance to deformation bending.
[049] The outer layer 250 may decrease the amount of pipe wall deformation
and improve pipe performance by increasing the pipe stiffness without
thickening the
pipe walls or using a stiffer material for the pipe walls. One way the outer
layer 250 may
accomplish this is by moving the centroid (or radius of gyration) of the pipe
wall 200
closer to the midpoint of the wall thickness.
[050] Figure 3 illustrates a portion of the pipe wall having a calculated
location
for the centroid 310 of a dual-wall pipe having no outer layer 250. The
calculated
location of the centroid 320 of a three-wall pipe having the outer layer 250
is also shown.
As depicted, the mass of the outer layer 250 may move the centroid of the pipe
wall
closer to the midpoint of the wall thickness, thereby providing a more uniform
stress
distribution resulting in a lower maximum stress during any deformation
bending.
[051] In one embodiment, the thicknesses of each of the outer layer 250 and
the
inner wall 210 may be adjusted by a similar amount in order to maintain the
location of
17
CA 02650182 2009-01-19
the centroid 320 relative to the midpoint of the three wall pipe thickness.
For example,
given a need to increase the thickness of the outer layer 250, the thickness
of the inner
wall 210 may be increased by the same amount to prevent the centroid of the
three wall
pipe from moving. The thickness of the outer wall 220 may also be adjusted in
a
manner that maintains the desired location of the centroid. By preventing the
centroid
from moving, the optimal stiffness of the three-wall pipe can be maintained.
[052] Moreover, just as the corrugations of known corrugated pipe may
comprise a sacrificial layer capable of deflecting to a certain extent in
order to
accommodate forces exhibited on the pipe in use, the outer layer 250 of the
present
invention may provide yet another sacrificial layer. Thus, in an exemplary
embodiment,
there may be two layers capable of deflecting to accommodate forces exhibited
on the
pipe in use to prevent those forces from deforming the inner wall of the pipe.
[053] The shape of the outer layer 250 may also advantageously increase the
soil bearing area of the pipe exterior, because the load on the pipe created
by backfill is
spread out over a greater exterior area of the pipe, thus reducing the load
per square
inch on the pipe exterior thereby reducing the maximum forces on the pipe from
the
backfill load.
[054] A further advantage of the presently disclosed three wall pipe is that
the
outer layer can be applied to or extruded with existing double wall corrugated
pipe
eliminating any need to redesign existing double wall corrugated pipe. The
outer layer
250 may be fused to the corrugated outer wall 220 where the convex portions
260 of the
outer layer 250 meet the crests 230 of the corrugated outer wall 220. The
inner and
outer walls 210, 220 may also be fused together by extruding the outer wall
220 onto
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CA 02650182 2009-01-19
the inner wall 210 while the inner wall 210 is still hot. Likewise, the outer
layer 250 may
be fused to the outer wall 220 by extruding the outer layer 250 onto the outer
wall 220
while the outer wall 220 is still hot.
[055] In a preferred embodiment, the manufacture of the three wall pipe
includes extruding the outer layer 250 out of a cross-head die and onto the
outside of
the outer wall 220 while the outer layer 250 is still hot. The three wall pipe
may then be
conveyed through a spray tank to water-cool the three wall pipe without being
first
conveyed through a vacuum sizing tank. Accordingly, the naturally occurring
concave
portions 270 of the outer layer 250 are allowed to form between crests 230 of
the
corrugated outer wall 220, without the time and energy consuming process of
vacuum
sizing.
[056] The layers of pipe may alternatively be co-extruded or adhered to each
other with a suitable adhesive after extrusion. The present disclosure also
contemplates a variety of methods for creating a pipe with an outer layer 250,
for
example by strapping the outer layer 250 to the outer wall 220 of the
corrugated pipe.
[057] In a preferred embodiment of the invention, the inner wall 210, outer
wall
220, and outer layer 250 of the pipe comprise a plastic such as high density
polyethylene (HDPE) or polypropylene (PP). The pipe may alternatively comprise
a
variety of other materials including, for example, other plastics, metals, or
composite
materials. For example, the inner wall 210, outer wall 220, and outer layer
250 of the
pipe could be comprised of different, but compatible, materials.
[058] Referring now to Figure 4, it is also contemplated within the present
disclosure to manufacture the pipe wall 200 having an in-line bell and spigot
coupling
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26934-29
formed therein. Figure 4 illustrates an exemplary, partial portion of three-
wall,
corrugated pipe during manufacturing of a coupling preform 411 prior to
cutting of the
pipe. Specifically, a coupling preform 411, including a bell portion 412 and a
spigot
portion 414, may be formed "in-line" with the rest of the three-wall
corrugated pipe.
Accordingly, Fig. 4 illustrates a coupling preform 411, having the bell
portion 412 and
spigot portion 414 of three-wall, corrugated pipe, after having been extruded
from a
cross-head die but before having been cut into separate portions. As
illustrated in Fig. 4,
a portion of the outer layer 250 constituting a spigot outer Wall 464 has been
drawn
down over, and fused or covalently bonded to, an intermediate corrugation 442
and
spigot corrugations 446. Moreover, the spigot outer wall 464 may be drawn down
adjacent to a spigot terminus 450, such that all three walls of the corrugated
pipe are in
contact between the spigot portion 414 and the bell portion 412 of the
coupling preform
411. Because the walls have been drawn down together, a scrap portion 456 of
the
coupling preform 411 (indicated by dashed lines on Fig. 4) may be easily
removed by
making cuts proximate to the spigot terminus 450, a bell terminus 452, and an
inner wall
terminus 454.
[059] Accordingly, the exemplary three-wall pipe having the inner wall 210,
the
corrugated outer wall 220 (having crests 230 and valleys 240), and the outer
layer 250
(having convex portions 260 and concave portions 270), may be cut into
discrete
sections and coupled together by the bell and spigot portions 412, 414.
[060] It will be apparent to those skilled in the art that various
modifications and
variations can be made in the gasket of the present invention and in
construction of this
gasket without departing from the scope of the invention.
= CA 02650182 2015-06-09
=
26934-29
[061] Other embodiments of the invention will be apparent to those skilled in
the
art from consideration of the specification and practice of the invention
disclosed herein.
It is intended that the specification and examples be considered as exemplary
only, with
a true scope of the invention being indicated by the following claims.
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