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Features, News, Q&A: GMA Techline | February 1, 2018 | By: ,

Determining quantity of recycled HDPE in a geonet

Q: I have a question about high-density polyethylene (HDPE) geonets. Is it possible, by thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) analysis, to determine the quantity of recycled HDPE present in a geonet? I have a geonet that, I think, is made with virgin and recycled HDPE, but I am not sure.

A: The issue of adding “rework” to a formulation (which is unused material of the same type and formulation) is generally allowed up to 10% as in our specifications. The use of “recycled” to a formulation (as in postconsumer plastics) is generally not allowed in any quantity.

Now to your question. To identify rework is virtually impossible. With only a single melt cycle having occurred during manufacture, and it being the same resin, I doubt that even gel permeation chromatography (GPC) could tell the difference. With recycled plastic being added, it is possible, particularly if it is a different resin type or density. Many tests in addition to TGA and DSC are candidate methods (e.g., Fourier transform infrared stereoscopy [FTIR], gas chromatography [GC]/liquid chromatography [LC], and melt index/density). The last two mentioned are the least expensive and most common.

Hope this helps.

Robert M. Koerner / GMA Techline

Estimating two-layer bearing capacity

Q: I have a two-layer soil system of weak bay mud (c = 275 to 400 psf) with 3 feet of clayey sand with gravel (I estimate c =350, phi = 28 unit weight, 115 pcf compacted to 88%) to be placed over this weak bay mud. The drilling rig (to place the piers) needs a bearing capacity of 600 psf. Is there any research done that I can use or a book to consult to estimate the improvement in the bearing capacity of this two-layer system? They do not want to import any rock, but they want to use the existing material, as the 3 feet of fill material to be placed is a Class 1 hazardous waste soil. They do not want to take it off-site.

A: High-strength geotextiles and geogrids have been used to construct embankments over very soft soils since the U.S. Army Corps of Engineers pioneered the concept in the 1970s. Close to what you seem to want is known as a “working blanket.” There are papers available, but for site-specific conditions. There are specialty-design consultants who do such designs. Several manufacturers of the materials also have excellent design staffs.

Robert M. Koerner / GMA Techline

Cutting rolls of geosynthetics

Q: I am wondering if you know a good way to cut rolls of woven geosynthetic fabric (for road-grade construction). I have rolls that are 17.5 feet long and would like to cut the entire roll (fabric and cardboard tube) in half to make two 8.75-foot-long rolls. I have used a chainsaw and a reciprocating saw, but neither work very well. Any help/ideas would be greatly appreciated.

A: We use a chainsaw, as mentioned, and it works quite well if kept sharp. Otherwise, a band saw works cleaner but lifting rolls up onto a table requires extra help.

George Koerner / GMA Techline

Geotextile tubes in wetlands

Q: I have a project where geotextile tubes may be useful. We have a wetland remediation project where we plan on capping some wetlands, work which includes doing some excavation. The excavation would be down to 6 inches. During the excavation, we really don’t want the sediment or soil to migrate into other areas. Would a geotextile tube be good for this application? We need the geotextile tube to be at least 4 feet above the ground surface and resistant to water and sediment loads (no tearing or leaking).

This will be placed in wetland soils with standing water. Let me know if geotextile tubes are the right way to go.

A: Thanks for your question and indeed a sediment-filled geotextile tube would be an excellent barrier against sediment or soil infiltrating through it and into your wetland. Using your dimensions, it would have to be substantial (i.e., 4.5 feet high plus any settlement of the underlying in situ soil it is sitting on). The latter might be significant. Certainly, cost it out with some geotextile manufacturers.

Robert M. Koerner / GMA Techline

Defining a high-strength geotextile or geogrid

Q: Is there a definition (by an agency, author, etc.) for a high-strength geotextile or geogrid? Of course, this is relative, especially with so many functions geosynthetics can serve. I am thinking geosynthetics for use in reinforcement applications.

A: We put a minimum value at 180 kN/m (1,000 lb/in.). At this value, one needs roller grips to do wide-width testing, and the fabric or grid inherently becomes “high strength” to us. The Grip Type Test Method is online at There you will see four categories of testing grips, one being for “high strength.”

Robert M. Koerner / GMA Techline

Fallow vs. CBR puncture resistance

Q: I’m referring to an example in your book, Designing with Geosynthetics (6th ed., 2012, Section 2.5.4). In example 2.10, Fallow is the ultimate puncture strength of 300 N according to ASTM D4833. Since manufacturer specifications now report the California Bearing Ratio (CBR) puncture resistance (ASTM D6241), I wanted to see if it is appropriate to use the CBR puncture resistance as Fallow or must it be determined only by ASTM D4833?

Thank you for your assistance.

A: The basis for the Fallow is ASTM D4833, which is the “pin” puncture using a beveled probe of 5/16-inch diameter. That said, there is a correlation with the CBR puncture ASTM D6241 where the probe is 2 inches. The correlation is Pin × 5.0 = CBR.

George Koerner / GMA Techline

Fracking flowback and geosynthetic liners

Q: We are working on making a surface impoundment liner system for fracking flowback water. I wanted to see if you have any information or interesting articles on how fracking flowback water chemically affects HDPE geomembranes and geocomposites?

A: The chemical compatibility (or resistance) of geosynthetics must be evaluated in the laboratory using the representative geosynthetic at incubation-specific conditions. ASTM is set up nicely in this regard with D5322 for the immersion and then D5747 for geomembranes, D6389 for geotextiles, D6388 for geonets and D6213 for geogrids. Incidentally, these tests supersede the depreciated EPA 9090 protocol.

There are several commercial labs doing this activity.

Robert M. Koerner / GMA Techline

Wick Drains and Clay Embankments

Q: If there is a very high positive pore water pressure inside a clay embankment that is constructed rapidly, and assuming an average degree of saturation of 0.94, would a wick drain still be an option (in unsaturated soil)? I’m wondering if you have any experience with this situation or know of a relevant case study.

A: The word “saturated” is, in my mind, unfortunate in this regard. As consolidation occurs, the saturation will increase, since air is expelled first. That said, as long as the water is continuous (even being slightly less than 100% saturated), consolidation will occur. Thus, wick drains should work. I don’t know the boundary, but I do feel it’s close to your estimate.

On a related note, many years ago I looked at the mechanism as being stochastic process. In particular, the air was expelled as a “simple death process” as compression occurs. Then Terzaghi consolidation takes over. My advisor scared me off of the concept but maybe it should be revisited. I did write a paper, but no one ever asked for it!

Robert M. Koerner / GMA Techline

Seaming HDPE and LLDPE

Q: I have a geosynthetics question for a current project. The project is utilizing a 40-mil linear low-density polyethylene (LLDPE) liner in a cap system. The specification stated that the extrusion rod material used for all extrusion welding was to be of the same resin as the geomembrane sheets. It was discovered that the installer was using HDPE extrusion rod material for welding. Here’s the question that came up as a result: Is the LLDPE sheet material compatible with HDPE extrusion rod material? Also, would using HDPE rod material provide a stronger, more superior weld than using LLDPE?

A: With HDPE having better long-term properties than LLDPE, you are conceptually okay (but not the other way around). That said, the welding window between the HDPE and LLDPE is very small, about 50°F (10°C). Thus, seaming is difficult—but not impossible. If you go with an HDPE welding rod, I would take more seam test samples, and I would also do many trial seams and test them to see that all aspects of the specification are met. This includes strength, locus-of-break and elongation/incursion.

Robert M. Koerner / GMA Techline

Failure for junction strength of geogrids

Q: What would you consider a failure for junction strength on a knitted geogrid: first peak when the material loses structure or maximum force?

A: For a knitted geogrid, we always take the first peak when the material loses its structure. This is usually the maximum force, but sometimes it is clouded if twisted yarns and/or coating knot up behind the junction after continued elongation. This is an artificial peak and well beyond the functional longevity of the flexible geogrid.

George Koerner / GMA Techline

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