PHOTO 1: Separation in the center of a 2.0 mm thick HDPE sheet

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Beverley Jones
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1 SIP in Geomembrane Liners: An Acceptable Condition? by Mark E. Smith, PE, GE Separation in Plane, or SIP, is a phenomenon where a material (a geomembrane in this article) shears parallel or sub parallel to the surface. In multiple ply sheet, such as Hypalon and reinforced polypropylene, one form of SIP would be delamination of the individual plys. Thus, the term delamination is also used for this condition. Photo 1 illustrates the condition better than words. By far the most commonly used geomembrane in mining applications is polyethylene, usually in the form of high density (HDPE) or linear low density (LLDPE) resins. SIP is essentially non existent in LLDPE but does occasionally occur in HDPE. This paper speaks to SIP in HDPE. PHOTO 1: Separation in the center of a 2.0 mm thick HDPE sheet How is SIP Detected? SIP is usually discovered during testing of the construction seams, since these are regularly subjected to peel and shear testing for specification conformance. Thus, it is usually the responsibility of the construction quality assurance personnel to first identify SIP. Most HDPE specifications are silent on in plane separation and thus a very real problem arises of whether SIP performance is acceptable. Since subjective interpretation of construction specifications is what drives many claims and disputes, anticipating this condition in the specifications can avoid costly construction delays and contract disputes. It has been estimated that about 1% of HDPE production exhibits SIP performance, almost exclusively in 1.5 mm (60 mil) and thicker material. There is

2 essentially no information correlating long term performance of HDPE either problems or lack of problems with in plane separation. This lack of information is a key problem, since it is ultimately the long term performance that we are most interested in and least able to test directly, especially during the construction phase. What Causes SIP? High density polyethylene geomembrane resin is created by mixing or compounding pure HDPE, usually some lower density polyethylenes to improve stress crack resistance and plant production, carbon black and other additives to provide UV light resistance and enhance other factors, and in some cases recycled liner, commonly called regrind. Some plants use exclusively precompounded resins, which are formulated and blended at the resin plant and provided ready to use. Other plants use some pre-compounded resins and some plant blends, which allows them to optimize physical properties, plant production and pricing. Once compounded, the resin is heated to a molten state, or about 230 C (450 F), then extruded through either flat or circular dies. With a circular die the liquid resin is extruded through the die and forms a vertical tube. The tube is pulled upwards and allowed to chill by circulating air. With a flat die the resin is extruded onto a chilling roller. HDPE transitions from an amorphous to a semicrystalline plastic at about 125 C (260 F). Slower cooling produces a higher degree of crystallinity, higher density and higher tensile strength. Faster cooling has the opposite effect. So, what causes SIP? On this there is no consensus in the industry. The opinions tend to cluster into these general areas: 1. Rapid cooling of the geomembrane as it is extruded in the plant, creating a crystallinity gradient through the cross section of the sheet and giving the core a different modulus of elasticity, density and tensile strength than the outer skin. 2. Elevated ambient temperature in the plant. While this would retard differential cooling it also promotes slower overall cooling. Which, in turn, promotes higher density, higher modulus and higher tensile strength across the sheet. It is the higher modulus that could contribute to SIP behavior. 3. Improper dispersion or mixing of carbon black and other additives, or improper mixing of the resin batch in total. 4. All of the above could be contributing factors. The first two points are supported by the idea that SIP is more common in thicker sheet and, for at least one manufacturer, it is a problem that occurs only during 2

3 the peak summer temperatures. The third point is supported by specific cases of SIP which have been directly attributed to the use of plant-blended resins, and because SIP is much more common in sheet with carbon black than in clear sheet. The author tends towards the fourth point, all of the above, believing that it is a critical combination of all of these factors that create this uncommon phenomenon. PHOTO 2: 2.0 mm HDPE with skin-core separation Is SIP Acceptable Behavior? The voice of geosynthetcis consensus in the Americas is the Geosythetic Research Institute. Specifications are issued by the Geosynthetic Research Institute after considerable committee research and membership review. These specifications are adopted directly by some design engineers, referenced or amended by others, and ignored by yet others. The American Society for Testing and Materials is the leading organization producing material testing standards. They have been issuing geosynthetics specifications since the industry started to produce the material. ASTM specifications are almost universally used by design engineers and manufacturers in North America and are commonly used in the balance of the Americas. GRI recently released seam specifications which address SIP (GRI-GM-13, 17 and 18 {draft} for HDPE, LLDPE and polypropylene). ASTM recently issued Test Method D-6392 Standard Test Method for Determining the Integrity of Nonreinforced Geomembrane Seams Produced Using Thermo-fusion Methods. Both of these documents refer to SIP and consider in plane separation an 3

4 acceptable break pattern. That is to say that a seam which separates by SIP in the base sheet is an acceptable seam. The logic here is that, since a seam can be no better than the base sheet, seam tests which terminate by separation through the sheet are considered passing seams. This has caused some confusion in the industry and some manufacturers have used these standards to imply that SIP in the base sheet is also acceptable. That is definitely not what the ASTM and GRI standards say. The purpose of these standard specifications is to set standardized pass/fail criteria for field welded seams. They do not speak to the issue of SIP in the base sheet. Neither ASTM nor GRI have yet to tackle the issue of whether SIP constitutes acceptable behavior of the base sheet. There is a consensus that SIP behavior in a field seam does not constitute a failing seam the seam is essentially as good as the base sheet. There is no consensus as to what significance SIP has on the base sheet performance i.e., does SIP behavior suggest a long term problem that may shorten its useful life? A very good question, especially when the intended purpose is a buried application such as a leach pad or tailings pond. The major manufacturers are united in their position that SIP is acceptable behavior and have issued position papers supporting this position. However, in each of these position papers it is argued that, for example, SIP is not a seam failure. One manufacturer attributes the behavior as normal and caused by the skin cooling effect. Another claims that it is high ambient temperatures and a third traces specific instances to use of in-plant blended resins versus precompounded resins. None have performed sufficient long term performance testing to show, either way, if there is a potential problem and if so under what conditions should those problems be expected. One of the oldest and most serious problems with HDPE is high crystallinity. Early liner had problems with fatigue failure, seam cracking and embrittlement because of the high crystallinity. Modern geomembrane resin is formulated for a significantly lower crystallinity and lower density than sheet of 20 years ago and these modern formulations have mostly eliminated the stress cracking problems. But, if SIP suggested excessive crystallinity or an excessive crystallinity gradient, then perhaps SIP is an early warning sign of stress cracking a condition that can take years to manifest in the field. Alternatively, if SIP is related to mixing and dispersion of the additives, as evidence clearly supports, then SIP might be an early warning of inadequate mixing, incomplete dispersion or poorly compounded resin. Or a warning of a problematic combination of imperfect blending and excessive crystallinity. Poor blending of carbon black and other additives can significantly reduce the resistance to UV light as well as cause other problems. 4

5 It is the author s opinion that there is inadequate understanding of the science of SIP to simply accept it without further consideration. For critical installations where the geomembrane cannot be directly monitored and cannot reasonably be replaced, such as most leach pad and tailings pond installations, the decision to use a liner with SIP behavior needs to be made carefully by the design engineer. The engineer s responsibility to make this decision should not be abrogated in favor of standardized specifications (assuming that they are promulgated) which can not anticipate project specific performance demands. On the other hand, rejection of material during construction because of SIP can be problematic if the specifications do not anticipate this, and many do not. Thus, the decision should be made before the specifications are written and the construction contracts awarded and then the acceptable behavior clearly defined in the specifications. Mark Smith is a geotechnical engineer with over 20 years in mining geotechniques and worldwide geomembrane experience. He is co-founder of Vector Engineering, Inc. (Grass Valley, California, USA) and is currently the operations manager for their South American operations, working and living in Lima, Perú. He can be reached via at: smith@vectoreng.com. Published in: Perú Minero, VII-no. 39, Nov-Dic., 2001 y The Latin America Mining Record, V. 8, No. 4, Jul-Aug,

GSI White Paper #25. The Separation-In-Plane (SIP) Mode of Failure When Testing Polyolefin Geomembrane Seams

GSI White Paper #25. The Separation-In-Plane (SIP) Mode of Failure When Testing Polyolefin Geomembrane Seams Geosynthetic Institute 475 Kedron Avenue Folsom, PA 19033-1208 USA TEL (610) 522-8440 FAX (610) 522-8441 GEI GRI GSI GAI GCI GII GSI White Paper #25 The Separation-In-Plane (SIP) Mode of Failure When Testing