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Features, News, Q&A: GMA Techline | August 1, 2017 | By:

Liner repair? Pullout tests? Sewn seams? Shear strength of sludges?

Geomembrane liner repair

Q: I have a facility that has submitted liner repair documentation for the repair of a high-density polyethylene (HDPE) liner for a leachate/stormwater pond at an industrial landfill site. The repairs were performed using either a patch or, for indentations and small holes, just a small bead of extrudate from the extrusion welder. The areas patched using the bead of extrudate all passed vacuum testing. It seems like this would be an adequate method of repair for small holes, but I wanted to verify if this was an acceptable/common practice. This was the first time I have come across this type of repair in my review. All previous repair work has been with a patch of similar geomembrane material. Thanks so much for your help.

A: You ask a dicey question and one that regulators, consultants, and quality assurance (QA) firms dance around regularly. Being none of the above, let me comment by separating small holes from indentations. For small holes (of size to be determined by the parties involved), grinding, welding a bead over the entire ground area, and vacuum box testing seems OK to me. Large holes, or groups of holes closely spaced, would require a cap strip over the entire area, extrusion fillet welded and vacuum box tested. I, perhaps naively, think an accommodation can
be made.

It is the indentations that give me concern, particularly with HDPE. If the deformation within the indentation area is deep, the strain can be large and, of course, stress comes along accordingly. If the resin itself does not have an adequately high stress-crack resistance, it could lead to a long-term hole in spite of the extrudate within it. With just a small bead of extrudate, it might not be acceptable. In the indentation situation, I would favor a quite generous cap-strip solution.

Bob Koerner  /  GMA Techline

Shear strength of sludges in disposal facilities

Q: I have been researching the shear strength of industrial sludges mixed with mechanically stabilized walls (MSW) for slope-stability analysis purposes. As I’m sure you know, there is plenty of literature on MSW shear strength, and some of the published in situ measurements likely included some sludge inherently. However, I cannot find literature that deals exclusively with industrial sludges. There is some literature on dewatered sludge from wastewater treatment, but I am more concerned about the industrial sludges, which are wetter. I’ve seen analyses that reduce the shear strength of MSW (phi=30 degrees, c=200 psf) by the proportion of estimated sludge (phi=0, c=100 psf, for example). Do you know of other approaches for accounting for sludge in waste strength?

A: There is indeed some literature on shear strength of MSWs. We will send you what
we have in this regard.

Now to the nasty question of its mixture with sludges. As you have found from your search, I also have nothing! Even reducing its intrinsic and dubious shear strength by a percentage of sludge is dicey in my opinion. My solution is to do (or commission) the laboratory test and see what results. (As an aside, we once did leachate saturated wood bark that was the culprit in a landfill failure, of which there is no available literature.) Be careful!

Bob Koerner / GMA Techline

Innovative application?

Q: I assigned a term paper this quarter in which I asked the students to come up with a novel or innovative application of geosynthetics. One student proposed a “preseeded and fertilized impregnated geotextile” for use with slope stabilization (an adaptation of old-fashioned jute-matting slope stabilization). Does GSI have any archival information on such an application?
Thank you.

A: The closest to what the student proposes is a geosynthetic clay liner (GCL) with the bentonite removed completely and replaced with a fertilized topsoil enclosed between the two geotextiles, needle-punched together.

I thought it was a great idea as “instant turf.” It was produced in Austria and rolled up like a GCL, only to find that the roll would not unroll in the field. It seemed that the roll bonded together and formed a massive
log instead!

Bob Koerner / GMA Techline

Compaction of marginal fills

Q: I have a question about compaction of marginal fills in reinforced slopes. When a contractor is using a fill material with a high clay content, they often want to use a sheepsfoot-type roller for compaction and as heavy a machine as possible. Most published guides I have seen advise not to use this type of roller on a geogrid-reinforced fill. Do you have any advice on whether this type of compaction is suitable on cohesive fills?

A: I fully agree that a sheepsfoot roller should not be used to compact fine-grained soils for MSE walls and slopes. The free boundary outside of the facing will likely bulge and go out of alignment. Damage or movement of the geogrid reinforcement is also likely.

Remember that a sheepsfoot roller compacts soil at depth by virtue of the penetrating feet. That said, a padded-foot roller might work, but I would like to see a test fill to be sure. A vibratory steel-wheel roller or rubber-tired roller is more customary and, with relatively thin lifts, should do the job.

Bob Koerner / GMA Techline

Pullout tests?

Q: Do you have any pullout tests on mechanically stabilized earth (MSE) wall geogrids within a sand mass and some tensile tests on the same geogrids themselves, without sand, that you could share with us? Alternatively, do you know where I could find that information? We need both types of results, as well as the geometric details to be able to complete the simulation mesh.

A: With the possible exception of direct shear testing, I think the pullout testing is the most common soil-to-geosynthetic testing that we know of. There are many theses and, indeed, there is a thriving business of such tests. As such, we at GSI have not done anything in this area since around 1985 or so. In short, it is a commodity business at this point.

Bob Koerner / GMA Techline

Landfill settlement

Q: One of our cap projects is experiencing some differential settlement that could potentially impact the cap system and, in particular, impede the drainage. The cost to repair the settled areas is more than what the owner is willing to pay, so our office is looking at cheaper fixes. One idea that came up was possibly using a foam-jacking firm to pump polyurethane foam under the HDPE, and try and lift this cover up and reduce the size of the depression. In theory, it sounds good, but in practice I’m not sure it would work, or could even be a good solution. Have you ever heard of this? If not, do you know of any other low-cost options?

A: Good to hear from you and your interesting possible solution to the differential settlement. In this regard, there have been many attempts at what was originally called “mud jacking” for slabs, pavements, etc. Materials generally used have been various soils and flowable grouts. As memory serves, it usually worked but many times only temporarily.

Grouting companies are well versed in this regard. I have not heard about the use of polyurethane foam, but it sounds interesting. I think that the permanence of the fix will be an issue, not only for the depressions, which are lifted, but for others not yet evident. So revisits might be an issue depending on the waste mass, its thickness, organics, and compaction.

Bob Koerner / GMA Techline

Geotextile sewn seams

Q: We have been discussing seaming of geotextiles because recently a question arose as to whether a single-prayer seam (one row of stitching) could be used instead of the double-prayer or single-J seams that we currently have in our specifications. Is a single-prayer seam better than a double-prayer seam? Does GSI have any recommendations supporting the use of a particular seam type? Any historical reasons for preferring one seam type over another?

A: Of the three styles (prayer, J, and butterfly), the prayer and J give approximately the same strength. The difference being that with the prayer
seam, failure can be in the geotextile (GT) on either side of the stitch, while with the
J, it is always in the GT that is not folded over on itself. The butterfly is stronger
since the folded-over geotextiles act as cushions for both GTs. Strength increases are 20–30% higher, but it is a more difficult seam to make in the field (i.e., an extra worker is needed.)

Regarding a single or double seam, there is no difference in strength since the double seam is a backup to the single one and they fail successively at the approximate strength of a single seam. The reason for making a double seam is to be sure there are no gaps in seaming by having the sewing yarn run off of the edges of the two GTs.

Bob Koerner / GMA Techline

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