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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of Ser. No. 13/764,305, filed Feb. 11, 2013, and entitled “Leak Detectable Geomembrane Liners and Method and Apparatus for Forming”, the disclosure of which is hereby incorporated by reference. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not. Applicable. 
       MICROFICHE/COPYRIGHT REFERENCE 
       [0003]    Not Applicable. 
       FIELD OF THE INVENTION 
       [0004]    The present invention relates to combining geomembrane liner panels to form geomembrane liners, and more specifically to a method and apparatus for sealing geomembrane panels together to form liners facilitating detection of leaks therein. 
       BACKGROUND OF THE INVENTION 
       [0005]    Lining systems for containment systems (e.g., systems which contain bodies of water such as ponds) and the like are used to provide an “impermeable” barrier between contaminants and the underlying ground. Generally, these liners are made of insulating material (such as high density polyethylene) which, even if thoroughly tested to be defect free when shipped, can be damaged during shipping and/or installation by, for example, heavy equipment, cutting tools, welding equipment, animals, and vandalism, necessitating that a final leak check be conducted after the liner is installed to locate leaks caused by any such damage. The liner can also be damaged after it is covered by soil and/or liquid, including during its service life as a result, for example, of stones, rocks and/or settlement. Detecting such leaks is important, particularly where hazardous materials are involved, as holes as small as 1.0 millimeter in diameter may cause leaks on the order of a couple of gallons per day with one foot of water pressure. 
         [0006]    Electrical leak location has heretofore been used which involves placing an electrical potential across a geomembrane and then locate the points of anomalous potential distribution where electrical current flows through leaks in the geomembrane. The electrical potential is typically applied utilizing a power supply with the positive electrode submerged in water or a soil layer above the geomembrane, and the negative electrode connected to the soil layer below. When there are leaks, electrical current flows through the leaks, which produces high current density and a localized anomaly in the potential distribution in the material above the geomembrane. Electrical measurements are made to locate those areas of anomalous signal at the leaks. ASTM D7002 and D7007, for example, include details pertaining to such tests. Such measurements have been made using a dipole or pole measurement configuration (though various types of data acquisition equipment can be used), with point by point measurements commonly made using either dipole or pole measurements along parallel lines on a grid pattern. 
         [0007]    In one such method of electrically detecting liner leaks, for example, 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 a potential of the opposite polarity is induced on the opposite side of the sheet, the resulting electrical field will be affected if there is any conductivity from side to side across the sheet, with the effects on the conduction monitored to detect the presence of a leak. Such a detecting method requires an electrically conductive media both above and below the liner, which can be provided by liquid or soil above the liner and good electrical contact with a conductive underlying soil. 
         [0008]    However, in some installations, electrically detecting leaks in the above described manner is unreliable. For example, if the liner is not maintained in good electrical contact with the earth (due to, e.g., use of double liners or other insulating materials, irregularities in the subgrade, and/or wrinkles in the liner) and/or the earth under the geomembrane is dry or not conductive or highly resistant (e.g., in a landfill or with a mining heap leach pad, secondary containment, or coal ash containment), reliable measurements of potential may not be obtained. Similarly, in some landfills, there is leak detection layer of either sand, gravel or geosynthetic product directly underneath the geomembrane for draining any leakage through the geomembrane to a detection site, which layer can inhibit or nullify the leak location survey due to the lack of conductivity of the material. 
         [0009]    One solution to this unreliability arising from possibly insufficient electrical conductivity on the underside of the liner was suggested in U.S. Pat. No. 3,252,155, which disclosed placing the liner over or even adhesively secured to a metal foil sheet, where the foil would provide the required underlying conductivity. That technique was not widely accepted in the industry, however, as such foil is expensive, securing the metal foil to the liner, whether adhesively or mechanically, is extremely difficult to achieve, and the exposed metal foil could severely degrade as a result of, for example, galvanic corrosion, at the construction site. 
         [0010]    Spencer U.S. Pat. No. 5,288,168 (the full disclosure of which is hereby incorporated by reference) has significantly improved upon the foil sheet suggestion by disclosing a liner having an electrically conductive layer provided by embedding conductive particles in the bottom of the layer. The integrity of the sheet is then monitored by establishing an electric field across the sheet and monitoring for sparks between a probe and the bottom, conductive plastic layer. Such spark testing has been accomplished, for example, with a test device that includes a high voltage power source with the positive lead attached to a brass brush and the negative lead attached to a conductive neoprene grounding pad laid on top of the geomembrane. See, for example, ASTM 7240. 
         [0011]    Spark testing of seams in particular has heretofore been done such as detailed in ASTM D6365, wherein conductive material is inserted into the seam just prior to or during fabrication of the seam, with the conductive material connected to a negative terminal of a test apparatus and a positive voltage applied across the seam edge such that a suspect area in the seam is indicated by a spark from the voltage source to the conductive material. 
         [0012]    While the Spencer &#39;168 invention significantly improved leak detection in testing panels, it should be appreciated that during construction of a lined pond, leaks may be caused in a geomembrane panel which was found by testing to have no leaks immediately after liner installation (e.g., by puncturing a liner when it is covered in place by soil and/or water). Moreover, since such lined facilities are typically constructed using a plurality of geomembrane panels heat welded together along seams, testing of the individual panels will not detect leaks at the seams of the panels, where false and anomalous readings have been found. Still further, the conductivity of individual liner panels is often still insufficient for reliable testing, particularly where the liner panel is not maintained in good electrical contact with the earth (due to, e.g., use of double liners, irregularities in the subgrade, and/or wrinkles in the liner) and/or the earth is dry or not conductive. 
       SUMMARY OF THE INVENTION 
       [0013]    In one aspect of the present invention, a method is provided for forming a geomembrane liner testable for leaks. The method includes the steps of providing a plurality of geomembrane panels having conductive lower surfaces, and securing adjacent panels together, wherein the conductivity of the lower surface of an overlying panel is broken along a line adjacent the panel overlapping edge, and the overlapping edges are sealed along the conductivity broken first line. 
         [0014]    In one form of this aspect of the invention, the conductivity along the first line is broken as the overlapping edges are positioned and sealed. 
         [0015]    In another form of this aspect of the invention, the conductivity is broken along the line while heating the lower surface of one panel, and the securing step presses the heated lower surface against the underlying edge of the other adjacent panel. 
         [0016]    In still another form of this aspect of the invention, the conductivity of the lower surface of the one panel is broken along a second line adjacent the first line, and the securing step further comprises sealing the one panel overlapping edge along the conductivity broken second line to the underlying edge of the other of the adjacent panels. 
         [0017]    In another aspect of the present invention, a method is provided for sealing a plurality of geomembrane panels together along adjacent edges, with at least one of the panels having a conductive lower surface for testing the sealed panels for leaks. The method includes (a) laying one of the panels on a surface, (b) breaking the conductivity of the first panel bottom surface across a first line adjacent the side of the second panel, and sealing the bottom surface of the first panel along the first line to the adjacent panel. 
         [0018]    In one form of this aspect of the invention, the sealing step further includes heating the bottom surface of the first panel along the first line; and pressing the first and second panels together along the first line to heat seal the bottom of the first panel on top of the second panel. 
         [0019]    In another form of this aspect of the invention, the heating step includes sliding the bottom of the first panel against a heated element along the first line, and the heated element includes a raised projection which interrupts the conductive lower surface of the first panel. 
         [0020]    In still another form of this aspect of the invention, the breaking step further breaks the conductivity of the first panel bottom surface along a second line adjacent the first line, and the sealing step seals the bottom surface of the first panel along the second line to the second one of the panels. 
         [0021]    In yet another form of this aspect of the invention, the providing step provides panels having a non-conductive member with an integral conductive media on the lower surface of the non-conductive member. 
         [0022]    In another aspect of the invention, a heat welder is provided for securing together overlapped edges of adjacent geomembrane liner panels having conductive bottom surfaces. A welder body defines generally horizontally oriented first and second slots with top and bottom walls extending between the welder body front and rear, wherein the first slot is open on one lateral side for receiving one of the panels and the second slot is below the first slot and open on the lateral side opposite the one side for receiving the overlapping edge of the second one of the panels, with the first and second slots merging at the body rear end. A drive moves the welder body forward. A heating unit is between the first and second slots and forward of the merged first and second slots, and defines a portion of a bottom wall of the first slot. At least one projection extends partially into the first slot for engaging the conductive bottom surface of the overlying panel to interrupt the conductive bottom surface along a line as it passes the projection. The merged first and second slots press together first and second liner panels to heat weld the first and second liner panels together along the line of the interrupted conductive bottom surface. 
         [0023]    In one form of this aspect of the invention, the heating unit is a wedge having first and second laterally spaced sections. Each section defines a portion of a bottom wall of the first slot and a portion of a top wall of the second slot, with each section also have one of the projections. The wall defining portions of each section are tapered together at their rear ends adjacent the merged first and second slots. The merged slots are adapted to heat weld first and second liner panels together along substantially parallel lines corresponding to projection interruptions in the conductive bottom surface of the first one of the panels. In a further form, the projections extend longitudinally toward the body rear from a pointed end. 
         [0024]    In another form of this aspect of the invention, the projection extends longitudinally toward the body rear from a pointed end. 
         [0025]    In still another form of this aspect of the invention, the projection is associated with and heated by the heating unit. 
         [0026]    In another aspect of the invention, a heating unit is provided for use with a heat welder for securing overlapping edges of adjacent geomembrane liner panels together. The heating unit includes a wedge between the first and second slots whereby the wedge defines a portion of an upper wall of the lower slot and a portion of the lower wall of the upper slot, and the defined upper and lower walls taper together to a merged end. At least one projection extends from the wedge defined lower wall of a welder upper slot sufficiently to interrupt a bottom portion of a liner panel passing over the projection while still maintaining the overall structural integrity of the panel. 
         [0027]    In one form of this aspect of the invention, a heating element heats the wedge wall defining portions and the projection. 
         [0028]    In another aspect of the invention, a heating unit is provided to secure overlapping edges of adjacent geomembrane liner panels together. The unit includes a heated wedge movable between overlapping edges of adjacent liner panels, with upper and lower walls tapered together at one end of the wedge. At least one projection extends from the wedge upper wall sufficiently to interrupt a bottom portion of a liner panel sliding along the wedge upper wall while still maintaining the overall structural integrity of the panel. Guides direct the overlapping edges of adjacent liner panels together at the wedge one end when the heating unit is moved between liner panel overlapping edges. 
         [0029]    In one form of this aspect of the invention, the heating unit is adapted to heat the overlapping edges of both adjacent liner panels when the heating unit is between the adjacent liner panels. 
         [0030]    In another form of this aspect of the invention, a heating element heats the wedge wall defining portions and the projection. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a partial perspective view of a heating element according to one aspect of the present invention; 
           [0032]      FIG. 2  is side view of the heating element of  FIG. 1 ; 
           [0033]      FIG. 3  is a cross-sectional view showing a seam between adjacent panels of a prior art containment system liner; 
           [0034]      FIG. 4  is a side diagram view of a heat welder according to the present invention, having the heating element of  FIG. 1 ; 
           [0035]      FIG. 5  is a top view of the heat welder of  FIG. 4  shown welding adjacent geomembrane panels; 
           [0036]      FIG. 6  is a cross-sectional view showing a seam between adjacent panels of a containment system liner formed according to the present invention; 
           [0037]      FIG. 7  is a perspective view of the heating element of  FIG. 1 ; 
           [0038]      FIG. 8  is a perspective view of the heat welder of  FIG. 4  with the housing broken away for clarity; 
           [0039]      FIG. 9  is a perspective view like  FIG. 8  but showing a bottom liner panel being fed into the heat welder; 
           [0040]      FIG. 10  is a perspective view like  FIG. 8  but showing a both the top and bottom liner panels being fed into the heat welder; 
           [0041]      FIG. 11  is a perspective view like  FIG. 8  but both liner panels continuing to be fed into the heat welder with a the panels passing out the rear of the welder having two seams formed therein according to the present invention; 
           [0042]      FIG. 12  is a cross-sectional view showing a seam between adjacent panels of a containment system liner formed according to another aspect of the present invention; and 
           [0043]      FIGS. 13 and 14  are partial and full perspective views, respectively, of a heat welder which may be used in accordance with the present invention, with the heat welder being shown in an open configuration with the contour rollers illustrated in phantom in their operative position when the welder is closed. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0044]    A heat welding apparatus  10  is disclosed in the Figures which may be used in accordance with the present invention to heat weld seams  14  between geomembrane panels  16 ,  18  (typically, rolls of plastic sheet) used to form a liner  20  for, for example, large containment areas, referred to herein generally as containment systems. 
         [0045]    The panels  16 ,  18  are geomembranes formed of a suitable leak proof non-conductive material having a suitable integral conductive lower surface  25 . The lower conductive surfaces of the individual panels  16 ,  18  may also be interconnected with a series of conductive geomembranes, wires, or other conductive media in a grid pattern, or other materials suitable for connecting individual panels. Moreover, in accordance with the present invention, the formed seams  14  between panels  16 ,  18  maybe be suitably tested for leaks even after covered with, for example, water and/or soil, allowing performance of a reliable leak location survey 
         [0046]    In particular, in accordance with one aspect of the present invention, seams  14  may be easily formed so as to avoid the anomalies found in testing liner seams heretofore. Specifically, as illustrated in  FIG. 3 , the geomembrane panels  16 ′,  18 ′ have heretofore been connected in prior art liners  20 ′ by overlapping two edges of the panels  16 ′,  18 ′ and then heat welding the overlapping edges together along a seam  14 ′. Even where two such seams  14 ′ are formed as illustrated to help to guard against leaks between the overlapping edge, the conductive lower surface  25  of the seam flap  26 ′ of the top panel  16 ′ will carry current from above the liner  20 ′ through the seams  14 ′ to the bottom of the liner  20 ′ (i.e., at the right side of  FIG. 3 ) where it is in contact with the underlying ground  27 . Such conductivity through the seams  14 ′ provides a false identification of a leak in the liner  20 ′ along the seam  14 ′. Moreover, ignoring such current flow as being anomalous (or as indicating a leak through the seam(s)  14 ′) could cause actual leaks through the bottom panel  18 ′ near the flap  26 ′ of the overlying edge of the top panel  16 ′ to be overlooked. 
         [0047]    In accordance with the present invention, the seam(s)  14  between adjacent panels may be advantageously heat welded continuously along the length of the overlapping edges of adjacent panels  16 ,  18  wherein the conductive layer  25  on the bottom of the top panel  16  is interrupted along the parallel lines of the seam(s)  14  during the heat welding process (see  FIG. 6 ). As a result, the seam(s)  14  between adjacent panels  16 ,  18  will not allow electric current to flow between the top and bottom of the system through the liner seam(s), and thus reliable leak test readings may be obtained even at the seam(s)  14 . 
         [0048]    The heat welding apparatus  10  and formation of the e seams  14  will now be described. 
         [0049]    Specifically, a heat welding apparatus  10  which may be advantageously used in connection with the present invention includes a body  40  having front and rear ends  42 ,  44 . As best seen in  FIG. 4 , the body  40  defines top and bottom slots  50 ,  52  extending between the front and rear ends  42 ,  44 , each slot  50 ,  52  being generally horizontally oriented and arranged to accept the overlapping edges of adjacent panels  16 ,  18 . 
         [0050]    As best understood from  FIGS. 4 ,  5  and  14 , the slots  50 ,  52  of the apparatus (heat welder)  10  are also open on opposite lateral sides of the body  40 , so that the welder  10  may be oriented so that it too overlaps with the overlapping edges of the panels  16 ,  18 . The apparatus is suitably supported by front and rear wheels  60 ,  62  so that it may move relative to the panels  16 ,  18  and may be suitably driven by nip rollers  64  (as indicated by the arrows  65 ) to pull the heat welder  10  along the panels  16 ,  18  in the direction of arrow  66 . 
         [0051]    It should be understood that while the apparatus slots  50 ,  52  may be described as extending horizontally, such horizontal orientation refers to the slots  50 ,  52  extending generally from the front to rear ends  42 ,  44 , with the slots  50 ,  52  providing a non-planar path which merges together at the rear end  44  of the apparatus  10 . 
         [0052]    Moreover, it should be understood that while the slots  50 ,  52  may be described as having top and bottom walls for simplicity of description, such description encompasses guiding members  70  such as contour rollers and/or partial walls. As such, “slots” as described generally herein would encompass any structure in which the edges of the panels  16 ,  18  may be moved through the apparatus while maintaining their generally horizontal orientation without buckling or folding. 
         [0053]    The welder  10  includes a heating unit  80  between the slots  50 ,  52  and forward of the merger of the slots  50 ,  52  at the apparatus rear end  44 . Advantageously, the heating unit  80  defines a portion of a bottom wall of the top slot  50  and a portion of the top wall of the bottom slot  52  and is wedge shaped so as to be tapered together at its rear end. The heating unit  80  is suitably heated so that the panels  16 ,  18  which pass over the heating unit  80  have their faces heated sufficiently so that when the panels  16 ,  18  are pressed together in the merged path at the apparatus rear end  44 , they are heat welded. 
         [0054]    As illustrated, the heating unit  80  includes two laterally spaced heating sections  82 ,  84 , for forming a seal having two parallel seams  14 , though it should be understood that it would be within the scope of the present invention to provide a single heat welded seam, or more than two seams if desired. 
         [0055]    Moreover, in accordance with the present invention, at least one heating section  82 ,  84  of the heating unit  80  also includes at least one projection or fin  90  extending partially into the top slot  50  from below. 
         [0056]    The fin  90  may advantageously be of any shape suitable to melt through the conductive thin layer on the bottom surface  25  of overlapping edge of the top panel  16  as it passes through the slot  50  and past the projection  90 . Moreover, while the fin  90  may advantageously be shaped as illustrated, with a pointed leading (forward) edge, the shape and size could vary while still providing at least some of the advantages of the present invention. 
         [0057]    Further, the fin  90  may be an integral part of the heating unit  80 , or it may advantageously be provided on an insert  92  in a recessed pocket in the heating unit  80  and removably secured therein by, for example, a countersunk screw  94 . Still further, for heating units  80  such as illustrated which have more than one heating section  82 ,  84 , it should be appreciated that a projection  90  may be provided on both sections  82 ,  84  to provide redundancy, although at least some of the advantages of the present invention could be provided with a projection  90  provided on only one of the sections  82 .  84 . 
         [0058]    It should thus be appreciated that as the two heated panels  16 ,  18  are pressed together behind the heating unit  80  by the nip rollers  64 , each of which have two sections aligned with the two fins  90 , respectively for forming the heat welded seams  14  along the length of the panels  16 ,  18 . The welder  10  will thus form a pair of parallel seals  14  between the overlapping adjacent panels  16 ,  18  wherein, as shown in  FIG. 6 , there is no conductive layer passing through either of the seams  14 —that is, there will be no current flow through across the seams  14  such as has heretofore provided anomalous and erroneous readings when leak testing. (It should be appreciated also that it would be within the scope of the present invention to form only one such seam  14 ). 
         [0059]    Yet another embodiment of the present invention allows for reliable leak testing of liners formed of a plurality of panels even when used in applications where the liner may not be not maintained in good electrical contact with the earth (due to, e.g., use of double liners, irregularities in the subgrade, and/or wrinkles in the liner) and/or the earth is dry or not sufficiently conductive. 
         [0060]    Specifically, as illustrated in  FIG. 12 , in accordance with this aspect of the invention, a conductive member  100  may be provided beneath adjacent geomembrane liner panels  116 ,  118  having conductive bottom surfaces  125 . As illustrated in  FIG. 12 , the conductive member  100  is an inverted section of a geomembrane liner panel with a conductive surface on one side—laid upside down with the conductive surface  130  on top so that it contacts the conductive bottom surfaces  125  of both of the liner panels  116 ,  118 . 
         [0061]    While the conductive member  100  may extend continuously underneath the adjacent liner panels  116 ,  118 , spanning across the two so as to place them in electrical contact with each other, it should be appreciated that the member  100  may also consist of spaced short sections or strips of conductive geomembranes conductively connecting the adjacent panels  116 ,  118  at spaced locations along the seam(s). In fact, it should be appreciated that virtually any conductive member  100  could be used, including a grid of spaced wires or other conductive media laid beneath the liner, so long as it allows for the individual panels to effectively provide a single conductive bottom surface across the plurality of panels defining the liner  120 . 
         [0062]    It should be appreciated that while  FIG. 12  illustrates this aspect of the invention with a seam incorporating the first aspect of the invention (i.e., with the conductive bottom surface  25  of the top liner panel  16  broken), the advantages of this second aspect of the invention (i.e., a conductive interconnection of the bottom surfaces of adjacent liner panels) could be provided with even prior art seams such as illustrated in  FIG. 3 . However, the full advantages of both aspects of the invention may be provided by the configuration illustrated in  FIG. 12 . 
         [0063]    As previously noted, leak detection sensitivity depends on the conductivity of the materials above and below the geomembrane. As also previously noted, standard leak detection tests may use either water or moisture in the soil to transmit voltage above the geomembrane, and standard testing may utilize water or moisture in the soil below the liner for a grounding source. If there is a hole in the geomembrane then the voltage introduced in the above material will flow through the hole and to the grounding source underneath the geomembrane creating a current for leak detections. However, as also previously noted, where the material underneath the geomembrane does not have enough (or consistent) moisture to provide a suitable grounding source, such leak location testing could not heretofore be suitably performed. 
         [0064]    With a liner  120  formed according to this aspect of the invention, leak surveys can be accomplish with direct connection to a minimum number of panels (i.e., any one of interconnected panels). The bottom conductive surfaces  125  of the electrically interconnected geomembrane panels (e.g.,  116 ,  118 ) provide a single grounding source underneath the liner  120  to allow the leak location survey to be performed over entire geomembrane surface. Since the conductive layer (bottom surfaces  125  and conductive member  100 ) is always in intimate contact with the geomembrane panels  116 ,  118 , and the conductivity is consistent regardless of the conductivity of the underlying layers, leak surveys can be more effectively performed when the conductive layer is utilized. 
         [0065]    It should also be appreciated that leak detection of liners  20  formed of a plurality of panels  16 ,  18  according to the present invention may be performed using a variety of leak testing methods, including spark testing according to ASTM 7240. Moreover, leak detection of the seams of liners  20  formed according to the present invention could also be accomplished by spark testing according to ASTM 6365, with conductive material inserted into the seal (e.g., between the seams  14 ) and spark testing performed in the area of the seams  14 . 
         [0066]    It should thus be appreciated that the present invention as disclosed herein allows for containment system liners to be more easily, economically and reliably inspected using an electrical inspection apparatus to detect leaks. Such inspections can be made without the need for maintaining good electrical contact with conductive natural surroundings outside the liner. Furthermore, other objects, features and advantages of the invention will become apparent from a review of the entire specification including any appended claims and drawings. 
         [0067]    It should be appreciated that the invention may include any or all of the above-described features, include only one of the above features, more than one of the above features, and any combination of the above features. Moreover, it should be appreciated that such features may be achieved by use of fewer than all of the above-described structural elements, including combinations less than all of the above-described structural elements.

Summary:
A method for forming a geomembrane liner testable for leaks by securing adjacent panels together with the conductivity of the lower surface of an overlying panel broken along a line adjacent the panel overlapping edge, and the overlapping edges sealed along the line. A heat welder has slots for the overlapping panel edges, with a heated wedge between the slots and having a projection to break the conductivity of the overlying panel bottom surface as it passes the wedge. The slots merge to press the liner edges together to heat weld them along the line of broken conductivity as the welder is moved along the panel edges.