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The GMA Techline

October 1st, 2018 / By: , , / GMA Techline

Toxicity of nonwovens?

Q: Are there any comprehensive tests (or a general synopsis) of the environmental toxicity of leachate from nonwoven polypropylene fabrics when aged in a freshwater or saltwater solution? Regulatory and municipal bodies in Europe and New Zealand are requesting more info about products that are used in riparian and coastal environments.

A: As you might be aware, there is nothing in polypropylene resin to cause toxicity; it is simply carbon and hydrogen in a repeating structure. Aging in fresh or salt water makes no difference.

However, manufactured fabrics also consist of an additive package containing carbon black (no problem) and antioxidants. The latter are very complex structures but only present in trace amounts, e.g., much less than 1%. I do not know if any residual can leach out of the structure and, if so, what concern they may be.

Of course, they 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 at an enormous cost.

Robert M. Koerner / GMA Techline


Maximum permeability for CCL?

Q: Could you please assist with a question that has come up? Is there any international literature that sets a maximum permeability for the soil layer below a composite high-density polyethylene/geosynthetic clay liner (HDPE/GCL) in a landfill barrier? There seems to be a rule of thumb that it should not be more permeable than 1 × 10-5 cm/s to prevent possible bentonite erosion issues and to generally facilitate some form of attenuation below the geosynthetic barrier.

A: Good question. At the root of the issue is that most (if not all) regulations call for a compacted clay liner (CCL) beneath a geomembrane (GM) (as in a composite GM/CCL), with the CCL having a maximum permeability of 1 × 10-7 cm/s. If a GCL is used, its permeability is from 1 to 5 × 10-9 cm/s. It (the GCL) being thin, however, is often backed up by a low-permeability soil layer but of what permeability? We believe that your question is how much higher than 1 × 10-7 can this soil layer be? Indeed, designers do use 1 × 10-5 cm/s and using Terzaghi & Peck’s multilayer soil approach, the net of the GCL and lower soil layer permeabilities comes out to be about 1 × 10-7 cm/s, which is what is called for with a CCL beneath the GM by itself. All of that said, the regulator has to buy into the concept and calculation.

Robert M. Koerner and George R. Koerner / GMA Techline


Comparing puncture test methods?

Q: I am using your book Designing with Geosynthetics, sixth edition, to verify a nonwoven geotextile regarding puncture protection to the adjacent geomembrane.

You reported the factors S1, S2 and S3 to correct the index test ASTM D4833. I have the data from the test ISO 12236 that is very close to ASTM D6241. Do you have some factors to correct ISO 12236 or can I use the same for ASTM D4833?

A: The puncture design in Designing with Geosynthetics (pp. 645–648) is based on ASTM 5514, which is the Pyramid Puncture test, and the various shape factors refer to its specific results. The ISO puncture test is CBR puncture probe, which is quite different. It uses a 2.0 mm flat probe. However, we do have a nifty correlation between the different puncture tests. Marilyn Ashley will send it to you.

That said, I am a bit uneasy in using empirical shape factors between different puncture test methods, but you might try and see if it produces reasonable results. Tell me about it if you care to.

Robert M. Koerner / GMA Techline


Geogrids and retaining walls

Q: Kind greetings. I am researching about geosynthetic grids (geogrids) that are used in reinforced segmental retaining wall systems, and I came across the attached file available on the Geosynthetic Materials Association website (www.GMANow.com).

To filter this extensive (very useful) table to the needs of my research, I need to clarify the following: first, is it a wall and second, with respect to the suggested application, which type of geogrid resin applies for a mechanically stabilized earth (MSE) retaining wall system?

A: Your topic and its cross section is definitely a wall (“W” in your listing). Now to the type of polymer used for manufacturing the geogrids, there are many to choose from, depending on the method of manufacturing, the subsequent properties and the cost. At this time, the most common types of resins are HDPE, PP, PET and PVA in no particular order.

Robert M. Koerner / GMA Techline


Remediation for MSE berm failure

Q: I am currently directing a design project for an MSE berm in a landfill expansion. We are working to answer the question of what is reasonable for a financial assurance requirement against possible failure. We have seen your work that indicates full replacement cost of up to four times the original construction costs, which I do not dispute. However, from a financial assurance perspective, the bonding costs will break the bank on most projects (including ours), and I wonder what is the likelihood of remediation costs.

A: In our webinar on “MSE Wall/Berm Remediation and Monitoring,” we show five cases of rebuilding of deformed walls costing 1.05, 1.64, 3.50, 1.46 and 1.36 ratios of rebuilt to original costs. We also show five cases of remediation of collapsed walls costing 4.66, 1.50, 1.43, 1.61 and 1.75 ratios of remediated to original costs. Site-specific conditions appear to have great variation in the remediation costs, but four times is at the upper end of our data.

Robert M. Koerner / GMA Techline

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