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Do you have experience with T seams?

Q&A: GMA Techline | August 1, 2022 | By: George R. Koerner

Q: I have a question about T-intersections where three panels of geomembrane meet. In my experience these intersections have all needed to be repaired with a patch. However, a recent project fusion welded straight through these intersections. Air channel testing was completed on the first seam prior to completion of the perpendicular seam. After the perpendicular seam was completed, air channel testing passed for that seam as well. My concern is that the area where the two seams interact will not have the same integrity as the rest of the geomembrane due to fusion welding three panels at once and the fusion welds intersecting previously welded material. Do you have any knowledge or experience with this that you could share?

A: Regarding your Techline question on “T” seams, see the paper, “Experimental Investigation of ‘T’ Seams in HPDE Geomembranes,” that Bob Whitfield and I wrote for the Seventh Geosynthetic Research Institute (GRI) Conference on Geosynthetic Liner Systems: Innovations, Concerns and Designs in 1994.  

As described in the paper, there are three ways to complete the detail: wedge, extruded “T,” and patch over the entire intersection, as shown on page 115. You are going to have to have a moment of trial and error with your installation crew for your application to find something that works well for a long period of time if there is not a specific design detail in the plans and specifications. Obviously, the wedge takes the least time and the patch over the entire intersection the most, with the extruded “T” somewhere in between. However, I have seen all the techniques work.  

A few additional things to consider when making “T” seams: 

  1. The overlap trim back is a critical part of the process of making a good “T” seam.
  2. When making the wedge “T” seam, one most often uses a solid wedge and not one with an air channel groove. 
  3. It is important to vacuum box and electrical leak location (ELL) test all “T” seams for leaks after they have been completed and before the facility is put into service.

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