CA2321808A1 - Composite layer wall construction for a dual containment vessel - Google Patents

Abstract

The invention comprises the composition of and the method for making a novel 3 layer dual containment vessel useable as a manhole, tank or pipe comprising; a flexible inner liquid-tight shell comprised of a thermoplastic; a thin annular space occupied at least in part by a porous geocomposite material that has been applied to the outer face of the flexible inner liquid tight shell; and a flexible exterior liquid-tight shel l comprised of a thermoplastic that has been helically wound upon the porous geocomposite material.

Description

.. CA 02321808 2000-09-28 COMPOSITE LAYER WALL CONSTRUCTION FOR
A DUAL CONTAINMENT VESSEL
BACKGROUND OF THE INVENTION
In dual containment vessels such as manholes, tanla, and pipes, the vessel provides a primary (inner) containment vessel surrounded by a secondary (outer) containment vessel that is separated by an annulus or gap layer. Typically, manhole and tank vessels have a cylindrical body section and use flat, conical, or domed shaped sections to close the ends of the cylinder. In the dual (secondary) containment vessel, typically but not always, the primary (inner) vessel contains the liquid, often a hazardous liquid. The secondary (outer) shell layer encloses the primary (inner) shell layer such that any leakage of liquid from the primary (inner) shell is contained, and prevented from leaking out and contaminating the external environment. An annulus layer between the primary and secondary containment layers generally provides for devices to monitor for leakage from the primary containment shell. Dual containment pipes are simply long cylinders of the same construction, but connecting between vessels or other piping appurtenances.
Currently, two construction methods are used for dual (secondary) containment vessels such as manholes, tanks, and pipes. One wall construction method used for dual 2o (secondary) containment vessels employs a construction where an inner vessel is fitted within an outer vessel, and there is a physical space or annulus between them.
See Figure 1. Generally, spaces or blocks placed in the annulus are used to position or stabilize the inner vessel.
In the Figure 1 method, the outer vessel and the inner vessel are structurally independent from each other; that is, neither layer can rely upon the strength of the other to resist applied loads or stresses. The inner vessel must be designed to withstand the internal loads and stresses of the application. In the current design, it is frequently necessary to add gussets and spacers between the vessels to stabilize the inner vessel. The outer vessel must be designed for both external loads and stresses and internal loads and stresses. In addition 3o to being the primary structural member for the dual containment vessel, the outer vessel becomes the containment for the internal fluid if the internal vessel fails.
Figure 2 illustrates a second method where field construction is used to provide secondary containment for a single-wall vessel. In this method, an outer layer of impenetrable clay soil is lined with a synthetic geomembrane. Inside the liner there is a layer of crushed stone surrounding a single-wall vessel. The geomembrane lined clay layer is the secondary containment, the crushed stone is the annulus, and the single-wall vessel is the primary containment shell. This complex, secondary containment method is constructed in the field.
Two patents that relate to double walled storage vessels are U.S. Patents 4,640,439 and 5,220,823 both herein incorporated by reference.
U.S.P. 4,640,439 relates to a double wall tank for the storage of liquids that is manufactured from a rigid single wall inner tank by applying a spacing material to at least 1 o a portion of the exterior surface of the inner tank and applying over that inner tank exterior surface and the spacing material a substantially rigid outer sheath of a material that is substantially liquid-tight. The patent discloses the use of a ri id inner tank and an outer sheath that is substantial liquid-tight.
U.S.P. 5,220,823 relates to an underground storage tank with a load-transmitting ~ s material in the annular space between inner and outer walls. The load-transmitting material passes aqueous liquid and the stored product and so may be used with either wet or dry alarm systems. This patent also is to a ri-~ vessel and discloses the material of construction as being reinforced resin material (fiberglass).
The present invention discloses a or flexible vessel that provides liquid-tight dual 2o containment and discloses a method for making the vessel. The vessel and method of the present invention provide advantages that are discussed below.
SUMMARY OF THE INVENTION
The present invention relates to a novel dual containment vessel useable as a 25 manhole, tank, or pipe and a method for making the vessel. The vessel comprises:
a flexible outer liquid-tight shell comprised of high-density polyethylene;
a flexible interior liquid-tight shell comprised of high-density polyethylene, said interior shell being spaced from said outer shell so as to create an annular space 3o therebetween;

-2-said annular space occupied at least in part by a porous geocomposite material , wherein said flexible inner and said outer shells are capable of transmitting compressive load to each other.
The present invention is a novel three-layer wall construction. See Figure 3.
An inner shell of polyethylene or other plastic or non-plastic material is wrapped with a porous material such as a synthetic geonet, or a geofabric, or a geocomposite material (from here on referred to as the porous geocomposite material), then an outer shell of polyethylene or other plastic or non-plastic material is applied. Generally, the entire three-layer construction is accomplished in a single manufacturing process, but it may also be produced by an assembly process.
Among other factors the present invention is based on the surprising conception that a dual containment vessel usable as a manhole, tank, or pipe can be made using a unique three-layer structure. The three-layer structure has a very thin but fairly uniform thickness of an annular space which is defined by a geocomposite material. The annular space allows for the free flow of liquids in the annular space facilitating leak detection while at the same time, given the flexibility of the thermoplastic inner and outer shells, allows the shells to transmit compressive load to each other and otherwise to provide structural benefits to each other. The structural synergy between the inner and outer layers allows the dual containment vessel of the present invention to be lighter and less expensive than it would otherwise have to be. It has further been found that the three layer structure of the present invention can be made using a devise for helically winding extruded thermoplastic where at least one or preferably both the inner and outer shells are fabricated using the helical winding method.
It has been surprisingly found that it is particularly advantageous to produce the three-layer construction in a single manufacturing process. Most preferably this single process has the following steps all performed on a single rotating mandrel/extruder devise:
a) Forming a flexible inner liquid-tight shell comprised of high-density 3o polyethylene that has been helically wound upon a mandrel;

-3-b) Wrapping the inner shell with a porous geocomposite material while the inner shell is still on the mandrel; and c) Forming a flexible exterior liquid-tight shell comprised of high-density polyethylene that has been helically wound upon the porous geocomposite material;
wherein an annular space is formed between the flexible inner liquid-tight shell and the flexible exterior liquid-tight shell. The porous geocomposite material occupies the annular space and as such allows for the free flow of liquid within the annular space.
The invention solves the inherent structural problems of the independent shell layer 1o method by having a thin annular space and by filling the thin annular space between the inner and outer shells with material so that internal and external stresses and loads are transferred between the inner and outer shells. Thus, the outer shell provides structural stability for the inner shell, and the inner shell provides structural reinforcement for the outer shell.
The design structural requirements of the outer shell are also reduced because the inner shell provides structural benefit to the outer shell. Unlike the independent layer wall construction, the invention's inter-dependent, three-layer construction does not require that the outer shell layer duplicate the structural requirements of the inner shell layer. This new construction is lighter, stronger, and less costly compared to conventional, independent layer wall construction.
The central layer (annular space) of porous material provides an annulus for leakage detection devices. Access to the central layer is provided simply by tapping through the inner layer or through the outer layer.
The invention solves the problem of complex, multiple material, multiple layer field construction by providing secondary containment within the wall of the vessel, rather than external to the vessel.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a prior art dual containment vessel with a large annular space defined by radial 3o gussets.
FIG. 2 is an alternative method of providing secondary containment.

-4-FIG. 3 is an example of a dual containment vessel within the scope of the present invention.
FIG. 4 is a prior art diagrammatic side elevational view of a device for producing a helically wound tube.
FIG. 5 is a top plan view of a device indicated in FIG. 4.
FIG. 6 is a enlarged side elevational and partially sectional view of a guide channel used in the device of FIG. 4.
FIG. 7 is a plan view of the guide channel shown in FIG. 6.
FIG. 8 is a section taken along the line A--A of FIG. 6.
DETAILED DESCRIPTION OF THE DRAWINGS
Figure 1 is a prior art dual containment vessel and is discussed in more detail in the background of the invention. A key feature of the dual containment vessel of Figure 1 is that the inner and outer vessels are separated by gussets therebetween.
Another key feature of the Figure 1 vessel is that the outer vessel and the inner vessel are structurally independent from each other, that is, neither layer can rely upon the strength of the other to resist applied loads or stresses. The inner vessel must be designed to withstand the internal loads and stresses of the application. In the current design, it is frequently necessary to add gussets and spacers between the shells to stabilize the inner shell layer. The outer vessel must be designed for both external loads and stresses and internal loads and stresses. In addition to being the primary structural member for the dual containment vessel, the outer vessel becomes the containment for the internal fluid if the internal vessel fails.
Figure 2 is an alternative underground dual containment vessel comprising the following features: The inner tank is intended as the primary containment vessel.
Surrounding the primary containment is a secondary containment area comprising 3o compacted crushed stone. A leak detection pipe is shown that can facilitate detection of a leak into the secondary containment area. Surrounding and defining the secondary

-5-containment area is a synthetic geomembrane intended to contain a leakage from the inner containment tank. An area of clay is shown surrounding the geomembrane.
Surrounding the area of clay is the insitu soil.
Figure 3 is an example of a dual containment vessel within the scope of the present invention. The drawing shows a dual containment vessel as well as an expanded view of the three-layer construction of the present invention. Note that the inner shell and the outer shell have different shell thicknesses because unlike the prior art vessel of Figure 1 the inner and outer shells can rely upon the strength of the other to resist applied loads or stresses.
t o Figures 4 through 8 illustrate a helical winding devise of the prior art that can be useful for making the dual containment vessel of the present invention.
Referring to the drawings in particular therein comprises a device for producing a helically wound tube comprising a rotating mandrel 5 around which the tube is wound. A profiled pressure member generally designated 6 forms a calibrating guide channel disposed alongside the mandrel 5. An extruder of a plastic strip generally designated 2 having an extrusion head 3 is disposed alongside the mandrel S and extrudes a continuous strip of plastic material through the guide channel 6 where it is directed at a selected pitch angle onto the mandrel 5. During the depositing of the strip material on the mandrel it is rotated and both the extruder tool and the guide channel 6 are mounted on a carriage 8 for movement relative to 2o the mandrel in respect to the axis 9 of the mandrel.
In accordance with the method of the invention, a continuous flat strip of material is first directed from the extrusion head 3 through the guide channel 6 which is oriented to deposit the strip of material onto the rotating mandrel 5 as the mandrel is rotated the extruder in the guide channel are moved along the length of the mandrel to deposit continuous windings of the strip material onto the mandrel. In accordance with the method it is also preferable to provide a reinforcement of the strip 4 which is achieved by directing a reinforcing member 14 through the extrusion head to cause the formation of a tubular reinforcement or portion 12 of the strip material leaving a base flange 11 on each side thereof as shown in FIG. 8. The device illustrated is designed for the production of a tube 1 3o by the helical winding of a thermoplastics profile section or strip 4 molded in an extruder 2 having an extrusion head 3 and directing it onto a mandrel 5. The mandrel 5 operates in _b_ conjunction with a profile pressure member 6 of a width exceeding the width of the plastics profile section 4, which welds together the obliquely abutting and/or overlapping edges 7 of the plastics profile section 4. The pressure member 6, together with the extruder 2 and its extrusion head 3, are movably mounted on a carriage 8 relative to the fixed mandrel 5, which merely rotates about is axis 9.
The extrusion head 3 is disposed with its longitudinal axis on a tangent 10 to the mandrel 5, corresponding to the pitch of the helix (FIGS. 4 and 5), and is adapted to extrude a plastics profile section 4 having two base flanges 1 I forming the longitudinal edges and an outwardly projecting tubular longitudinal reinforcement 12 between them to (FIG. 8). Gusset-shaped longitudinal recesses 13 run between the base flanges 11 forming the longitudinal edges and the longitudinal reinforcement 12. The longitudinal tubular reinforcement or portion 12 accommodates the supporting member or tube 14, which is introduced from the rear into the extrusion head 3 of the extruder 2 (FIGS. 4 and S).
The pressure member 6 is formed by a calibrating slide having two portions 15,16, having a guide channel 17 for the longitudinal tubular reinforcement 12. The calibrating slide is disposed coaxially with the longitudinal axis of the extrusion head 3 and has suitable coolant passages 18 through which a coolant is circulated.
Corresponding to the longitudinal recesses 13 in the plastics profile section 4, the calibrating slide 6 has guide rails 19 which protrude into the longitudinal recesses 13. Reference number 20 in FIG. 8 2o indicates a space by which the tubular longitudinal reinforcement 12 in the calibrating slide is distanced further from the base flanges 11 forming the longitudinal edges than within the extrusion head 3, to compensate for undesirable subsequent shrinkage forces.
Furthermore, FIGS. 6 and 7 show that looking in the direction of advance of the plastics profile section 4, the calibrating slide 6 has an entry portion of length 15 and constant cross-section (see also FIG. 8) leading into a laterally wedge-shaped exit portion of length 1 S
(FIG. 6 only) having a guide channel 17 of gradually decreasing cross-section. Towards the free end 21 of the exit portion 16 the guide channel 17 runs out. As the arrows 22 and 23 in FIG. 4 indicate, the calibrating slide 6 can be advanced or retracted along its axis away from or back towards the extrusion head 3 (arrow 22) and is also adjustable at least with respect to 3o its vertical distance from the mandrel S (arrow 23).

The functioning of the device just described is readily apparent from the figures.
The mandrel 5 is rotated under power at a peripheral speed corresponding to the extrusion speed and the calibrating slide 6 is advanced until the free end 21 of its exit portion is in the welding position. At the start, material is extruded without a supporting tube 14. As soon as a few turns have been formed, the supporting tube 14 is introduced into the extrusion head 3 and the calibrating slide 6 is advanced again so that the entry portion 15 reaches the welding position. As extrusion commences, the carriage 8 on which the extruder 2 and the calibrating slide 6 are mounted is sent in synchronous motion.
While specific embodiments of the invention have been shown and described in 1o detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
DETAILED DESCRIPTION OF THE INVENTION
The composite layer wall construction is used in cylindrical, flat, conical, domed, t 5 and other shapes for above ground or buried dual (secondary) containment vessels including manholes (typically vertical applications), tanks (horizontal or vertical applications), and pipes for the transport, collection, storage, and containment of liquids, especially hazardous liquids. The composite wall construction consists of (a) an inner shell layer, usually of polyethylene but also of other plastic or non-plastic material; (b) an 2o annulus layer that is can be filled with synthetic geonet, a geofabric or a geocomposite material, and (c) an outer shell layer, usually of polyethylene, but also of other plastic or non-plastic material.
The preferred thermoplastic useful in the present invention is high-density polyethylene (HDPE) however other flexible thermoplastics can be used in this invention.
25 Other flexible materials useful in the present invention include but are not limited to styrene-elastomer block copolymers (such as styrene butadiene block copolymers), acrylonitrile-butadiene-styrene (ABS), other styrene-elastomer copolymers, polypropylene, high impact polystyrene (HIPS), other polyethylene copolymers, Polyvinylchloride (PVC), crosslinked PE (PEX), etc.
3o In a preferred embodiment of the present invention the inner shell of the vessel is formed by helically winding extruded high-density polyethylene upon a rotating mandrel _g_ as disclosed in U. S. Patents 4,466,854, 4,510,004, 4,544,435, and 4,826,423 all of which are hereby incorporated by reference.
In a particularly preferred embodiment the flexible shells can be made of more than one layer of high-density polyethylene. This is accomplished by winding additional layers s of HDPE.
It is particularly preferred that the HDPE used in the process of the present meets ASTM D-3350 which is herein incorporated by reference. It is also particularly preferable that one or both of the inner and the outer shells meet ASTM F-894 specification which is also herein incorporated by reference.
1 o In the present invention the annular space is very thin, less than 1'/z inches thick, preferably between 1/32 and 1 inches in thickness, more preferably between 1/32 and 5/8 inches in thickness, still more preferably between 1/8 and'/2 inches in thickness. Having a very thin but fairly uniform thickness of the annular space allows for the free flow of liquids in the annular space facilitating leak detection while at the same time, given the ~ s flexibility of the high-density polyethylene inner and outer shells, allows the shells to transmit compressive load to each other. When it is stated in this patent application that the annular space is a within certain thickness range it is meant that at least 80 % of the annular space as measured by surface is within the given range. There may be portions of the vessel that are outside the given range due to imperfections of the manufacturing 2o process or due to other factors.
In the preferred process of the present invention the double walled dual containment vessel useable as a manhole, tank , or pipe is of a roughly cylindrical shape.
The dual containment portion of the vessel is the side of the cylinder. The ends of the vessel can be single walled, or double walled depending upon the requirements of the 25 particular service for which the vessel is intended. The ends of the vessel can include conventional dual containment features such as gussets with holes for free flow in the annulus, and the thickness of this portion of the annulus can be much thicker than the side of the cylinder as shown in Figure 3.
In a particularly preferred embodiment of the present invention the double walled 30 dual containment vessel is useable as a manhole. The dual containment vessel of the present invention is particularly well suited for use as a manhole or manway.
The flexibility of the thermoplastic elastomer (preferably high-density polyethylene) used in the present invention is particular durable and forgiving of accidents. The impact strength of the preferred high-density polyethylene allows it to survive rough treatment without damage, rupture, or breakage. Heavy tools and equipment are often used and dropped in manholes. Prior art fiberglass materials often crack when impacted by a heavy tool that has been dropped. The dual containment vessels of the present invention have proven much more able to withstand accidental impacts without damage. Manholes are defined in ASTM F 1759 which is herein incorporated by reference.
Another key feature of the present invention is the flexibility and durability of the 1 o dual containment vessels of the present invention. The flexibility of the three-layer dual containment vessel of the present invention facilitates soil support when the vessel is buried. The flexibility is critical to allow soil support. Depending upon soil support allows the tank to be lighter and less expensive than if it were to be entirely self supported in an application where the vessel is underground.
Still another key feature of the present invention is the ability of the dual containment vessel to maintain the leak detection ability of the annular layer even if the vessel is distorted when for instance the vessel is buried. The porous geocomposite material useful in the present invention serves to keep the annular space open to pass liquids even if the vessel wall is pushed in or otherwise distorted. If the annular space did 2o not contain the porous geocomposite material such a distortion (particularly given the flexibility of the vessel of the present invention) could have the effect of pushing walls together and isolating some areas of the annular space. Such an event could prevent liquid from flowing freely within the annular space and thus could prevent early detection of any leak by the leak detection equipment thus defeating one of the purposes of dual containment. It has been found that in the vessel of the present invention the geocomposite material serves to define the annular space and maintain the dual containment leak detection ability of the vessel.
Another significant benefit of the use of the geocomposite material is that the annular space can be very thin. The ability to use a very thin annular space due to the 3o geocomposite material allows the inner and the outer shells to depend on one another for compressive support and other structural support. This in turn allows each shell to be thinner and lighter than it would otherwise have to be if it were not depending on the other layer for support. This feature of the present invention is a significant advantage and allows the dual containment vessel of the present invention to be lighter, use less materials, take up less space, and be less expensive than alternative dual containment vessels.
Another feature of the present invention is that each of the flexible shells can be made of one or more layers of the thermoplastic material. Making a dual containment vessel where one or both of the shells is made up of to or more layers of thermoplastic can be accomplished by laying down a first layer of thermoplastic following by additional layers layered on top of the first layer. In the particularly preferred embodiment of the present invention multiple layers of the preferred thermoplastic, high-density polyethylene, can be applied by helically winding as many layers of thermoplastic as desired one on top of another. Typically for a large vessel such as a manhole each shell will comprise two or more layers of thermoplastic.

The following example is intended to help illustrate production of a dual containment vessel of the present invention. The example is not intended to in any way limit the invention.

20 To produce a large dual containment vessel multiple layers of thermoplastic may be required to achieve the desired strength. In such a vessel the first layer of the inner shell is extruded onto the mandrel as discussed in the detailed description of Figure 4. An additional layer is formed by extruding the continuous strip of thermoplastic material onto the first layer while the first layer is still on the mandrel. This procedure can be repeated 25 for as many layers as are desired for the inner shell. The number of layers, and thus the thickness of the inner shell is a function of the size of the vessel and the potential load and pressure requirements for said vessel. After the inner shell is complete it can be wrapped with the geocomposite material while still on the mandrel. A geocomposite material that is useful for this purpose is Tensar DC6205E66AA2 Geotextile Composite. This material 3o was obtained from Tensar Environmental Systems of 1108 Citizens Parkway, Marrow, GA, 30260. The geocomposite material can be held in place on the inner shell by any means such as tying on with strapping line or tape as needed. The outer shell is then made by helically extruding a layer of thermoplastic onto the geocomposite. It is preferable that the vessel be on the mandrel for the entire construction process. Additional layers of thermoplastic can be helically wound onto the vessel as desired. The vessel is allowed to harden then is removed from the mandrel.
It has been found that there are significant advantages to producing a vessel using the procedure outlined above in one continuous process. It has been found that when the geocomposite material is wrapped on to the inner shell when the inner shell is still hot the geocomposite conforms to the contours of inner shell better than if the inner shell is 1 o allowed to cool. Also, depending on the type of geocomposite used, the geocomposite material may stick to the inner shell if wrapped when the inner shell is still hot and somewhat soft. This can facilitate ease of wrapping of the inner shell with the geocomposite material. A more significant advantage of producing the vessel in one continuous process is that the heat from the inner layer may also facilitate addition of the outer shell by helping the outer shell conform to the contours of the geocomposite material layer.
It may be observed that there is sagging of the outer shell shortly after the outer shell is completed. Generally, as the outer shell cools and hardens the sagging is eliminated as the thermoplastic contracts or shrinks upon hardening.

Claims (22)

1. WHAT IS CLAIMED IS:
   A double walled dual containment vessel useable as a manhole, tank or pipe, comprising:
   a flexible outer liquid-tight shell comprised of high-density polyethylene;
   a flexible interior liquid-tight shell comprised of high-density polyethylene, said interior shell layer being spaced from said outer shell layer so as to create an annular space therebetween;
   said annular space occupied at least in part by a porous geocomposite material; and wherein said flexible inner and said outer shells are capable of transmitting compressive load to each other.
2. 2. The double walled dual containment vessel of claim 1 wherein said annular space is between 1/32 and 1 1/2 inches in thickness.
3. A method of making a double walled dual containment vessel useable as a manhole, tank, or pipe comprising the following steps:
   forming a flexible interior liquid-tight shell of said vessel from a material comprising high-density polyethylene;
   applying a porous geocomposite material having an inner surface and an outer surface to at least a portion of the outer face of the interior liquid-tight shell; and forming a flexible exterior liquid-tight shell layer of said vessel from a material comprising high-density polyethylene;
   
   wherein the porous geocomposite material defines an annular space between said interior liquid-tight shell and said exterior shell, said flexible inner and said flexible outer shells are capable of transmitting compressive load to each other, and said porous geocomposite material being of a nature such that liquids will flow within said annular space.
4. 4. The method of claim 3 wherein the flexible interior liquid tight shell is made by helically winding the high-density polyethylene upon a mandrel.
5. S. The method of claim 4 wherein the flexible exterior liquid-tight shell is made by helically winding the high-density polyethylene upon the porous geocomposite material.
6. 6. The method of claim 5 wherein the flexible exterior liquid-tight shell is made by helically winding the high-density polyethylene upon the porous geocomposite material before the flexible interior liquid tight shell is removed from the mandrel.
7. 7. The method of claim 3 wherein the double walled dual containment vessel is a manhole.
8. 8. The double walled dual containment vessel of claim 1 wherein the vessel is a manhole.
9. 9. The double walled dual containment vessel of claim 1 wherein the flexible outer liquid-tight shell and the flexible inner shell are comprised of high-density polyethylene meeting ASTM specification F-894.
10. 10. The double walled dual containment vessel of claim 1 wherein said annular space is between 1/8 and 1/2 inches in thickness.
11. 11. The method of claim 3 wherein the double walled dual containment vessel is a manhole wherein said manhole meets the definition in ASTM F 1759.
12. 12. The double walled dual containment vessel of claim 1 wherein the flexible inner liquid-tight shell is comprised of at least two layers of high-density polyethylene.
13. 13. The double walled dual containment vessel of claim 1 wherein the flexible inner liquid-tight shell and the flexible outer liquid-tight shell are each comprised of at least two layers of high-density polyethylene.
14. 14. The method of claim 3 wherein both the interior liquid-tight shell and the exterior liquid-tight shell are made by helically winding the high-density polyethylene.
15. 15. A double walled dual containment vessel useable as a manhole, tank or pipe, comprising three layers, said vessel comprising:
    a) a flexible inner liquid-tight shell comprised of a thermoplastic;
    b) an annular space occupied at least in part by a porous geocomposite material that has been applied to the outer face of the flexible inner liquid tight shell;
    c) a flexible exterior liquid-tight shell comprised of a thermoplastic that has been helically wound upon the porous geocomposite material;
    wherein said annular space is defined by the flexible inner liquid-tight shell and the flexible exterior liquid-tight shell.
16. 16. The double walled dual containment vessel of claim 15 wherein the flexible inner liquid-tight shell is comprised of high-density polyethylene that has been helically wound upon a mandrel.
17. 17. The double walled dual containment vessel of claim 16 wherein the vessel is a manhole.
18. 18. The double walled dual containment vessel of claim 16 wherein the flexible inner liquid-tight shell and the flexible outer liquid-tight shell are each comprised of at least two layers of high-density polyethylene.
19. 19. The double walled dual containment vessel of claim 16 wherein the vessel is left on said mandrel until the exterior liquid-tight shell is completed.
20. 20. The double walled dual containment vessel of claim 16 wherein the annular space is between 1/32 and 1 inch in thickness.
21. 21. The double walled dual containment vessel of claim 15 wherein the vessel is intended for use underground.
22. 22. The double walled dual containment vessel of claim 1 wherein the vessel is intended for use underground.