Areas to be seamed shall be free from dust\debris and dry. In no case shall HDPE Sheet overlap be less than 75mm HDPE Geomembranes sheets are to be sufficiently overlapped up to 150mm to secure sufficient good weld.
Weather conditions can be of extreme influence to the weld quality; therefore welding or Seaming is prohibited during rain without special protection. Seaming is usually prohibited at temperatures of less than + 4°c and above + 75°c. Subgrade must be maintained in good condition to guarantee permanent bearing capacity (in case welding equipment is dependent on ground pressure). Prior to commencement of welding, test welds are to be executed and tested to all weld parameters.
No vehicular traffic shall travel on the HDPE geomembrane sheet other than an approved low ground pressure Vehicle or equivalent
Sand bags or equivalent ballast shall be used as necessary to temporarily hold the geomembrane material in position under the foreseeable and reasonably - expected wind conditions
Geomembrane placement shall not be done if moisture prevents proper subgrade preparation, sheet placement, or sheet welding.
The HDPE geomembrane shall not be allowed to "bridge over" voids or low areas in the subgrade.
The HDPE geomembrane shall rest in intimate contact with the subgrade.
Welding or Seaming shall primarily be performed using automatic fusion welding equipment and techniques. Extrusion welding shall be used where fusion welding is not possible such as at pipe penetrations, patches, repairs and short (less than a roll width) runs of seams.
Welding or seaming of HDPE Geomembrane is a thickness dependant technique that requires a minimum material thickness to create an effective weld without distortion. Extrusion welds in sheet less than 1.0 mm thickness are not recommended. Extrusion welds in 1.0 mm thick HDPE Sheet show some distortion and can sometimes be very difficult to prepare around intricate pipe penetrations and mechanical attachments. 1.5 mm thick HDPE Geomembrane sheet can be reliably extrusion welded in most situations and is recommended in most applications. Welds in 2.0 and 2.5 mm thick HDPE Sheet are excellent and are recommended in applications that require exceptional durability. Extrusion welds are very slow and are typically used only for repairs and details.
Hot wedge welders have become the technique of choice for HDPE installers over the past several years. Wedge welders are fast, produce excellent welds in HDPE sheet from 1.0 mm to 2.5 mm thickness, and can be easily tested. Hot wedge welders today produce two weld tracks separated by a small unbonded channel. By sealing off both ends of this channel, and then pumping it full of air, entire seams can be checked quickly and effectively.
Fishmouths or excessive wrinkles at the weld or seam overlaps shall be minimized and when necessary cut along the ridge of the wrinkles back into the sheet so as to effect a flat overlap. The cut shall be terminated with a keyhole cut nominal 10 mm diameter hole so as to minimize crack/tear propagation. The overlay shall subsequently be welded. The key hole cut shall be patched with an oval or round patch of the same base HDPE Sheet geomembrane material extending a minimum of 150 mm beyond the cut in all directions.