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From the GMA Techline
Note: This is an appendix to the article, “GMA Techline’s 3rd 500 Q’s-and-A’s.”
Question 1
Q: I am trying to find a way to predict the size and shape of a gas bubble that might form under a geosynthetic liner in a pond lining system—specifically, the relationship between its vertical height and its width, and the variables that would be used to calculate it.
Any help you can give me would be appreciated.
A: What an unusual question and one in which I have no standard answer. I say this because the size of such bubbles varies from enormous to small. Regarding dimensions, the height will invariably be the depth of the water plus a hemispherical section above the water level. Regarding width, there is nothing consistent since the submerged shape will be like a golf ball on a tee ready to be driven. The water pressure will squeeze the geomembrane bubble as it grows and only when it is finally above the liquid level will it expand in an unconfined mode.
Question 2
Q: I saw your mention of “whales” or “hippos” in the recent article in Geosynthetics about shale-gas prospects. Whales in geomembranes could be a very interesting study.
Here is my hypothesis:
A significant-sized whale in a single-lined liquid waste impoundment is likely a sign that the geomembrane is doing little to prevent leakage from the impoundment. The whale is caused when liquid leaking through a geomembrane enables biological gas generation. The gas migrates to high spots such as wrinkles and relative maxima in the geomembrane. When sufficient leakage causes enough gas, the whale breaches the surface of the water. When a whale is present, the pressure under the geomembrane must be equal or greater than the pressure on top of the geomembrane. The pressure on top of the geomembrane is equal to the head of water. Therefore, the water under the geomembrane is under the same pressure as the water in the impoundment. Therefore, any leakage under the geomembrane is subject to the same or greater pressure as the head of water in the impoundment. That is the same condition (or worse) as an impoundment without a geomembrane.
Personnel performing geomembrane leak location surveys while wading can often feel a “waterbed” under a single geomembrane, which confirms that there is water as well as gas under the whales.
A drainage system under the geomembrane will help relieve the whales. But relieving the whales does not relieve the leakage.
A: There are numerous hypotheses as to the formation of whales, one being leakage as you suggest. Others are elevated temperature in the subgrade, decaying vegetative matter and rising water levels in the subgrade. Whatever, the necessity of an underdrain system is my focus and it is a design issue. To do something after whales occur is extremely difficult and costly… Thanks for your comment.
Question 3
Q: We are proposing to use a 1.5mm HDPE liner over a GCL for a waste emplacement cell into which we will place cement stabilized contaminated river sediments. The cell will have a drainage layer and leachate collection system.
The main contaminants in the sediments are polycyclic aromatic hydrocarbons. Total PAH concentrations are expected to be 3000 micro grams per liter, predominately naphthalene. Additionally individual organic chemicals detected include anthracene, benzo(a) pyrene, fluoranthene, benzene and total petroleum hydrocarbons (TPH) C6-C9, C10-C14, and C15-C28. Preliminary results from leachate test indicate that high concentrations of naphthalene could be present in the treated sediment and pH could exceed 12. Temperature of the treated sediments is expected to not exceed 30deg C. After placement of the treated sediments the cell will be covered using an LLDPE liner and soil layer approximately one meter thick.
I am finding it difficult to find definitive information relating to the chemical resistance of HDPE and am hoping that you may be able to direct me to appropriate sources. Any information that you are able to provide will be greatly appreciated.
A: Thanks for your e-mail in which you ask a relatively common question. About the only waste liquid which degrades HDPE geomembranes are hydrocarbons which constitutes some of your waste stream. For this specific reason the U.S. EPA created its 9090 test protocol which has been superseded by ASTM D5322 followed by ASTM D5747. The first is the incubation and the second for the actual testing. Thereafter one must assess the nature of the changes (if any) and the level of acceptance.
The actual answer to your question is ugly in that you have to do yourself or commission the test to be done by others. There are several commercial labs which do this type of testing. Look on our website at www.geosynthetic-institute.org in our lab accreditation section for such labs. I am afraid the literature is very lean in this regard since the results of such past testing are proprietary and simply not available. Even if they were, the liquid would probably be different than yours.
Question 4
Q: I am e-mailing you with a question regarding the issue of wick drain pinching. I am working on a relatively large project in Alberta, Canada, where we are anticipating installing approximately 800,000 linear meters (2.5 million linear feet) of wick drains.
A typical subsurface profile at the site would consist of an upper 10-foot thick soft Holocene clay layer (Su in the order of 200 to 300 psf) over 30 feet of medium stiff to stiff Pleistocene clay (Su generally varying between 500 to 1500 psf), which in turn is underlain by competent glacial till. The upper part of the Pleistocene clay is desiccated, which results in a sharp contrast of stiffness at the interface between the Holocene and Pleistocene clays (let’s say a Su of 200 psf versus 1000psf).
A tailings dam will be constructed in the future on top of this material, and as a result ground improvement was required to ensure stability of the 60 to 90 foot high starter dyke that will be constructed over a relatively short period of time. The wick drain solution was found to be the cheapest improvement method.
Given the contrast in stiffness at the interface between the two clay units, the concern that comes to mind is whether differential lateral deformations along the wick drains would cause the core of the wicks to pinch along that interface. While we have a good handle on the strength and deformation characteristics of the Pleistocene clay, we do not unfortunately have a very detailed characterization of the upper Holocene clay.
A: You ask a reasonable question and one in which I believe almost all of the wick drain shortening will indeed occur in the softer clay. Just how the wicks deform within the layer is very difficult to predict. The worst case scenario is if they kink (or crimp) in a short distance. We have tested about thirteen different wicks to assess their flow rate behavior before and after kinking. We used both a rounded and pointed kinking setup.
That said, the behavior of the wicks surprised us. We initially placed them in three categories; stiff, flexible and intermediate. We thought that the stiff ones would perform the poorest. As you will see, this is not the case and things are pretty well all over the place. It is indeed a difficult situation to predict.
Question 5
Q: How do you achieve a satisfactory extrusion weld between a 1mm LLDPE geomembrane and a 2mm HDPE geomembrane? What constitutes a satisfactory extrusion weld?
A: You have two things going against you in your situation; thickness differences and material differences. Regarding the thickness situation, 1mm is difficult not to overgrind in order to have the extrudate to be placed upon it.
You will need to put your best personnel on this task. It is difficult but possible. Regarding the material differences, LLDPE has a broader, and somewhat lower, melting window than HDPE, but they do overlap somewhat. In this regard, wedge welding of the two different resins is more difficult than extrusion welding.
I don’t think this is as much a problem as is dealing with the thinner sheet. Clearly many trial (or test) strips will be necessary on your project.
You also ask what would be an acceptable extrusion weld. If you go to our website at www.geosynthetic-institute.org under “specifications” and look at GRI-GM19 you will find that 1.5mm HDPE requires 525N in shear and 340N in peel. Conversely, the thinner and weaker 1.0mm LLDPE requires 263N in shear and 250N in peel. I am afraid that all you can ask for in this case is the lower values associated with the 1.0mm LLDPE.
Question 6
Q: Thank you for your prompt response. Our landfill project was brought to a standstill today by the CQA engineer. We are attempting to weld a textured 1 mm LLDPE cap to a textured 2mm HDPE basal liner at the crest of a cell slope using LLDPE welding granulate. Production weld samples are passing normal peel and shear destructive tests (according to me) because the 1mm liner is ribboning out and ultimately breaking and the weld remains intact (an SE 2 type break). The CQA engineer however is insisting on destructively testing the bond between the LLDPE extradite and the HDPE liner. The resulting extradite to liner destructive peel test is resulting in an AD1 adhesion failure so he is then determining that the weld failed and must be repaired. We have argued all day but he is sticking to his opinion.
What do you think? Are there any industry guidelines relating to the welding and testing of HDPE to LLDPE connections? Would it be better to use HDPE welding granulate?
A: There are no criteria to my knowledge for welding dissimilar geomembranes of any type including LLDPE to HDPE. This is what I wrote to you some days ago and the best you can probably get is the average of the two values for strength. Extrudate-wise, I would certainly try a HDPE rod and I think it would have been my first choice. The temperature control will be critical and get it just high enough to melt the rod but not the LLDPE.
Follow-up Comment to Answer: Thank you for your help. We switched from using LLDPE welding granulate to HDPE welding rod and the test samples are now passing—Thank God!—and everybody is happy!
Question 7
Q: We are approximately 70% complete on capping a waste rock stockpile area (220,000m2), the earthworks contractor has begun placing the 700mm thick layer of cover material (silty sand with gravel site material) over the liner system which consists of the 60mil liner then a one sided composite geonet/geotextile (textile on top), the maximum slope over the stock pile area is 5%. We are now noticing that some wrinkles are present within the liner system or being generated into the liner system due to the soil cover placement. There seems to be three different conditions being observed:
- The geocomposite has some slight to significant wrinkles (upwards of 200mm high), with minimal wrinkles in the HDPE liner.
- The geocomposite has some slight to significant wrinkles with virtually no HDPE liner wrinkles.
- The geocomposite has some slight to significant wrinkles with matching wrinkle size in the HDPE liner.
Most of the conditions on site are the first condition noted above. A large portion of the wrinkles seem to be a cause of the soil cover placement operations, we are now keeping a closer eye on the soil placement and trying to guide the machine operators to help avoid causing wrinkles to form. Some of the wrinkles are forming from thermal expansion of the materials, as the HDPE is in a fairly flat condition in the morning, but wrinkles form through-out the day as the ambient temperature rises.
My question to you is, is there a point at which the wrinkle height of the HDPE liner is considered to be too high and remedial work should be done (cutting of liner to remove the wrinkle?) I would assume if the liner is able to fold over onto itself it could affect the integrity of the liner system. I’m hoping you have time to reply with some suggestions as to how I should approach this problem; any assistance you could give me would be greatly appreciated.
A: Fair question, but only subjective answers. Let me explain.
- HDPE wrinkles or waves of even the slightest amount if backfilled will never flatten out. We have experimented with all height of waves and even the smallest of 15mm did not flatten over time. If you backfill them they are there to stay.
- The wave forms change as they are backfilled. They go from nice sine-shaped forms when exposed to flattened forms when backfilled which mobilizes stresses where the curvature is abrupt. At the top (180deg fold) the stresses are the highest while at the two 90 deg folds they are somewhat less.
- The worst case is when the flattened waves bend over (as you mention) and then you end up with two 180 deg folds.
Years ago, I had written that waves were OK as long as the wave height-to-width at the base was less than 50%. Having done the above study (actually the dissertation of Te-Yang Soong), I cannot make such a recommendation. The waves simply have to go!
Question 8
Q: Regarding geotextile filters and landfill design, the seemingly un-measurable biofouling issue may be real for landfill subdrains and leachate collection layers. It appears that flow rate and permittivity are the two properties that may fall prey to this. Do designers just add safety factors of 2, 5 or 10 etc. to the computed required values?
A: Fair question and if you go to our website at www.geosynthetic-institute.org under the item of White Papers, look over Item #4; “Reduction Factors for GS Design.” There you will see how we reduce the ultimate values to allowable ones. The reductions for hydraulic design on flow rate and permittivity are extremely high. Regarding the “load side” of the equation, I would always use very conservative, i.e., worse case values.
Question 9
Q: Most of the tests based on ASTM geotextiles provide for conditions of temperature and specific humidity. Do these parameters greatly influence the test results if you change or vary them?
A: I believe that all of the standard test methods require controlled temperature and humidity during conducting the various geosynthetic tests. That said, temperature is very important in the various physical, mechanical and hydraulic tests and must be controlled within the tolerances given in the standard. Humidity control, in my opinion, is less critical and for hydraulic testing is not even a factor. Hope this helps.
Question 10
Q: I have a cow packing plant client who has a large brine lagoon with some holes in the liner (60-mil HDPE, 15-20 years old). They used a leak detection company from the USA that has determined the locations of the holes. They do not have any place to get rid of the brine solids or liquid so the plan is to fix the holes with the solids/liquid still there. They do not want to put in a cofferdam as they don’t think that there is enough other space available to pump the solids and liquids to. So the plan is to:
- Dredge the solids from the north side of the lagoon to the south end of the lagoon. The objective is to get as close to the liner as possible and clean it. We have mounted a squeegee on the bottom of our cutter head so that we can get right to the bottom without causing damage to the HDPE liner.
- The next step is to lower a ¼-in.- thick steel cylinder 7ft in diameter and 10ft in height directly over the breach.
- The brine solution would be pumped out from inside the cylinder so that a dry spot can be established so the repair can be performed.
- Once the repair is completed, the cylinder is flooded to reduce the pressure so the cylinder can be lifted out without sucking up the liner with it.
The obvious problem is: how do we obtain a seal with the bottom of the cylinder and the liner? We are talking to a rubber company about some of the products they have that we can bond onto the bottom of the cylinder. I am also thinking that we would line the inside of the cylinder base perimeter with an absorbent boom to control small leaks that could come under the cylinder. We do not know exactly how consistent the lagoon floor is and what is the quality of the 60-mil HDPE liner or if there are any folds, etc. In other words we’re flying blind with what the bottom is really like. We also don’t know the quality of the cell construction prior to the HDPE first being put down (i.e. soft vs. hard).
The questions I have are:
- Do you think this concept has a chance of working?
- How wide should the brim be on this cylinder to distribute some of the force over a larger area?
- How much pressure do you think the HDPE liner can take before our cylinder would cause it to make a hole in the liner?
- Can you think of a product that would help us obtain a seal between the cylinder and the HDPE?
- What else am I not considering?
A: You certainly do have an interesting project. Unfortunately, you have eliminated the cofferdam option which I am told has been successfully used by others. That said, your cylinder option does sound “doable” but not without difficulty. I would suggest a flange around the bottom circumference of the cylinder in order to reduce the point stresses and also to affix a thick and flat rubber cushioning gasket. You are clearly on the right tack in this regard.
My main concern is over possible folds in the geomembrane. They will be problematic as far as leakage and further damage to the liner itself. If the stress crack resistance is poor to begin with or has decreased over time, you may induce cracking along the folds.
For all of the above, it might help to put plugs in the holes rather than try to weld patches over them. This will allow less than perfectly dry surfaces to work on. There are several variations available in this regard. Hope my comments help and best wishes on a most difficult task.
Question 11
Q: Can a geotextile under a geomembrane really convey some gas and preclude the formation of “whales”? Are gas and water under membrane really only a concern before material is place above the liner?
P.S. – We have no reason to anticipate much gas at this site, which is a storage pond for ash. Not a landfill or WWTP; not near a swamp; dikes constructed of sand/gravel with clay liner on the inside; bottom right at GW table. Ultimately we felt we would be okay with the 8oz/yd2 geotextile. Although we’ve since proposed adding one panel of composite at each of the eight vent locations (we’re filling the anchor trench with native sand, sealed by clay that is penetrated by vent pipes at eight strategic locations, spaced roughly 500ft apart.
A: While needlepunched nonwoven geotextiles are generally tested for their water transmissivity, they are excellent in their gas transmissivity. I had a student do such testing years ago and he found that the gas transmissivity was about 1,000 times greater than with water. Even for a partly saturated geotextile, the gas gets through by a phenomenon called “permselectivity.”
To use a geotextile under a geomembrane to prevent so-called whales is an excellent strategy. In fact, in the next edition of “Designing with Geosynthetics,” I will make it the recommended strategy for all ponds and surface impoundments.
Question 12
Q: What, if anything, do wall contractors typically do to attach their wall facing to concrete culverts that intersect the face of an MSE wall? In this instance, we’re dealing with rock-filled baskets on the face.
A: You ask a very contentious question. To answer you directly, the pipe simply juts through the facing in this case a gabion basket system. I say contentious because I would never bring drainage through the reinforced soil zone of a MSE retaining wall. We have so many failures of walls from leakage of these pipes and inlets that it is a tragedy. Please don’t even think about doing what you propose…Definitely not recommended.
Question 13
Q: I have been working on a problem of surface flaking of a fPP floating cover installed on several reservoirs by a water company. I believe some photos have been provided to you. I have a copy of some information you presented at a recent ASTM workshop which was most interesting as the mode of failure was virtually identical to the problem we have.
I was wondering if you know what disinfectant was being used in your case history? The disinfectant used in our situation was initially chlorination which then changed to chloramination which is known to be more aggressive to some polymers and rubber materials.
I also note in your discussion there was the thought that freeze/thaw may have contributed to the release of the flake. While I agree, this could exacerbate the release there is no such freeze/thaw with the installations but there is still significant release of the flake material.
A: e do not know the precise chemicals used and the owner is reluctant to discuss the situation. Also, the freeze-thaw hypothesis is somewhat questionable since the flaking was also in the bottom liner which does not freeze. We are still evaluating this situation but have no clear explanation at this time. Sorry in this regard.
Question 14
Q: We have a dirt area at our Recycling Center that large trucks drive on, and during normal conditions, the soil is stable. However, problems arise after heavy rains and it becomes difficult for the truck to drive on.
So, we are looking for a geomat that we can roll out for reinforcement only when needed, and then roll it back up and store it when the ground dries out. A geogrid is not an option because we are looking for something that does not need additional material or aggregate put on top of it. My boss mentioned that 10-15 years ago they used to use a metal mesh type mat (sorry I do not have information), and they are looking for something of a similar nature, that can just be used as needed and stored the rest of the time.
A: Your boss has a very good memory. Congratulate him/her for me and watch out yourself in this regard!!!
In the first edition of “Designing with Geosynthetics (1984),” I mention a steel strand reinforced high strength fabric manufactured by Robusta of Holland under the trade name of “Mommothmat.” While various tensile strengths can be manufactured, one of them developed 4500 lb/in. (800 kN/m) by 11,200 lb/in. (2000 kN/m). I wrote, “This particular fabric is capable of being used in stabilization of poor soils without the need of any aggregate surfacing layer. It has been used as such for military construction and for oil exploration projects over soft clay and peat.”
The above said, it is probably quite expensive and I don’t think you can seam the edges and ends, so to prevent the soft soil from squeezing through you probably have to put a light weight fabric beneath it.
Thanks for asking, it was fun to look back in time…
Question 15
Q: We have been contacted by an engineer regarding geomembrane compatibility with Marcellus-related, hydrocarbon-impacted soils. Of course, we’re not geomembrane guys, so we couldn’t comment. The engineer sounds like he’s in a pinch, so we thought of asking you.
Analytical data is shown in the attached table. Total Petroleum Hydrocarbons (TPH) ranges from <170 mg/kg to 65,000 mg/kg. One sample was tested for only Diesel Range Organics (DRO), with a result of 118,000 mg/kg.
It is our understanding that TPH is the sum of three different ranges: Gasoline Range Organics (GRO), Diesel-Range Organics (DRO), and Motor Oil-Range Organics (ORO), and that each of these groups covers hydrocarbons mixtures within a certain range of carbon atoms [i.e., GRO (C6-C12), DRO (c10-C20), and ORO (C20-C28)].
What are your thoughts about the compatibility of polyethylene geomembranes with this waste?
A: I am following the brine back-wash from the Marcellus shale operations quite closely. The brine is quite brackish being about five times the level found in seawater. It, per se, is no problem for geomembranes. It also contains low levels of many heavy metals which are also of no concern.
The diesel, oil and gasoline are another matter altogether. As a group hydrocarbons cause swelling of geomembranes and can lead to eventual degradation depending on the type and level and exposure duration. I cannot comment on the data provided and would resort to laboratory incubation and subsequent testing. ASTM is set up nicely insofar as the protocol is concerned. The tests are D5322 for incubation followed by D5747 for geomembranes.