GMA Techline

October 1st, 2017 / By: / Feature, GMA Techline

Welding HDPE to PVC and more

Q: I would appreciate any input you have on these questions:

Can a high-density polyethylene (HDPE) geomembrane (GM) be welded to a PVC geomembrane?

I am looking to connect a geomembrane to a building foundation. Based on some research, I have found the following options: bolt the geomembrane to the building (and use an epoxy sealant for watertightness) or cast a geomembrane strip into the concrete and weld the outside geomembrane to the cast-in-place geomembrane strip. Do you have any other thoughts?

Is there a particular type of geomembrane that you would recommend using in roadway cross sections beneath the asphalt?

A: Let me answer the questions in the order you list them:

HDPE cannot be welded to PVC. The melting temperatures are so different that you will melt one and not the other and vice versa.

GM connections to concrete are nicely made during construction, wherein an insert is placed in the form before pouring and, when cured, the form is removed and the strip (called a “batten strip”) remains exposed for the GM to be welded to it. Most GM manufacturers have information on this. If the concrete is already poured, it is ugly! Adhesives will stick to concrete but not very well to GMs, especially HDPE GMs. One might come up with a hybrid batten strip and attach it to the hardened concrete and the GM on it, but the details promise to be dicey.

Most GMs will work under asphalt since the asphalt temperature during placement will not melt the resin. The questions that then remain are durability and mechanical properties. They are site specific.

Hope this helps but get back to me if you have further questions.

Asperity height

Q: I am delighted to see the Geosynthetic Research Institute (GRI) GM13 revision addressing asperity height, which we as a regulator pursue. Some have pursued a different height because they undertook an interface shear test using two different geomembranes once upon a time. Clearly not every designer appreciates the caution you have placed on record so often with respect to testing the actual materials.

What led to the asperity height decision? I am trying to undertake a small testing program on a complex issue of asperity height and density/distribution influence with a single soil of clay liner specification. (I do believe the GRI standard to be correct largely due to interface transmissivity importance, and noting that we use the minus 0.425-mm fraction of soil for linear shrinkage and laboratory soils test.)

A: Our original asperity height was very low since we attempted to include the spray-on method, which impinges very fine particles on the sheet. It is used in Central Europe. For blown film and structured surfaces, however, it was indeed too low. After it was decided to increase it, we had to make the decision as to how high. Discussion then focused on blown-film texturing and with increasing asperity height came the possibility (even likelihood) of decreased core thickness, which requires additional polymer so as to maintain the required core thickness. Thus, we came to the present 0.40-mm height as a “negotiated” value.

To go to 0.50 mm or even 0.60 mm will give slightly higher interface strengths, but it certainly is not linear and has a diminishing return in this regard. This is not the most rigorous answer, but the best we can offer.

Reducing geogrid ultimate strengths for stiffness

Q: I hope I find you well and that you continue to enjoy working with us mere mortal geotechnical engineers.

I followed your recommendations for reduction of the ultimate strength of geogrids to obtain an allowable stress and finally a design strength. For numerical modeling, we also need to model the stiffness of the geogrid, both in the short term and in the long term (design life). Most people I have seen doing these types of analyses have not reduced the stiffness (comparing with case histories, where measurements were done immediately after construction). Reducing the stiffness with only the reduction factor for creep RFcr may not be enough. Should we reduce the stiffness by the same ratio as you do the strength? I know we may not have enough information on this, or, if we do, I am obviously not aware. Any suggestions would be welcome.

A: Thanks for reaching out in this regard. I would be cautious about using reduction factors on ultimate strengths for stiffness due to nonlinearity of the response curve and the time frames involved. More specifically, I do think that installation damage applies to stiffness, but I have reservations on degradation and creep since both are long-term issues. My temptation is to use full installation damage values for the others.

Teaching about geosynthetics as a construction material

Q: I have been asked to give a series of lectures in a class on construction materials, and I picked geosynthetics as the topic. The students have a background in cementitious materials. They seem very interested in knowing more about these construction materials. I think in engineering education in the United States, geosynthetics should be included with wood, steel and concrete as construction materials.

A: We certainly agree that geosynthetics are a bona fide civil engineering construction material. Dr. Y. Grace Hsuan of Drexel University has taught geosynthetics as such for at least five years and maybe ten. She has integrated it nicely with the other traditional materials. If you want ideas on her experiences in this regard, do ask her accordingly.

HDPE geomembrane shear strength

Q: I am investigating the soil geomembrane shear strength for different thicknesses of HDPE geomembranes. In the lab, I am using the large direct shear machine to find out the interface friction between Ottawa sand 20/30 and HDPE 120 mil.

I spent more than four months repeating tests because I had some major errors in the machine. I have attached a file in Microsoft® Excel that shows the plot that I obtained from the test. Based on your experience, does the curve look okay? Also, I did not understand why residual values go down like I have found. I checked many publications and found nothing similar to what I have. What would you suggest in such a case?

I appreciate your time and help in advance.

A: Your direct shear curve looks excellent. However, you will need to test out to about 100-mm deformation before getting to residual strength. Most people call it “large deformation” since true residual is beyond most device’s capabilities. That said, you mentioned that you were investigating the shear effects resulting from various HDPE thicknesses. I don’t think you will see much in the way of differences using a smooth sheet. More interesting would be to use textured HDPE by differing asperity heights and types of texturing. In so doing, the differences should be large, and an interesting story should result. Nevertheless, best wishes as you go forward.

Anchor trench for a geosynthetic pond liner

Q: Thank you very much for your assistance. While in a perfect world, the geosynthetic pond liner we are using may not see any tensile forces, why would you not design an appropriate anchor trench just in case? Also, I have concerns about not including a geosynthetic clay liner (GCL) in the anchor trench and the cited friction angles are on the high end of the range. I have typically seen interface shears in the 19- to 23-degree range, not the 30- to 32-degree range depicted in the calculations. Finally, the article of yours that they cite is for landfill veneer stability applications and not a reservoir or pond liner.

Any insight you can provide on our approach would be greatly appreciated.

A: Thanks for your question. The deep and narrow anchor trench as designed will certainly contain the liner system, as opposed to pulling it out. It is very conservative, for a landfill or pond, in this regard. Also, to fold all three components down and along the base of the anchor trench (and with HDPE) will be difficult to construct. Otherwise, it is up to the designer of record to decide on the degree of conservativeness. That said, I did not recalculate to this situation, but the paper cited has been used since its publication some years ago, and no one has challenged the methodology.

The difference between a landfill and pond is indeed something to consider. In the case of a pond, I would not expect tension stresses other than a thin veneer of cover soil (if present) and construction and maintenance equipment.

Hope this helps.

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