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The GMA Techline

August 1st, 2019 / By: / GMA Techline

Moderated by Robert M. Koerner, George R. Koerner and Y. “Grace” Hsuan

A geotextile for unpaved driveway/road?

Q: Is there any point in using a 150 g/m2 (4.4 oz/yd2) nonwoven geotextile in building an unpaved gravel driveway or gravel road? If so, what should be the most important characteristics of the fabric and what are
the benefits? The site conditions are average: the soil is not sandy, no high clay content, no high moisture, no slope.

Could you, please, recommend any resources to find answers to the questions asked by customers when I sell geotextiles?

A: The answer to your question is definitely “yes.” The geotextile, even though it is very lightweight, will act as a separator, keeping the subgrade soil from contaminating the overlying gravel, which, in this case, is the wearing surface. This maintains the base course’s permeability and drainage to the sides of the roadway. It is extremely important in winter to prevent heave within the soil-contaminated saturated base course (without the geotextile) from creating uneven surfaces and/or potholes. It’s the least expensive “insurance” one can get.

For further reading on geotextiles as separators, look at the book Designing with Geosynthetics, available through Amazon or Barnes & Noble or through us at www.geosynthetic-institute.org/publications.htm

P.S. If the base course is quarried gravel (hence, angular) and is thin (6 inches [15 cm] or less), I would go to a heavier geotextile than you suggest.


Asperity height in GRI-GM13

Q: I have a question about the GRI-GM13 standard, and I am glad to have this channel for clarification.

In 2014 the minimum geomembrane asperity height was changed from 0.25 to 0.40 mm in the GRI-GM13 standard for high-density polyethylene (HDPE) geomembranes.

I did not find more details about this revision in the GSI White Paper #32 “Rationale and Background for the GRI-GM13 Specification.” Is there any report with the reasons why it was concluded that an asperity height of
0.25 mm was not enough? I’m looking for the rationale of this change.

A: Thank you for your question regarding asperity height in our GRI-GM13 specification. The original height was indeed set at 0.25 mm, which we thought gave a reasonable interface friction value for most situations. As time progressed higher asperity heights, particularly for geomembranes produced by flat die methods, came into being. While possible to achieve in blown film production, there is a tendency to decrease core thickness unless more polymer is used. This trade-off led to the 0.40 mm height and is admittedly difficult, but we wanted to accommodate both types of texturing.

The above said, if an interface with a particular material is satisfied at the 0.25 mm asperity height, one should indicate that the geomembrane meets GRI-GM13 (modified), where the modification is substantiated by site-specific direct shear tests. As such, we don’t mind modifications to our specifications if properly explained.


Size of the U.S. geotextile market

Q: Hello, I am an MBA student conducting research on the current geosynthetic market in the U.S. I’ve found some interesting information regarding geotextiles and their use in highway construction and erosion control. I’m having trouble finding a number to gauge the size of this industry. I understand this isn’t a technical question, but I saw your note about this contact being open to the world and thought I would ask. If you know of any resources that would help, I’d love to be pointed in the right direction.

A: The last time the Geosynthetic Institute surveyed the industry insofar as geotextiles are concerned was in 2012. At that time the worldwide market was approximately 1.4 billion m2 at approximately $0.75/m2 for a total of $1.05 billion. You can increase those numbers by about 3–6% per year to bring things up to date. 


GM, GCL or both?

Q: I am reaching out to you to get clarity on pond waterproofing material(s).

I am an architect and we are designing a large school campus for a social organization in India. On the campus, the client wishes to have a pond of around 27,000 square feet (2,500 m2). We have a limited budget, as in all projects.

The vendor is suggesting a geomembrane over geosynthetic clay liner. They say this will result in less chance of leakage and be longer lasting. We are planning to bury the entire base surface of the geomembrane with 12–18 inches (30–46 cm) of soil covered with 2 inches (5 cm) of sand, for plants to grow into the water.

What do you think would be the most appropriate material to use: a geomembrane, a geosynthetic clay liner or is it a substantially good investment to have both?

I appreciate your online material, and I am looking forward to hearing from you.

A: Clearly your vendor is aware of the liner status for landfills and ponds containing unusual or hazardous liquids. In such cases, a geomembrane (GM) over a geosynthetic clay liner (GCL) is the preferred strategy. Any leakage passing the GM is captured (attenuated) by the bentonite of the GCL. The two materials together are what is called a composite liner system. For a pool situation you, as the designer, have to assess how the GM will be placed, the possibility of leakage, the loss of water, erosion beneath the GM, the possibility of interaction with subsoil constituents, the possibility of bubbles (called whales or hippos) lifting the GM, damage from maintenance equipment during service lifetime, etc. As such, it is dicey to use the GM by itself. If the budget is available, a GM/GCL composite is the way to go.


Reconstructing a fusion seam

Q: I was discussing with my boss the viability of reconstructing a fusion seam that failed destructive testing by cutting off the overlap and extrusion welding over the edge of the existing weld. My boss said the second heating of the material ultimately causes a weakening of the bond between the adjacent panels, but if I could find anything to the contrary, he would take it into account. Are there any recent studies that show it is acceptable to extrude a fillet weld over an existing fusion one, and does this vary between 40 and 60 mil (1 and 1.5 mm) thicknesses of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE)? 

Thank you for your consideration.

A: The concern over the regularly used technique of extrusion welding over a fusion seam previously made by a wedge welder has been discussed often in light of weakening of the original seam, its geomembrane on either side of the fusion weld, or if the geomembrane on either side of the proposed extrusion weld has been raised. However, this is not from the concern over weakening the bond, but instead over crystallizing the adjacent welding area, possibly leading to stress cracking sensitivity.

We did some testing after several melt cycles and found no degradation of the HDPE in the area of the fusion seams. Since LLDPE is less crystalline than HDPE, I would think it is not sensitive either. While not common, and if acceptable by the construction quality assurance (CQA) inspector, I would say okay. Regarding the testing of the new seam, the vacuum box method could be upgraded to using a copper wire in the weld area and passing an electric wand over it, which might be more comforting to all involved. Best wishes.