The GMA Techline

Vegetation causing gases under geomembrane?

Q: I have a site where we are installing a high-density polyethylene (HDPE) liner in an existing pond. Because of some residual heavy metals in the top layer of soil, I would like to strip off the top 3–6 inches (7.6–15.2 cm) of soil and vegetation. Alternatively, we could mow and remove vegetation, disk, grade and compact the pond surface. We will install a geotextile under the liner, have vents at the top of slope above the water surface and a slope of 0.006 ft/ft on the bottom of the pond. Do you think we would have a problem with gases forming under the liner due to not fully removing the vegetation?

A: There are at least two reasons why you should not have grasses, or other vegetative growth, beneath the geomembrane (and its underlying geotextile). The first is that there is no light of any kind and the second is that it is in an oxygen-starved atmosphere. In the latter regard, you might get some off-gases from the existing grasses but the geotextile should allow for the escape (it’s carbon dioxide and methane) through the vents at the top of the slope. Once such degradation is complete, there should be no issue remaining.

Direct shear testing of geogrids

Q: We are investigating the behavior of geogrids in interaction with soil through triaxial shear tests. According to your experience, could you recommend some topics that could be of interest regarding the behavior of the geogrids?
A proposal that I have is to evaluate the arrangement of the geogrids, two geogrids in a triaxial cell; could it be of interest in the analysis of reinforced walls? Thanks in advance.

A: Thank you for reaching out; however, you have a topic about which we have concern. The triaxial cell must be extremely large (make that “huge”) so as to eliminate scale effects of the geogrid to the soil. Even further, the “dead zones” top and bottom of the test setup are troublesome insofar as interpretation is concerned. Far better, in my opinion, is to perform direct shear tests, which simulate how geosynthetics generally perform in practice. Research is even available on multiple geosynthetic layers tested as a composite and certainly could include soil layers (albeit quite thin ones). In this regard, the variables are enormous; however, I think that the data will be much better received than with triaxial testing data.

Installing liners in cold temperatures

Q: Good afternoon. I have several questions regarding fabric-formed concrete and liner installation during winter months. This is for a project where the frost line will be about 2 feet (0.6 m) below the ground surface, and the material on-site is a sandy soil. Please let me know if there are any additional precautions, criteria or recommendations for installing liners during winter months.

1. Can a liner be installed on low temperature/frozen subgrade? Can a liner be welded in low temperature/winter conditions?
2. Should additional trial welds and field destruct samples be taken if welding in winter conditions? If so, how many additional samples would you recommend taking?
3. Can fabric-formed grout be placed in low temperature/winter conditions? If so, would the mix design of the grout need to be changed?
4. Can fabric-formed concrete be installed on top of the liner if the subgrade has frozen?

A: Thanks for your questions and here are our answers:

1. A liner can be installed in freezing conditions but with caveats. The welding device should be enclosed in a movable tent. See GSI-GM9, “Practice for Cold Weather Seaming of Geomembranes.” Wind is a major issue as well as installers’ performance under such conditions. Snow and other moisture are both detrimental to quality seaming.
2. Additional trial welds are definitely recommended—at least hourly and tested immediately on-site. Additional production weld samples will be dependent on the results of the trial seams.
3. All cement mixtures are sensitive to freezing conditions. When necessary, additives are included to protect the mix until proper setup time is realized. Contact the concrete supplier in this regard.
4. The subgrade can be frozen but not rutted. Remember that moisture in the area to be welded is disastrous to good quality. That said, the construction crew must be really good at their art.

Pipe penetration through geomembranes

Q: Please let me know if the Geosynthetic Institute (GSI) has a specification guide or white paper that covers the subject of pipe penetrations in HDPE geomembranes. I sincerely appreciate your assistance.

A: The answer to your question is, “We have nothing written on the topic whatsoever.” That said, we are remiss in this important topic. George and I will get together and see what we can do in the future. There are several important papers in the literature, e.g., Wells (1993) and Thiel and De Jarnett (2009), among others. Contact us if interested in copies. I should also mention that there are great sketches and drawings by several manufacturers on the internet. Just google “pipe penetration through geomembranes.” Bear in mind that such sketches are easier to draw than to make in the field.

On pressure

Q: We are investigating the design of a landfill where the side slopes will be built into an existing quarry, made up of 32-foot (10-m) high benches at a slope angle of 4H to 1H, i.e., 76˚.

On the floor, the protection geotextile will be designed to carry the normal load on the base, calculated by multiplying the unit weight by the expected waste depth. The question has come up, on the steep side slopes, if the normal load on the base is still the correct load to use when looking at the design of the protection geotextile, or if this should instead be considered as a passive pressure (normal pressure multiplied by a coefficient less than 1.0)? If it is, what would be a sensible earth pressure coefficient for waste? Thanks again.

A: Let’s go back to basics—in this case, back to lateral pressures on walls. In backfilling a wall, it is assumed that the wall will move away from the backfill, thereby decreasing the lateral earth pressure into its active state. Therefore, Ka would be used in calculating the lateral earth pressure. At the same time (it actually takes more movement) if soil is against the toe of the wall, it increases the lateral earth pressure into its passive state. Therefore, Kp would be used in calculating the earth pressure.

That said, I think that you have neither condition since the rock “wall” of the quarry just sits there as it probably has for decades. Ka is too low and Kp is too high; however, Ko (I feel) is appropriate. If you remember your soil mechanics, the Jaky formula is Ko = 1 – sin (Φ) where phi is the friction angle of the waste material, which is the computational variable. Also important to remember is that the force vector is not normal to the face of the rock but inclined at a downward sloping angle (Φ) due to waste settlement over time. You might recall that Terzaghi said many years ago to use 2/3 (Φ) as an estimate—the maximum angle being (Φ).

Hope this helps, but remember it’s strictly an opinion.

Testing textured geomembranes

Q: Could I please ask you some questions regarding textured geomembranes?

1. If the (embossed) structure is formed by different elements (both on one and the other side of the geomembrane), how shall the samples for the tensile tests be taken?
2. How shall the results be generally expressed if the results for the break elongation range from 100% to 1,000%?
3. Shall all the samples taken for the tensile tests (or the tear resistance/or the puncture resistance) be the same?

Thank you very much in advance for your comments.

A: Nice to hear from you. The following are in regard to your email questions:

1. One measures both one-sided and two-sided textured geomembrane via ASTM D5994. We do not recommend using a smooth platen or pressure foot when one of the sides is smooth. That yields usually high results particularly with stiff material.
2. Report the results as generated per the ASTM D6693 method. Please note GRI-GM13 specification values for break elongation are very different for smooth versus textured geomembranes.
3. One needs to make a very clear distinction between a roll, sample, coupon and specimen. The sample for geomembranes is typically 1 meter wide by roll width. From this sample, coupons are taken evenly spaced along the width, per the ASTM or ISO method. Specimens are then cut out of the coupon in both machine and cross machine directions, etc. Specimen sizes for the various test methods are clearly stipulated in the standard being used.

Hope this helps.