WO2000001895A1

WO2000001895A1 - Multilayered conductive geomembrane

Info

Publication number: WO2000001895A1
Application number: PCT/US1999/013673
Authority: WO
Inventors: Michael Mathieson
Original assignee: Gse Lining Technology, Inc.
Priority date: 1998-07-07
Filing date: 1999-06-17
Publication date: 2000-01-13
Also published as: AU4691199A

Abstract

A liner (12) includes an intermediate flexible conductive layer (24). The conductive layer (24) is sandwiched between a pair of nonconductive layers (22, 34). Either or both of the outwardly facing surfaces of the liner may be textured or smooth surfaced.

Description

MULTILAYERED CONDUCTIVE GEOMEMBRANE

Background

The invention relates generally to conductive plastic liners which may be utilized to line fluid containment areas, waste disposal areas, landfills and any other application where seamed plastic liners are utilized. A lining system provides a generally impermeable barrier between contaminants and the groundwater. The liners may be made of electrically insulating material such as high- density polyethylene.

Synthetic liners, manufactured under stringent quality control standards, are thoroughly tested to be defect- free at the time of shipment. However, during installation, a liner is exposed to a wide spectrum of hazards, such as heavy equipment, cutting tools, welding equipment, animals and vandalism. Even the most stringent quality control program cannot protect against every hazard. Therefore, a final leak check may be conducted after the liner is installed to locate leaks caused by construction damage. These leak tests are designed not only to determine whether or not a leak exists, but they also hopefully succeed in locating the leak so that it may be repaired.

Conventional leak detection and location techniques include smoke, traceable gas, electrical surveys and the like. However, in the case of smoke and traceable gas, since there can be no positive assurance that the indicating medium has reached the entire surface of the liner, holes that may cause leaks may go undetected. Electrical surveys require the liner to be in contact with an electrically conducted medium, both above and below the liner. This normally means that not only must the liner contain a liquid, but also the soil upon which the liner rests must be conductive. The resulting water pressure on top of the liner generally maintains the liner in contact with the subgrade. However, contact with the earth is not always maintained because of irregularities in the subgrade and wrinkles in the liner.

In a commercially available apparatus for electrically detecting liner leaks, a potential is induced across the thickness of a liner. If a potential of one polarity is induced on one side of the sheet and the potential of the opposite polarity is induced on the opposite side of the sheet, the resulting electric field will be affected if there is any conductivity from side to side across the sheet. The effects of the conduction can be sensed to detect the presence of a leak. U.S. Patent No. 5,288,168 to Spencer assigned to the assignee of the present application, discloses a liner with an integral conductive layer. The liner includes an upper layer which could be formed of conventional liner material and a lower layer which is adapted to sufficiently conduct electricity to enable the detection of pinhole leaks in the liner. This product has been used with considerable success around the world for several years. Despite the success of this product, there are a number of issues with the current product. The product must be installed with the non-conductive layer facing upwardly. If the product is inadvertently seamed to adjacent sheets in an improper orientation, the installation must be removed and repaired. Because only one side of the current conductive liner is made of the conventional plastic, the product can only have one textured side. In addition, it may be more difficult to weld the two layer product in some instances because a conductive layer is welded to a non-conductive layer in lapped seam. In addition, in some cases, unless a hole penetrates completely through the nonconductive layer, it is not detected with the current product. Thus, there is a continuing need for an improved conductive liner for facilitating the electrical detection of liner leaks.

Summary

In accordance with one embodiment, a liner may include an outwardly facing first plastic layer and an outwardly facing second plastic layer. A conductive plastic layer is located between the first and second plastic layers.

Brief Description of the Drawings

Figure 1 is a sectional view of a landfill, hazardous waster disposal pond, reservoir or lagoon in accordance with one embodiment of the present invention;

Figure 2 is an enlarged cross-sectional view of an alternate embodiment; and

Figure 3 is an enlarged cross-sectional view of still another embodiment of the present invention.

Detailed Description

Referring to Figure l,a hazardous waste or outdoor fluid containment area 10 may contain a number of water soluble pollutants, such is liquid, sludge, solids, or combinations thereof. The containment area 10 may be lined with a water impermeable plastic sheet or thermoplastic liner 12 having a bottom and sidewall 16. The sidewall 16 covers a berm area 18 of the containment area 10 and the bottom 14 covers the planar portion 20 of the containment area 10. A liquid (not shown) or other material may be contained by the water impermeable flexible plastic liner 12 constructed in accordance with one embodiment of the present invention.

The thermoplastic liner 12 may be of a uniform density material having an upwardly facing layer 22 and a downwardly facing layer 34 in contact with the containment area 10. An intermediate layer 24 is made of a conductive plastic designed to conduct electricity. The layers 22, 24 and 34 may be integrally bonded together. Through the use of the conductive layer 24, electrical testing for holes can be easily accomplished using well known equipment.

The thermoplastic liner 12 may be manufactured using standard thermoplastic techniques such as co-extrusion. For example, a conductive plastic stream may be co- extruded with a pair of conventional plastic streams to form an integral sheet. With a co- extrusion technique, the conductive plastic seamlessly bonds with the conventional plastic in laminar fashion to form a solid, continuous plastic sheet of a desired thickness containing upwardly and downwardly facing insulating layers sandwiching an intermediate conductive layer. While a joint "A" is shown in Figure 1 for illustration purposes, with a co-extrusion technique there may be no such distinct boundary between layers. In this way, the upper and lower layers 22 and 34 provide the strength and integrity normally required to function as a lining for reservoirs and containment areas and the like. At the same time, the intermediate layer provides a conductive plate to facilitate in situ electrical analysis of the sheet. Because it is not responsible for providing structural integrity, the intermediate layer may be relatively thin, for example, on the order of 5 mils. In accordance with one preferred embodiment of the present invention, the conductive layer is less than 20% of the thickness of the overall liner and preferably is about 5 to 10% of the overall liner thickness. The layers 22 and 34 may be the same or different thickness.

The conductive layer 24 may be formed in a variety of ways. For example, a doping material may be added to the type of plastic that is normally used to form liners, so that a chemically inert, conductive layer may be formed. In this way, the doping material may be maintained in isolation from the materials associated with the containment area. A variety of particles may be used including powders, flakes and fibers. For example, the conductive layer 24 may be doped with conductive particles such as carbon black, metal particles, or other materials to make the layers sufficiently conductive to allow the detection of leaks regardless of the electrical characteristics of the surrounding material or soil. Advantageously, the conductive particles are nonmetals, and particular advantages may be achieved by using particles that are themselves chemically inert.

In one embodiment, the conductive filler constitutes more than 5% by weight of its layer 24. In another embodiment, a conductive material constitutes between about 10 and 25%) by weight of the its layer 24. For example, in an embodiment where the doping material is carbon black, carbon black may make up between 10 and about 15% by weight of its layer 24. One useful carbon black is furnace type carbon black such as Black Pearls 2000 available from Cabot Carbon of Billerica, Mass. This carbon black has a nitrogen surface area of about 1475 m.²/gm.

The doping material, such as carbon black, may be thoroughly intermixed with a plastic stream which is then co-extruded between the layers 22 and 34. Alternately, a plastic layer containing the conductive material may be secured to another sheet using conventional techniques such as adhesive or heat to form the composite. In any case, the conductive layer advantageously has a volume resistivity of less than 280 ohm-cm. A desirable sheet has a volume resistivity of between 10 and 60 ohm-cm. The use of particles with surface areas greater than 1000 m.²/cm. is also advantageous. Although medium or high density polyethylene or other plastic materials, such as polyvinyl chloride or polypropylene may be used to form the layers 22 and 34 and the conductive layer 24, it may be desirable to form the conductive layer from low density polyethylene to provide additional elasticity. For example, one useful resin is linear low density polyethylene such as that available from Union Carbide, DGDA-7028, having a density of about 0.85 gm/cm ^'

With the present invention, pinhole leaks may be quickly located using electrical monitoring techniques since the more conductive layer quickly and reliably draws electricity through the hole. A spark discharge probe 26 may be moved across the liner 12 to detect a spark discharge between the probe and the conductive layer 24. A source of potential 28 is connected is connected to the layer 24 and the probe 26. A conventional detector/alarm indicates when a hole is detected. While the probe 26 may take a variety of forms, the probe 26 is illustrated as a brush with brass bristles 32. The bristles 32 are at a relatively high potential so that when they pass over a hole, a spark jumps between the layer 24 and the bristles. This spark can be detected by an operator or by conventional equipment.

The following example illustrates one embodiment of the present invention. A pair of 30 mil high density polyethylene liners may be co-extruded with a 6 mil high density polyethylene conductive layer which contains 15%> by weight of Cabot Corp. Black Pearls 2000 furnace type carbon black. The carbon black may be mixed into the plastic by using a screw type blender to obtain an A-1 dispersion. The carbon black has a nitrogen surface area of 1475 m.²/g. A 12,000 volt spark tester made by Pipeline Inspection Co., model 725, may be connected with the cathode attached to the conductive layer and the anode connected to a two foot wide brass brush.

The liner 12 may be formed of a number of large flexible sheets aligned in abutting or overlapping fashion. Adjacent sheets are connected at seams using conventional welding techniques, such as extrusion or wedge welding.

Referring to Figure 2, one technique for butt seaming together adjacent layers in abutting fashion involves extrusion welding layers in and end to end arrangement. In this way, the outwardly facing layers 22 and 34 of each of the sheets 12a and 12b are butt seamed together while the conductive layers 24 of each of the sheets 12a and 12b are similarly connected in an end to end alignment.

Referring to Figure 3, the layers 22a and 34a may be formed with roughened exterior surfaces 38. The exterior roughening may be achieved using any conventional technique. Because of the presence of two nonconductive layers 22a and 34a, both sides may be made roughened and the liner may be placed with either side facing up.

The embodiment shown in Figures 1 through 3 has a number of advantages. The product can be produced with texturing on both sides so that either side may be placed in an upwardly facing configuration. In addition, there are some applications in which it is advantageous to have both sides of the liner being textured. In a liner made of only two layers, this was not possible, since it was not feasible to texture the conductive layer.

In any case, regardless of whether the outwardly facing surfaces are textured, with the embodiment shown in Figures 1 through 3, either liner surface can be the upwardly facing side. This simplifies installation. With a two-layer liner, it was required to maintain the conductive layer in a downward position. In addition, the product is easier to weld because it is possible to butt weld the surfaces together. Generally, with the two-layer embodiment, the upper layer being nonconductive and the lower layer being conductive, it was necessary to lap weld the structures together. It was then advantageous to use a conductive welding material to electrically connect the conductive layers of adjacent sheets.

Because the layers 22 and 34 are generally thinner in an embodiment containing only two layers, a hole does not have to penetrate the whole sheet. With the embodiment shown in Figures 1 to 3, scratches which do not penetrate all the way through to the conductive layer may be located. In addition, the embodiment shown in Figures 1 to 3 can be produced on a calendar line.

While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as falls within in the true spirit and scope of the present invention. What is claimed is:

Claims

1. A geomembrane comprising: an outwardly facing, nonconductive liquid impermeable first layer; a second outwardly facing nonconductive liquid impermeable second layer; and a conductive layer co-extruded between said first and second layers.

2. The geomembrane of claim 1 wherein said conductive layer is adapted to sufficiently conduct electricity to enable detection of pinhole leaks in the geomembrane by establishing a potential difference between a probe on one side of the geomembrane and the conductive layer.

3. The geomembrane of claim 2 wherein said geomembrane is adapted to be maintained for flexure in use over a supporting surface so as to at least approximately conform to the shape of the supporting surface.

4. The geomembrane of claim 3 wherein the liner contains two opposed, outwardly facing surfaces, each of said surfaces being textured.

5. The geomembrane of claim 4 wherein said geomembrane is adapted to cover outdoor earthen fluid containment areas.

6. The geomembrane of claim 5 wherein said conductive layer contains conductive particles embedded within the layer.

7. The geomembrane of claim 6 wherein said first and second layers are formed of high density polyethylene.

8. The geomembrane of claim 1 formed of at least two sheets, said sheets being butt welded together.

9. The geomembrane of claim 1 adapted to cover outdoor earthen fluid containment areas.