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From a presentation at the Geosynthetics 2013 conference
By Abigail Beck
Editor’s note: Proceedings from the Geosynthetics 2013 conference are available at IFAI
A standing-room-only crowd gathered April 3, 2013, for the panel discussion “How to specify a geoelectric leak location survey,” presented during the Geosynthetics 2013 conference April 1–4 in Long Beach, Calif.
The varying viewpoints of the panel members and the challenging questions and comments from the audience made for lively discussion. Forums such as these are critical for sharing knowledge and improving industry standards and methods.
Tim Bauters (Golder Associates) kicked off the presentation by sharing some alarming photos of leaks that would have remained in the installed geomembrane if they had not been detected by liner integrity surveys (LIS). The rest of the 90-minute session was shaped by questions and topics posed to the panel, which included Glenn Darilek (Leak Location Services Inc.), Ian Peggs (I-Corp Intl. Inc.), Abigail Beck (TRI Environmental), and André Rollin (Groupe Alphard). Each member of the panel was given the opportunity to provide a response or comment, followed by questions and comments from the audience.
The questions and topics are presented here, followed by a synopsis of the subsequent discussions.
Q: What should one know about liner integrity surveys during the liner system design? How should one specify a liner integrity survey (LIS)? How should one perform construction quality assurance (CQA) for a liner integrity survey?
A: The panel pointed out that, foremost, a liner integrity survey needs to be considered in the design phase. Boundary conditions such as material conductivity and survey area isolation are important to a successful survey. Several case studies were mentioned—at one site the geocomposite was too dry for a successful survey, at other sites it was sufficiently wet and leaks were located. If the requirement for watering the geocomposite is part of the project specifications, dry geocomposite can be avoided.
The same was noted with subgrade conditions. The consequences are greater, however, since the subgrade cannot be watered after the geomembrane has been placed. Desiccated subgrade and encapsulated GCL can prevent the ability to survey an installation. If this is not considered in the design phase, the survey may not be successful.
Design engineers should contact an experienced leak location company for design guidance and they should understand that they have the option of educating themselves in liner integrity surveys.
Panel members commented that it would be quite complicated to produce a complete design guidance document, given the wide range of technologies and site and design-specific conditions. The panel noted that survey companies chosen to perform LIS work need to be experienced, but that the minimum qualifications should be sensible for the method used. One useful tool for CQA is the blind actual leak placement—a hole drilled by the site engineer, owner, or CQA agent to check the functionality of the LIS.
GPS-based mapping can be used with the dipole survey method as a valuable CQA tool to provide documentation that the survey was completed and that the method was properly applied.
In conclusion to this portion of the discussion, the audience expressed the desire for a more comprehensive liner integrity survey guidance document for design engineers.
Q: Talk about challenging cases relating to site isolation from your field experience.
A: The importance of perimeter isolation was stressed, particularly in terms of cleanliness and dryness. Grounded objects inside the survey area also require isolation. In addition to the perimeter site isolation, excessive leaks through the liner itself can reduce survey sensitivity.
However, even if isolation or excessive leakage compromises the sensitivity of a survey, the survey should be done because it is better than not doing a survey at all. The survey will still locate larger leaks. In the case of adverse site conditions, ASTM does allow latitude in the sensitivity.
Q: What new technologies (materials and equipment) and standards should the field look to for performing or improving LIS sensitivity and success?
A: A GPS-based 3-D voltage mapping system for soil-covered dipole surveys was presented. This method provides meaningful CQA documentation. In addition, the equipment used for GPS-based mapping can be used for simultaneous multidirectional surveying. This would increase dipole survey sensitivity and quality.
A method for cancelling the background noise for the dipole method was also presented. This technology would be applied to sites with electrical noise from sources such as pumps, power lines in the surrounding area, and objects in the survey area that create localized voltage potentials. It has shown to increase the signal created by a leak by reducing the electrical noise created by other sources.
The introduction of a new ASTM standard for conductive geotextile was announced.* The technology for creating conductive geotextile has also been significantly advanced.
*(See the complete news article regarding the proposed ASTM WK42576, “Practice for Electrical Leak Location on Exposed Geosynthetics.”
This proposal is currently under development and will be a topic for discussion at the Jan. 29–31, 2014, biannual meetings of the ASTM Committee D35 on Geosynthetics in Houston, Texas.
Q: Where are we with regard to destructive testing of liners on site?
A: The requirement for destructive testing was discussed, particularly the frequency. The panel’s first comment was that the frequency of destructive sampling is excessive. Several states allow a decrease in destructive seam sampling if a liner integrity survey is performed. It was mentioned that there are other methods of seam analysis that are nondestructive. The concept that it might make more sense to only do destructs at the end of the seams where patches will be anyway was mentioned.
J.P. Giroud offered a reason for the current 500-ft. frequency. He stated that in 1983 the frequency was arbitrarily created and was meant as an interim requirement until a more sensible one could be implemented, but that value has never changed. Another audience member commented that the ability to randomly sample for testing has made a positive impact on overall seam quality, calling it an important CQA tool to be able to require a destruct sample in questionable areas.
In conclusion, a comment was made from the audience that it shouldn’t be a question of replacing one form of testing for another; all of the methods should be used to complement each other to create a holistic evaluation of installation quality.
Q: Should artificial leaks be standardized?
A: Artificial leaks are conductive circular disks meant to simulate actual leaks. They are electrically isolated around the circular conductive disk and an insulated wire can then be grounded to the earth at the perimeter of the survey area. This allows the liner integrity survey to check for survey sensitivity without actually creating a leak in the geomembrane.
In response, the panel commented that the signal from a leak depends more on hole contact than on hole size. The artificial leaks can be thought of as a way to check equipment functionality. It was noted that ASTM already provides specific details for artificial leaks, and this approach has worked well for 23 years.
It was stressed that artificial leaks should be set up to conservatively estimate an actual leak signal so the sensitivity test results do not make the survey sensitivity appear greater than in actuality. One panel member said that it is always better to drill a hole if possible to verify survey functionality; another panel member disagreed, since the ASTM method is meant as a nondestructive test. The drilling of holes can be costly, since someone will have to pay to patch it, especially if no other holes are found during the survey except for the drilled holes. The ASTM standard explains the advantages of artificial vs. actual leaks and some owners do not want to pay to have a liner crew mobilized to repair an actual leak.
Q: Should blind actual leaks be used?
A: The question was posed whether it is a good idea for owners, designers, or CQA agents to make blind actual leaks. These types of leaks are intentionally put in for the leak location contractor to locate as a means to verify the functionality of the methods that contractor will use on the site.
One panel member said that at least one blind leak per project should be made since the owner, designer, and CQA agent’s confidence in LIS technology goes up when blind holes are drilled and located.
Yet, two case studies were described where blind holes were not drilled correctly and multiple leak location companies were unable to locate them. These situations resulted in project delays and significant financial loss for the leak location contractors involved.
It was noted that guidance for creating blind actual leaks will soon be an ASTM standard. This should mitigate many of the problems encountered by poorly executed blind leaks. The ASTM standard is currently under review for approval.
The panel made the final comment that since the spirit of liner integrity methods is that they are nondestructive test methods, blind actual leaks should not be required. The audience responded with the comment that in Europe, it is the standard protocol to place at least three blind leaks per project, but it is imperative that a blind actual leak placement protocol is provided.