By far the most commonly used geomembrane in waste-disposal applications is polyethylene, usually in the form of high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE) resins.

By Mark E. Smith and Scott Elfen

Separation in plane (SIP) is a mode of tensile failure where a geomembrane shears parallel or subparallel to the surface. In a multiple-ply sheet, such as reinforced polypropylene, one form of SIP would be delamination of the individual plies.

The issue of in-plane separation of HDPE geomembranes has been discussed in literature (Smith 2001; Smith 2003). SIP is essentially nonexistent in LLDPE but occasionally occurs in HDPE. This article speaks only of SIP in HDPE, as a follow-up to the previously mentioned original publications.

Whatís New?

Recent testing on several large mining projects in South America shows a new trend: separation initiation at very high strains. In some cases, SIP does not manifest until uniaxial strains exceed 700%. In other cases, separation occurs at double-digit strains. Certainly these two behaviors constitute different levels of risk. SIP at a low strain is clearly a problem, since low-strain conditions are common in real-world installations and since a separated geomembrane does not have the same resistance to puncture, chemical attack, and weathering (not to mention to seam performance issues) as an intact sheet does. There is, however, an upper limit to the strains an installed geomembrane will see. If SIP first manifests comfortably above such a limit, is it a problem? Good question. We have no answers today.

Perhaps more important is the apparent increase in occurrence of this mode of failure. Earlier estimates put the occurrence rate at about 1% of total HDPE production. Information from the first half of 2003 suggests that the current rate is higher by perhaps more than 5%, at least in the Americas. Further, SIP might be even more common than the data suggest. Separation usually is detected as part of the normal quality-control process, where samples of base sheet or seams are tested in uniaxial tension, using either 6-millimeter-wide "dog bones" or 25-millimeter-wide uniform strips. In some cases, the testing laboratory does not report SIP behavior even when it is observed since it is not a specified test. Nor does any standardized test method or reporting procedure yet exist. There is also evidence that the narrow dog-bone specimens (used for tensile strength of the sheet) are less reliable in detecting SIP than the uniform strips (used for seam strength testing) are.

Is SIP Acceptable?

We still donít know what causes SIP behavior. The best current thinking is that it has to do with the resin and additive packages or the incomplete mixing thereof. It also might be related to the use of recycled or "regrind" material, especially if that material has a history of SIP behavior.

The Geosynthetic Research Institute (GRI) and the American Society for Testing and Materials (ASTM) both have standard test methods for geomembranes and their seams. Both consider SIP to be nonfailure of the seams. This is logical; the purpose of seam testing is to verify that the seam has (typically) a specified percentage of the base sheetís strength. If the base sheet exhibits SIP behavior, so too will the seams. Contrary to the claims of some manufacturers, however, this does not mean that SIP is an acceptable factor in the base sheet. Both GRI and ASTM remain silent on this issue, and very few publications are available on the topic.

Recommendations

There is not enough understanding of the science of SIP and its long-term implications to simply dismiss it out-of-hand. For critical installations, such as most landfill installations, where the geomembrane cannot be monitored directly and cannot be reasonably replaced, the decision to use a geomembrane liner with SIP behavior needs to be made carefully. On the other hand, rejection based on SIP can be problematic if the specifications do not anticipate this, and many do not. Recent data showing that in some cases SIP only occurs at very high strains complicate the matter further. One recommendation is to reject any geomembrane exhibiting SIP behavior unless it first manifests at strains high enough not to affect field performance. What constitutes "high enough" is a project-specific decision to be made by the design engineer. Furthermore, the manner in which SIP is to be handled should be decided before the specifications are written and the construction contracts are awarded.

References

Smith, M.E. "Separaciónen Planos, Una Condición Aceptable?" Peru Minero, November/December 2003.

Smith, M.E. "SIP in Leach Pad and Tailings Pond Liners: An Acceptable Condition?" The Latin America Mining Record, July/August 2001.

Mark Smith is vice president of South American operations for Vector Engineering Inc. Scott Elfen is engineering manager for Vector Perú SAC.

MSW - September/October 2004