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The GMA Techline

Q&A: GMA Techline | June 1, 2019 | By:

Who is doing the testing?

Q: This question relates to the number of rolls to be selected as the lot sample for specification conformance testing and refers to ASTM D4354. When I look at this standard, it appears the values in Table 1 of GRI’s White Paper #10 refer to the number of units to be selected as lot sample or manufacturing quality control (MQC), in Table 1 of D4354. Please, will you clarify which figures I should use? If 200 rolls of a geosynthetic are supplied to a site, should one or six rolls be tested for field specification conformance?

A: The answer to your question depends on who is doing the testing. If you are the manufacturer and performing MQC at the factory, you need to test six rolls of the lot of 200; i.e., Table 1 of ASTM D4354. If you are the construction quality assurance (CQA) technician on-site where the rolls have been delivered and are performing conformance testing at a third-party lab, you need to test one roll of the lot of 200; i.e., Table 3 of ASTM D4354.

Veneer stability

Q: Good afternoon. I was looking to see if you will still provide the veneer stability spreadsheet as talked about in the GRI White Paper #11, or if GRI has a more up-to-date spreadsheet/calculation. Thanks.

A: First of all, you mentioned GRI White Paper #11, which has to do with various interpretations of the shear strength parameters friction and cohesion. That white paper stands alone and we have nothing further on that subject other than the various interpretations stated therein. 

Your email also speaks about our veneer stability spreadsheet, which has associated with it a number of computational options. Those options follow directly the attached paper titled “Analysis and Design of Veneer Cover Soils.” The paper and the spreadsheet are coordinated with one another for each of the options that are analyzed. It’s self-explanatory, but give a call if you want further clarification.

Nonwoven vs. woven geotextiles in filter applications

Q: I was rewatching your webinar on geotextiles in filter applications and I came up with what I think would be a superiority of using nonwoven geotextiles. Namely, since an AASHTO M288 Class 2 nonwoven geotextile will have lower strength but also higher elongation at failure (i.e., a much lower modulus), I would think that if it is used around a crushed-stone underdrain, there would be a much greater chance to achieve intimate contact, since it would stretch more when backfill is placed above it. Am I correct in this understanding? If so, am I also correct that as long as it meets the AOS and permittivity requirements for the base soil that, other things being equal, the nonwoven would be preferred over the woven for this application?

A: Clearly, needlepunched nonwoven fabrics will deform around crushed stone quite readily, the amount depending on the specific normal stress applied from the surrounding soil. Depending on its mass per unit area, the elongation at break is more than 80%. For heat-bonded nonwovens it
is slightly lower, but not much.  

In contrast, the wovens, both monofilament and slit films, have deformation at break values of less than 30%. As a result, AASHTO M288 makes the distinction at 50% without overtly stating woven and nonwoven. Marilyn Ashley will send you a set of experimental curves illustrating the above behavior for different geotextile types.

She will also send a really old paper I wrote titled “Should I Specify a Woven or Nonwoven?” Interestingly, it was written in 1984. It covers all of the possible functions but is at this point in time of historical interest only. Thanks for your question.

The stronger the fabric, the lower the seam efficiency

Q: Can you recommend some papers that discuss seam strength efficiencies for high-strength woven geotextiles? Thank you.

A: There is available literature as to how to make geotextile seams, but little published as to the results. Some limited data is in the book Designing with Geosynthetics, 6th edition, giving results from two field projects, one using high-strength polyester fabrics, the other using high-strength polypropylene fabrics. Results are shown on Figure 2.10 of the book along with the accompanying test. As expected, the stronger the fabric, the lower the seam efficiency. Further, factory versus field seaming is illustrated.
The information is attached.

Interface friction values

Q: Do you have a reference for interface friction values at low normal loads between any of the following?:

Smooth HDPE geomembrane vs. finished concrete

Smooth HDPE geomembrane vs. smooth HDPE geomembrane

Smooth HDPE geomembrane vs. rubber matting

A: The interfaces you ask for are quite unusual. We suspect that smooth to smooth high-density polyethylene (HDPE) will be the lowest of the three, but at low normal pressures, the finished concrete will be low also. The rubber interface will likely be the highest of the three interfaces but be highly dependent on the durometer. Textured HDPE will obviously increase the values considerably, particularly against rubber. Best to do a few tests and, as a suggestion, you might get reasonably good values using a small shear box. 

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