Pipes and tubular conduits – Flexible – Corrugated
Reexamination Certificate
2001-11-09
2003-11-11
Brinson, Patrick (Department: 3752)
Pipes and tubular conduits
Flexible
Corrugated
C138S122000, C138S173000, C138S177000
Reexamination Certificate
active
06644357
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to pipe, and more particularly, to a corrugated plastic pipe or corrugated metal pipe which is formed having improved overall profile stability.
BACKGROUND OF THE INVENTION
This invention is directed to the providing of corrugated pipe, and more particularly to the providing of corrugated pipe of the type used in the drainage of soil and transportation of surface water by gravity for agricultural, residential, civil construction, or recreational purposes. In these particular markets, concrete, steel or clay pipe was traditionally utilized. In practice, the drainage pipe was installed through the placing together of relatively short sections of the pipe to construct the desired drainage system. Such sections or pieces could range from a few feet to more than a couple of dozen feet.
Consequently, construction was extremely time consuming and relatively expensive because of the number of pieces and total weight associated with the components of the drainage system. Additionally, the relatively large number of pipes which were required to be installed for such systems necessitated proper positioning of an extremely large numbers of pipe in order to insure minimal leakage.
Corrugated metal pipe was first developed in the late 1800's utilizing a simple sinusoidal shape. More recent designs have used a box shape corrugation, which is known to be unstable and which imparts very high strain in the outer wall of the corrugation during manufacturing processes. This reduces the effective strength and the tolerance to deflection of this product.
More recently, plastic began to be used as a construction material for drainage pipe. Originally, sections of the plastic pipe or tubing were provided in predetermined lengths or sections which then were interconnected by using separate couplers or coupling units. It was found that corrugated pipe possessed a more desirable structural integrity than regular smooth-walled pipe. The corrugated pipe which currently exists typically has a profile in which the sides are fairly steep and the top or crest is fairly flat. In a typical plastic corrugated pipe, the ratio of the height of a corrugation to the width of an entire corrugation, which latter dimension is also referred to as the pitch, is approximately 0.9:1.0.
Several patents disclose corrugated plastic pipe representative of the prior art. Examples of patents disclosing plastic corrugated pipe include Hegler, Pat. No. 5,071,173; Hegler, U.S. Pat. No. 4,779,651; Shroy et al, U.S. Pat. No. 3,926,222; Schmunk et al, U.S. Pat. No. 3,929,359; Bonnema et al, U.S. Pat. No. 4,969,670; Ström, U.S. Pat. No. 4,202,568; and Sting et al, U.S. Pat. No. 4,082,327. These plastic corrugated pipe patents represent inventions associated with four well-known plastic pipe companies around the world. Each patent discloses pipe having the general profile discussed above, namely, corrugations with relatively steep sides and fairly flat tops.
Tests have been developed by the American Society of Testing & Materials (ASTM) and the American Association of State Highway and Transportation Officials (AASHTO) to evaluate, for example, the performance of corrugated plastic pipe designed for use in hydraulic applications. In one such stiffness test, a pipe must possess a certain minimum stiffness in order to pass. This test is conducted by applying a load at a prescribed rate to a section of the pipe to determine the stiffness per unit length of pipe at 5% deflection. Currently, if the load at 5% deflection is less than the specified minimum, then the pipe fails this test requirement.
Another important deflection criteria seeks to determine the amount of deflection the pipe can withstand without wall buckling, cracking, encountering what is known in the trade as dimpling, or loss of ability to carry increased load. In order to pass this test, the pipe must be able to withstand substantial deflection without any of the aforementioned failures occurring. Currently, in order to pass the test requirement, the pipe must be able to deflect 20% of its original diameter.
While both tests are important, with current designs the second is more difficult to pass than the first. To aid in the strengthening of the pipe and to reduce local buckling in the corrugation crown during deflection testing, some manufacturers of plastic pipe have put ridges along the top surfaces of the corrugations. These additional ridges tend to improve the tendency toward local buckling in the crown of this particular type of pipe, but they do not improve the global buckling limits and can actually increase the local bending stresses and strains leading to localized cracking. Hegler et al, U.S. Pat. No. 5,071,173 and Hegler et al, U.S. Pat. No. 4,779,651 both disclose pipe having such an additional corrugation present on the crest.
However, concerns about the strength of corrugated pipe still exist, especially if the pipe is made of plastic. Many current corrugation or wall profile designs tend to be focused on increasing the effective moment of inertia of the pipe wall (increasing the pipe stiffness while limiting or reducing the pipe weight per unit length) with little attention paid to the stability of the wall design. This has resulted in the development of pipe capable of passing the prescribed stiffness tests in the laboratory, but which exhibit dimensional instability when installed in the ground. This dimensional instability can lead to premature failure or excessive deflection. Furthermore, existing pipe profiles do not permit deflection significantly above the current performance requirement of 20%.
A recently developed lab test, which tests the wall section in bending and compression simultaneously, has shown the profile type designed in accordance to this methodology to be substantially more stable than current designs. In bending, one measure of the stability of the pipe wall is the reduction of wall thickness. Wall sections representing this design approach is reduced in thickness approximately 20% of comparable current design.
It is thus apparent that the need exists for an improved corrugated pipe which provides for improved overall profile structural stability, and thus improved in-service performance.
SUMMARY OF THE INVENTION
In accordance with this invention a double-walled corrugated pipe is provided having a pipe structure having an inner wall having an interior surface and an exterior surface, and an outer wall having corrugations. Each of the corrugations has an interior surface and an exterior surface, and each corrugation has a crown. The pipe structure has a trough between each corrugation, each trough having a center-point, with the distance between the center-points of adjacent troughs defining the pitch of the pipe. The pipe structure having at each crown a first distance extending in a straight line from the crown exterior surface to the interior surface of the inner wall, with the distance defining the pitch being a second distance, and with the ratio of the first distance to the second distance being less than 0.8:1.0.
In the preferred embodiment of the invention, the ratio of the first distance to the second distance is about 0.75:1.0. Furthermore, the pipe could be plastic or metal, but if plastic, the pipe upon the application of pressure can deflect to greater than 30% of its original diameter without exhibiting imperfections of structural failure, with this number exceeding 35% in the preferred embodiment of the invention.
The outer wall has a sidewall portion intermediate the crown and the trough, with the sidewall portion being inclined to the inner wall in the range of 75-80°, and with the sidewall being inclined to the inner wall in the preferred embodiment of the invention about 78½°. The inner wall is smooth.
There is also disclosed a double-walled corrugated pipe having a pipe structure, with the pipe structure comprising a smooth inner wall having an interior surface and an exterior surface, and an outer wall having corrugations, with each of t
Advanced Drainage Systems, Inc.
Brinson Patrick
Kremblas, Foster Phillips & Pollick
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