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The GMA Techline

Q&A: GMA Techline | August 1, 2020 | By:

Seaming dissimilar geomembranes

Q: I kindly request your technical support in the questions below:

A nonpolyethylene geomembrane was used in phase 1 of the tailings dam waterproofing system of a gold mine. For the expansion (phase 2), the use of a high-density polyethylene (HDPE) geomembrane is being evaluated. How can the HDPE geomembrane join the bituminous geomembrane from the previous phase? What is the recommended procedure in cases like this? 

A: The seaming of dissimilar geomembranes is typically achieved with a stability berm that delineates the two projects and geosynthetic clay liner (GCL) between the two geomembranes. One may need to run stability analysis if this connection is on a side slope. 

IAGI certification

Q: What is IAGI certification and can we get it through GSI?

A: Nice to hear from you and thanks for reaching out to us at the Geosynthetic Institute (GSI). Unfortunately, I think you want to get in touch with the International Association of Geosynthetic Installers (IAGI) instead of GSI. They have two different programs, which are described below,
that may be of interest to your firm.

Program #1: IAGI’s Certified Welding Technician (CWT) program recognizes the knowledge, experience and skill of those technicians who hold the certification.

Engineers benefit from IAGI’s CWT program because certification verifies that the welders on their job have experience in geomembrane welding and meeting industry standards of skill for those geomembranes they are certified in. IAGI encourages all engineers
to require that any welding done on their jobsite be done with CWTs.

Member companies who have invested resources in training and testing their welding technicians take pride in the skill of their welding technicians. IAGI encourages you to use companies that have made this commitment to quality on your next job.

Program #2:  IAGI’s Approved Installation Contractor recognizes geosynthetic installation companies that meet a minimum level of professionalism, ethics and business practices. Approved Installation Contractors must meet requirements in the following areas: corporate history and business practices, insurance verification, safety training, and professional competence and experience. Click on the links below for additional AIC program information.

If you have any additional questions or comments regarding the IAGI International program, please contact the IAGI office at or +1 (720) 353-4977. 

Ultraviolet radiation and geomembranes

Q: We are currently assessing some geomembrane products for simulated lifetime exposure for use in central Australia and have been comparing values with the Arizona example provided in your multiple geomembrane test studies (GRI Report #44 and #47).

I have a query related to the value adopted for the site-specific ultraviolet (UV) radiation.

Our source for our site indicates approximately 20 MJ/m2/day (Source:

I understand the source adopted for the GRI example is

Where the Arizona figure indicates approximately 7.9 kWh/m2/day, which equates to approximately 28.1 MJ/m2/day.

I wish to understand how the value of 28 MJ/m2/month (as used in your example) is obtained. If you could please provide some clarity on the matter it would be greatly appreciated.

A: In short, these values have been handed down by the equipment manufacturers of weathering devices. Attached is GSI’s best present correlation. 

Lifetime Equivalency from Laboratory Weatherometers

GM1: UV fluorescent
half-life in lab=2100 hours
field lifetime=2.2 years
Thus: 1000 hours QUV=1.05 years
field (9.2 ×)

GM2: UV fluorescent
half-life in lab=8700 hours
QUV 1000 hours=1 year in field
field lifetime=8 years
Thus: 1000 hours QUV=0.92 years
field (8.1 ×)

GM3: Xenon arc
half-life in lab=4500 hours
Xenon 1000 hours=0.5 year in field
field lifetime=2.2 years
Thus; 1000 hours xenon=0.49 years
field (4.3 ×)

Conclusion: UV fluorescent is twice the strength of xenon arc 1000 hours in QUV at ASTM D7238 conditions equals a year in Phoenix, Ariz.; 1000 hours in xenon arc at ASTM D4355 conditions equals a year in Phoenix, Ariz., field conditions.

Geotextile tubes with flocculants

Q: I just listened to the webinar today on geotextile bags. We are planning a pilot study for March for a paper mill site that is a Superfund. Our plan currently adds the flocculant before the bag and then we go through bag filters before going through carbon treatment. The idea of adding the carbon to the bags and potentially eliminating another step is very appealing. So, I was wondering if the carbon addition that you did was in isolation or combined with a flocculant? We will be using a line addition and will be using CO2 to adjust the pH after flocculation before the carbon treatment. The focus for the carbon is driven by PFAS/PFOA. So, if you could point me toward any additional information on what has been tried with respect to the carbon addition to the geobags, that would be appreciated.

A: Thank you for your GMA Techline questions related to geotextile tubes and polyfluoroalkyl substances (PFAS). PFAS are a group of human-made chemicals that includes PFOA, PFOS, GenX and many other chemicals. PFAS have been manufactured and used in a variety of industries around the globe, including in the United States since the 1940s. There is evidence that exposure to PFAS can lead to adverse human health effects, and its cleanup is one of the hot issues of our day. There have been three areas related to slurry additives with geotextile tubes: activated carbon, nano clays and organo clays.

The idea of dewatering and decontamination can be accomplished with geotextile tubes used in conjunction with the above additives. Success depends on type and concentration of the pollutant. The key parameters are solubility and sorption. 

Much of the effort with geotextile tubes has focused on river and harbor sediments. I would recommend the following two references:

Moo-Young, H. K., Gaffney, D. A., and Mo, X. (2002), “Testing procedures to assess the viability of dewatering and geotextile tubes,” Jour. of Geotextiles and Geomembranes, 20(5), 289–304.

Mori, H., Miki, H., and Tsuneoka, N. (2002), “The geo-tube method for dioxin-contaminated soil,” Jour. of Geotextiles and Geomembranes, 20(5), 281–288.

Activated carbon treatment has been studied extensively for PFAS/PFOA removal. Activated carbon is commonly used to absorb natural organic compounds and synthetic organic chemicals in drinking water treatment systems. The problem with adding it to the geotextile tube is that the fluid is turbulent and not clean enough for it to be effective. To work well it needs to be added during finishing or a tertiary stage of water treatment. Otherwise, one is using a lot of it inefficiently. If the surface area of the activated carbon gets coated with too many “other things,” it won’t capture the PFAS/PFOA well. It is also very light and does not distribute well in a slurry without help.

Geonet-to-geonet contact

Q: I have come across an interesting scenario on-site; however, this time it is to do with geocomposites and how we connect their end seams. 

What I have noticed is that the way in which the geocomposite is manufactured causes the geotextile to have a strong bond to the geonet core at the ends (sides are not bonded). This is for good reason. However, it then creates a difficult scenario on-site as that bond needs to be broken to create the overlap. I have noticed that process is very difficult and, in some circumstances, tears the geotextile, thus compromising the design. Practically speaking I can see it costing the industry a lot of time and money.

To ensure the geocomposite is allowing the fluid to flow as designed, isn’t being compromised during installation and to assist the workers on-site, I have been hypothesizing a slight change in the design and would very much appreciate your feedback. In our scenario, we have an HDPE geomembrane underneath the geocomposite and soil placed on top. I’m of the opinion that installing the geocomposite would still allow fluid to flow with a similar velocity.

 Your feedback would be appreciated. 

A: I disagree. One needs geonet-to-geonet contact along the butt seam overlap (L=about a foot for a 3:1 slope) to maintain high flow rates. The geotextile at the exit of the upslope geocomposite will be a very high correction factor (DCF) and be susceptible to clogging. Your second scenario is not recommended for the northeastern U.S. where we get significant rain events. We have seen this scenario fail in the past.

Geotextiles as reinforcement

Q: I read a research paper on strengthening of clay beds by providing granular piles reinforced with horizontal geotextile. But there is no mention of the mechanism of geotextile, which decreases settlement. Please help me in this regard. 

A: The geotextiles over granular piles act as reinforcement. The geotextile and geogrids are shaped like a catenary spanning the columns and supporting the soil above. One can search load transfer platform, basal reinforcement or geosynthetic encased column for more. In regard to specific references on the subject, I would search Carlsson 1987, Kempfer 2004 or Collin 2005.

A key concern with the technology is the stress concentration and the pile/column caps. I would suggest you consider a rounded precast pile cap on rigid piles or shafts. Such caps are not necessary on granular piles, which do not have defined boundaries.

Building from the bottom up

Q: I am currently detailing a 45° reinforced slope for a highway overpass (26 feet [8 m] high). We have designed this without a facing system to allow the contractor to overfill and then trim back to the design profile.

I have not personally seen this done, but standards and text suggest it to be practical. Do you have any examples of this being successful and what provisions are worth considering for protection of the grid during the subsequent trimming operation? 

Thanks in advance.

 A: If you are detailing (i.e., building) a 45˚ (1:1) reinforced 26-foot highway slope, why wouldn’t you build it to grade (use GPS or lidar survey to assure location of geosynthetic reinforcement) with a long track hoe, gradall or stone slinger (conveyor)? Build from the bottom up after you set (backfill) the anchor trench or plinth. 

Please, do not overbuild the toe and cut with a dozer as you suggest. I have seen this done on glacial tills and CCL for landfill liners of 2:1 slopes (26.5˚), not 1:1 slopes (45˚). For health and safety concerns, please be very careful with that approach.

You may want to use “L” frame wire mesh face supports, prebuild facings units or a two-pronged compaction approach to facilitate construction. All are tried and true techniques used for building geosynthetic-reinforced green walls.

Required degree of survivability

Q: I am working as a quality assurance/quality control (QA/QC) engineer for infra projects. I had many discussions with our project consultant regarding all parameters required for the geotextile, precisely, the nonwoven used for road and trenches.

I believe that every project was guided by its project specs and the country specs, but really, I had some point that I need some clarification on it. Specifically, for the nonwoven and the use on the infra project, which class is advised? What are the critical parameters and what are the performance parameters? Burst strength: is it required for a geotextile?

Based on my experience, for every test we should think about its purpose, and then we will decide technically if it complies or not.

A: Thank you for your GMA Techline question in regard to geotextile survivability. 

Please consult Table 3 of GRI-GT13a for the required degree of survivability based on ground pressure of construction equipment and subgrade conditions. Geotextiles are typically tested for three survivability parameters: ASTM D4632 Grab, ASTM D4533 Trapezoidal Tear and ASTM D6241 CBR Puncture. Burst is no longer specified due to test variability.

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