Abstract:
A pipe having an axially extending bore defined by a smooth inner wall fused to a corrugated outer wall is provided. 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 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. 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 is also provided.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/078,323, which was filed on Mar. 14, 2005, now U.S. Pat. No. 7,484,535 and which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     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 
     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, it 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. 
     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 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. 
     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. 
     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. 
     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. Pat. 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&#39;s inner wall, in the range of 75-80°. This ratio allows the pipe to deflect to greater than 30% of its original diameter without exhibiting imperfections associated with structural failure. 
     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. 
       FIG. 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 . 
     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  FIG. 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. 
     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 
     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. 
     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 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. 
     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. 
     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. 
     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 including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  illustrates a cross-section of a sidewall of one type of prior art double-wall corrugated pipe; 
         FIG. 2  illustrates a cross-section of a sidewall of an exemplary embodiment of a three-wall, corrugated pipe consistent with the present invention; 
         FIG. 3  illustrates a chart comparing an outside linear thickness of an outer layer of a pipe to a percent increase in pipe profile area; 
         FIG. 4  illustrates a chart comparing an outside linear thickness of an outer layer of a pipe to a percent increase in pipe stiffness; 
         FIG. 5  illustrates a chart comparing an outside linear thickness of an outer layer of a pipe to a load per length; 
         FIG. 6  illustrates a chart comparing a corrugated outer wall thickness of a pipe to a percent increase in pipe profile area; 
         FIG. 7  illustrates a chart comparing a corrugated outer wall thickness of a pipe to a percent increase in pipe stiffness; 
         FIG. 8  illustrates a chart comparing a corrugated outer wall thickness of a pipe to a load per length; 
         FIG. 9  illustrates another chart comparing a corrugated outer wall thickness of a pipe to a load per length; 
         FIG. 10  illustrates a chart comparing a corrugated outer wall thickness of a pipe to a percent change in buckling load; 
         FIG. 11  illustrates a partial cross-section of the sidewall of  FIG. 2 , depicting the location of the centroid before and after addition of the outer layer; and 
         FIG. 12  illustrates a cross-section of the three-wall, corrugated pipe including an in-line bell and spigot formed therein. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 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 . 
     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 . 
     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 0.052 inches and an outer wall  220  may have a material thickness of approximately 0.08 inches to approximately 0.09 inches. In some cases, the thickness of the walls may not be completely uniform. The thickness of the outer layer  250  may be approximately 0.052 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 0.25 inches. In some cases, the thickness of the outer layer  250  may not be completely uniform. 
     For an exemplary embodiment of forty-two inch diameter corrugated pipe, an inner wall  210  may have a thickness of approximately 0.111 inches and an outer wall  220  may have a material thickness of approximately 0.15 inches to approximately 0.16 Inches. In some cases, the thickness of the walls may not be completely uniform. The thickness of the outer layer  250  may be approximately 0.1123 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 0.25 inches. In some cases, the thickness of the outer layer  250  may not be completely uniform. 
     The following chart provides some exemplary dimensions of a greater variety of pipe sizes: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 Pipe 
                   
                 Inner 
                 Outer 
                 Outer Layer 
               
               
                 Diameter 
                 Pipe 
                 Wall 
                 Layer 
                 (250) 
               
               
                 (inside 
                 Diameter 
                 (210) 
                 (250) 
                 Corrugation 
               
               
                 bore) 
                 (exterior) 
                 Thickness 
                 Thickness 
                 Height 
               
               
                   
               
             
             
               
                 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″ 
               
               
                   
               
             
          
         
       
     
     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. 
     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. 
     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: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                 Outer Layer 250 
                   
               
               
                 Case 
                 Corrugation Height 
                 Outer Layer 250 
               
               
                 Number 
                 (inches) 
                 Thickness (inches) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 0 
                 0.12 
               
               
                 2 
                 0 
                 0.16 
               
               
                 3 
                 0 
                 0.20 
               
               
                 4 
                 0 
                 0.24 
               
               
                 5 
                 0 
                 0.28 
               
               
                 6 
                 0.125 
                 0.12 
               
               
                 7 
                 0.125 
                 0.16 
               
               
                 8 
                 0.125 
                 0.20 
               
               
                 9 
                 0.125 
                 0.24 
               
               
                 10 
                 0.125 
                 0.28 
               
               
                 11 
                 0.25 
                 0.12 
               
               
                 12 
                 0.25 
                 0.16 
               
               
                 13 
                 0.25 
                 0.20 
               
               
                 14 
                 0.25 
                 0.24 
               
               
                 15 
                 0.25 
                 0.28 
               
               
                 16 
                 0.375 
                 0.12 
               
               
                 17 
                 0.375 
                 0.16 
               
               
                 18 
                 0.375 
                 0.20 
               
               
                 19 
                 0.375 
                 0.24 
               
               
                 20 
                 0.375 
                 0.28 
               
               
                   
               
             
          
         
       
     
     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 shown in  FIG. 3 . 
     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 shown in  FIG. 4 . 
     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. 
     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.  FIG. 5  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). 
     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. 
     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 layer  250  of approximately 0.16″. The twenty different cases are represented in the table below: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                 Outer Layer 250 
                   
               
               
                 Case 
                 Corrugation Height 
                 Outer Wall 220 
               
               
                 Number 
                 (inches) 
                 Thickness (inches) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 0 
                 0.180 
               
               
                 2 
                 0 
                 0.200 
               
               
                 3 
                 0 
                 0.220 
               
               
                 4 
                 0 
                 0.237 
               
               
                 5 
                 0 
                 0.260 
               
               
                 6 
                 0.125 
                 0.180 
               
               
                 7 
                 0.125 
                 0.200 
               
               
                 8 
                 0.125 
                 0.220 
               
               
                 9 
                 0.125 
                 0.237 
               
               
                 10 
                 0.125 
                 0.260 
               
               
                 11 
                 0.25 
                 0.180 
               
               
                 12 
                 0.25 
                 0.200 
               
               
                 13 
                 0.25 
                 0.220 
               
               
                 14 
                 0.25 
                 0.237 
               
               
                 15 
                 0.25 
                 0.260 
               
               
                 16 
                 0.375 
                 0.180 
               
               
                 17 
                 0.375 
                 0.200 
               
               
                 18 
                 0.375 
                 0.220 
               
               
                 19 
                 0.375 
                 0.237 
               
               
                 20 
                 0.375 
                 0.260 
               
               
                   
               
             
          
         
       
     
     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 shown in  FIG. 6 . 
     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 shown in  FIG. 7 . 
     The results indicate that increasing the thickness of the corrugated outer wall  220  increases the overall pipe stiffness. 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 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. 
     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 shown in  FIG. 8 . 
     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%. 
     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 of the corrugated outer wall  220  at a given outer layer  250  corrugation height (“wave height”), as shown in  FIG. 9 . 
     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%. 
     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  FIG. 10 . 
     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%. 
     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. 
     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  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. 
     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. 
     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. 
       FIG. 11  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. 
     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 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. 
     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. 
     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. 
     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 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. 
     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. 
     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. 
     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. 
     Referring now to  FIG. 12 , it is also contemplated within the present disclosure to manufacture the pipe wall  200  having an in-line bell and spigot coupling formed therein.  FIG. 12  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. 12  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. 12 , 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 wails 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. 12 ) may be easily removed by making cuts proximate to the spigot terminus  450 , a bell terminus  452 , and an inner wall terminus  454 . 
     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 . 
     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 or spirit of the invention. 
     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 and spirit of the invention being indicated by the following claims.