Moderated by George R. Koerner and Y. “Grace” Hsuan
Q: I am aware of the section in your book regarding geomembrane “permeability” and get from it that it aims to calculate an equivalent Darcian permeability using vapor diffusion via Fick’s law. Can one use the derived permeability then when calculating flow using Darcy’s law? A 60 m head gives a hydraulic gradient equal to 60/0.0015 = 40000! Using this gradient, the resulting leakage is still small but by no means negligible. Maybe I am overanalyzing the matter or fail to appreciate that it is inferred that the derived Darcian permeability applies to Darcian flow. Further to the above, can one argue that it is then clean water only and that the majority or perhaps all dissolved metal salts such as Ca, Na stay behind?
A: Yes, the Darcian permeability goes right into Darcy’s formula for the resulting flow values. The high gradient (for your case) coupled with the low diffusion value works out reasonably well. Incidentally, any liquid diffusing through a geomembrane as a vapor only to recondense on the other side as a liquid is the cleanest liquid ever produced! The contaminants simply don’t travel along with the vapor.
Q: Are there any comprehensive tests (or a general synopsis) of the environmental toxicity of leachate from nonwoven polypropylene fabrics when aged in freshwater or saltwater solution? Regulatory and municipal bodies are requesting more information about products that are used
in riparian and coastal environments.
Thank you for your time.
A: As you might be aware, there is nothing in polyethylene resin to cause toxicity. It is simply carbon and hydrogen in a repeating structure. Aged in fresh- or saltwater makes no difference. However, most geosynthetic materials are formulations of the parent resin and an additive package containing some carbon black (no problem) and antioxidants. The latter are very complex structures but only present in trace amounts, e.g., less than 0.5%. I have no idea if they can leach out of the structure and, if so, what concern they may be.
The above said, all geosynthetics can be evaluated by elevated temperature incubation in deionized, fresh- or saltwater baths and subsequent testing of the incubating liquid, but it will take years
for results and at an enormous cost.
GEOTEXTILE FILTERS AND RECYCLED CONTENT
Q: The final version of the ICOLD Bulletin 55 on Geotextiles as Filters in Dams is receiving one last check for editing before being sent for translation and publication. In that document, your Geosynthetic Research Institute (GRI) Report has been referenced as a design method for estimating geotextile service life in particular. (Thousands of people owe you thanks.)
Two comments from the Technical Committee on Embankment Dams were about nomenclature and recycled materials. The first comment on nomenclature questioned the use of “geotextile filters” versus “geofilters,” and it is resolved to use the former to align with International Geosynthetics Society considerations (but your opinion is also welcome). The second issue is on durability and questions to what extent the use of postconsumer recycled polymer (polypropylene or other) influences the durability estimate compared to virgin material used in manufacture. Your guidance on any quantitative response would be highly appreciated. I have been unable to find an appropriate reference in the member section and apologize if I am overlooking the obvious.
A: Regarding your questions:
“Geofilters” is a trademark of a commercial manufacturer, so do use “geotextile filters,” which is completely generic.
The second question is more difficult to answer, and I take our past geomembrane discussions to give insight. We feel that a certain percentage of “rework” is okay in a formulation, providing it is of the exact same formulation and is “clean.” We ended up specifying a maximum of 10%, although even 50% did not significantly affect properties. Regarding “postconsumer plastics,” the variability is so great that we decided to eliminate even a very small percentage of this very variable material.
Hope this helps but do ask if anything is needed at any time.
GROOVE IN THE GEOMEMBRANE
Q: We have encountered a matter that we have not experienced before, and we are not sure how to proceed.
We are currently building an industrial waste facility designed with a double composite barrier system. The barrier consists of a 60-mil (1.5-mm) secondary double textured high-density polyethylene (HDPE) geomembrane and an 80-mil (2.0-mm) primary double textured HDPE geomembrane.
During the installation of the primary geomembrane, we noted a groove being created during the seaming process by the wedge welder. The installer stated that the groove is created by the “drag bar” of the wedge welding machine. Upon cutting open one of the seams we see the groove is created in both the bottom geomembrane, which forms the base of the air channel that is later air pressure tested, and the top geomembrane, which forms the air channel’s cover. The groove created is approximately 0.3 to 0.5 mm deep.
The installer states that the seams pass according to our specification because they pass the shear and peel tests.
We are concerned because we have specified a GRI-GM13 specification geomembrane that calls for 80-mil (2.0-mm) thick geomembrane. The effect of this groove, although located within the air channel of the seam, reduced the thickness locally to 60 mil (1.5 mm).
Unfortunately, we have picked up that this is occurring after several panels have been installed, and we have to make a call on whether those seams should be cut out and repaired or not. We have instructed the installer to stop installation until other wedge welding devices are brought to site that won’t cause the groove.
Have you encountered this before?
A: Most dual track wedge welders have a follower (sometimes called drag bar or toggle) within the recessed central portion of the wedge. This is to be sure that the air channel is kept open for the subsequent air pressure test.
In your case, we agree with your installer and feel that the follower is rubbing against the upper and lower sheet, thereby abrading the inside air channel. Since the sheets are hot, it doesn’t take much friction to do so, and we feel that you are seeing the resulting outcome. It is not acceptable as you state the situation and certainly must be remedied accordingly.
REACTION OF NEOPRENE TO LOW-PH ACIDIC WATER
Q: Although we have numerous geomembrane-lined lagoons containing polluted water in the mining industry, there are a few that contain acidic water of pH at 3.5 to as low as 2.0. In a couple of these facilities, there is a drainage pipe penetration through the liner system that has a make-off between liner and pipe. Two systems of make-off are used—either a bolted stainless steel baton holding down the HDPE geomembrane to a concrete make-off with a neoprene gasket on either side of the geomembrane, or the geomembrane is welded to a factory-formed boot that is clamped to the pipe with a stainless steel circular clamp. The first option
is preferred, especially for large facilities.
Are you aware of any research results that define the durability of neoprene rubber when exposed to low pH acidic water (varying from arsenic acid to sulfuric acid and occasionally hydrochloric acid)?
The follow-up question is whether it is appropriate to provide buffer protection by covering the exposed gasket with a bitumastic material?
I hope to hear from you at your earliest convenience but hope even more that we will meet again in the not-too-distant future.
A: Regarding your question on acid reaction of neoprene to very low pH liquids, the closest we have to neoprene insofar as actual data is ethylene propylene diene monomer (EPDM) to both organic and inorganic acids in temperatures of 100˚F (38°C) and 158˚F (70°C). Both are thermoset plastics and the EPDM has excellent resistance in all cases. As a result, I think you are clear, but it is a slightly different material.
Lastly, covering with a bituminous might look comforting, but I don’t think it’s very permanent. Otherwise, very best regards.