Cutting holes for testing vs. testing for holes


When it comes to geomembrane quality assurance, are destructive testing practices as effective as basic leak detection technologies?

The function of a geomembrane liner is to prevent liquid flow to the environment. CQA measures should be judged on how effective they are in meeting this requirement. This article critically examines the industry’s current means of providing confidence that final installed geomembranes do not leak.

Destructive seam tests: CQA cornerstone or millstone?
In the early days of geomembrane installation, research and the resulting technical guidance put a heavy emphasis on how to create and check for good seams. The underlying assumption was that seams were the “weak link” because they were often created under less-than-optimal field conditions by technicians subject to human error. Indeed, the landmark textbook on designing with geosynthetics (Koerner 1994) states in a discussion of the most important aspects of geomembrane construction, “Seams should be at the top of everyone’s list.” From that perspective, the industry responded by instituting a plethora of required CQA documentation in the form of seaming logs, panel logs, temperature measurements, start-up procedures, non-destructive testing procedures, and destructive testing procedures. Much of this was promulgated by the U.S. EPA (1991, 1993).

The seaming of polyethylene geomembranes - by far the most common type in the waste containment industry - requires trained personnel and special equipment. These specialized installation requirements tend to create a mystery surrounding the welds, which then seems to compel organizations to do even more testing.

The rationale and justification for requiring even more scrutiny of field seams was fueled by the phenomenon of stress cracking in HDPE geomembranes, where it was noted that most of the stress-cracking problems were immediately adjacent to and in field seams. Since polyethylene in the early days was already prone to stress cracking wherever stress concentrations occurred, field seaming activities greatly exacerbated the issue by both introducing stress concentrations and degrading the polymer with heat in the vicinity of the seams. Dramatic field failures, some continuing even to this day from older installations, led to more and more recommendations regarding verification and field testing of seams in an effort to minimize the potential for stress cracking. In fact, the greatest strides in the stress cracking arena came not so much from improved seaming methods and quality assurance, but rather from the polyethylene resin manufacturers. The fact is, the stress-cracking problem was aggressively addressed and, in the authors’ opinion, largely solved, at least with the high quality resins being used in North America. Any specification following GRI GM-13 should provide a satisfactory material resistant to stress cracking. Of course, good seaming practice is still important, especially for exposed conditions. Nonetheless, the intense scrutiny over field seaming procedures, voluminous record keeping, and intensive destructive testing begins to feel like overkill.

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